The object behind the echo: dolphins (Tursiops truncatus)...

Behavioural Processes 58 (2002) 1–26

The object behind the echo: dolphins (Tursiops truncatus)perceive object shape globally through echolocation

Adam A. Pack a,b,c,*, Louis M. Herman a,b,c, Matthias Hoffmann-Kuhnt a,d,Brian K. Branstetter a,b

a Kewalo Basin Marine Mammal Laboratory, 1129 Ala Moana Boule�ard, Honolulu, HI 96814, USAb Psychology Department, Uni�ersity of Hawaii, Honolulu, HI, USA

c The Dolphin Institute, 420 Ward A�enue, Suite 212, Honolulu, HI 96814, USAd Institut fur Verhaltensbiologie, Freie Uni�ersitat Berlin, Haderslebener Str. 9, 12163 Berlin, Germany

Received 6 June 2001; received in revised form 8 October 2001; accepted 15 October 2001

Abstract

Two experiments tested a bottlenosed dolphin’s ability to match objects across echolocation and vision. Matchingwas tested from echolocation sample to visual alternatives (E–V) and from visual sample to echolocation alternatives(V–E). In Experiment 1, the dolphin chose a match from among three-alternative objects that differed in overall(global) shape, but shared several ‘local’ features with the sample. The dolphin conducted a right-to-left serialnonexhaustive search among the alternatives, stopping when a match was encountered. It matched correctly on 93%of V–E trials and on 99% of E–V trials with completely novel combinations of objects despite the presence of manyoverlapping features. In Experiment 2, a fourth alternative was added in the form of a paddle that the dolphin couldpress if it decided that none of the three-alternatives matched the sample. When a match was present, the dolphinselected it on 94% of V–E trials and 95% of E–V trials. When a match was absent, the dolphin pressed the paddleon 74% and 76%, respectively, of V–E and E–V trials. The approximate 25% error rate, which consisted of a choiceof one of the three non-matching alternatives in lieu of the paddle press, increased from right to center to leftalternative object, reflecting successively later times in the dolphin’s search path. A weakening in memory for thesample seemed the most likely cause of this error pattern. Overall, the results gave strong support to the hypothesisthat the echolocating dolphin represents an object by its global appearance rather than by local features. © 2002Elsevier Science B.V. All rights reserved.

Keywords: Dolphin; Echolocation; Cross-modal matching; Global shape perception; Representation

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1. Introduction

What does a dolphin perceive when it inspectsan object through echolocation? Does it perceiveonly the raw acoustic characteristics of the echoes(e.g. amplitude, frequency spectra), and subse-

* Corresponding author. Tel.: +1-808-591-2121; fax: +1-808-597-8572.

E-mail address: pack@hawaii.edu (A.A. Pack).

PII: S0376-6357(01)00200-5

A.A. Pack et al. / Beha�ioural Processes 58 (2002) 1–262

quently relate those through associative learningto some visually perceived object? Alternatively,does it perhaps perceive only some particularlocal feature, such as the presence of an openarea or of a protruding arm? Or, does it directlyperceive the object globally, as an integratedwhole? This is the problem of object recognition,how objects are perceived and how they are rep-resented in memory. This problem has been stud-ied extensively within the field of visualperception (e.g. Tarr and Bulthoff, 1998). Hu-mans, and likely many other animals, have theremarkable ability to recognize objects visuallyafter a single glance, based on their shape (Bie-derman, 1995). Some examples from research onanimal visual perception include findings that pi-geons are sensitive to the spatial configuration ofa picture’s features (Wasserman et al., 1993;Kirkpatrick-Steger et al., 1998; cf. Cerella, 1980),and that chimpanzees (as well as humans) dis-criminate visually between complex figures by at-tending to their outer contours (i.e. their overallshape) (Tomonaga and Matsuzawa, 1992; seealso Fagot and Tomonaga, 1999 for global prece-dence effects in these species). Clearly, directshape perception characterizes much of visualperception.

Of course, the question of the basis of objectperception is germane to any sensory system ca-pable of shape perception. Recent cross-modalmatching-to-sample studies at our laboratoryhave suggested that dolphins can directly perceiveobject shape through echolocation. These studies(Pack and Herman, 1995; Herman et al., 1998)demonstrated that a bottlenosed dolphin (Tur-siops truncatus) could spontaneously (i.e. on thefirst trial, without reliance on associative learn-ing) recognize an object through echolocationalone that it had inspected previously throughvision alone. The reverse was also true. Perfor-mance was at or near ceiling levels in both direc-tions, indicating a high degree of integration ofthe percepts arising from these two senses. Pop-per et al. (1997) argued that ‘raw’ acoustic cuespresent in the echoes returning from targets in-spected by dolphins through echolocation (suchas, highlight structure, echo spectrum, and targetstrength) were insufficient to explain the cross-

modal matching results reported by Pack andHerman (1995). However, raw acoustic cues, inconjunction with the learning of associations be-tween these cues and reward contingencies, haveoften been invoked to explain dolphin capabili-ties for object discrimination through echoloca-tion (e.g., see summaries in Au, 1993). Incontrast, the data from our cross-modal match-ing studies support the hypothesis that the dol-phin is capable of directly forming ‘object-based’representations through echolocation (i.e. repre-sentations having characteristics of objects) with-out the necessity for associative learning (Packand Herman, 1995; Herman et al., 1998; Pack etal., in press).

In these prior cross-modal studies, the dolphinchose between two-alternative objects, one ofwhich matched the ‘sample’ object inspected pre-viously. Theoretically, the dolphin could haveperceived the sample at different levels of com-plexity, ranging from attention to a single localfeature to apprehending the object’s overall(global) shape. In the present studies, to deter-mine whether the dolphin represents an object byits global shape, or attends primarily to localfeatures, we examine its matching accuracy andits search behavior among objects that differ inshape but share particular local features. Supposethat following sample exposure the dolphin fo-cuses on a local feature. If so, either of the twosearch behaviors might result. One would becharacterized by considerable vacillation beforechoosing, as the dolphin goes back and forthbetween the two-alternatives, each of which con-tains the key local feature. Alternatively, the dol-phin might simply choose the first object itencountered in its search path that contained thekey feature. In either case, over a large numberof trials, matching accuracy would be at chancelevels, assuming that the location of the correctalternative along the dolphin’s search path is arandom variable. However, if the dolphin repre-sented the sample by its global shape, and assum-ing the objects are clearly perceptually different,there should be little vacillation, and perfor-mance should be at levels well above chance.

In the new cross-modal matching studies pre-sented here, we contrasted objects sharing ‘local

A.A. Pack et al. / Beha�ioural Processes 58 (2002) 1–26 3

class features’ to determine whether the dolphinsearched among the alternative objects for a lo-cal feature, or features, present in the sample, orsearched for a global match. A class feature wasdefined as a general characteristic common to atleast two different objects. For example, an ‘L’and a ‘l’ share the same class feature ‘verticallinear orientation’. A circle and a square sharetwo class features, ‘symmetry’ and ‘closure.’ Sev-eral previous studies have demonstrated thatdolphins can be taught to attend to specific classfeatures such as symmetry (von Fersen et al.,1992) linear orientation (Hoffmann-Kuhnt,1994) and closure (Herman et al., 1984). De-pending on the degree to which the dolphin’srepresentation of the sample in the cross-modaltask preserves information about its global spa-tial structure, the dolphin may or may not con-fuse objects of different shape but withoverlapping class features.

In our previous cross-modal studies, matchingperformance was well above chance levels oninitial trials with novel objects, and vacillationwas extremely rare. However, the use of onlytwo-alternatives limited any detailed investiga-tion of the dolphin’s search strategy, and its useof local features versus global shape. Theoreti-cally, the dolphin could make an A/not A typeof judgment (i.e. if the first alternative does notmatch the sample, the second one must). Exami-nation of the second alternative was not neces-sary. In the current cross-modal study, weinstead used three-alternati�e objects taken froma set with overlapping class features (Experiment1), or three-alternative objects plus a fourth pos-sibility that we called, ‘none of the above’ (Ex-periment 2). With three-alternative objectspresent, if the first alternative is rejected, thedolphin must examine at least one more alterna-tive. It cannot simply assume the next object tobe correct. This procedure can help reveal thedolphin’s search strategy (for example, randomversus systematic, partial vs. exhaustive). Inas-much as the local features of the non-matchingalternatives overlap with those of the sample,the three-alternative procedure can also help todetermine the dolphin’s reliance on local fea-tures versus global shape. The latter should re-

sult in high matching accuracy, while theformer, in a substantial number of errors byselection of non-matching objects when they areencountered prior to the match. The logic ofadding the fourth ‘null’ possibility rests on theassumption that if the dolphin were searchingfor a local feature or features, it should find itor them, in one or more of the offered alterna-tives and select that alternative even though nei-ther it nor the other alternatives are an exactmatch for the sample. On the other hand, if thedolphin were searching for a particular globalshape, the mere presence of particular local fea-tures should not be sufficient for response. Thedolphin should, therefore, reject all alternativeswhen no match is present, choosing instead‘none of the above’. In effect then, when noalternative matches the sample object, the dol-phin must examine all three to conclude reliablythat no match is present. Its search strategy canthen be delineated in detail, and conclusionsabout reliance on local features versus globalshape can be made more definitively.

In Experiment 1, we developed the three-alter-native cross-modal procedures and, after train-ing, tested the dolphin with objects that differedin global shape but had overlapping class fea-tures. In Experiment 2, we extended the cross-modal matching task to the fourth possibility,allowing the dolphin to press a paddle if it de-cided that a match was not present among anyof the three-alternatives. For these tests, we usedeight objects of abstract shape, all constructedof the same polyvinyl chloride (PVC) materials.The eight had been experienced previously bythe dolphin (Herman et al., 1998), but only asfour fixed pairs (i.e. only four pairings weretested out of the possible 28 that can be con-structed among eight objects). The dolphin haddemonstrated that it could match these shapes,as well as others (Pack and Herman, 1995), sig-nificantly above chance on the first trial that itencountered new objects, and hence without re-liance on associative learning. For the currenttests, all three-way combinations of objects werenovel to the dolphin, having never been encoun-tered previously.

Fig. 1. Computer schematic (not to scale) of an exploded view of the anechoic box used for presenting sample objects to thedolphin’s echolocation sense in echoic–visual (E–V) matching. The box measures 1.1 m wide×0.6 m deep×1.0 m high, and iscomposed of a PVC frame with angled redwood slats attached to each side and the back. The front Plexiglas panel is partially opento reveal an object suspended inside from monofilament line. The bottom of the box is covered by a thin sheet of PVC with a smallopening at the back to allow seawater to fill the box when suspended from the tank wall. Two of the three opaque screens that arenormally attached to the sides and front of the top half of the box have been retracted to reveal the interior. The mirror used toview the dolphin touching the Plexiglas panel is attached to the top of the box. Anechoic boxes used in visual–echoic (V–E)matching are similar in construction to the box used in E–V, except that no mirror is present, and a response paddle is attachedto the side of the box.

2. General method

2.1. Subject

The subject throughout was an 11-year-old fe-male bottlenosed dolphin (T. truncatus) namedElele. She was housed with three other dolphins intwo interconnected seawater pools (each 15.2 m indiameter and 1.8 m in depth) at the Kewalo BasinMarine Mammal Laboratory (KBMML) in Hon-olulu, HI, USA. All matching tests were con-ducted in one of these pools. Elele was fedapproximately 9.1 kg of a combination of smelt,herring, sardines and capelin daily, a normal di-etary intake. A portion of her feed was availableduring test sessions. Elele was the subject in theprior referenced cross-modal studies (Herman andPack, 1992; Pack and Herman, 1995; Herman et

al., 1998). She was, therefore, highly experiencedin two-alternative echolocation-to-vision (E–V)matching-to-sample (MTS) and in vision-to-echolocation (V–E) MTS, as well as in within-modal visual (V-V) MTS and echoic (E-E) MTS.

2.2. Apparatus

2.2.1. Exposing objects for echolocation inspectionalone

As in our previous studies (e.g. Pack and Her-man, 1995), we exposed objects to the dolphin’secholocation sense alone by presenting themwithin customized sound attenuation (anechoic)boxes. Fig. 1 shows a schematic of an ‘exploded’view of an anechoic box. The sides and back ofthe box consist of a series of angled redwood slatsthat when soaked in water disperse and attenuate

echoes. The critical feature of the box is itsfront panel, a visually opaque but acousticallytransparent 0.32 cm-thick black Plexiglas sheet(shown partially open in Fig. 1). In water, thethin Plexiglas sheet transmits sound well andconsequently, allows objects suspended withinthe box to be inspected through echolocation.The dolphin’s echolocation signals reflect off theobject suspended in the water column inside thebox and return as echoes.

Depending on the particular matching task ineffect (e.g. E–V or V–E), one or more anechoicboxes were suspended inside the dolphin’s poolfrom the wall and filled with seawater throughan opening at the bottom. Objects were sus-pended underwater within the box from thinmonofilament lines. Black plywood blinders at-tached to the front and sides of the box (twoare shown retracted in Fig. 1) prevented the dol-phin from seeing objects as they were placed inthe box. The box used to expose a sample ob-ject (Fig. 1) was fitted with a mirror at its topto allow an assistant located behind the box tosee the dolphin’s rostrum (beak) when it wasdirectly in front of the Plexiglas sheet. Each boxthat was used to expose an alternative objectwas fitted with an underwater response paddlethat when pressed deflected an attached arm.The arm extended in air and ended in a redbuoy, making deflections highly visible from thesurface.

2.2.2. Exposing objects for �isual inspection aloneObjects (either the sample or the alternatives)

to be presented strictly to the dolphin’s visualsense were held in air, a medium in which dol-phin echolocation is ineffective (Pack and Her-man, 1995). Objects were held approximately 30cm above the water surface by assistants donnedin white shirts (to present a uniform back-ground) and wearing opaque goggles (to preventany view of the dolphin). The assistant display-ing the sample object had no knowledge of thelocation at which the matching alternativewould appear, and the assistants displaying thealternatives had no knowledge of whether theobject they held was a match or not for thesample object.

2.3. Objects

2.3.1. Baseline objectsThere were six ‘baseline’ objects that were

highly familiar to Elele, a terra-cotta flower pot,a green fiberglass grating, a sand-filled PVCcross, a letter ‘I’ constructed from aluminum I-beams, a stainless-steel bowl, and a black hardrubber roller. They differed from each other insize, material composition, and shape, and hadbeen experienced previously by the dolphin inwithin-modal and cross-modal matching tests(e.g. Herman and Pack, 1992). The baseline ob-jects were used in part to train the three- andfour-alternative cross-modal tasks, and were alsointerleaved during transfer tests among trials us-ing the less familiar PVC transfer objects. Base-line objects were easy discriminations for Elele,and were employed throughout to help insure areasonably high level of performance when sheexperienced new contexts or new procedures,thus guarding against negative emotional behav-ior that might obscure her potential.

2.3.2. PVC training objects.A set of six pairs of 12 objects constructed

exclusively of PVC pipes and fittings were usedtogether with the baseline objects to teach thethree-alternative task. Computer-scanned imagesof the PVC objects and pairings appear in Fig.2. Elele had previously experienced these PVCobjects (Pack and Herman, 1995; Herman et al.,1998). Each object within a pair was constructedfrom schedule 40 and/or 80 PVC pipe andfittings, ranging in diameter from 0.5 to 1.5 in(1.27–3.81 cm). Within pairs, the same diame-ters of PVC pipe were used, and the surfaceareas of the two objects were equated to within4%. Objects were filled with dry sand to achievenegative buoyancy for immersion in water andto reduce internal reflections, to emphasize sur-face features.

2.3.3. PVC transfer objectsThe transfer objects were eight PVC objects

used in Herman et al. (1998), Experiment 2 (seeFig. 3). Each object had been experienced previ-ously by Elele exclusively as a member of a par-

Fig. 2. Computer-scanned images of pairs of the PVC training objects numbered 1 through 6. Both a frontal view (shown to thedolphin) and a 45° view of the objects are displayed to reveal their three-dimensionality.

ticular test pair for 48 trials each in E–V andV–E MTS and for 24 trials each in V–V andE–E MTS. These previous pairings were: object 1versus 2, 3 versus 4, 5 versus 6, and 7 versus 8. No

other pairings of these objects had been usedbefore the current study, and Elele’s previousexperience with these objects was limited to thenumber of trials noted. Further, there was a hia-

Fig. 3. Computer-scanned images of the PVC transfer objects numbered 1 through 8. Both the frontal view (shown to the dolphin)and the 45° view of the objects are displayed to reveal their three-dimensionality. The row adjacent to each object’s image indicatesthe presence (by a ‘+ ’) or absence (by a ‘0’) of six class features: vertical piping, horizontal piping, diagonal piping, enclosed space,symmetry, and protrusions.

tus of 20 weeks since the last use of these eightobjects by Herman et al. (1998) and their use in thecurrent study.

The eight PVC transfer objects were constructedin the same way and of the same materials as thePVC training objects. Additionally, each objectsubtended a 43 cm2 area. We classified each transferobject in terms of the presence or absence of sixlocal class features: vertical piping, horizontal pip-ing, diagonal piping, enclosed space (closure), hori-zontal symmetry, and protrusions (Fig. 3).

3. Experiment 1

Experiment 1 was conducted in three parts. Inpart A, we tested Elele in the three-alternative E–VMTS procedure. We used the baseline and PVCtraining objects to teach her the new procedure, byextending several of the already familiar two-alter-native E–V MTS procedures. We then proceededto formal tests of three-alternative E–V MTS usingthe PVC transfer objects. In part B, we tested Eleleon three-alternative V–E MTS. Again, we used the

baseline and PVC training objects to teach thethree-alternative V–E procedure, and then con-ducted tests with the PVC transfer objects. Fi-nally, in part C, we evaluated Elele’s searchstrategies among the three-alternatives in eachMTS paradigm to examine whether her behaviorwas consistent with a search for local class fea-tures or global shape.

3.1. Part A: three-alternati�e E–V MTS

3.1.1. MethodA single anechoic box was placed at the east

end of the dolphin’s pool. Three-alternative ob-jects were presented in air by assistants located atthe west-end of the pool (ca. 15.2-m diameterdistance from the east end). Objects were sepa-rated from each other by 2.5 m, with the centralalternative located directly opposite the positionof the sample box. At the beginning of each trialthe dolphin remained with her trainer at a remotestation 14-m circumferential distance from theanechoic box. While the dolphin’s head was heldout of water, to prevent any use of echolocation,an assistant immersed the sample object in theanechoic box. A second assistant simultaneouslyimmersed and then immediately withdrew two‘masking’ objects. The masking objects were al-ways identical to the two non-matching objectspresented as alternatives, and served to precludeany potentially useful sound cues made by thesample object as it was immersed in the water.When the sample object was in place, the trainersignaled the dolphin to approach the anechoicbox. The dolphin approached and inspected theobject inside through echolocation. When the firstassistant, looking through the mirror, viewed thedolphin’s rostrum in front of the box for approxi-mately 2 s, he/she verbally signaled the assistantson the opposite side of the pool to display theiralternative objects. The assistant’s loud vocaliza-tion also signaled the dolphin to swim across thepool and choose among the alternatives by sta-tioning in front of one of them for 3 s. Anobserver having no knowledge of the correctchoice timed the dolphin’s stationing responseand verbally indicated which alternative was cho-sen (by calling out ‘right’, ‘center’, or ‘left’). A

correct choice was followed by a whistle sound,fish reward, and social praise from the trainer.These were omitted following an incorrect choice.

3.1.1.1. Training. Elele was first familiarized overtwo sessions (39 trials) with the new location ofthe alternative objects at the west end of the pool(with the sample at the east end) using the base-line objects and two-alternative E–V MTS. Theselocations differed from those used in Elele’s pre-vious experience with two-alternative E–V MTS(e.g. Herman et al., 1998). Elele remained error-less throughout the two sessions. In each of thefollowing two sessions, Elele was exposed to 12trials of three-alternative E–V MTS using thebaseline objects interleaved among 12 trials oftwo-alternative E–V MTS using the PVC train-ing objects. She was again errorless. Thus, shetransferred her matching concept spontaneouslyfrom two to three-alternatives in the E–V MTStask.

3.1.1.2. Testing. Elele was next tested on three-al-ternative E–V MTS with the eight PVC transferobjects (Fig. 3). There are 336 unique permuta-tions of three-alternative objects taken from thepool of eight PVC transfer objects. The permuta-tions represent the six possible arrangements ofeach set of three as right, center, and left alterna-tive object. Further, any of the three could be thematching alternative, so that altogether therewere 1008 (336×3) unique trial types that couldbe tested. From these, we chose a one-third sam-ple (336 trials), such that all 56 combinations ofthree-alternative objects were tested, with eachobject serving as sample in two of the six possibletrial configurations. Over all transfer trials, ob-jects sharing local class features were, therefore,contrasted as sample and non-match on numer-ous occasions (see Fig. 3).

Each of the 28 three-alternative E–V MTS testsessions consisted of 12 transfer trials using thePVC transfer objects interleaved among 12 trialsusing the baseline objects. Trials were pseudo-randomly ordered with the restriction that nomore than three successive identical locations forthe matching alternative could occur. Over twoconsecutive sessions, each of the eight PVC trans-

Table 1Experiment 1: the number of correct first matches per sample object for each unique combination of sample object and twonon-matching alternative objects (n=168) in E–V and V–E MTS using the PVC transfer objects for partial and full transfer tests(see text)

Task Sample object 1 2 3 4 5 6 7 8 Total

9 8 9 8 8 9 9 9 69 (95.8%)E–V Partial transfer tests (n=9)12 12 12 11 12Full transfer tests (n=12) 12 12 12 95 (99.0%)

9 5 ns 9 9 9V–E 8Partial transfer tests (n=9) 9 8 66 (91.7%)11 10 12 9 12Full transfer tests (n=12) 12 11 12 89 (92.7%)

ns, not significant; all other cell values significant at P�0.005 (cumulative binomial test, chance=0.33).

fer objects appeared three times as sample, andeach of the six baseline objects appeared fourtimes as sample (twice per session). Within-trialprocedures remained as described earlier.

3.1.2. Results and discussionElele matched the echoic sample correctly on

331 (98.5%) of the 336 test trials using the PVCtransfer objects, and was errorless on the 336baseline trials. Table 1 shows the number of cor-rect first-trial matches with each object acting assample (i.e. the first occurrence of each uniquecombination of sample and alternative objects).The results are shown separately for ‘partial’transfer tests and ‘full’ transfer tests. Partial trans-fer tests were those trials in which two of thethree-alternative objects had been tested togetherpreviously in two-alternative E–V, and the thirdobject had been tested for an equal number oftrials as a member of a different pairing (Hermanet al., 1998). There were nine partial transfer testtrials per sample object. Elele was correct on eightor nine of these tests (P�0.001, cumulative bino-mial test, chance=0.33) for all eight PVC transferobjects acting as sample.

Full transfer tests were those trials involvingcompletely novel combinations of objects (i.e.combinations whose members had never beentested together pair-wise previously). There were12 full transfer test trials per sample object. Elelewas errorless on full transfer test trials for sevenof the eight sample objects, and made only asingle error on the eighth (P�0.001, cumulativebinomial test). Thus, she demonstrated sponta-neous transfer of her cross-modal matching con-cept to completely novel three-way combinations

of familiar objects in the three-alternative E–Vtask.

3.2. Part B: three-alternati�e V–E MTS

3.2.1. MethodTo present the echoic alternatives in V–E MTS,

three anechoic boxes fitted with response paddleswere suspended from the pool wall at the east endand separated from each other by 2.5 m. Anassistant wearing opaque goggles presented thevisual sample at the west end of the pool. On eachtrial, the assistant stood in view of the dolphinwith the object hidden behind the wall. The dol-phin stationed in front of the assistant in responseto a signal from the trainer. Upon hearing averbal signal from a coordinator located on anelevated observation platform, the assistantshowed the sample object to the dolphin, bybringing it forward over the pool wall and hold-ing it stationary about 30 cm above the watersurface. After approximately 5 s, the coordinatorverbally signaled the assistants behind the ane-choic boxes to suspend their objects inside theboxes. As in E–V MTS, each alternative objectwas plunged into a box simultaneously with twomasks that were then immediately withdrawn.The same three objects were plunged into eachbox. The dolphin swam to the boxes, inspectedthe alternatives echoically, and pressed the paddleon the selected box. Reinforcement contingencieswere identical to those used in E–V MTS.

3.2.1.1. Training. Using the V–E MTS procedure,Elele was first familiarized over seven sessions(139 trials) with the new location of the sample

object (previously, in Herman et al., 1998, Experi-ment 2, it had been shown at the remote station).She was also made aware that there were nowthree-alternative boxes rather than two. Thesefamiliarization trials used baseline objects only.However, the alternative objects were placed inonly two of the three boxes (randomly chosen).The third remained empty. Initially, some cueingwas used to teach Elele how to use the responsepaddle on the central anechoic box. Overall, Elelewas correct on 128 (92.0%) of the uncued trials,including 45 (93.8%) of the 48 uncued trials oc-curring in the final two sessions.

In the subsequent two sessions we presentedmixed trials of three-alternative V–E MTS usingthe baseline objects and all three anechoic boxes,and two-alternative V–E MTS using the PVCtraining objects placed in two of the three boxes(randomly chosen), with the third remainingempty. Elele was correct on 23 (95.8%) of 24 trialsin each condition.

3.2.1.2. Testing. Elele was next tested on three-al-ternative V–E MTS with the eight PVC transferobjects (Fig. 3). The same experimental designused for testing three-alternative E–V MTS wasused again, but with a different ordering of the336 test trials and the 336 baseline trials.

3.2.2. Results and discussionElele chose the matching echoic alternative on

316 (94.0%) of 336 transfer trials in V–E MTSand made but a single error on the baselineobjects. Table 1 shows the number of correctfirst-trial matches with each object acting as sam-ple for both partial transfer tests (N=9 trials persample) and full transfer tests (N=12 trials persample). For partial transfer tests, Elele was cor-rect on eight or nine trials (P�0.001, cumulativebinomial test, chance=0.33) for seven of eightsamples. She was correct on only five of ninepartial transfer trials with Object 2 as sample. Forfull transfer tests, Elele was correct on nine ormore trials (P�0.005, cumulative binomial test,chance=0.33) for all eight sample objects.Clearly then, Elele performed at near ceiling levelsin both the three-alternative E–V and V–E MTStasks using novel three-way combinations of fa-

miliar objects having overlapping local class fea-tures. Her performance accuracy on the first trialsof completely novel combinations of objects (i.e.full transfer tests) did not differ significantlyacross E–V and V–E MTS (�2 [1,192]=3.26,P�0.05).

3.3. Part C: analysis of search strategies

The dolphin’s search behavior, together with itsperformance accuracy, can provide informationabout its representation of the sample. To investi-gate whether Elele was searching among alterna-tives for a local class feature or features in parts Aand B, we analyzed the experimenter’s hand-writ-ten records of each transfer trial for the order inwhich Elele searched among the alternative ob-jects before finally selecting 1. During trials, theexperimenter had recorded the order in whichElele viewed each visual alternative object in E–VMTS (e.g. right, then center, then left, etc), andthe order in which she pointed her rostrum at thedifferent alternative anechoic boxes in V–E MTS.This latter measure reflects findings that echoloca-tion signals of Tursiops are directed forward ofthe head in a narrow beam (e.g. see Au, 1993).We analyzed search path for each first trial inwhich Elele experienced each unique combinationof sample and two non-matching alternative PVCtransfer objects in E–V and V–E MTS. Thesearch path was available for 286 PVC transfertrials, 150 in E–V MTS and 136 in V–E MTS.Fig. 4 represents Elele’s search path when choos-ing objects at the right, center, and left positions.For example, within a bar, ‘RCL’ means thatElele first searched the right position, then thecenter, and finally chose left. It is evident that onthe majority of trials Elele performed a right-to-left serial non-exhaustive search among the alter-natives. This strategy, summarized as ‘search onlyR, if R is a match, if R is not a match, search C,and if C is not a match, choose L,’ accounted for90.0% of the trials in E–V MTS and 76.5% of thetrials in V–E MTS. These percentages are wellabove what might be expected from a randomsearch strategy, which should yield an approxi-mately equal number of first approaches to eachbox, and no systematic path thereafter. The low

Fig. 4. The percentage of times the dolphin used particular search paths among the alternatives in three-alternative E–V and V–EMTS tasks with baseline objects when it selected the right, center, or left object. Search path is read from left to right where R,inspection of right object; C, inspection of center object; and L, inspection of left object.

incidence of vacillation in Elele’s search amongthe alternatives combined with her high perfor-mance accuracy does not support a search forlocal features. The sequential search path typicallyfollowed, R, then C, then L, if necessary, clearlyshows that Elele rejected non-matching alterna-tives, even though they shared local class featureswith the sample. Indeed, of the 48 permutationsof sample and non-matching alternative that Elelehad never experienced previously, she correctlyrejected the non-match on its first encounter dur-

ing her search on 47 occasions in E–V MTS andon 46 occasions in V–E MTS.

4. Experiment 2

The serial non-exhaustive search strategy usedby Elele in Experiment 1, and the high level ofperformance she achieved in the presence of manyoverlapping local features between sample andalternative PVC objects, together suggested that

Elele operated on the basis of a global ‘searchimage’ (Tinbergen, 1960; Vreven and Blough,1998) that corresponded representationally to thestructure of the sample object. It appeared thatshe searched serially among the three PVC alter-natives until an exact match was found, and thenstopped. Of course, a matching object was alwayspresent and Elele may well have developed thatexpectation. But what if none of the alternativeswas an exact match for the sample? Would Elelebe able to indicate the absence of a match? If shecould, this would provide compelling evidencethat her search was not for a specific local featureor features, inasmuch as a correct rejection of allPVC alternatives would mean that: (a) all alterna-tives had been inspected (which was not necessaryin Experiment 1 given her serial, non-exhaustivesearch strategy) and (b) that despite many over-lapping local features among the three-alterna-tives, and between the alternatives and thesample, she recognized that no object whollymatched the sample (i.e. she was not simply gen-eralizing to the ‘best-fit’ match from among theavailable alternatives).

To examine this issue, in Experiment 2 weadded a fourth alternative, in the form of a pad-dle that Elele could press if none of the three-al-ternative objects matched the sample. We termedthis paddle the ‘none of the above’ or NA paddle.If Elele were searching for the global shape of aspecific object, and it was not present among thethree-alternative objects, she should reject allthree and press the NA paddle. Alternatively, ifshe were searching for a local class feature orfeatures, she should select one of the objects thatcontained that local feature(s), even if no objectwas identical to the sample.

Experiment 2 was carried out in three parts.The baseline objects and the PVC transfer objectsfrom Experiment 1 were again used. The PVCtraining objects from Experiment 1 were nolonger used. In part A, we familiarized Elele withthe use of the NA paddle in E–V and V–E MTS.In Part B, we tested whether Elele would demon-strate spontaneous and appropriate use of the NApaddle in E–V and V–E MTS with the PVCtransfer objects. Inasmuch as the PVC transferobjects had many overlapping local features, if

Elele could correctly select a matching alternativeobject, or correctly reject all objects if nonematched the sample, it would provide compellingevidence that she was not representing the sampleas a local class feature or features, but as a globalshape. Finally, in part C, we examined Elele’ssearch strategies among the four alternatives ineach matching task of part B.

4.1. Part A: using the NA paddle with baselineobjects

In this Part, Elele was introduced to the NApaddle and attendant procedures using the E–Vparadigm. After training and testing within E–VMTS was complete, we introduced the NA paddlein the V–E paradigm and again tested herperformance.

4.1.1. E–V matching with the NA paddle present

4.1.1.1. Method. The same anechoic sample boxfrom Experiment 1 was used here. The NA paddleconsisted of a black oval float attached to a 22-cmPVC arm and frame. The frame was suspendedfrom the side of the pool, allowing the float torest just above the water surface.

Only the six baseline objects from Experiment 1were used when the NA paddle was present.When it was absent, either baseline objects or thePVC transfer objects from Experiment 1 wereused. This latter condition thus partially repli-cated that of Experiment 1, part B.

The NA paddle was introduced using the two-alternative E–V MTS paradigm with the sampleobject at the east end of the pool and the alterna-tive objects located at the west end and separatedby 2.5 m. When present, the NA paddle waslocated 7.0 m to the left of the left alternativeobject (from the dolphin’s perspective). After fa-miliarizing Elele to the two-alternative E–V con-dition, plus the NA paddle, we graduallytransitioned to the use of the NA paddle in three-alternative E–V MTS. The NA paddle continuedto be located to the left of the alternative objects.Elele indicated her choice of an alternative object,as she had in Experiment 1, by stationing in frontof it for a 3 s hold. Alternatively, to indicate her

Table 2Experiment 2: results of E–V matching tests for using the ‘None-of-the-Above’ (NA) paddle to report on the absence of a matchamong the alternatives for a baseline object appearing as sample

Match presentNumber of alternatives Match absent

Number correctNumber of trials Number of trials Number correct

8(a) Two objects+NA 88 88 99 9(b) Three objects+NA

(c) Three objects only 3940 – –

choice of the NA paddle, Elele pressed it. Thematching alternative object, if present, appearedequally often, or nearly so, in each availableposition. An observer having no knowledge of thecorrect solution judged all of Elele’s responses.

Three sessions were run, the first of 26 trials,and the remaining two of 24 trials each. Eachsession consisted of mixed trials of three types,representing three different arrangements of alter-natives. These were: (a) two-alternative objectsplus the NA paddle, (b) three-alternative objectsonly, and (c) three-alternative objects plus the NApaddle. Over the three sessions, there were 16trials of Type a, half containing a matching alter-native object, and half not, all using the baselineobjects only. There were 40 trials of Type b, 3using the baseline objects and 37 using the PVCtransfer objects. Finally, there were 18 trials ofType c, using the baseline objects only, given as‘probes’ in increasing density over sessions. Therewere two ‘probe’ trials during Session 1, fourduring Session 2, and 12 during Session 3. Forhalf the probes, one of the alternative objectsmatched the sample and for the remaining half,none did.

To acquaint Elele with the use of the NApaddle, it was suspended from the pool wall. Thetrainer, using gestures familiar to Elele, signaledher to touch the paddle with her rostrum. Eleledid so, for which she received social praise and afish reward. This was the full extent of her train-ing to use the NA paddle. The remaining trials ofthe session, using mixed trial types, began imme-diately thereafter.

4.1.1.2. Results and discussion. Elele successfullypressed the NA paddle on the first NA trial of

Session 1 in which there was no match for thesample among the two-alternative objects. Table 2summarizes the results over all three sessions. Itcan be seen that Elele made no errors on Type atrials, a single error on Type b, and a single erroron Type c (in all cases, P�0.001, cumulativebinomial test). Thus, Elele spontaneously used theNA paddle to report correctly the absence of amatch for the sample from among either two orthree-alternative baseline objects.

4.1.2. V–E matching with the NA paddle presentImmediately after completing E–V testing, we

examined whether Elele could transfer her facilitywith the use of the NA paddle to the V–Eparadigm.

4.1.2.1. Method. For the initial transfer test weused two anechoic boxes, located at the pool’seast end and separated by 2.5 m. The visualsample was located at the west end. The NApaddle, when used, was located 7.0 m to the leftof the left box (from the dolphin’s perspective).As in the earlier E–V test, only the six baselineobjects from Experiment 1 were used on V–Etrials in which the NA paddle was present. Afterthe assistant exposed the sample object in air toElele, the alternatives were presented in the ane-choic boxes (along with their masks, which wereremoved promptly). If one of the alternativesmatched the sample, Elele was required to pressthe response paddle on the box containing thematch. If neither matched the sample, she wasrequired to press the NA paddle. Both the base-line objects and the PVC transfer objects fromExperiment 1 were used on trials in which the NApaddle was absent. On these trials, a match for

Table 3Experiment 2: results of V–E matching tests for using the ‘None-of-the-Above’ (NA) paddle to report on the absence of a matchamong the alternatives for a baseline object appearing as sample

Match absentNumber of alternatives Match present

Number correct Number of trialsNumber of trials Number correct

A4Two objects+NA 44 1

B30 30 29One object+NA 3018 1818 18Two objects+NA

Three objects+NA 18 17 18 18

A, initial V–E test results; B, V–E tests with 1, 2, and 3 objects following brief training.

the sample was always present in one of the twoboxes.

Prior to any trials, the NA paddle was sus-pended from the pool wall and Elele’s trainersignaled her to press it. She pressed the paddlewithout hesitation and a single session of 24 trialsthen followed immediately. On eight trials the NApaddle was present. On four of these trials amatch was present in one of the two boxes(equally distributed between both boxes), and onfour trials a match was absent. These eight trialswere interleaved among 16 trials in which the NApaddle was absent. These latter trials used eitherthe baseline objects (N=3 trials) or the eightPVC transfer objects (N=13 trials).

Due to this initial transfer test showing onlymarginal success, we instituted brief NA paddletraining during the following V–E session of 18trials, using only a single anechoic box and withthe NA paddle present on all trials. Only baselineobjects were used. On four trials, a match for thesample was present in the box, and on 14 trials itwas not. To assist Elele in correctly choosing theNA paddle when a match was absent, the ane-choic box was empty on some trials. Elele thenspontaneously pressed the NA paddle. If the boxcontained an object, an assistant cued Elele topress the paddle by ‘jiggling’ it. This type ofassistance or cueing occurred on five of the first12 trials. In the final six trials of the session, nofurther assistance or cue was given.

Three 20-trial V–E MTS sessions using a singleanechoic box with the NA paddle present were

then conducted. Across these sessions, using thesix baseline objects only, each of the 30 permuta-tions of sample and non-matching alternative wastested once, and each of the six combinations ofsample and matching alternative was tested fivetimes for a total of 60 test trials. Each sessionconsisted of 10 match trials and 10 non-matchtrials. These three sessions were then followed byan additional four 24-trial sessions using the base-line objects only, during which the number ofanechoic boxes was increased to two, and then tothree. Within each of these sessions, half the trialscontained a match and half did not.

4.1.2.2. Results and discussion. Table 3A showsthe results of Elele’s initial V–E MTS test usingthe NA paddle with the baseline objects. Eleleselected the box containing the matching objecton all four trials in which a match was present,but pressed the NA paddle on only one of thefour trials when a match was absent. Her correctresponse occurred on the first ‘match-absent’ trial.On the remaining trials she inspected and rejectedthe right box and then selected the left box, whichwas closest to the NA paddle.

During the subsequent 18-trial training sessioninstituted with baseline objects to improve herV–E performance with the NA paddle present,Elele was cued on only five trials in which a matchwas absent. She responded correctly on a total of11 of 13 uncued trials (P=0.011, cumulative bi-nomial test), including correct responses on sevenof nine trials in which a match was absent and all

four trials in which a match was present. Elele thusdemonstrated the appropriate use of the NA paddlewith the baseline objects after only brief trainingwith minimal cueing.

Table 3B summarizes Elele’s subsequent perfor-mance with one, two, or three anechoic boxes. Shemade only two errors throughout, one on a match-present condition and one on a match-absentcondition (P�0.0001, cumulative binomial test).

4.2. Part B: testing use of the NA paddle withPVC transfer objects

In this part, we tested Elele’s ability to use theNA paddle appropriately when making discrimina-tions among the PVC transfer objects. These testsagain contrasted objects with overlapping localclass features to determine more conclusivelywhether Elele was searching among alternativeobjects for particular local features or for globalshape. Trials specifically germane to this goal werethose in which none of the alternatives matched thesample, but nevertheless shared one or more localfeatures with the sample.

4.2.1. Method

4.2.1.1. Objects. Six of the eight PVC transferobjects from Experiment 1 (Objects 3–8, see Fig.3) were tested in four-alternative E–V and V–EMTS. The transfer set was limited to six objects tokeep the overall number of trials manageable.Although the transfer objects were familiar to Elele,they had not been used previously in trials in whichthe NA paddle was available.

4.2.1.2. Procedure. The apparatus and procedureswere the same as in the final E–V and V–E MTSconfigurations of Part A. We tested 30 of the 120possible permutations of the six transfer objectstaken three at a time. Each permutation of three-al-ternative objects was tested on four trials. For threeof these trials one of the alternative objects matchedthe sample (i.e. each alternative object was thecorrect match once). For the fourth trial, none ofthe alternatives matched the sample, requiringtherefore a press of the NA paddle. Thus, a totalof 120 trials were run (30 permutations× four trials

per permutation) in E–V MTS and 120 in V–EMTS. Sessions of E–V and V–E MTS were coun-terbalanced to control for order effects. The loca-tion of particular alternatives at the right, center,and left positions was balanced across trials.

At each of 20 sessions, there were 12 transfertrials using the PVC transfer objects interleavedamong 12 trials using the 6 baseline objects. Thebaseline trials were organized under the same rulesand constraints described above for the transfertrials (i.e. 30 permutations× four trials per permu-tation per matching task). Within each session, forboth transfer trials and baseline trials, each of thefour alternative choices (right, center, left and NApaddle) was correct 25% of the time. Over thecourse of two consecutive sessions, each matchingobject appeared once in each alternative positionand also once as sample in trials in which there wasno matching alternative. The same control proce-dures described in Experiment 1 were used here.Research sessions were videotaped for subsequentdetermination of the dolphin’s search behaviouramong alternatives.

4.2.2. Results and discussion

4.2.2.1. Accuracy. Fig. 5 summarizes Elele’s perfor-mance when the matching alternative was at theright, center, or left position, and when it wasabsent. For both E–V and V–E, Elele reported thepresence of a match with high accuracy, 97% orbetter with baseline objects and 93% or better withPVC transfer objects. Elele made no errors inreporting that a baseline object was absent. How-ever, for PVC transfer objects, she reported absencecorrectly for 77% of E–V trials and 73% of V–Etrials. Inasmuch as chance accuracy is 25% (assum-ing a random choice among the four alternatives),her performance in reporting absence of PVCmatches was nonetheless highly significant (P�0.0001, cumulative binomial test).

An analysis of variance compared conditions(E–V vs. V–E), object type (baseline vs. PVC) andalternative position (R, C, L or NA). There was nosignificant difference between conditions. Overallperformance with baseline objects signifi-cantlyexceeded that with PVC objects (F(1,464)=16.38,P=0.0001), and the effect of alternative position

Fig. 5. Percent correct responses in four-alternative echoic-visual and visual-echoic matching with baseline objects (gray bars) andtest objects (striped bars) for trials in which either a match was located in the right, center, or left position, or no match was present(absent). Each bar represents 30 trials. A chance performance line is shown at 25%.

was significant (F(3,464)=4.92, P=0.0023). Al-though Elele performed equivalently reportingthat a match was present in each of the alterna-tive positions (i.e. right, center, or left), shewas less accurate reporting a match’s absenceon the NA paddle trials (Duncan’s MultipleRange, �=0.05; df=464; MSE=0.051; critical

range=0.057 for two means, 0.060 for threemeans, and 0.062 for four means). In addition,the interaction of object type and position wassignificant (F(3,464)=7.32, P=0.0001), and ascan be seen in Fig. 5, was driven by the loweredperformance with the NA paddle with the PVCobjects.

Table 4Experiment 2: performance accuracy on the first trial in which a particular sample was displayed and none of the alternatives wasidentical to the sample (chance=25%) in E–V and V–E four-alternative MTS

4Object 53 6 7 8 Number correct

0 + 0E–V ++ 0 3V–E + + + 0 + + 5

8aTotal

Objects are as shown in Fig. 3. Note: ‘+’ indicates correct NA response, ‘0’ indicates false alarm (i.e. choice of an object when nomatch was present).

a P�0.005 by the cumulative binomial test.

4.2.2.2. Spontaneous reports of absence. As notedearlier, the PVC transfer objects had never beenused previously in the four-alternative cross-modaltask. In Table 4, we examined the details of Elele’suse of the NA paddle on the first trial given witheach PVC sample in which a match was absent.Elele was correct on eight of 12 first trials in E–Vand V–E MTS combined (P=0.0028, cumulativebinomial test, chance=0.25). Elele’s spontaneousand appropriate use of the NA paddle in these casesindicates that the presence of shared local features(Fig. 3) between alternatives and the sample gener-ally was not sufficient to induce the selection of analternative not wholly matching the sample. Thatis, when confronted with a situation in which noneof the alternatives matched the sample, Elele didnot simply respond to the ‘best fit’ match. Rather,her overall high levels of rejection of non-matchesin favor of the NA paddle indicate that her repre-sentation of the sample was highly faithful to theoverall spatial structure of that object.

4.2.2.3. False alarms and missed detections. Table 4showed four false alarms among the 12 initialmatch-absent trials with each PVC sample object.A false alarm occurred on a match-absent trialwhen Elele incorrectly selected a non-matchingalternative object instead of pressing the NA pad-dle. Overall, false alarms occurred on 15 of 60 NAtrials. These 60 trials actually represented 180individual comparisons of the dolphin’s representa-tion of the sample and a non-matching alternativeobject, inasmuch as there were three objects avail-able for comparison at each trial. False alarms didnot appear to be random selections among thealternative objects. Table 5 shows the identities of

the sample object and selected non-matching alter-native on false alarms. Although there were 15 falsealarms, they occurred with only six of 15 possiblecombinations of sample object and non-matchingalternative. Six of the false alarms (40%) occurredbetween Objects 3 and 6. Fig. 3 shows that theseobjects shared three of the six local class features(diagonal piping, symmetry, and protrusions).However, Objects 3 and 5 shared these samefeatures but were never confused. Thus, the basisfor these few false alarms remains in question. Itis interesting that in 11 of 15 false alarms the sameconfusions occurred in both E–V and V–E (Ob-jects 3 vs. 6, 4 vs. 7, and 6 vs. 5). This symmetryprovides additional evidence for the functionalequivalence of the representations of objectsderived through echolocation and vision, as firstproposed by Pack and Herman (1995).

A missed detection occurred on a match-presenttrial in which Elele incorrectly pressed the NApaddle when a match was actually present amongthe alternatives. Only four missed detections oc-curred in 180 match-present trials (combining E–Vand V–E), significantly fewer than the number offalse alarms (Fishers exact test, N=240, P�0.0001). The PVC samples associated with the fourmissed detections were Objects 4 (three misseddetections) and 6 (one missed detection).

4.3. Part C: analysis of search strategies with NApaddle present

We analyzed the archival videotape records ofeach available trial in Experiment 2, Part B, todetermine Elele’s search behavior among the fouralternatives after her inspection of the sample

Table 5Experiment 2: the number of times a particular non-matching alternative was selected in error for a particular sample object (i.e.a false alarm) when none of the alternatives matched the sample in the four-alternative E–V and V–E MTS

Selected alternative Number times selected (N=3)Task Positions of false alarm objectSample

E–V 3 6 1 C6 23 L, CV–E

6E–V 3 1 LV–E 6 3 2 L, R

7 14 LE–V4V–E 7 2 L, C

5 1E–V C65 16 LV–E

4E–V 3 1 C3 1V–E L86 28 L, CE–V

Objects are designated as in Fig. 3. Note, R, right; C, center; and L, left.

object. In Experiment 1, Elele most often con-ducted a right-to-left serial non-exhaustive searchof the alternative objects, both in E–V and V–EMTS. However, dfletailed inspection of the leftobject was not in theory necessary inasmuch as amatch for the sample always appeared among thealternatives. Consequently, when the match waspositioned left, the dolphin could have adopted astrategy of ‘If not right, and not center, then chooseleft.’ This strategy would not have been effective inExperiment 2 in the presence of the NA paddle.

4.3.1. MethodTwo raters independently viewed taped E–V

and V–E MTS trials to determine Elele’s searchpath, and to measure her response time afterpresentation of alternative objects. For E–VMTS, without knowledge of the sample object,the raters recorded the order in which Eleleviewed each alternative object. For V–E MTS,without knowledge of the identity of alternatives,they noted the order in which Elele pointed herrostrum at each anechoic box (as a measure ofechoic inspection strategy, see Experiment 1).

The raters used the digital tape’s chronometerrecord to determine response time. For E–VMTS, they measured the time from the box assis-tant’s verbal command ‘In’ (signaling the assis-tants on the opposite side of the pool to presentthe alternative objects in air) to the moment whenthe dolphin either stationed in front of the object

of its choice or touched the NA paddle. For V–EMTS, they measured the time from the experi-menter’s verbal command ‘In’ (signaling the assis-tants to suspend the alternative objects in theanechoic boxes) until the dolphin began its pressof a response paddle (either one of the paddles onthe anechoic boxes or the NA paddle).

4.3.2. Results and discussionEach rater independently examined 427

videotaped trials to determine the dolphin’ssearch pattern. Overall, the raters were in exactagreement on 377 trials (88.3%). This included98.2% agreement on 109 of 111 E–V PVC testtrials, 96.3% agreement on 103 of 107 E–V base-line trials, 84.0% agreement on 89 of 106 V–EPVC test trials, and 73.8% agreement on 76 of 103V–E baseline trials. Each of the 50 trials in whichthere was disagreement was then viewed indepen-dently by a third rater who had no knowledge ofthe previous judgements. If two of the threejudges were in agreement, this was selected as thefinal description of search behavior. There wasagreement between the third rater and one of thetwo original raters on 36 trials, leaving 14 trialson which none of the raters agreed. The total of413 final descriptions (377+36) are summarizedin Fig. 6, which shows the percentages of differentsearch strategies used by Elele when responding toeach position (i.e. right, center, left, or NA pad-dle). Results are shown for both PVC transfer

‘O’

Fig. 7. The dolphin’s response times in E–V and V–E MTS for baseline and test trials as measured from the time of the presentationof the alternative objects until the dolphin’s response to a particular position (R, C, L, or NA). Each bar represents mean responsetimes for a particular alternative selected by the dolphin. Range data is shown within each bar, as is the number of trials that aparticular alternative was selected. The linear regression line and the regression equation based on all the response data are shownfor each set (test and baseline trials) and each task (E–V and V–E MTS). All regression lines showed a positive gradient(P�0.0014).

objects and baseline objects, in both E–V and V–EMTS. As in Fig. 4, the nomenclature used todescribe the particular search strategy is shownwithin each bar and reads from left to right. Theletter ‘O’ indicates that there was no agreementbetween the three raters and therefore the searchbehavior was deemed ambiguous. As in Experi-ment 1, Elele most often performed a serial non-ex-haustive search among the alternatives, startingfrom the right-positioned object and working herway left until either a match was found, in whichcase she ceased searching, or no-match was found,resulting in a press of the NA paddle. There weresome exceptions, most notably in V–E MTS withthe baseline objects. Here, she sometimes appearedable to make her echolocation decisions from

slightly less than the midway point in the pool,making it difficult for the rater to judge thedolphin’s rostrum orientation. Additionally, withthe baseline objects she sometimes appeared eitherto examine the contents of the center box before thecontents of the right box, or on her way to the rightbox, cocked her head toward the center box andthen back to the right box. Nevertheless, on major-ity of V–E trials with the baseline objects (84.5%),she appeared to follow the serial non-exhaustivesearch strategy. Furthermore over all tasks vacilla-tion between alternatives was relatively rare.

An analysis of Elele’s response times largelyconfirmed her serial search strategy. A total of 427trials were used to measure response times. Acomparison of the raters response time measure-

ments for each trial revealed significant inter-raterreliability (r=0.973, P�0.0001). We thereforeused the mean of the rater’s response time mea-surements for each trial. If Elele performed aright-to-left serial non-exhaustive search (as indi-cated by the qualitative judgements, Fig. 6), wewould expect her response time to be shortestwhen responding to the right-positioned objectand longest to the NA paddle. Fig. 7 showsElele’s response times as a function of the alterna-tive selected (R, C, L, and NA) for both PVC testand baseline trials in E–V and V–E MTS. Therewas a significant positive slope for each plot (P�0.0014) from R to NA, providing support for theserial non-exhaustive search strategy commencingwith the alternative at the right.

Elele’s search strategy on NA paddle trialshelped to clarify the nature of her representationof the sample. Recall that Elele committed 15false alarms on NA paddle trials. These errorswere not evenly distributed between the three-al-ternative positions containing a non-match. Elelecommitted eight false alarms to the non-matchpositioned left, six false alarms to the non-matchpositioned center, but only one false alarm to thenon-match positioned right. Indeed a greaternumber of false alarms occurred to the non-matchin the left position (n=8) than to the non-matchin the right position (n=1), the first alternativeobject examined during the Elele’s search (N=120, P=0.032, Fishers’ exact test). Given thatevery sample/non-match permutation of the sixPVC transfer objects was represented in eachposition, Elele’s correct rejection of right-posi-tioned non-matches on 59 of 60 trials (29 in V–Eand 30 in E–V) provides substantial evidence thather representation was not of a local feature orfeatures, but rather of the overall global shape ofthe sample. That is, she did not simply select thenon-match at the right even though it containedoverlapping local features with the sample and noexact match was present among the alternatives.

5. General discussion

The findings from the current studies supportour earlier contentions (Pack and Herman, 1995;

Herman et al., 1998) that dolphins can form apercept or internal representation of the spatialstructure of an ensonified object. The major dis-tinguishing features of our current approach were:(a) the use of a cross-modal matching-to-sampleprocedure that required the matching of objectsacross the senses of echolocation and vision inboth directions, echolocation to vision (E–V) andvision to echolocation (V–E); (b) the use, intransfer tests, of multiple objects of complex, ab-stract shapes, all constructed of the same PVCmaterials and all having almost the same degreeof reflective surface and the same peripheral ex-tents; (c) the requirement (in Experiment 1) thatthe dolphin select a match for the sample objectfrom among three-alternative objects with over-lapping features with the sample; and (d) the use(in Experiment 2) of a unique fourth alternativethat allowed the dolphin to indicate that none ofthe three offered alternative objects matched thesample object. Items a and b also characterizedportions of our earlier studies, which had used atwo-alternative matching-to-sample procedure(Pack and Herman, 1995; Herman et al., 1998).Items c and d were unique to the current studies.Item a insured that raw acoustic cues could not beinvoked as an explanation of object matchingperformance (Popper et al., 1997; cf. Au, 1993), asthere are no inherent physical correlates betweenacoustic and visual cues. Further, inasmuch asobjects were never exposed to the dolphin’secholocation and visual senses contemporane-ously, and if successful matching occurred signifi-cantly often on the first trial with new objects (asoccurred in the studies reported by Herman et al.,1998), associative learning could be ruled out asan explanation for the dolphin’s performance.Item b helped to insure that the only reliabledifference among objects was shape. Further, theabstract and complex shapes of the objects usedmeant that generalization from previously en-countered familiar shapes was quite unlikely.Items c and d allowed us to examine the dolphin’ssearch strategy among alternatives and to deter-mine whether that search was for some localfeature or features or for the overall (global)shape of the object. A search with little vacillationamong alternatives having many overlapping fea-

tures, coupled with a high level of matching accu-racy and correct rejection of all alternatives if nomatch were present, would argue strongly againsta feature-based interpretation of the dolphin’ssearch strategy and for an interpretation based onrepresenting the sample object by its global shape.

The results of Experiments 1 and 2, includingsearch strategy and matching accuracy, clearlysupported the latter hypothesis. In the three-alter-native condition, the dolphin’s search strategy wasbest characterized as serial and non-exhaustive. Forboth E–V and V–E MTS, the dolphin typicallybegan its search with the right-most alternative,then the center, and finally the left, stoppingimmediately when a match was found. Exceptionsto this pattern were relatively rare, as was retracingof the search path in reverse order, or travelingback and forth between alternatives (vacillation), asmight be expected in a search for local features thatwere present in more than one alternative. Theright-to-left search strategy was most clearly seenduring Elele’s inspection of the visual alternatives(E–V matching), because she approached the alter-natives closely, sometimes looking at them throughthe water–air interface as she swam by, and some-times raising her head above the surface (bot-tlenosed dolphins have good visual resolutionacuity both underwater and in air—Herman et al.,1975). The right to left search strategy was stillapparent during Elele’s search among echolocationalternatives (V–E matching), though more difficultto classify on some trials because objects weresometimes inspected from 1 to 3 m distance.

Clearly, in the three-alternative condition, andgiven the dolphin’s search strategy, not all alterna-tives need be inspected for the dolphin to make adecision. This was not the case in the four-alterna-tive condition when no matching object waspresent. In that instance, the dolphin most oftencontinued its search path, right, center, left, NApaddle, in that order, despite the presence of manylocal features among the alternatives that over-lapped those of the sample object. When falsealarms occurred (on 15 of 60 match-absent trials)they most often were in choosing a non-match ineither the center (n=6) or left position (n=8). Afalse alarm occurred to the non-match at the rightposition only once. What factors might explain this

pattern of false alarms? Consider the dolphin’s taskwhen a match is absent, in light of her typical searchstrategy: examine the right-hand alternative first,then the center, and finally the left. During thatprocess, two conditions affecting memory for thesample can occur. The first is, hypothetically, atime-dependent weakening of the integrity of therepresentation of the sample object as the dolphinproceeds along her search path. The second isretroactive interference, arising from the successiveinspection of each alternative. Theoretically, then,we would expect false alarms to be rare to theright-hand alternative, and to increase for thecenter and left-hand alternatives, which is preciselywhat occurred. It is significant that missed detec-tions, pressing the NA paddle when a match wasin fact present, were extremely rare, occurring ononly four of 180 trials, suggesting that the dolphin’sdecision criterion was set low to favor detections.

Elele’s matching performance also gave strongsupport to the hypothesis that the sample objectwas represented in memory by its global shape. Thethree-alternative procedure of Experiment 1demonstrated that the dolphin was able to select thematching alternative with high reliability eventhough there were many overlapping featuresamong alternatives and between the alternativeobjects and the sample object. During the PVCtransfer tests, Elele chose the matching alternativeon 94% of V–E trials and on 98% of E–V trials.Elele had never experienced many of the combina-tions of alternatives previously, yet her perfor-mance remained at a high level and includednumerous successes on the first occasion that aparticular combination appeared.

These results were strongly reinforced by findingsfrom the four-alternative procedure. Here, for thefirst time, the dolphin was able to reject all alterna-tives (by pressing the NA paddle) despite thepresence of local features among the alternativesthat were also present in the sample object. WhenElele was presented with a trial in which noalternative exactly matched the sample, she did notsimply choose the ‘best fit’ match. Nor did shevacillate between alternatives. Instead, she mostoften examined each non-matching alternativeonce and then pressed the NA paddle. When amatch was present among the alternatives during

the PVC transfer tests, Elele chose it on 94% of V–Etrials and on 95% of E–V trials, results very similarto those of Experiment 1. When a match was absent,and a press of the NA paddle was thereforerequired, performance declined to 74% and 76%,respectively, in V–E and E–V. We noted earlierthat the decline was possibly associ-ated withuncertainty of the memory for the sample.

There are several possibilities for how anechoically inspected object may be represented bythe dolphin. At a complex level, the representationmay contain information about the spatial arrange-ment of object features relative to each other, whichin essence preserves the shape of the object (seeBiederman, 1987 for a model of visual objectrecognition that contains similar properties). How-ever, simpler levels are possible theoretically, oneof the most elementary being the representation ofone or more perceived features (we have focused on‘class features’ here). The findings of the presentstudy indicate that echoically inspected objects arenot represented simply as one or more features.Elele’s lack of vacillation among alternativessharing class features with the sample, combinedwith her high levels of accuracy in reporting thepresence or absence of a match from amongthree-alternatives, provide substantial inferentialevidence that her representation of the sampleobject was of its global shape. A theory of featurerepresentation would predict that the dolphin se-lects the first alternative object along its search paththat contains the key feature or features. This wouldbe the right-most object, given the dolphin’s typicalsearch path, and the abundance of overlappingfeatures among the alternatives and the sampleobject. However, when a match was present, errorswere rare overall, and were no more frequent to theright-most object than to the objects in the remain-ing positions. When a match was absent, errors wereleast frequent to the right-most object.

Another possibility is that the dolphin representsa sample object as an unorganized list of featuresthat it matches against the features of an alternative.When a discrepancy is found, the alternative isrejected. If all features in the list match the sample,the alternative is selected. We have limited evidencearguing against this theory from pairings of Objects3 and 5 (Fig. 3). Each of these objects contained

the same four class features. Nevertheless, theobjects were never confused. This suggests that thelocation of features and their spatial relationshiprelative to each other may be preserved in thedolphin’s representation of echoically perceivedobjects.

Further evidence supporting this theory comesfrom our earlier findings of spontaneous cross-modal recognition between televised miniaturizedimages of PVC objects presented as samples to thedolphin’s visual sense and ‘live’ alternatives unal-tered in size presented to the echolocation sense(Pack and Herman, 1995). Four pairs of PVCobjects had overlapping features and had neverbeen exposed previously in the television condition.The televised images of the samples were 40–49%smaller than the live objects making the actuallengths and widths of PVC components differentbetween sample image and matching alternativeobject, although the relationship between the struc-tural features of the objects remained unaltered.Nevertheless, Elele spontaneously matched the tele-vised sample to the correct echoically perceivedalternative on first trials without error across fourPVC pairs, including one with which she had noprevious cross-modal experience.

If, as suggested, the dolphin represents the struc-tural relationship between features in an echoicallyinspected object, an obvious question is whatmechanisms might allow for this representation.The question is similar in some respects to thedilemma of how in vision a three-dimensional objectis recognized from a two-dimensional retinal arrayof photon activated cells that varies over differencesin viewpoint, object size, and illumination (Marrand Nishihara, 1978). In the case of the echolocatingdolphin, a shape percept must be constructed fromreturning echoes that may vary significantly withangle of ensonification and changes in object size,orientation, and material composition. Herman etal. (1998) hypothesized that echoic shape perceptionis a result of specialized peripheral and centralprocessing, arranged hierarchically and resultingultimately in a representation of the contours of anobject. Theoretically, the dolphin can take advan-tage of both spatial and temporal information in theechoes returning from an ensonified object to createa spatial-temporal representation that is then inter-

preted by central auditory mechanisms as a three-dimensional shape.

Pack and Herman (1995) and Herman et al.(1998) suggested that fine range-difference resolu-tion and cross-range (angular) resolution capabili-ties might be important auditory processes con-tributing to shape perception. Range-differenceresolution is a function of the ability to perceivetemporal differences in echo arrivals. Echoesreflecting from different points along an object willarrive at slightly different times resulting in atemporal array that represents the array of differentrange values along that object (cf. Simmons, 1989).Range-difference resolution capabilities as small as0.9 cm at a range of 1 m were reported in a dolphinsubject by Murchison (1980) who trained the dol-phin to respond to the closer of two identical foamspheres separated in azimuth by 40°1. To achievethis level of resolution, it is theoretically necessaryfor the dolphin to detect echo-delay differences ofapproximately 12 �s, applying the same model asSimmons (1989). This value is in close agreementwith the time resolution constants of 12 to 15 �sfor Tursiops reported by Au (1993) (Table 10.1),which translate to resolvable ranges of 1.0 to 1.1cm between point targets. Murchison’s results arealso consistent with those of Au (1993) who showedthat a dolphin could perceive time differencesbetween echo highlights as small as 500 ns, a valuethat translates to a resolvable distance of 0.76 cm.

Cross-range (angular) resolution has recentlybeen obtained for the bottlenosed dolphin.Branstetter (2001) reported a horizontal angulardifference threshold of 1.6° by an echolocatingdolphin. The echolocating dolphin was required todiscriminate horizontal angular differences be-tween two arrays of vertically oriented PVC pipes,in a procedure similar to that used with echolocat-ing bats (Simmons et al., 1983). Branstetter’s resultsagree well with those from passive sound localiza-tion experiments conducted by Renaud and Popper(1975) who found a minimum audible anglethreshold of approximately 0.9° for clicks sounds

and 2.1° for pure tones. Clearly, the dolphin’srange-difference resolution and cross-range resolu-tion capabilities are fine enough to provide signifi-cant temporal and spatial information for the directperception of the types of relatively large complexshapes used in our cross-modal studies.

Movement by the dolphin during object ensonifi-cation may also enhance shape perception (Azzaliet al., 1995; Pack and Herman, 1995; Herman et al.,1998). During our cross-modal tasks, the dolphinis free to echolocate at any time as it approachesthe anechoic box containing an object. Echoloca-tion from different aspects relative to the frontpanel of the box provides some degree of differentviewpoints of the object suspended within the box.

The central neuronal processes involved in cross-modal recognition by the dolphin remain specula-tive. However, there are several intriguingpossibilities. Herman et al. (1998) referred to mul-tisensory neurons (see Stein and Meredith, 1990,1993) that respond to both visual and auditorystimuli as a possible mechanism for integratingvisual and auditory input. For example, Maunsellet al. (1989) found that neurons at level V4 of thevisual cortex of the monkey that are tuned to aparticular line orientation respond regardless ofwhether the line orientation is examined visually orhaptically. Although some evoked potential studieshave reported on the principal sensory projectionzones in the dolphin’s cerebral cortex (e.g. seeRidgway, 1986), none of these studies havesearched for the presence of multisensory neuronsthat respond to both visual and acoustic input.Supin et al. (1978) noted, however, an unusualorganization of the dolphin brain in that auditoryand visual cortex lie immediately adjacent to eachother. This proximity may promote auditory-visualintegration.

Knudsen and Konishi (1978) demonstrated thatnerve cells in the midbrain auditory nucleus of thebarn owl (Tyto alba) had spatial receptive fieldsthat were topographically organized. Adjacentsound sources in auditory space trigger corre-sponding adjacent areas in the midbrain, thuscreating a physiological map of auditory space. Inaddition, Knudsen (1982) discovered spatiotopicneurons in the tectum of the owl that respondedto both visual and auditory stimuli. The spa-

1 Au (1993) (p. 206) considered the study by Murchison(1980) to be better characterized as a range difference discrim-ination experiment rather than a range resolution experiment,because ‘true’ range resolution involves resolving targets atdifferent distances along the same line of acoustic propagation.

tiotopic map responded to either light or sound inspecific locations in space and had the same recep-tive fields, thus suggesting that areas of the tectummay provide an amodal representation of space.There appear to be no studies that have examinedcomparable spatial mapping in the dolphin brain,but it is clear that such amodal mapping couldbenefit cross-modal matching.

In summary, our studies have revealed that thedolphin is a sophisticated perceiver of objectshape. Our previous work (e.g. Pack and Herman,1995; Herman et al., 1998) provided compellingevidence for direct perception of shape throughvision, through echolocation, and across thesesenses. The findings of the current study extendour understanding of what aspects of object shapeare perceived, represented, and transferred acrossthe senses of echolocation and vision. These find-ings indicated that the dolphin’s representation ofthe sample object was highly faithful to the over-all spatial structure of the object inspected, andnot easily confused with alternative objectssharing features with the sample. Consequently, itappears that objects were represented globally(holistically) by the dolphin whether perceivedthrough vision or through echolocation. Similarly,visual object perception research with humans,chimpanzees, and pigeons has in many cases alsofound reliance on global appearance rather thanon local features (e.g. Navon, 1977; Tomonagaand Matsuzawa, 1992; Wasserman et al., 1993;Kirkpatrick-Steger et al., 1998; Fagot andTomonaga, 1999; cf. Cavoto and Cook, 2001).The current finding that E–V and V–E perfor-mances closely mirrored each other is consistentwith our earlier results (Pack and Herman, 1995;Herman et al., 1998) and supports the theory thatecholocation and vision are functionally equiva-lent in their representation of object shape. Fi-nally, perceptual mechanisms, in particulartemporal acuity and azimuthal acuity, are avail-able to the dolphin that appear to have sufficientresolution for representing faithfully the spatial/temporal array of returning echoes, which in turnpreserves the global shape of the ensonifiedobject.

Acknowledgements

This study was supported in part by grant IBN009-07-44 from the National Science Foundation,from Earthwatch and its Research Corps, by sup-port from The Dolphin Institute and its members,and by donations from Apple Computer, Inc, andMitsubishi, Inc. We thank all of the staff, stu-dents, interns, and Earthwatch and Dolphin Insti-tute participants at Kewalo Basin MarineMammal Laboratory who contributed to this pro-ject. Lara Roefer and Judith Leone were veryhelpful assisting with Experiment 1. Craig Holtand Pete McDougall assisted with subsequentanalyses of search strategies. We are grateful toDarlene Randalls and Beth Fletcher for acting as‘raters’ in Experiment 2. Additionally, AlisonCraig provided valuable assistance with analysisof response data in SAS, Dave Matus donated theuse of his photo printer, and Lori Marino helpedin discussions of the dolphin brain. This study isdedicated to Elele.

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