Memory & Cognition1975, VoL 3 (5),519·526

The effects of contextual sceneson the identification of objects

STEPHEN E. PALMERUniversity of California, San Diego, La Jolla, California 92037

This experiment demonstrates the influence of the prior presentation of visual scenes on theidentification of briefly presented drawings of real-world objects. Different pairings of objects and sceneswere used to produce three main contextual conditions: appropriate, inappropriate, and no context.Correct responses and confusions with visually similar objects depended strongly on both the contextualcondition and the particular target object presented. The probability of being correct was highest in theappropriate context condition and lowest in the inappropriate context condition. Confidence ratings ofresponses were a function of the perceptual similarity between the stimulus object and the namedobject; they were not stronl{ly affected by contextual conditions. Morton's (1970ll-"logogen" modelprovided agood quantitative fit tothe response probability data.

Figure I. Examples of drawings used in the experiment. Thepresentation of the seene establilhes the context for one of thethree drawings used as tuget objects for this scene: a loaf ofbread (A), a mailbox (8), or a drum (C). See text for anexplanation of contextual conditions formed by pairing scenesand objects.

similar stimuli. The paradigm also allows a quantitativetest of Morton's (1969, 1970) "logogen" model forsensory-contextual interaction.

In this experiment, the context for an object isestablished by showing a visual scene that depicts alocation where one would expect to find certain kinds ofobjects. The basic experimental task is illustrated inFigure 1. The subject is shown two slides: the first(left-hand drawing in Figure 1) represents a contextualscene, and the second (Drawing A, B, or C in Figure 1)depicts the target object. After a brief presentation ofthe target item, the subject's task is to name the objectand rate his confidence in his response.

Contextual conditions are varied by different pairingsof scenes with target objects. Consider, for example, thekitchen counter scene in Figure 1. When this scene ispaired with the loaf of bread (Drawing A), the scene isan appropriate context for the target object. When thesame scene is paired with the mailbox or drum

We find bread in kitchens, watches on wrists, andtelevision sets in living rooms. People know a good dealabout where objects are likely to be found in the world,and it is plausible that they use this knowledge inidentifying objects they see. If so, a loaf of bread shouldbe more easily identified when it is on a counter in thekitchen than when it is on a post in the front yard.These are context effects: effects of the environment ofan object on the perception of that object, independentof the intrinsic properties of the object itself.

Many theories of perception stress this interaction ofsensory and contextual information in identifyingsensory input (e.g., Bruner, 1957; Morton, 1969, 1970;Neisser, 1967; Norman, 1968). While theorists differsubstantially in their formulations of how contextaffects perception, they agree that appropriate contextshould aid identification and that inappropriate contextshould hinder it. This general result has been establishedin the recognition of words using sentences as contexts(e.g., Tulving & Gold, 1963; Tulving, Mandler, &Baumel, 1964), and in the recognition of letters usingwords as contexts (e.g., Reicher, 1969; Wheeler, 1970).Related effects are present in visual search phenomena.For example, less time is required to find a target objectwithin an appropriate scene than within an inappropriatescene (Biederman, 1972; Biederman, Glass, & Stacey,1973). The present experiment demonstrates theimportance of contextual scenes for object identificationin terms of correct responses and confusions with visually

This research was supported by a Graduate Fellowship fromthe National Science Foundation to the author and by ResearchGrants NS-Q7454.()7 from the National Insti:tute of Health andG8-32235 from the National Science Foundation to Donald A.Norman. The author gratefully acknowledies KathleenO'Connor's assistance in performing the experiment and DonaldA. Norman's guidance and support in all phases of this work.Additional thanks are due Norman H. Anderson, Lynn A.Cooper,~ David E. Rumelhart for their helpful comments on

. this researCh and earlier drafts of this paper.Dr. Palmer is now at the Department of PsYcholollY,

University of California, Berkeley, California 94720.

CONTEXTUAL SCINI TA.on OIJltT

519

520 PALMER

(Drawings B and C), the scene is an inappropriatecontext for the target object. Additionally, when a blankscene is paired with any object, there is no context forthe target object. If the contextual scenes influence theidentification of objects as sentences influence theidentification of words, then performance should be bestwhere there is an appropriate context biasing responsestoward the presented object, intermediate when there isno context at all, and worst when there is aninappropriate context biasing responses towardcontextually appropriate objects which were notpresented.

The inappropriate context condition is divided intotwo subconditions in order to study the interaction ofsensory and contextual factors in greater detail. In onesubcondition, the target object is shown after aninappropriate context, but is visually similar to an objectwhich is appropriate to the scene. In Figure 1, forexample, the mailbox (Drawing B) is inappropriate tothe kitchen counter scene, but is similar in shape to theappropriate object, the loaf of bread (Drawing A). In theother subcondition, the target object is not similar to theappropriate object, as illustrated by the drum (DrawingC) when paired with the kitchen counter scene. If thetarget presentation is brief enough that only partialinformation is extracted about the target object, and ifthat information is consistent with the sensoryproperties of an appropriate object, then responses arebiased toward the object which is consistent with boththe sensory and contextual information available, eventhough a different object was actually presented. Thus,there should be fewer correct responses in thesubcondition in which the target is visually similar to thecontextually appropriate object than in the subconditionin which the target is not similar to the contextuallyappropriate object. Furthermore, more errors of namingthe contextually appropriate, visually similar object areexpected in the former subcondition than in the latter.

METHODStimuli and Apparatus

Twelve pairs of objects were chosen so that the two items ofeach pair were similar in shape; one such pair is the loaf of breadand the mailbox shown as A and B in Figure 1. Some of the pairsof objects were approximately the same size in the real world;others were quite different in size. All objects were representedas line drawings of about the same size. For each object, a scenewas drawn to provide an appropriate context. The scene did notcontain the target object, but if it had, the appropriate positionfor the object would have been in the center of the scene.

The drawings of both scenes and target objects were projectedfrom 35-mm black and white negatives onto a light beige wallby a random-access projector. Thus, the pictures appeared aswhite line drawings on a dark gray background. The projectedimage of the contextual scenes sub tended approximately 12 degof visual angle (2 x 3 ft at a distance of 10 ft from the subjects),and the image of the target objects subtended approximately 2deg of visual angle (averaging 3 x 5 in. at 10ft). The exposureduration was controlled by a mechanical shutter. Dim roomillumination was provided by four red lights so that subjectscould see their answer sheets.

DesignFour contextual conditions were constructed by painng

objects with scenes: Appropriate context (A) in which theobjects were paired with their own contextual scenes, Nocontext (N) in which the objects were paired with blank scenes,Inappropriate context, similar object (Is) in which the objectswere paired with the scenes appropriate to the other object ofthe visually similar pairs, and Inappropriate context, differentobject (ld) in which the objects were paired with scenesappropriate to other objects in the experiment, but outside thevisually similar pairs.

All of the 24 objects and 18 of the 24 scenes (plus six blankscenes for the no context condition) were shown to each subject.Thus, for any given subject, six objects appeared in each of thefour contextual conditions. The 24 objects were divided intofour "object groups" which always appeared together in thesame condition. Different groups of subjects saw the objectgroups in different conditions, so that across all subjects, eachobject group (and, therefore, each object) appeared in eachcondition an equal number of times. This is the pairings factor:each of the. four object groups was paired with one of the fourcontextual conditions to form a particular pairing for a group ofsubjects. A Latin square design was used to assign the objectgroups to the four contextual conditions for the four pairingsrequired. This entire design was repeated with different groupsof subjects at four exposure durations: 20, 40, 60, and 120msec,

ProcedureWritten instructions were given to the subjects, informing

them of their task and explaining the appropriate, inappropriate,and no context conditions. Both the written instructions and alater verbal reminder strongly encouraged the subjects to guess.Each subject was given 24 trials composed of the followingsequence: (a) the presentation of a contextual scene or blankslide for 2 sec; (b) a delay of 1300 msec while the slide changed;(c) the presentation of the target object for either 20,40,60, or120 msec (always the same for any given subject); and (d) a20-sec interval during which each SUbject wrote the name of theobject he or she perceived and gave a confidence rating on afive-point scale. The order of presentation for each group ofsubjects was randomly generated by a PDP-9 digital computerwith the constraint that objects within the visually similar pairsnot appear with less than six trials between them. The computeralso co n trolled the random-access projector and thetachistoscopic shutter.

SubjectsSeventy-one students at the University of California, San

Diego, participated in the experiment as part of theirintroductory course requirements. The data from seven subjectswere eliminated: three due to equipment malfunctions, two forfailing to follow instructions, and two because they forgot theirglasses. All subjects were tested in groups of four or less.

RESULTS AND DISCUSSION

Each response was classified as being one of threetypes: the name of the presented object, the name of theother object of the visually similar pair (i.e., within-pairerrors), or the name of any other object. Nonresponseswere placed in the last category, but accounted for lessthan 0.1%of the data. All responses that were synonyms(e.g., "garbage can" and "trash can") or the names ofvery closely related objects (e.g., "pen" and "pencil")were classified together. From these response categories,

CONTEXT IN OBJECT IDENTIFICATION 521

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EXPOSURE DURATION (msec.) CONTEXTUAL CONDITION

Figure 2. Probability of naming the presented object as afunction of exposure duration for each contextual condition:appropriate context (A), no context (N), inappropriate context,different object Od). and inappropriate context, similar object(Is>. Theoretical curves represent the best fit solution for amodel of contextual effects.

th e pro babilities of correct identifications andwithin-pair errors were computed.

Correct ResponsesThe data on correct identifications are shown in

Figure 2 for all contextual conditions and exposuredurations. It is evident that the probability of correctlyidentifying an object is highest following an appropriatecontext (A), lower following a blank slide (N), and lowerstill following an inappropriate context (Ict) and (Is). Inaddition, performance in the Is condition (where thetarget is visually similar to the contextually appropriateobject) is inferior to performance in the Id condition(where the target is visually different from theappropriate object). This follows the pattern of resultsobtained in similar experiments using linguistic materials(e.g., Tulving & Gold, 1963; Reicher, 1969). It is quiteprobable, then, that the process through whichcontextual information affects perception is the samefor a variety of types of stimuli.

Performance improves as exposure duration increases,though the effect is not as strong as expected. Therelatively small influence of exposure duration may bedue to the lack of a masking field. If the afterimage islong relative to the actual exposure duration, its effectwould tend to mask the small differences in physicalexposure.

A three-way analysis of variance (Exposure by Objectsby Contextual Conditions) was performed on these data,pooled over subjects and pairings with objects treated as

Figure 3. Probability of naming the presented object in eachcontextual condition (A, N, !ct, and Is; see text) for four groupsof objects based on their overall level of recognizability: high(H), medium high (M+), medium low (M-), and low (L).Theoretical curves represent best fit solution of a model forcontextual effects.

a random variable. All main effects were significant:exposure, [F(3j69) = 9.31, P < .01]; objects,[F(23/1152) = 28.97, P < .001]; and contextualconditions, [F(3/69) = 43.30, P <.00Il.The interactionof objects with contextual conditions was alsosignificant, [F(69/1152) = 2.19, p < .01].

It seemed likely that the interaction of objects withcontextual conditions was due to a ceiling effect. Thatis, some of the objects were so recognizable that thedifferent contexts had little effect. To examine thispossibility, the 24 objects were rank ordered accordingto the proportion of correct responses for each objectover all exposures and contextual conditions. They werethen divided into four groups: high (H), medium high(M+), medium low (M-), and low (L), based on thismeasure of overall recognizability. The probability of acorrect response was then calculated for each group ofobjects in each contextual condition. These data areplotted in Figure 3. In addition to the large effect of thedifferent object groups, it is evident that both floor andceiling effects are responsible for the interaction. Theeffect due to different objects may be caused byvariations in a number of factors: the contrast of theslides, the dimensions of the drawings, the familiarity ofthe object depicted, or even the "typicality" of thedrawing as an instance of the intended object (cf. Rosch,in press).

A second analysis was performed using a four-factordesign in which exposure and pairings are factoriallycombined as between-subjects variables (subjects being

5:: PALMER

nested within the Exposure by Pairings design) andcontextual conditions as a within-subjects variable. Forthis analysis. the data were pooled over the objectsfactor. The results supported those of the previousanalysis. Because both subjects and objects are properlyconsidered random variables (see Clark. 1973), min F'was calculated for exposure and contextual conditions.Using this test, contextual conditions remained a highlysignificant factor. [min F' (3;103) = 29.34. P < .OOl},while exposure proved only marginally signiticant, [minF'(3/l01) = 3.69, p < .05].

Within-pair ErrorsThe effects of context can be further analyzed by

looking at the incorrect responses. In particular, theprobability of responding with the name of the otherobject of the similar pairs is shown in Figure 4. Asexpected, this kind of error is very unlikely with anappropriate context (Condition A), but is more likely inthe other conditions. The probability of a within-pairerror when there is no context (Condition N) gives ameasure of the visual confusability of the pairs of similaritems. Note that the probability of a within-pair error inthe Id condition is approximately the same as in the Ncondition. Thus, the Id contexts did not bias responsesaway from this kind of error whereas the A contexts did.The overwhelming effect, though, is that subjects madewithin-pair errors very frequently in the Is conditionwhere the error is consistent with the context as well asthe overall visual characteristics of the presented object.In fact. within-pair errors were made with about thesame frequency as correct responses in this condition.

The statistical analyses performed on the within-pairerrors were the same as those performed on correctresponses. The first analysis (Exposure by Objects byContextual Conditions, pooled over subjects and

pairings) indicated significant main effects forexposure.[F(3/69) = 3.13, P < .05],objects,[F(23/1l5 2) =21.43, P < .001] , and contextualconditions,[F(3/69) = 32.33, p < .001]. Both exposureand objects produced significant interactions withcontextual conditions: [F(9/207) =2.43, P < .05] and[F(69/1l52) = 2.58, p<.Ol], respectively. Theinteraction between exposure and contextual conditions

. is evident in the nonparallelism of the curves in Figure 4.To illuminate the source of the interaction betweenobjects and contextual conditions, the 24 objects wereagain divided into four groups based on overallrecognizability-the same groups used in analyzing thecorrect responses (see Figure 3). The probability ofresponding with the name of the other object in thevisually similar pair was then calculated for eachcontextual condition. These data, as plotted in Figure 5,reveal the strong floor effect responsible for theinteraction, and illustrate the considerable differencesbetween the particular objects used in the experiment.

The second analysis (including exposure, pairings,subjects, and contextual conditions, pooled over objects)supported the results of the first. Combining the resultsof the two analyses (with both subjects and objectsbeing treated as random variables) showed thatcontextual conditions was a highly significantvariable,[min F'(3/l29) = 22.33, p < .001], whileneither exposure nor the interaction of exposure withcontextual conditions reached statistical significance.

Confidence RatingsConfidence ratings were treated in two ways. First,

the raw confidence ratings were recorded. Then theconfidence ratings were signedi+ or -), conditional onthe category of the response. In analyzing for correct

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Figure 4. Probability of naming the other object of a visuallysimilar pair as a function of exposure duration for eachcontextual condition: appropriate context (A), no context (N),inappropriate context, different object (ld), and inappropriatecontext, similar object (IS>. Theoretical curves represent best fitsolution of a model for contextual effects.

CONTEXTUAL CONDITION

Figure S. Probability of naming the other object of a visuaUysimilar pair in each contextual condition (A, N, Id, and Is; seetext) for four groups of objects with different levels ofrecognizability: high (II), medium high (M+), medium low (M-),and low (L). Theoretical curves represent best fit solution of amodel for contextual effects.

CONTEXT I~ OBJECT IDE~lIFICAnON 523

A MODEL FOR CONTEXTUAL EFFECTS

where s, c, n > 1 and n represents the size of theresponse set.

To test this model against the present data, a numberof simplifying assumptions will be made. First, assumethat the context Ck affects only the logogen Rkcorresponding to the contextually appropriate object,Sb defined by the experimenter. Further, assume this

contextual scene might be inappropriate for thepresented object.

Statistical analyses support these observations.Approximate analyses of variance were performed oncell means using two designs. In the first analysis. the 64subjects were divided into four groups (balanced forexposure duration), yielding four observations for eachcell in Table I. In the second analysis, the 24 objectswere divided into four groups (balanced for level ofrecognizability), also yielding four observations per cell.The grouping of subjects and objects was necessitated bylow response frequencies in certain cells. The combinedresults of the two repeated measures analyses show astrong main effect of response category [minF'(2/ 11) = 30.08, P< .01], but no significant effectsdue to contextual conditions [min F'(3/11)= 1.47,P > .10] or to the interaction between responsecategories and contextual conditions [min F' < IJ.

This experiment shows that both the context and thesensory characteristicsof the presented object determinethe response made by the subject. A quantitativeassessment of this interaction between sensory andcontextual information is possible with the "logogen"model developed by Morton (1969, 1970). Mortonassumes the existence of internal informational units,called logogens, which are activated by a prioriexpectations. context, and the similarities of theirperceptual encoding with the item presented. In thepresent experiment, if context Ck and sensory signal Sjare presented, the activation of the logogen unit R, isdetermined jointly by the contextual biasing of Ck onRi, denoted by the variable Cki, and the sensoryactivation of Sj on Rio denoted by the variable Sji' Thetotal activation of a logogen is given by the product ofthe sources of information; that is, the joint effect of Ckand Sj on R, would be SjiCki. The probability of selectinga given unit for a response follows Luce's (1959) choiceaxiom: the probability is the ratio of the activation of aparticular unit to the sum of the activations of all units.Thus, for the presentation of context Ck and sensorysignal Sj, the probability of response R, would be

(1)p(Ri ICk,~) = ---n

~ Sjhckhh=l

Table 1Mean Confidence Ratings (and Frequencies) for Response

Categories and Contextual Conditions

Contextual Correct Within-Pair OtherCondition Responses Errors Errors

A4.53 3.67 2.13(319) (27) (38)

N4.15 3.28 2.10

(241) (72) (71)

Id4.05 3.62 2.37

(188) (65) (131)

Is4.20 3.78 2.49(153) (166) (65)

responses, confidence ratings were multiplied by +I ifthe response was correct, and by -I if the response was awithin-pair error or any other error. Similarly, inanalyzing for within-pair errors, confidence ratingsweremultiplied by +I if the response was a within-pair error.and by -I if the response was correct or any other error.These signed confidence ratings are, in effect, augmentedmeasures of the probability of correct responses andwithin-pair errors.

Signed confidence ratings essentially duplicated thepattern of results for response probabilities reportedabove. Ratings conditionalized on correct responses werehighest in the A condition, intermediate in the N and Idconditions, and lowest in the Is condition. Confidenceratings conditionalized on within-pairerrors were highestin the Is condition, intermediate in the N and Idconditions. and lowest in the A condition. Analyses ofvariance for signed confidence ratings yielded resultsnearly identical to those for response probabilities.

Raw confidence ratings were also analyzed byresponse categories and contextual conditions. Mean rawconfidence ratings were calculated for correct responses,within-pair errors, and other errors in each of the fourcontextual conditions. As indicated in Table 1,confidence ratings are highest for correct responses,lower for within-pair errors, and lowest for other errors.Within response categories, however, the differentcontextual conditions produce only slight perturbationsin confidence ratings. This is in contrast to the largedifferences between contextual conditions in thefrequencies of response types (Table I, parenthesizednumbers). These results indicate that contextualinformation is important in selecting a response,but notin rating confidence in that response. Once the responsehas been made, confidence is judged by assessing the"goodness of the fit" between the perceptual dataextracted from the stimulus and the perceptual dataencoded in the internal representation of the namedobject. It is important to note, however, that this is areasonable strategy, given that subjects know the

Xote-Numbers in parentheses denote response frequencies forthe cells.

524 PALMER

contextual activation has a constant value, c, for allappropriate object-scene pairs, and a value of I (i.e., noeffect) for all other pairings. Thus, for example, thekitchen counter scene in Figure I exerts an activation ofc units on the logogen corresponding to the response"loaf of bread" and activations of I on the logogenscorresponding to any other response. Second, assumethat the stimulus Sj affects only two logogens: Rj whichcorresponds to the presented object and Rj, whichcorresponds to the other object of the visually similarpair in the experiment. For mathematical simplicity, wefurther assume that these sensory activations areconstant for all presented and within-pair objects,denoted by the parameters SI and 52, respectively. Thus,the presentation of the loaf of bread (Figure I, DrawingA) exerts a sensory activation of SI units on the logogenfor "loaf of bread", S2 units on the logogen for"mailbox", and I unit on all other logogens. Similarly,the presentation of the mailbox (Figure I, Drawing B)exerts sensory activations of SI, 52, and I on thelogogens for "mailbox," "loaf of bread," and any otherresponse, respectively.

Now, it is easy to derive the equations for theresponse probabilities in the present experiment. Bymaking the stated simplifications of Equation Iaccording to the experimental situations in each of thefour contextual conditions, we derive the eightequations required to provide a quantitative assessmentof the data for correct responses and within-pair errors.These equations are presented in Table 2.

Exposure Duration and Contextual ConditionsThe first set of data to be predicted is shown in

Figures 2 and 4, representing the effects of exposureduration and contextual conditions on the probabilitiesof making correct identifications and within-pair errors.There are four basic parameters to the model equations:SI, S2, c, and n. The sensory parameters, Sl and 52, areexpected to vary with the amount of stimulusinformation extracted and, therefore, with exposureduration. Thus, we require four separate estimates of SI

and S2 for the four exposure durations, yielding eightsensory parameters. However, the context parameter, c,and the response set size, n, are expected to remaininvariant over different exposure durations. Altogether,then, 10 parameter estimates are required to fit the data.According to the psychological interpretations of theparameters, SI should increase monotonically withexposure duration as more sensory information isextracted. However, S2 should begin to decrease (afteran initial increase) as specific, discriminating features ofthe presented object are extracted, thus decreasing thesimilarity between the extracted information and theencoding of the other object of the visually similar pair.

The 10 model parameters were estimated by acomputer application of a minimum chi-square

procedure. The best fitting solution, as shown in Figures2 and 4, exhibits a good match to the data, yielding[X2 (22) = 22.90, P >.40 and a root mean squareddeviation (RMSD) of .036. In addition, the sensoryparameters behave in the expected manner: SI increasesmonotonically with increasing exposure and S2 shows aninitial increase followed by a decrease at the longestexposure (see Table 3A). The only difficulty withinterpreting the parameters is that the lowest chi-squareis obtained with the response set size, n, estimatedbetween .four and five. This value is much too small torepresent the number of possible responses. Theinconsistency between n's estimated value and itspsychological interpretation will be discussed after thesecond test of the model is presented.

Object Recognizability and Contextual ConditionsThe basic model can also be used to predict the data

for the interaction of objects with contextualconditions, as shown in Figures 3 and 5. Recall thatthese data were compiled by dividing the 24 objects intofour groups according to their overall level ofrecognizability: high (H), medium high (M+), mediumlow (M-), and low (L). Response probabilities were thencomputed for each level of object recognizability withineach contextual condition.

The model analysis for these data is similar to thatjust described. The main change is that differentestimates of the sensory parameters are made for each ofthe four object groups (rather than exposure durations).Thus, there are 10 parameter estimates required to fitthe data. Naturally, we expect that the values of SI willincrease with increasing recognizability of the objectgroups. It is also likely (though not necessary) that thevalues of S2 will decrease with increasing recognizabilityof the object groups.

The 10 parameters were estimated for the 32

Table 2Equations for Response Probabilities

No Context Condition (N)CR * P(RiICo ,Si) = SI/(SI + S2 + n - 2) (2)WPEt P(RiICo,Si)=S2/(SI +S2 +n-2) (3)

Appropriate Context Condition (A)CR P(RjICj,Si) = SIc/(St C + S2 + n - 2) (4)WPE peRi IChSi) = S2 /(SI C + S2 + n - 2) (5)

Inappropriate Context/Similar Object Condition (Is)CR P(RiICj ,Si) = SI/(SI + S2 C + n - 2) (6)WPE P(RjICj,Si) = S2 c/(si + 82C + n - 2) (7)

Inappropriate Context/Different Object Condition (ld)CR P(RiICk,Si) = SI/(SI + S2 + C + n - 3) (8)WPE P(RjICk,Si) = sz/(St + Sz + C + n - 3) (9)

Note-Subscripts i and j denote objects (and their associatedcontextual scenes) within Visually similar object pairs. Subscriptk denotes objects (and scenes) outside these pairs. Co is ablank scene..Com!ct responses fWithin-pairerrors

CONTEXT IN OBJECT IDENTIFICATION 525

Table 3Parameter Estimates

A. Exposure Duration and Contextual ConditionsParameter n c s, s. 5, s. S2 S2 S2 S2 X2 RMSDtExposure All All 20 40 60 120 20 40 60 120Estimate 4.53 3.37 5.22 5.73 9.52 10.33 1.83 2.41 2.87 2.32 22.90* .036

B. Object Groups and Contextual ConditionsParameter n c s, s, S, SI S2 S2 S2 S2 x2 RMSDtObj. Group All All H M+ M- L H M+ M- LEstimate 4.98 4.69 63.28 14.39 4.84 2.23 2.00 2.00 2.31 4.00 30.60** .034

fRoot mean squared deviation "df = 22, p > .40 **df = 22, p > .10

experimental points using a minimum chi-squareprocedure. Inspection of Figures 3 and 5 shows that thebest fitting solution of the model again matches the dataquite closely, RMSD = .034. The statistical fit for thesedata, [X2 (22) = 30.60, p > .10], is not as good as forthe previous data, but the difference is due to the largernumber of points approaching the extreme probabilitiesof zero and one.

As expected, the SI parameter increases substantiallywith increasing recognizability of the object groups(seeTable 3B). In contrast, the S2 parameter remainsrelatively constant, increasing only for the leastrecognizable group of objects (L). The fact that theestimate of S:2 is greater than that of SI is nottroublesome because the "sensory parameters" SI and S2

in this simplified application of Morton's model actuallyincorporate any a priori biases which exist (due tofrequency, for example). In comparing the two analysessummarized in Table 3, note that the estimates of theresponse-set size, n, and contextual activation, c, agreewell.

Sizeof the Response SetThe estimated valueof n is approximately five in both

analyses. This is a significant problem for a model inwhich n represents the total number of responses. Onepossible explanation is that the model may be quiteinsensitive to the size of n, even though the best fittingestimate is small. Additional analyses, however, showedthat acceptable fits could not be obtained with n fixedat values largeenough to be interpreted as the number ofpossible responses. Another possibility is that theassumptions made in simplifying the general model areresponsible for the low estimate of the response set size.

A simpler solution to the problem, however, is toreinterpret n as the size of the "candidate set" assuggested by Rumelhart (in press; Rumelhart & Siple,1974). With this interpretation, n represents the numberof responses under consideration after eliminatingresponses which are contradicted or unlikely given theinformation gathered to that point.The good fit of themodel to the present data with a low value for npresumably indicates that at the exposure durations used(without a masking field), enough sensory information

has been gathered to eliminate all but a few of thepossible responses.

Given the high ratio of model parameters to datapoints, the success of the model must be interpretedwith some caution. I do not claim that the logogenmodel is the only one which can account for the presentdata or even that it is the best model for these data.However, it has been applied to a variety of contextualeffects with creditable success (Morton, 1969, 1970).The present results simply add to existing evidence thatthis type of formulation is viable as an abstractcharacterization of how contextual and sensory data areintegrated during perceptual identification.

A more complete characterization of perceptualidentification requires the elaboration of the abstractmodel in terms of information structure and processing."Logogens" may be further specified in terms of theirrepresentational format (features, propositions, orwhatever) and informational content. The relationshipsbetween particular logogens encode specific worldknowledge-e.g., that a loaf of bread is likely to belocated on a counter in a kitchen and that a mailbox islikely to be located in the front yard of a house. Theserelational interconnections, then, are the mediumthrough which contextual information exerts itsinfluence on perceptual interpretation. An informationprocessing model of perception along these lines ispresented in Palmer(in press).

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(Received December 21. 1973.Revision received November 18. 1974.)