/. exp. Biol. 149, 1-19 (1990)Printed in Great Britain © The Company of Biologists Limited 1990

A DIRECTIONALLY SELECTIVE MOTION-DETECTINGNEURONE IN THE BRAIN OF THE LOCUST:

PHYSIOLOGICAL AND MORPHOLOGICALCHARACTERIZATION

BY F. CLAIRE RIND

Department of Biology, The University, Newcastle upon Tyne, NE1 7R U

Accepted 25 October 1989

Summary

1. The anatomy and physiology of a directionally selective motion-detecting(DSMD) neurone in the locust are described. The neurone was descending, withthe cell body in the protocerebrum. The axon lay in the dorsolateral quadrant ofthe nerve cord and has been traced as far as the metathoracic ganglion. Itarborized, ipsilateral to the cell body, from the dorsal intermediate tract (DIT) inthe suboesophageal and thoracic ganglia.

2. The neurone was binocular and sensitive to motion in the horizontal plane. Ithad a preferred direction backwards over the ipsilateral eye and forwards over thecontralateral eye. Motion in the opposite direction suppressed the discharge,which had a frequency of 5-20 spikes s"1 at resting membrane potential.

3. The neurone showed a clear directional response to stimuli with temporalfrequencies between 0.7 and 44Hz, with a peak response at 11-22 Hz. Itresponded with spikes to light ON and light OFF.

4. The neurone responded directionally to spatial frequencies of 0.28 cyclesdegree"1 (3.7° stripe period) to above 0.025 cycles degree"1 (40° stripe period).The maximum response was at around 0.035 cycles degree"1 (29° stripe period).

5. No evidence of adaptation was seen in the responses of the neurone to real orapparent continuous horizontal motion in either the preferred or the nulldirection.

Introduction

Neurones in the visual system which are excited by movement in one particulardirection (preferred direction) and inhibited by movement in the oppositedirection (null direction) have been found in a wide range of invertebrate andvertebrate species. Such directionally selective motion-detecting (DSMD)neurones are thought to underlie an important class of visually guided responses,termed optomotor responses, whereby an image is stabilized in a fixed position onthe retina. In insects, a role in optomotor behaviour for descending DSMD

Key words: identified neurone, directionally selective motion detector, descending inter-neurone, visual system, locust.

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neurones (those that have axons descending from the brain) is supported by directphysiological observations in the moth (Rind, 1983a,b), by anatomical and indirectphysiological evidence in the fly (Strausfeld and Bassemir, 1985; for a summary ofthe physiology see Rind, 19836), and by indirect physiological evidence in thelocust (Kien, 1974a,6, 1977).

A mathematical model has been proposed for the types of computationaloperations which might occur in the central nervous system of a beetle to controloptomotor behaviour (Hassenstein and Reichardt, 1956) and has subsequentlybeen found to describe direction-selective motion detection in other insects and inhumans (Poggio and Reichardt, 1973; van Santen and Sperling, 1984, 1985). Themodel involves a correlation between neighbouring input channels. The inputsignals from two retinal sampling stations are multiplied after the signal from oneof the stations has been delayed or low-pass filtered with a characteristic timeconstant. As a result of these operations, the response of the detector depends notsimply on the velocity of the stimulus, but also on the spatial frequency of thestimulus. The response typically shows a maximum for a certain temporalfrequency. The determination of this response optimum allows an estimation ofthe time constant of the low-pass filter. Also, the spatial frequency response of thedetector can be used to reveal the spatial organization of the detector inputs. Theaverage response of the detector becomes negative (that is, directional selectivityis reversed and the null direction becomes the preferred) when the spatialwavelength of the stimulus becomes smaller than twice the angular separation ofthe sampling stations, owing to geometric interference. This reversal has beenobserved in the response of DSMD neurones in the fly lobula and occurs at theangle separating individual ommatidia, implying that photoreceptors withinneighbouring ommatidia form the sampling stations (Eckert, 1973; Buchner,1984).

The correlation model introduced by Reichardt and coworkers describes thedirection-selective process at a phenomenological level, not at the level of synapticinteractions between identified neurones in the visual system (for a review, seeReichardt, 1987). This approach has provided a framework for hunting for specificoperations between neurones. In the locust, responses from DSMD neurones havebeen described at three levels in the visual system: in the second optic neuropile(the medulla, Osorio, 1986), from the stalk of the optic lobe (Kien, 19746, 1977)and from the circumoesophageal connectives (Kien, 19746,1977). Recording fromvertically sensitive DSMD neurones in the medulla has shown that the interactionconferring a directional response occurs between adjacent units or channelsseparated by a single interommatidial angle from one another and depends, atleast in part, on an inhibitory interaction between adjacent channels sensitive toluminance change (Osorio, 1986). This supports the proposal of a vetoingoperation, proposed by Barlow and Levick (1965) to explain in cellular terms theprocesses underlying directionally selective motion detection in retinal ganglioncells of the rabbit. In contrast, in the fly eye, by stimulating neighbouringrhabdomeres numbers 1 and 6 within a single ommatidium (and hence two

Locust directionally selective movement detector 3

neighbouring cartridges), Riehle and Francheschini (1984) found evidence that theresponse of the lobula HI neurone (Hausen, 1976) depends on an excitatoryinteraction between adjacent channels. The DSMD neurones of the distal locustmedulla project to the protocerebrum without arborizing in the lobula. They arelocalized to the lateral ventral parts of the medulla and are sensitive to verticalmovement (Osorio, 1986).

An adaptation in response to continuous movement is shown by all DSMDneurones identified in the fly lobula, and in the cat cortex, and by those DSMDneurones whose responses have been measured indirectly in the human, usingpsychophysics (Hausen, 1982; Maddess and Laughlin, 1985; Greenlee andHeitger, 1988; Maddess et al. 1988). Maddess and Laughlin (1985) suggest that thisadaptation sharpens the response of the neurone to changes in temporalfrequency. Adaptation has been shown to shift the time constant of thedirectionally selective motion-detection process (de Ruyter van Steveninck et al.1986). The time constant of a motion detector describes the limits of resolvabletemporal frequency, and thus any process which has been shown to change thetime constant has important functional consequences for the detection of motion(Egelhaaf and Reichardt, 1987; Guo and Reichardt, 1987).

In this paper the morphology of a unique descending DSMD neurone in thelocust brain is described. The responses of the neurone to real and apparentmovement are used to test the apparent generality of the direction-selectivemotion-detection process, and also to test the fitness of the Hassenstein andReichardt correlation model of directionally selective motion detection to describethe direction-selective response of the locust horizontal-motion detectors.

Materials and methods

Adult Locusta migratoria were purchased from Animal Magic, Brighton.Experiments were performed at 17-19°C, using two different procedures.

Response characteristics recorded from the axon in the nerve cord

The locust was mounted intact, dorsal side up, and a dorsal midline incision wasmade along the thorax. The thorax was gently opened, the gut was removed, and aplastic-coated platform was manipulated under the cervical connectives. Record-ings were made with glass electrodes, filled with 2 mol 1 ~1 potassium acetate, whichhad d.c. resistances in saline of 10-20MQ. Recordings were made from the axonof the neurone either intracellularly or just extracellularly, and window circuitswere used to discriminate extracellularly recorded action potentials. The locustviewed a television screen (Joyce Electronics, Cambridge) placed either 15 or30cm from the eye. At 15 cm, the pattern on the screen subtended 90°x60.5° atthe eye, and was aligned parallel to the flat sideways-looking ommatidia of oneeye, in the equatorial region. The blank screen had a luminance of 160 cd m~2. Thepattern viewed during experiments was a sinusoidally modulated luminanceprofile of maximum luminance 160cdm~2 and Michelson contrast of 0.5. The

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profile was produced and controlled by a PDP 11/73 computer and had a framerepetition rate of 200 Hz. The luminance profile (stripes) was aligned vertically,and moved horizontally across the screen at temporal frequencies between 0 and62 Hz. The spatial frequency of the stripes ranged from 0.025 to 0.8 cyclesdegree"1. The angular subtense of the pattern could be reduced horizontally orvertically by windowing.

In addition to the Joyce Electronics screen, two arrays of 18 small, greenrectangular LEDs, each subtending 5°x8° at the eye, were also used. The LEDswere used to produce apparent movement of a striped pattern. The two arrayswere each aligned horizontally over an 8°x90° arc just ventral to the equatorialregion of the eye. Within each array, the LEDs were controlled to produceapparent movement of a striped stimulus, each LED within the arc representing astripe. Six LEDs per array were illuminated simultaneously, each separated bytwo unilluminated LEDs. Apparent movement consisted of sequential illumi-nation of the six neighbouring LEDs. The sequential illumination of the LEDs wasrecorded as a stepping trace, each step indicating extinction of six LEDs andsimultaneous illumination of their neighbours. The sequential illumination orextinction proceeded from left to right for eight sequences, and then reversed foreight. In all figures, an upward step indicates an anticlockwise apparent movementto the left (backwards over the left eye, forwards over the right) and a downwardstep indicates a clockwise apparent movement to the right. Apparent movement,such as that produced by LED illumination, has been found to be effective inexciting motion-detecting neurones in the fly (Pick and Buchner, 1979; Riehle andFranceschini, 1984) and, more recently, in the locust (Osorio, 1986; Rind, 1987).

The protocerebral, descending direction-selective motion-detecting(PDDSMD) neurone was identified in these experiments by the position of itsaxon in the connectives and by its characteristic directionally selective response toapparent horizontal movement over both eyes. In particular, the neurone wasidentified by its discrete response to each apparent movement in the preferreddirection, and its silence when movement was in the null direction. A neuroneidentified in this way, in a subsequent experiment, was filled via its axon in thecervical connectives and was found to have the same morphology in the brain anda similar axon position in the suboesophageal connectives to those PDDSMDneurones filled and identified after recording from their cell bodies in the brain.

Response characteristics recorded intracellularly from the cell body in theprotocerebrum

The locust was dissected and the brain exposed following the method describedby Rind (1987). Intracellular recordings were made using glass capillary microelec-trodes filled with a saturated solution of hexamminecobaltic chloride. Electrodeshad resistances, in saline, of 30-60MQ. All 18 neurones characterized werestained using 15 nA pulses of positive current every second for 1 h. The brains werefixed in 10% formaldehyde buffered to pH7. Stained neurones were intensified(Bacon and Altman, 1977) and later drawn in whole-mounts of the brain and optic

Locust directionally selective movement detector 5

lobe. Some preparations were embedded in Spun or LEMIX (EMscope) resin.The suboesophageal, prothoracic and mesothoracic ganglia were serially cross-sectioned (20 fan sections) to determine in which tract the axon was located, andwhere the branches projected within each ganglion. In addition, cross-sections,5/zm thick, were taken of the axon in the suboesophageal connectives. In severalpreparations, neurones on both sides of the same preparation were characterizedand stained.

Apparent movement of a striped pattern was produced by two arrays of 18LEDs, as described in the previous section. The two arrays were placed andcontrolled independently of each other. In most experiments, each array wasaligned horizontally over an 8°x90° arc in the equatorial-dorsal region of the eye.The arrays could also be aligned vertically to test sensitivity to vertical apparentmotion.

Results

Identification of the neurone: its response to horizontal movement

The protocerebral DSMD neurone was identified in these experiments by theposition of either its axon in the cervical connectives or its cell body in theprotocerebrum, combined with its characteristic directionally selective response toapparent horizontal movement over both eyes. In particular, the neurone wasidentified by its discrete response to each apparent movement at a velocity of90° s"1 and below in the preferred direction, and by its silence in response tomovement in the null direction (Fig. 1). The neurone whose response is shown inFig. 1 had a cell body in the right protocerebrum. Spikes and EPSPs were recordedin the cell body of the neurone. Depolarizing the neurone using 1-5 nA of currentinjected at the cell body led to spikes being generated by the EPSPs. In theabsence of stimulus movement, the neurones had a resting discharge of 5-20spikes s"1. Apparent movement in the preferred direction (Fig. 1 backwards overthe ipsilateral, forwards over the contralateral eye) increased this resting dis-charge, whereas movement in the null direction (Fig. 1 forwards over theipsilateral, backwards over the contralateral eye) suppressed the discharge. Theexcitatory input in the preferred direction and the suppression in the null directionwere received from both eyes (Fig. 1C,D). At 17-19°C this response had a latencyof 65-80 ms from the apparent movement step to the first upward inflection in theresponse at the neurone. No IPSPs were observed in response to movement in thenull direction over either eye, even when the neurone was depolarized by injectedpositive current to accentuate IPSPs. The response of the neurone in the preferreddirection did not wane over the course of the experiment. Horizontal directions ofmovement produced much stronger responses than those produced by verticalmovements.

On 17 separate occasions, neurones with the response characteristics describedabove were stained intracellularly, and the following morphological characteristicswere consistently revealed. In one preparation (Fig. 2) the neurones on both sides

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jluim5mV

Fig. 1. Response to movement of a visual stimulus, recorded Lntracellularly from thecell body of the PDDSMD neurone in the right protocerebrum. The response of theipsilateral cell (after Rind, 1987) was monitored indirectly via an extracellularrecording from the axon of the DSMD neurone in the cervical connectives (arrow intraces A,B,D)- The non-directional LGMD1 is not excited by stimuli which excitePDDSMD. (A) Resting discharge in the absence of apparent movement;(B-D) responses of the neurone to apparent horizontal movement at 180° s"1 to theright (downward stepping on bottom trace) and the left (upward stepping on bottomtrace) (B) over both eyes, (C) over the right eye and (D) over the left eye.

of the same preparation were filled. It was concluded from the combinedmorphological and physiological evidence that there was only one such neurone oneach side of the animal.

Identification of the neurone: its morphology

The 25-30//m diameter cell body of the protocerebral, descending direction-selective motion-detecting neurone (PDDSMD neurone) was located in a cortexof cell bodies on the posterior slope of the protocerebrum. The cell body lay withinthe V formed by the tracheae entering the posterior slope at the protocerebrum,50 ton ventral and 30 /xm more medial to the conspicuous DCMD (O'Shea et al.1974) cell body (Figs 2, 3). A 5-10 /j,m diameter neurite emerged from the cellbody and projected ventrally for 90 /zm, before giving off a series of five brancheswhich projected medially for 100 ̂ m. No branches crossed the midline of thebrain. The first large (>7/xm) process projected laterally for 200 [xm towards theipsilateral optic lobe. A second, and sometimes a third, large process emergedfrom the neurite 20-50/xm after the first, and projected both laterally towards the

Locust directionally selective movement detector 7

Fig. 2. Morphology of the left and right PDDSMD neurones in the brain. (A) Cameralucida drawings from whole-mounts of two neurones stained in the same preparation.The brain is viewed from behind. (B) Cross-section through the circumoesophagealconnective showing the location of the stained axon (arrowhead) of the rightdirectionally selective motion-detecting neurone drawn in A.

ipsilateral optic lobe and anterioventrally towards the deuterocerebrum. Theseanterior processes did not enter the antennal lobe of the deuterocerebrum. Themain process of the neurone narrowed slightly after the emergence of the second(or third, if present) large branch, and bent 25° towards the midline of the brainbefore producing 5-7, 1/xm diameter processes in the tritocerebrum, whichprojected 20-30/an from the axon of the neurone. These processes were thickerthan the processes of the neurone in the protocerebrum. The 10-15/zm diameteraxon of the neurone then projected to a position in the dorsolateral quadrant of

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Fig. 3. Morphology of the left PDDSMD neurone in (A) the brain, and (B) thesuboesophageal, (C) the prothoracic and (D) the mesothoracic ganglia. Camera lucidadrawings from whole-mounts of brains viewed either (A) from behind or (B-D)dorsally. Scale bars, 100 fan (A), 200/xm (B-D).

the ipsilateral connective (Fig. 2B). In Fig. 2, the left and right neurones havebeen filled in the same preparation. The inset shows a 5/an diameter sectionthrough the right connective, at the level indicated by the arrow. The stainedprofile of the neurone is indicated by an arrowhead. The dorsally located DCMDaxon is the largest profile in the nerve cord. The PDDSMD neurone has been filledfrom the cervical connectives as far as its projections into the mesothoracicganglion (Fig. 3). Its branches in the suboesophageal, pro- and mesothoracicganglia were all restricted to the dorsal, ipsilateral half of the ganglion (Fig. 3).The axon projected through the suboesophageal, prothoracic and mesothoracicganglia in the lateral, dorsal quadrant of the dorsal intermediate tract (DIT)(Tyrer and Gregory, 1982). The mediolateral extent of the arborizations in the

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Fig. 3. Morphology of the left PDDSMD neurone in (A) the brain, and (B) thesuboesophageal, (C) the prothoracic and (D) the mesothoracic ganglia. Camera lucidadrawings from whole-mounts of brains viewed either (A) from behind or (B-D)dorsally. Scale bars, 100/an (A), 200/xm (B-D).

the ipsilateral connective (Fig. 2B). In Fig. 2, the left and right neurones havebeen filled in the same preparation. The inset shows a 5/an diameter sectionthrough the right connective, at the level indicated by the arrow. The stainedprofile of the neurone is indicated by an arrowhead. The dorsally located DCMDaxon is the largest profile in the nerve cord. The PDDSMD neurone has been filledfrom the cervical connectives as far as its projections into the mesothoracicganglion (Fig. 3). Its branches in the suboesophageal, pro- and mesothoracicganglia were all restricted to the dorsal, ipsilateral half of the ganglion (Fig. 3).The axon projected through the suboesophageal, prothoracic and mesothoracicganglia in the lateral, dorsal quadrant of the dorsal intermediate tract (DIT)(Tyrer and Gregory, 1982). The mediolateral extent of the arborizations in the

Locust directionally selective movement detector 9

suboesophageal, pro- and mesothoracic ganglia exceeded the dorsoventral extentby a ratio of 3:1.

Temporal frequency response

To quantify the response of the PDDSMD neurone to real movements, spikeswere recorded from its axon in the cervical connectives in locusts with intact heads.A pattern subtending 90°x60.5° at the eye could be made to move at variousvelocities across a television screen, viewed by one of the locust's eyes. First, theoptimum spatial frequency was selected for each neurone. Next, the responses(spikes s"1) to stripe movements at a series of seven different temporal frequencieswere recorded (Fig. 4). Each temporal frequency was presented 10 times, inrandom order (that is N=\Q). Temporal frequencies in the range 0.71-5.5 Hz werepresented for 2.5 s and the mean spike rate sampled at 10-ms intervals over 1.75 s.Temporal frequencies between 7.8 and 62.4Hz were presented for Is and themean spike rate sampled at 10-ms intervals over a 0.5-s period. The meanresponses of the neurone during the sampling period (and standard error of themean) were plotted against temporal frequency (Fig. 4A,B). The response to atemporal frequency of 5.5 Hz measured throughout the presentation time of thestimulus is shown for motion in the preferred (Fig. 4Ci) or null directions(Fig. 4Cii) or in the absence of movement (Fig. 4Ciii). There was no modulationof the response to movement in either the preferred or null directions at any of thetemporal frequencies tested. The peak in the response at temporal frequencies of11-22 Hz and the lack of a directional response above 44 Hz were consistent inseven preparations. The response was consistent between the two neurones oneither side of an animal. A clear directional response was given at the lowesttemporal frequencies tested (0.7Hz).

Spatial frequency response

The responses to a series of different spatial frequencies of pattern(0.025-0.8 cycles degree"1) were recorded from the PDSMD axon in the cervicalconnectives (Figs 5,6). The selected velocity of pattern movement was 15.6Hz, atwhich the neurone gave an optimum response in the preferred direction. Patternswere moved in either the preferred or the null direction for 0.5 s. Each separateexposure to one spatial frequency was given 10 times in random order. Thesubtense of the pattern on the screen could be varied, as could the position of thescreen relative to the locust's eye. The screen was either 15 cm (Fig. 5) or 30cm(Fig. 6) from the eye. Thus, the range of spatial frequencies was 0.025-0.4 cyclesdegree"1 (Fig. 5) or 0.05-0.8 cycles degree"1 (Fig. 6). The response of theneurone to the largest pattern at 15 cm from the eye is given in Fig. 5A. Theresponse in the preferred direction increased as the spatial frequency of thepattern decreased, with a maximum at spatial frequencies of 0.035 cycles degree"1

(i.e. 29° stripe period). There was no clear directional response to spatialfrequencies of 0.4 cycles degree"1 (i.e. 2.5° stripe period) and above. Reducingthe amount of pattern shown (Fig. 5B,C) lowered the response of the neurone to

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6Or A

O Preferred direction• Null direction<i> No movement

coa.

1.0 1.4 2.0 2.8 4.0 5.5 7.8 11.0Temporal frequency (Hz)

15.6 22.0 31.2 44.1 62.4

200

100

0

_ 200i

I 100'B." 0

200

100

0

Ci— Preferred direction at 5.5 Hz

Ci. 0 5• Null direction at 5.5 Hz

1.0 1.5 2.0 2.5

Cii,No movement

1.0 1.5 2.0 2.5

1 , . . L ^ i . iwi n.l.Rn-* _ nfi- n _

0.5 1.0 1.5Time (s)

2.0 2.5

Fig. 4. Temporal frequency response of the PDDSMD neurone. The mean spike rateis plotted as a function of temporal frequency. Stimuli of optimal spatial frequency forthis neurone (period of 29°) were used. Each temporal frequency was presented 10times in random order. The stimuli were presented as two series of seven temporalfrequencies. (A) A low temporal frequency series in which each frequency waspresented for 2.5 s and spike rate sampled over a 1.75-s period. Standard errors are alsoshown. (B) A high temporal frequency series in which each frequency was presentedfor Is and spike rate sampled for 0.5s. Even at the lowest temporal frequencies thelocust viewed an entire luminance profile cycle. As shown in C for a temporalfrequency of 5.5 Hz, the response of the neurone is unmodulated. Each bin is 10 ms inwidth and represents the total number of spikes s"1 for the 10 presentations of eachtemporal frequency. Recordings were made from the axon of the neurone in thecervical connectives.

5 25

ex1/5

Locust directionally selective movement detector

50r- A 90°x60.5°

11

« 25

o Preferred direction• Null directiono Resting discharge

J0.025 0.035 0.05 0.07 0.1 0.14 0.2 0.28 0.4

50r- B 90°x8° 50i—

25

45°x8°

J0.0250.035 0.05 0.07 0.1 0.14 0.2 0.28 0.4 0.0250.035 0.05 0.07 0.1 0.14 0.2 0.28 0.4

Spatial frequency response (cycles degree"1)

Fig. 5. Spatial frequency response of the PDDSMD neurone. Mean spike rate isplotted as a function of spatial frequency (cycles degree"1). The pattern subtense overone eye was (A) 90°x60.5° (B) 90°x8° or (C) 45°x8°. The pattern had a temporalfrequency of 15.6Hz (the optimum for this neurone; see Fig. 6 from the sameneurone). Each point is the mean response to 10 presentations of each spatialfrequency for 0.5 s and given in a random order. Standard errors (which were small)are also shown.

pattern movement. It also revealed an apparent crossing over of responses tomovement in the preferred and null directions at spatial frequencies around0.28 cycles degree"1 (i.e. 3.7° stripe period). This apparent crossing over wasinvestigated further by increasing the distance from the eye to the screen (from 15to 30 cm) and, hence, changing the range of spatial frequencies (Fig. 6). When thiswas done, the spatial frequency at which there was no clear directional responsewas 0.4 cycles degree"1 (i.e. 2.5° stripe period). At 0.56 cycles degree"1 (1.9°stripe period) the response to motion in the preferred direction and null directionalso appeared to reverse.

Adaptation of the response

Responses recorded intracellularly from the cell body in the brain, or from theaxon in the neck in a minimally dissected locust, showed no sign of adaptation tomaintained moving stimuli. Fig. 7A,B shows the response of PDDSMD in a

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251— 53°8'x32°42'

_'S.

O Preferred direction

• Null direction

A Resting discharge

0.05 0.07 0.1 0.14 0.2 0.28 0.4 0.56 0.8Spatial frequency response (cycles degree"1)

Fig. 6. Spatial frequency response of the PDDSMD neurone. Mean spike rate isplotted against spatial frequency (cyclesdegree"1). The pattern subtended was53°8'x32°42' over one eye and had a temporal frequency of 15.6Hz. See text forfurther details of stimulus presentation. Standard errors are shown.

minimally dissected locust to real movement at speeds of 450° s"1 (16 Hz) using asinusoidally modulated luminance profile (stripe period) with a spatial period of29° (an optimum period for this neurone) viewed on a Joyce screen. The patternwas a bright (160cdm~2) high-contrast (0.5) pattern subtending 55°x40° over oneeye. First, the optimal spatial and temporal frequency of the pattern weredetermined for each neurone. Then followed three possible 20-s blocks ofstimulus. Each of the three possible blocks was repeated 10 times in randomsequence. The initial 5 s of the 20-s block consisted of one of three adaptingstimuli: (1) the optimal stimulus in the preferred direction; (2) the optimalstimulus moving in the null direction; (3) a blank screen. Each of these 5-sadapting exposures was immediately followed by a 5-s test exposure to the optimalstimulus moving in the preferred direction, then by a 10-s recovery period beforethe next adapting stimulus was delivered. The response was plotted during each0.5 s of the 5-s test exposure, and a comparison was made between the responsesfollowing the three possible adapting stimuli. The first data point in each graph wastaken from the last 0.5 s of the adapting stimulus. There was neither a clear trendin firing rate during the course of the test stimulus, nor any consistent difference inthe response to the test stimulus following the adapting stimuli. During theseexperiments, a clear ON response was produced by changing from a blank screento either the null or the excitatory stimulus. This response accounts for the highresponse to the first 0.5 s of the test stimulus following the blank-screen adaptingstimulus. Fig. 8 shows the intracellularly recorded response to 18 s of continuousapparent movement, at 90°s"' , of a pattern subtending 8°x90° over each eye.

Locust directionally selective movement detector 13

50 r— A

25

tPreceding 5-sadapting stimulus was:

O movement in the preferred direction,

• movement in the null direction,

O no movement (blank screen).

50 r— B

25

I

v V V *•* Preceding 5-sadapting stimulus was:o movement in the preferred direction,

# movement in the null direction,

O no movement (blank screen).

1 2 3 4 5Time (s)

Fig. 7. (A,B) Lack of adaptation in the response of PDDSMD. Spike rate is plottedagainst time during 5 s of optimal stimulus in the preferred direction, which followed 5 sof optimal stimulus in the preferred direction, or in the null direction or a blank screen.The first data point was from the last 0.5 s of the adapting stimulus. A 10-s rest occurredbetween each trial. (A) Recordings were made from the axon of the PDDSMDneurone in the left connective. Figs 4 and 5 show temporal and spatial frequency plotsusing data from the same neurone. (B) Recordings were made from the axon of thePDDSMD neurone in the right connective of the preparation. Standard errors areshown.

The interpretation of the intracellularly recorded response to apparent movementis complex. In the preferred direction, PDDSMD is responding to each apparentmotion step rather than to continuous motion, whereas in the null direction theresting discharge is continuously suppressed. Clearly the response in the nulldirection does not adapt.

Discussion

The protocerebral, descending DSMD (PDDSMD) neurone has not been

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D

A/UIJNJAA^AJI/LJ^5mV

200 ms

Fig. 8. Lack of adaptation of the intracellularly recorded response to maintainedapparent movement, first in the preferred and then in the null direction. Records A-Care part of a continuous recording; 12 s separate the end of A from the start of B, and afurther 12 s separates B from C. The resting discharge was recorded in the absence ofstimulus movement. The recording is from the cell body of the left DSMD neurone.Upward stepping on the lower trace represents apparent movement at 90 's ' 1 , to theleft, over both eyes. These records are from the same neurone as those in Fig. 4.

identified previously. The cell body is a member of the protocerebral posteriorpars intercerebralis group of Williams (1975), although it has not been describedindividually. Kien (19746, 1977) recorded extracellularly from DSMD neurones(termed Bl and B2) with axons in the circumoesophageal connectives. LikePDDSMD, these neurones gave a directional response to horizontal movement,with the left neurone having a preferred direction backwards over the ipsilateraleye and forwards over the contralateral one, and the right neurone having theopposite preferred directions. Kien concluded that neurones with responseproperties matching those of the descending DSMD neurones Bl and B2produced the optomotor torque response monitored from neck muscles (50 and51) and from the motoneurones innervating them. Like Bl, B2 and PDDSMD, thefast and slow motoneurones innervating the neck muscles respond to velocities ofmovement of 475°s~1 and above. In Kien's study this represents a temporalfrequency of 25 Hz, because a 19° stripe period was used. The optimum temporal

Locust directionally selective movement detector 15

frequency cannot be estimated from the data of Kien, because the response did notpeak and was still increasing at temporal frequencies of 25 Hz. Neurones like Bl ,B2 and PDDSMD could contribute to the optomotor response in the horizontal(yaw) plane.

Other motion-sensitive visual interneurones in the locust include the 'deviationdetectors'. Identified deviation-sensitive neurones DNM, DNI, DNC, TCG andPl(2)5 are excited by specific combinations of sensory modalities, all signallingdeviation from a straight flight path (Simmons, 1980; Mohl and Bacon, 1983;Reichert et al. 1985; Hensler, 1988). DNI, DNC and Pl(2)5 are all protocerebral,descending neurones and, like PDDSMD, they are direction-selective and aresensitive to horizontal movements over the compound eyes (Reichert et al. 1985;Hensler, 1988). The axons of these descending neurones project in distinct tracts inthe thoracic nervous system. PDDSMD and TCG have axons in the DIT tract(Bacon and Tyrer, 1978), whereas DNI, DNM, DNC and Pl(2)5 project in aseparate tract - MDT (Gris and Rowell, 1986; Hensler, 1988).

Such descending neurones influence motoneurones in two ways. In the moth,for example, descending DSMD neurones contribute directly to optomotorresponses by monosynaptic excitation of motoneurones to specific flight muscles(Rind, 1983ft). However, multimodal deviation-sensitive neurones in the locust,such as DNM, DNI and DNC, make both direct connections with specificmotoneurones to flight muscles capable of producing a turn during flight(Simmons, 1980) and indirect connections via premotor interneurones. It has beensuggested that the indirect connections act in flight, during each wing beat cycle, toalter activity in flight motoneurones capable of correcting a perceived deviationfrom a straight flight path (Reichert et al. 1985).

Spatial frequency response

In the present investigations, the DSMD neurone gave a directional response tospatial frequencies down to periods of between 2.5° and 3.7°, values equal to twicethe interommatidial angle in this region (Horridge, 1978). Light-adapted locustphotoreceptors in the equatorial region of the eye have acceptance angles of 1.4°(width of the sensitivity function at half of maximum; Wilson, 1975). This suggeststhat the direction-selective motion-detection process can occur between samplingbases beneath adjacent ommatidia. The flat screen upon which the stimuli werepresented will introduce a distortion factor. From an angle of 45°, a 3° stripe willhave an apparent width of 2°. Increasing the distance of the screen from the locust(Fig. 6) or decreasing the extent of the pattern (Fig. 5B,C) minimizes thisdistortion but does not change the observed limit of spatial resolution. Further-more, the experimental results suggest a crossover in the directional response (thepreferred direction becomes the null direction) to movements of patterns with aperiod of around 3.7°. This was particularly clear when the extent of the patternwas reduced, lessening the pattern distortion both because of gradual changes inhorizontal alignment of rows of ommatidia over the eye and because of a decreasein the apparent spatial frequency of a pattern with increasing viewing angle of the

16 F. C. R I N D

stimulus from the eye (Figs 5B,C, 6). The crossover in response was notstatistically significant but was observed consistently. Further experiments arenecessary to confirm the crossover statistically.

Temporal frequency response, time constants and adaptation

The PDDSMD neurone shows a response maximum which depends on thetemporal frequency of the stimulus (Fig. 4A,B)- This indicates that it receivesinput from motion detectors of the correlation-type and not from detectors using adifferent algorithm, leading to the functional characterization of the directionallyselective motion detectors in the locust visual system as correlation motiondetectors. Furthermore, the unmodulated response (Enroth-Cugell and Robson,1966) of PDDSMD suggests that a non-linear process, such as the multiplicativestage of the Reichardt correlation motion detector, underlies the frequencyresponse of PDDSMD.

The optimum temporal frequency, for a pattern repeat of 29°, was 15.6Hz. Theneurone responded directionally to temporal frequencies ranging from around31 Hz down to 0.75 Hz. In flies, the optimum temporal frequencies have beenestimated for DSMD neurones (HI and HSE) in the lobula, for the optomotorresponse and also for the behavioural landing response. The responses of lobulaneurones HI and HSE and the optomotor response show temporal frequencyoptima in the range 1-10Hz (Gotz, 1964; Eckert, 1973, 1980; Mastebroek etal.1980; Hausen, 1982; Buchner, 1984; Maddess and Laughlin, 1985). Maddess andLaughlin (1985), Egelhaaf and Reichardt (1987) and Guo and Reichardt (1987)suggest that this range is an underestimate of the optima, particularly because atime-averaged response was taken so that any process of adaptation would beignored. The unadapted temporal frequency optimum, when measured, is at8-10Hz (HI, Maddess and Laughlin, 1985; HSN HSE, HSS, Hausen, 1982). Thisvalue is close to the temporal frequency optimum of the landing responsemeasured as the shortest latency to landing (Borst and Bahde, 1986). Bycomparison, the optimum of the locust PDDSMD neurone response occurs athigher temporal frequencies, suggesting that the filter time constant of thedirectionally selective motion-detecting process is shorter, in the locust, than thatmeasured for either the landing response or for direction-selective motiondetection in the fly. Recently, the time constant of the direction-selective motion-detecting process has been measured both behaviourally and from the response oflobula DSMD neurones without using the temporal frequency optimum. The timeconstants of the processes underlying the behavioural response and of theresponse of the lobula neurones were found to be similar, and were less than 20 ms(Egelhaaf and Reichardt, 1987; Guo and Reichardt, 1987). The time constant ofthe landing response is 10-30 ms (Borst and Bahde, 1986). Egelhaaf and Reichardt(1987) state that temporal frequency optima can only be used as a rough guide tothe time constant of the directionally selective motion-detecting process as theyare typically broad about their peak. However, they give a range of observedtemporal frequency optima between 9 and 90 ms. The time constant of the

Locust directionally selective movement detector 17

directionally selective motion-detecting process in the locust, estimated from theoptimal temporal frequency response of the PDDSMD neurone followingEgelhaaf and Reichardt (1987), would be shorter than these estimates (7.5 ms).This may be an underestimate of the filter time constant, as the contrastmodulation of the stimulus (0.5) may have introduced a saturating non-linearityinto the process. In the fly, introducing such a non-linearity leads to a lowerestimate of the filter time constant, but only when it is combined with velocities ofmotion above about 10°s"1 (15 Hz Fig. 3; de Ruyter van Steveninck etal. 1986).In summary, this means that the locust PDDSMD neurone is tuned to higherfrequencies than are fly lobula plate cells, a finding which is particularlynoteworthy given the comparatively slow impulse response of the locust retinulacells (Howard etal. 1984).

One of the most striking features of the response of the PDDSMD neurone is itslack of adaptation to continuous motion, at high photometric contrasts, over awide area of the visual field and at high temporal frequencies. These conditionswould be expected to accentuate any adaptation. This lack of adaptation may bepartly responsible for the short time constant, estimated for the DSMD processfrom the temporal frequency optimum, because the optimum frequency of thetemporal frequency response reduces during adaptation (HI, Maddess andLaughlin, 1985; HSN HSS and HSE, Hausen, 1982). However, the temporalresolution of HI, measured by its ability to follow sudden movements (velocitycontrasts), has been found to increase with adaptation (Maddess and Laughlin,1985; de Ruyter van Steveninck et al. 1986). Adaptation is a general feature of theresponse of the DSMD neurone in the fly eye. It has been suggested that theadaptation process, which may be governed by temporal frequency, increases theneurone's ability to signal changes in temporal frequency (Maddess and Laughlin,1985). Adaptation to a continuous stimulus in the fly lobula motion detector, HI ,also shifts the time constant (Maddess and Laughlin, 1985; de Ruyter vanSteveninck et al. 1986), as does temporal modulation of luminance without motion(Borst and Egelhaaf, 1987). In the locust PDDSMD neurone, which was found notto adapt, it is not clear how the time constant could be shifted. This raises thequestion of the nature of a direction-selective motion-detecting process which doesnot adapt and which cannot, therefore, use a changed time constant to signalcontrasts in the velocity of motion. The lack of adaptation shown by the locustdescending DSMD neurone may be correlated with the lower overall spike rates(50-60 spikes s"1 optimal response to movement in the preferred direction, cf.[JOOspikess"1 in HI: Maddess and Laughlin, 1985). In other words, in theexperiments reported here the locust PDDSMD may not have been sufficientlyexcited to cause any adaptation. This explanation seems unlikely, as the size of thescreen used to stimulate the DSMD neurone with real movement was much largerthan that used by Maddess and Laughlin (1985) to demonstrate adaptation in HI(90°x60° compared with 58° diameter). In addition, fly lobula plate neurones(HSN, HSE, HSS) also receive binocular inputs and show adaptation whenstimulated using a single monocular 36.9° diameter pattern (Hausen, 1982).

18 F. C. RIND

I am grateful to Andrew Derrington in whose laboratory most of theexperiments were conducted and the data processed. Tracey Pickering helped toperform the experiments. Peter Simmons read the manuscript critically and gavehelp and support during all facets of the work. I was supported by an SERCAdvanced Research Fellowship and an SERC Project Grant.

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