eye movements elicited by transparent stimuli

Exp Brain Res (1994) 98:314-322 �9 Springer-Verlag 1994

T. Niemann - U.J. Ilg - K.-P. Hoffmann

Eye movements elicited by transparent stimuli

Received: 17 June 1993 / Accepted: 18 November 1993

Abstract A transparent motion condition occurs when two different motion vectors appear at the same region of an image. Such transparency during self-motion has shown demonstrable effects on perception and on the underlying neurophysiology in the cortical and subcortical structures of primates. Presumably such stimulus conditions also influence oculomotor behavior. We investigated smooth-pursuit performance, using a transparent stimulus consisting of two oppositely-moving patterns. We found slight reduction in the mean eye velocity tracking a transparent pattern, compared with that when tracking a unidirectional pattern. Additional- ly, we investigated the behavior of the optokinetic system to transparency, demonstrating that it elicits antagonistic optokinetic nystagmus, with distinctly reduced gain of the slow phases. Furthermore, we observed, during optokinetic stabilization of transparent stimuli, directional dominances demonstrating that subjects preferably followed one direction. Presenting a transparent stimulus with oppositely moving patterns and different velocities we found a general velocity dominance demonstrating that patterns with a certain velocity are preferred. Performing all experiments under dichoptic conditions produced results comparable with those found under transparent stimulus conditions.

Key words Transparency �9 Smooth pursuit Optokinetic nystagmus �9 Antagonistic OKN Dichoptic presentation �9 Human

T. Niemann . K.-P. Hoffmann ([~) Allgemeine Zoologie und Neurobiologie, Ruhruniversitfit, D-44780 Bochum, Germany

U.J. Ilg Neurologie, Neurologische Universit/itsklinik, D-72076 T/ibingen, Germany

Introduction

In natural environments transparency occurs as a con- sequence of self-motion or motion between objects. This means that at a specific retinal location two different motions occur simultaneously. For example, it occurs when gaze direction is perpendicular to the linear movement of the observer: objects behind the fixation point move in the direction of the observer, whereas objects in front of the fixation point move opposite to the observer. Other common examples of transparency are snow- drift, or moving raindrops on the windshield of a moving car, or shadows moving over a textured surface. Psy- chophysical experiments indicate inhibition in the perception of transparent stimuli: superimposing two patterns moving in different directions results in a falsifica- tion in the perceived direction of each (Marshak and Sekuler 1979). Mather and Moulden (1983) have demonstrated that motion detection of two motion directions requires higher contrast threshold than that for one direction.

Recent neurophysiological investigations have re- vealed substantial effects of transparent stimuli on cortical and subcortical motion-sensitive structures. Snow- den et al. (1991) have observed that direction-selective cells of the middle temporal area (MT) showed sup- pressed responses whereas cells in area 17 (V1) typically responded well to their preferred direction, despite the transparent stimuli. In anesthetized monkeys Hoffmann and Distler (1989) have demonstrated that cells in the nucleus of the optic tract (NOT, in the pretectum) and the dorsal terminal nucleus (DTN, of the accessory optic tract) show inhibited responses when, in addition to the preferred, ipsiversively moving random-dot pattern, a second contraversively moving pattern was presented. Interestingly, cells in MT at the cortical level, and in NOT/DTN at the subcortical level, showed similar modulated responses to transparent stimuli. As MT and NOT participate in the generation and control of eye movements, we may hypothesize that transparent stimuli could have an influence on the oculomotor behavior.

Several oculomotor investigations have been performed, observing smooth-pursuit behavior under conditions similar to that of transparent stimulation. Con- tradictory results were found by Murphy et al. (1975) and Collewijn and Tamminga (1984). While Murphy et al. (1975) did not find any effect of a stationary structured background on the velocity of smooth-pursuit eye movements, Collewijn and Tarnminga (t984) found a slight reduction in the mean eye velocity. According to Kowler et al. (1984) these contradictory results may be due to differing physical parameters of the target and the background. The measurements were thus repeated by Kowler et al. (1984), with two identical, superimposed random dot patterns, serving as stimulus and background. They found no, or a negligible, reduction of mean eye velocity, thus confirming the results of Mur- phy et al. (1975). The negligible reduction of mean eye velocity in the smooth-pursuit performance may be due to the attention of the observer (Kowler et al. 1984). As smooth-pursuit performance in the latter study was tested only at a target velocity of 1.2~ and the reduction in smooth-pursuit performance may be detectable only at higher stimulus velocities, we considered this issue worthy of reexamination. We tested smooth-pursuit with a transparent stimulus (where target and background have the same physical characteristics), but at higher stimulus velocities. We were able to demonstrate a slight, but significant, reduction of mean eye velocity during pursuit.

Furthermore, we tested the influence of transparent stimuli on the optokinetic system. In 1963 Merrill and Stark presented two patterns of stripes moving in opposite directions and found that the subjects direction of optokinetic nystagmus (OKN) alternated. Howard and Gonzales (1987) showed that the optokinetic nystagmus was significantly disrupted when a stationary display of dots was superimposed on a moving display of dots. Our transparent stimulus, with oppositely moving dis- plays, was also able to elicit an alternating OKN. Addi- tionally we examined the mean eye velocities of the pursuit phases and found them distinctly reduced. More- over, we observed individual directional dominances and, presenting a transparent stimulus with oppositely moving patterns and different velocities, we found a general velocity dominance. All experiments were also performed under dichoptic conditions, yielding results very similar to those found under transparent stimulus conditions.

Materials and methods

Subjects

Six subjects took part in the experiments, ranging in age from 23 to 30 years, all subjects naive to psychophysical octflomotor experiments.

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Eye movement recording

Horizontal eye movements were measured with an infrared eye- tracker (Ober2), spatial resolution 0.25 ~ and the sampling rate of the analog to digital converter was 100 Hz. The head was stabi- lized using chin and :forehead support, and the ambient luminance of the laboratory was _<0.01 cd/m 2.

Stimuli

The stimulus was either a unidirectional, horizontally moving random-dot pattern, or two superimposed random dot patterns moving in opposite horizontal directions (transparent condition), generated by a computer (Compaq Deskpro 286). The size of the patterns was 108 ~ x 50 ~ each consisting of t00 dots (dot size was 30'), with a mean neighborhood distance of 7.35 ~ The luminance of the dots was 0.1 cd/m 2 in the smooth-pursuit experiments and 2.65 cd/m 2 in the optokinetic experiments, and background luminance was _<0.01 cd/m 2. A videoprojector (Electrohome ECP 3000, connected to the computer) projected onto a transparent foil while the subject viewed the stimuli from the opposite side, from a distance of 57 cm. For the dichoptic condition, the colours of the dot patterns were diftbrent (red/green), and the subject wore spec- tacles with red/green filters (Kodak Wratten gelatin filters No.25/ No.58).

Procedure

All trials were viewed binocularly, except for the unidirectional stimulus under the dichoptic condition, which was viewed monocularly. At the beginning of each session the infrared eye-movement recording system was calibrated by instructing the subject to alternately fixate two dots 10 ~ horizontally apart, and the calibra- tion factor was fed into the controlling software. This procedure was repeated in cases of misalignments of the eye tracker.

Four conditions were tested under normal and dichoptic conditions, differing according to the stimulus and instructions:

1. Active, selective smooth-pursuit of a unidirectional stimulus: the subject was instructed to follow a single dot or a group of dots from the pattern. Both horizontal directions were tested.

2. Active, selective smooth-pursuit of a transparent stimulus: the instructions were the same as in condition 1.

3. Passive optokinetic tracking of a unidirectional stimulus: the subject was instructed to passively follow the moving pattern as a whole, but not to fixate any special details. Both horizontal directions were tested.

4. Passive optokinetic tracking of a transparent stimulus: the subject was instructed to passively view the transparent stimulus and not to fixate any particular area or detail.

All conditions were tested with different stimulus velocities of 6~ 12~ and 24~ Each subject performed at least six measurements for each condition and velocity, each measurement lasting 20 s. Gain (eye velocity/stimulus velocity) was calculated using the mean eye velocity of pursuit, or slow phases of the OKN, over the whole trial period (six measurements of 20 s).

Data analysis

Eye velocity was obtained using digital filter techniques. Mean eye velocity and gain were calculated within user-defined time intervals of unidirectional pursuit. Catch-up saccades were excluded from evaluation of eye velocity. Only those bits of data were in- cluded in the analysis with distinct pursuit in the appropiate stimulus direction (see Fig. 1C as an example; data evaluated for mean eye velocity are marked with bars in the eye position trace).

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Fig. 1A-C Horizontal eye position (upper trace) and velocity (lower trace), during active, selective pursuit of the right-moving pattern of a transparent stimulus (A), during passive optokinetic tracking of a unidirectional, right-moving stimulus (B), and during passive optokinetic tracking of a transparent stimulus (C). The sequences marked with horizontal bars in the eye position trace of C were evaluated for mean eye velocity. All patterns had a velocity of 12~ (OKN optokinetic nystagmus)

Results

Each subject described the transparent stimulus as one consisting of two planes of moving dots at different depths. Depending on the instructions to the subject

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(active or passive) the elicited eye movements were distinctly different.

Active, selective smooth-pursuit of single targets

Only a small difference in gain was observed at low stimulation velocities to unidirectional targets and transparent stimuli. A slight but significant reduction in gain (Dixon and Mood 1946; cited in Sachs 1969) was seen for smooth-pursuit of transparent stimuli at higher velocities. Figure 1A shows a typical example of smooth-pursuit eye movements during a transparent stimulus, actively following one of the two patterns. Smooth pursuit eye movements were sometimes inter- rupted by forward saccades. No difference between the gain of smooth pursuit to the right or left was observed in the subjects, and therefore leftward and rightward data were pooled.

Since the reductions in gain were minimal and the individual gains differed distinctly, we calculated a relative reduction in gain of the smooth pursuit during transparent stimuli, as shown in Fig. 2. Positive values represent a gain reduction with transparent stimuli. Ev- ery stimulus velocity was tested in three subjects. At a stimulus velocity of 6~ no significant reduction in gain was observed. One subject even showed a gain improve- ment but none of the values at 6~ were significant (Dixon and Mood 1946). At 12~ two subjects (T.W. and G.S.) exhibited small but significant reductions in gain (P_<0.01), whereas at 24~ the gain was significantly reduced (P_< 0.01) for all three subjects when stimulated with transparent simuli. The relative percentage values of reduction in gain ranged from 0.35-4.45% for stimulus velocities of 12~ and from 3.42-6% for 24~

The smooth-pursuit tracking was never perfect, and the resulting retinal error was usually compensated by small "catch-up" saccades. Thus we hypothesized that transparent stimulation with a slightly reduced gain should result in increased retinal error and therefore a

higher number of catch-up saccades. Indeed, we detect- ed a significant increase (P_<0.01; Dixon and Mood 1946) in the mean number of catch-up saccades at 24~ for transparent stimuli, in that the mean catch-up saccades for the unidirectional stimulus was 15 (SD 4.5) saccades per measurement (20 s), which increased to 19 (SD 4.8) saccades per measurement for the transparent stimulus. This result confirmed earlier findings made by Howard and Marton (1992), showing an increased frequency and magnitude of saccades when pursuing a target over textured background compared with a non- textured background.

Active, selective smooth-pursuit under dichoptic conditions

The influence of dichoptic stimuli with oppositely moving patterns on smooth-pursuit eye movement was investigated to determine any parallels or differences to the monoptic condition. The smooth-pursuit gain of a monocularly presented stimulus was compared with the gain, while pursuing one of two oppositely moving patterns with each eye. No significant differences were found between the gains during nasotemporal or temporonasal stimulation, and thus the data were pooled.

Figure 3 shows a slight but significant reduction in gain (P_<0.01) for two subjects, to a given dichoptic stimulus (oppositely moving patterns), when compared with the gain in unidirectional, monocularly presented pattern. This result, resembling the effect for the transparent condition, provides evidence that the inhibitory influence of oppositely moving patterns appear similar for transparent and dichoptic stimulation.

Passive optokinetic tracking

Transparent stimuli showed a distinct reduction of gain in the slow phases during OKN. Figure 1B shows typi-

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cal eye position and velocity during OKN elicited by a unidirectional stimulus. After prolonged stimulation, we were able to elicit an optokinetic afternystagmus (OKAN). Additionally, the subjects reported the feeling of circular vection. Figure 1C shows eye position and velocity elicited by transparent stimuli. The slow-phase directions obviously alternate between the two random- dot pattern directions, resembling an antagonistic OKN.

Figure 4 shows the percentage reduction in gain with transparent stimuli, compared with the gain with unidirectional moving patterns. When the subject's attention was directed to the entire pattern, a distinct and always significant (P<0.01) reduction in eye velocity was observed for transparent stimulation as compared to unidirectional stimulation. This reduction increased with stimulus velocity. Transparent patterns seem to have an inhibitory influence on eye velocity.

Passive optokinetic tracking under dichoptic conditions

The elicited eye movements with dichoptically presented, oppositely moving patterns resembled those shown in Fig. lB. This antagonistic OKN, or optokinetic rivalry, has been reported for humans and monkeys (e.g., Enokkson 1961,1963,1968; Fox et al. 1975; Logothetis and Schall 1990). We compared the gain of the slow phases with the monocular unidirectional pattern with the gain of the slow phases during dichoptical stimulation with oppositely moving patterns. Both nasotemporal and temporonasal directions were tested. A distinct and significant reduction in gain (P_< 0.01) for the slow phases with oppositely moving dichoptic patterns was found (Fig. 5). This reduction seems to be dependent on the stimulus velocity, but as only two different velocities were measured, this result is preliminary. The results again resemble those obtained with transparent stimuli.

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Fig. 5 Relative gain-reduction of passive optokinetic tracking of dichoptic stimuli with countermoving patterns, compared with the passive optokinetic tracking of unidirectional stimuli. The values for subjects J.L. (O) and A.T. (&) are significant, with P _< 0.01, also for T.W., with P_<0.05. Values obtained with temporonasal and nasotemporal stimulation have been pooled

This indicates that countermoving patterns have a strong inhibitory influence on the optokinetic system, independent of whether they are viewed binocularly or dichoptically.

Directional dominance

Figure 1C presents a typical example of an antagonistic OKN for transparent stimuli, wherein three intervals can be distinguished: (1) pursuits to the left, (2) pursuits to the right, and (3) time periods with no pursuit. After evaluation of the different intervals, it was noticed that some subjects preferably followed one direction when exposed to oppositely moving patterns. Figure 6 shows an example of the relative frequencies of pursuits to the left, right, and no pursuit for three subjects during OKN, elicited by transparent and dichoptic stimuli of different velocities.

Subject J.L. exhibited more pursuit eye movements to the right. This preference was independent of na- sotemporally or temporonasally moving stimuli under dichoptic conditions and was also independent of velocity. The difference between the frequency of pursuits to the left and right was found to be significant, as indicat-

ed under the corresponding section of Fig. 6. Subject T.W., on the other hand, showed a preference to pattern movements to the left, during OKN with countermoving patterns, and the difference between left and right was found to be significant. Subject A.T. showed no significant preference.

In summary, when presenting oppositely moving patterns, directional dominances or preferences can occur, which, although specific to the subjects, stay consis- tent over a range of different experimental conditions. No relationship between this and handedness or eye dominance was observed.

We also observed an influence of a preceeding unidirectional moving stimulus on the directional dominance. Viewing a unidirectional pattern for approxi- mately 1 min before viewing the transparent stimulus facilitated tracking in the previously presented direction. A given directional dominance was never reversed but significantly shifted to the conditioned side (P<0.01). Frequency of pursuits increased approxi- mately, 10-30% in the conditioned direction.

Velocity dominance

We further examined the influence of different velocities in one transparent stimulus on the OKN. Transparent stimuli with velocity pairs of either 6 and 12~ 12 and 24~ or 6 and 24~ were tested. Each condition was tested over six measurements, with the direction of the different pattern velocities changing for each measurement. The overall eye movement patterns elicited equalled those during OKN with transparent stimuli of identical pattern velocities. All velocity pairs elicited an antagonistic OKN. The gain of the slow phases was distinctly reduced compared with the gain for unidirec- tionally moving patterns.

When evaluating the time periods of slow phases to the left and to the right no significant directional dominances were found, although two subjects had slight but significant directional dominances under the "normal" transparent stimulation (Fig. 7; for subject A.T. see Fig. 6).

Preferences for certain velocities were found by evaluating the time the subjects took to follow one of the two stimuli (Fig. 8). All subjects appear to preferably

Fig. 6 Relative frequencies of the slow phases during passive optokinetic tracking of trans- ~6 ,-. parent and dichoptic stimuli o "-" for three subjects. Slow phases ~ to the left (1), right (r), or no ~ eye movements (diagonal line), are indicated under each bar. ,',

Significant differences between ~ z~ the frequencies of the slow :~ ~ phases to the left and right are marked under the corre- tv sponding figures

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pursue the pattern moving at 12~ when viewing a transparent stimulus with a velocity combination con- taining one pattern moving at 12~ Presenting a stimulus with the velocity combination of 6 and 24~ the subjects preferably followed the slower stimulus. Any individual directional dominance seemed to be either canceled under these experimental conditions or coy-

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ered by a stronger velocity dominance. The similarity of the results of different subjects could be an indication that the preference for a particular stimulus velocity may be a general function of the optokinetic system.

Discussion

These results indicate that transparent stimuli influence oculomotor behavior. The effects are relatively small when attention is focused on single elements of the transparent stimulus (active smooth-pursuit), but be- come pronounced when attention is directed to the entire pattern disregarding specific detail (passive optokinetic stabilization).

The observation of a slightly reduced gain during smooth-pursuit performance of transparent stimuli is similar to that found by Collewijn and Tamminga (1984). The latter tested smooth pursuit of a single target moving across a stationary, structured background. Pursuing the target resulted in a shift of the textured background, across the retina, in the opposite direction

Fig. 8a-i Relative frequencies of slow phases during passive optokinetic tracking of transparent stimuli, with different velocities of the countermov- 100 ing patterns. The velocity of "6 ,-, the pattern followed is g ~,~ marked under the correspond- az co

ing frequency bar (6, 12, 24); ~ no eye movement is indicated ~ ~ 50 by a diagonal line. Significant c~ differences between the fre- | quencies of the slow phases ~ |

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to that of the target. Under this condition, a reduction in gain of about 10% was found; more pronounced than in our data of only 0.5-6%. Thus a countermoving pattern seems to have a smaller inhibitory influence on smooth- pursuit performance.

Kowler et al. (1984) have hypothesized that inhibitory influences of the background on smooth pursuit are due to physical differences in the target and the background. Using a stimulus with identical physical characteristics of both target and background, they found only a "negligible" influence of the background on smooth- pursuit performance. This assumes that a reduction in gain is not due to an involuntary oculomotor mechanism that integrates information from target and background, but can be attributed to insufficient attention or effort (Kowler et al. 1984). Our transparent stimulus also used identical physical characteristics for the target and background and we found, in most cases, a small but significant influence of the background on the smooth- pursuit performance. Also Kowler et al. (1984) observe a very small influence of the background of 1-4%, which is strikingly similar to our findings. The difference in the gain reduction between our's and that of Kowler et al. (1984) to those of Collewijn and Tamminga (1984) may be still due to the different characteristics of the targets used. Compared with the single target used by Collewijn and Tamminga, our full-field pattern may have been a more efficient stimulus for pursuit. With a full-field pattern a synergistic interaction between smooth pursuit and optokinetic system could possibly explain the slight differences in results of Collewijn and Tamminga's and our studies. Recent neurophysiological data of single NOT neurons seem to support the idea of such an interaction (Ilg and Hoffmann 1991). We therefore believe that the degree of attention or effort, although an important factor, is not sufficient to explain the measured reduction in gain. Target and background seem to be integrated by an involuntary oculomotor or motion analyzing mechanism. The smooth-pursuit system is large- ly, but not completely, effective in eliminating the influence of a structured background.

It has been shown that the suppressive interaction between patterns that move in orthogonal directions is canceled when one pattern moves slowly and the other pattern moves fast (Snowden 1990). This may be a rea- son why smooth pursuit is only weakly affected by a structured background, because once pursuit is under way the target being pursued should have little retinal velocity, whereas the background should have a large retinal velocity in opposite direction.

Keller and Khan (1986) and Mohrmann-Lendla and Thier (1992) recently have shown similar behavior in the eye movements of monkeys in the presence of a textured background. The gain of pursuit were marginally reduced (about 7-11%). This is very close to the background-induced reduction in gain observed in humans. While Keller and Khan (1986) and Kimmig et al. (1992) demonstrate that the initial eye acceleration is distinctly impeded by a structured background, Mohrmann-

Lendla and Thier (1992) found only a slightly inhibitory effect. Pursuit during initial eye acceleration is open- loop, and the decrement here reflects more directly the way motion is analyzed in the visual system.

Even the dichoptic experiments demonstrate a small- reduction in gain during smooth pursuit, indicating inhibitory mechanisms in motion analysis under dichoptic stimulus conditions too, although the perceptual state for the two types of experiments are quite different. In the transparent condition, both moving patterns can be seen binocularly, whereas under the dichoptic condition an alternation in the perception of the direction of motion is observed. This provides further evidence that structures analyzing motion and involved in eye movement control are situated at higher neuronal levels, where signals are mainly converged from both eyes and analyzed binocularly.

When the subject's attention was directed to the transparent pattern as a whole, the elicited eye movements resembled an antagonistic OKN with distinctly reduced gain of the slow phases. The characteristics of eye movements during unidirectional stimuli were similar to those elicited in a typical "stare nystagmus" with observed OKAN and subjective circumvection of the subjects (Ter Braak 1936; Cheng and Outerbridge 1974; Brandt et al. 1973; Cohen et al. 1977), thus indicating the optokinetic system as the generator of eye movements. Transparent stimuli lead to OKN in both directions. Under dichoptic experimental conditions this antagonistic OKN has been called "optokinetic binocular rivalry" (e.g., Enoksson 1961,1963,1968; Logothetis and Schall 1990). In both the dichoptic and the transparent conditions the antagonistic OKN reflects the rivalry of the two different movement directions.

The countermoving pattern always appeared to have a strong inhibitory influence on the slow phases of OKN. This can be seen in the distinctly reduced gain during transparent and dichoptic stimulus conditions as well. The reduction in gain under dichoptic conditions with countermoving patterns has also been reported for monkeys: Logothetis and Schall (1990) observed a reduction in gain of 40% at stimulus velocties of 4-12~ corresponding well with our results.

Again, the striking parallels in the results of the transparent and dichoptic conditions show that processing of motion signals and the resulting eye movements are similar despite different perceptual states. The perceptual alternation or binocular rivalry, which is quite common under dichoptic stimulation (e.g., Fox et al. 1975; Walker 1978; Blake 1989), seems to exhibit a suppressing effect which occurs on a rather high level in the visual system, and does not affect motion processing or eye movements.

Recent electrophysiological recordings from MT neurons of the alert monkey (Snowden et al. 1991) support our psychophysical data. Transparent stimuli reduced the activity of MT cells by 40% during fixation. MT and the medial superior temporal area (MST) are important structures for the smooth-pursuit system

(e.g., Komatsu and Wurtz 1988). They are parts of the parietooccipito-ponto-cerebellar circuit for smooth pursuit. MST cells discharge in relation to smooth-pursuit eye movements as well as to image motion during fixation (Newsome et al. 1988). Wurtz and Newsome (1985) have shown that MT and MST give direction selective responses during smooth-pursuit. Additional- ly, structured backgrounds can have inhibitory effects on the response of MT neurons during smooth pursuit (unpublished observations). The neurophysiological data at the cortical level thus support the finding of smooth-pursuit behavior in viewing transparent scenes.

On the subcortical level, the NOT, which has an important role for the generation of the slow phases of the OKN, showed modulated responses to transparent stimuli. Hoffmann and Distler (1989) have demonstrated a reduced response for NOT neurons in anesthetized monkeys, when presenting stimuli moving in opposite directions.

Many psychophysical measurements provide evidence that the suppressive effects in area MT and NOT to transparent stimuli are due to a cooperate-competi- tive network between neurons with different preferred motion directions (Levinson and Sekuler 1976; Mar- shak and Sekuler 1979; Chang and Julesz 1984; Mather and Moulden 1983; Snowden 1989). In this network the output of motion detectors with similar motion preferences faciliate one another, whereas different motions inhibit one another. A given transparent stimulus would then result in a mutual inhibition of the direction-selective filters. Our optokinetic results seem to provide additional psychophysical support for this model. The mutual rivalry between the two coherent, but opposite, motions results in an antagonistic OKN, with alternating periods of dominance of the two directions. Despite periods of dominance, the nondominant countermoving pattern distinctly reduces the gain. The directional dominance may disclose cortical asymetries in motion analysis. Direction-selective filters may inhibit one another, but to unequal extent. This could result in a higher frequency of slow phases to one side during antagonistic OKN.

Psychophysical and neurophysiological results indicate that the optokinetic system may have a nonlinear velocity sensitivity, with an optimal retinal slip velocity in a range between 10 and 30~ (Maioli 1988; Hoffmann and Distler 1989; Ilg 1991). Our observation of velocity dominance, during passive optokinetic tracking of transparent stimuli with different velocities, may support this idea. Before the onset of the antagonistic OKN the observer has the "choice" of following either the pattern with the velocity of 12~ or the countermoving pattern with a retinal slip of a different velocity (6~ or 24~ Twelve degrees per second appears to be the optimal stimulus velocity that may result in prevailing pursuits of this pattern. As this observation was made for all subjects tested, it may be a general function of the optokinetic system.

Transparent stimuli have demonstrable effects on

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perception, neuronal processing, and elicited eye movements. As transparent stimuli are quite common under natural conditions, especially during self-motion, the primate visual system may have had to find ways of coping with this stimulus configuration. Snowden et al. (1991) have proposed that the inhibitory interaction of opposing motions may contribute toward a mechanism for smoothing or averaging the velocity field to reduce noise and interpolate surfaces from moving objects. Such mechanism would then result in impaired eye-stabilization reflexes but quite normal pursuit.

Acknowledgements This work was supported by ESPRIT basic research INSIGHT.

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