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Formation Of Visual Images: Studies Of Stabilized Retinal Images

In this condition, the matching task involved two steps: Because the scanned field as well as the black stimulus were generated by modulating the scanning laser, their positions were also encoded directly onto the video. These videos were used to evaluate the eye motion during the task and to assure the fidelity of the eye tracking and delivery of the retinal stimulus.

Movies 1 through 3 in Appendix 3 show actual videos recorded for each experimental condition. Estimating eye motion during the actual matching task was not straightforward because we could not be certain of the times when the subject was actually attending to the task.

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To get the most accurate measurement, we evaluated eye motion for the first 3. Because the subject initiated each trial, we felt that these early periods were when the subject was most likely attending to the task. Two parameters were extracted from the eye motion traces. First, we determined the average standard deviation of fixation from the videos. Second, we estimated the number of microsaccades per second.


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A microsaccade was defined as a jump of more than 3 min of arc over a 33 msec interval. To test whether the different experimental conditions affected the eye's fixation behavior, we performed a series of longer trials on one subject in experiment condition C in which we recorded eye motion during each trial.

Motion-matching estimates for each individual showed similar trends but had different magnitudes between the three subjects Appendix 1. To account for that, we normalized each subject's results before combining them. These gain and angle settings were selected for normalization because they were conditions for which reliable motion estimates were made and no fading occurred.

For experimental condition C, each subject's motion estimates were divided by the average motion estimates for the field under the stabilized condition.

The experiments revealed that the percept of motion bears a more complex relationship to actual eye motion than we expected. We found that stimulus motions with directions that are consistent with eye motion but largely independent of amplitude of that motion produce the most stable percepts.

As long as all objects in the field slip in roughly the same direction, opposite eye motion, they are perceived as stationary despite having different speeds of retinal slip. This phenomenon is revealed under experimental conditions A and B, and the data are shown in Figure 2. Even though some relative motion is seen for all gains other than 0, the cardioid shape of the function is maintained for gains as high as 2, showing that the perceived motion is much reduced when retinal slip is opposite the eye's drift.

This means that the visual system works to stabilize its percept of stimuli moving opposite to eye motion even if they are moving across the retina more than the eye's own motion would make them move. Results from experimental conditions A and B. Polar plots of perceived motion versus gain and angle. Bar charts of the same data.

The average responses based on five trials from each of three subjects were first normalized then averaged as described in the Methods section. Error bars indicate standard deviation of normalized motion estimates. Actual motion estimates from each individual subject are provided in Appendix 1. Because the large, bright raster field itself was used as the stimulus in condition A, no fading was observed even when the field was stabilized on the retina small dotted circles on polar plots. The smaller stimulus in condition B, however, often faded whenever the stimulus was close to stabilized i.

For conditions in which fading occurred, the percentage of faded trials is listed above the bar on the figure. Because of the smaller stimulus size, more extreme gains could be tested under condition B. Preliminary trials indicated that the full range of results is essentially symmetrical around the 0— axis, so data was collected for only one half of the range.

The polar plots show that data reflected around the 0— axis using dashed lines, leading to the exact symmetry of the polar figures. If responses were based simply on relative motion of target and fixation, data should fall on a circle centered on the origin in the polar plots and have equal height bars in the histograms. We could reject the null hypothesis for effects of gain on motion, of angle on motion, and for the interaction of gain and angle with motion for both conditions A and B.

Condition C Figure 3 gave the most striking outcome: If two stimuli moving with different retinal speeds have directions that are both opposite the direction of eye drift, they appear fixed with respect to each other even though their veridical position relative to each other is constantly changing. The subject has an illusion of relative stability. This phenomenon is revealed best in experimental condition C in which the subject sees three objects, each moving with a different gain category 4 in Figure 3. The fixation cross has a gain of 0 and therefore slips across the retina in a direction and at a rate that is consistent with eye motion.

The retinal slip of the field and the stimulus are doubled angle , gain 1 and tripled angle , gain 2 , respectively, relative to eye motion. Despite the fact that these three objects are moving at different rates, they all appear to be fixed and moving very little relative to each other. If the same relative motions of stimulus and field are presented with, instead of against, the eye motion direction category 2 , the motions are clearly perceived.

Introduction

Results from experimental condition C. Perceived motion of the field red and the relative motion of the stimulus within the field black. Data from each individual subject are provided in Appendix 1. Error bars indicate standard error of the mean of the 15 normalized motion estimates for each category. As expected, a stimulus that is not moving relative to the raster appears as such first and third categories. However, when the field was moving at two times the retina slip and the stimulus was moving within it at three times the retinal slip rightmost category , there was nearly no apparent relative motion.

The same relative motion is perceived very differently, depending on its overall relationship to eye motion. Results from all three conditions show that the perception of relative motion of retinal images is dramatically reduced whenever the direction of motion of those images is opposite to ongoing eye motion direction, regardless of gain. Our results with the small drifts of the eye during fixation contribute to the larger body of evidence that the visual system incorporates estimates of eye motion into the perception of motions and positions of world objects.

Studies of this phenomenon have included perceived position of objects flashed at various time points relative to saccadic shifts of gaze Bridgeman et al. These larger eye motions are typically imperfectly compensated, resulting in misperceptions of position or motion under conditions designed to reveal them Raghunandan et al.

Here we add to this body of work the finding that the minute drifts of fixation are also compensated in perceived motion of targets. The results show that retinal slips that are roughly consistent with moment-to-moment eye motion during fixation are not perceived as object motion. Our results for fixation drift show compensation for a small range of motion directions relative to eye motion but a relatively large range of speeds in that direction. In the case of these larger, faster eye movements, some combination of efference copy and retinal image motion may also be responsible for the compensation.

Insensitivity to retinal slip in only the appropriate direction has obvious functional advantages. Complete suppression of small motions, regardless of direction, would leave us with no percept of target motion until its motion exceeded a certain threshold, and then it would appear to start moving with a combination of eye motion and its own motion.

Because fixation jitter direction is approximately random, this means any objects moving in the world ought to be perceived even if occasionally they are aligned with eye motion—induced slip. We thus retain an exquisite sense of actual motion of objects in the world despite continual fixation eye motion.

Not only did we find that the perceived stability of retinal images moving in a direction consistent with eye motion held for a range of amplitudes of motion, but objects moving with different gains were perceived as stable with respect to each other. We call this the illusion of relative stability. In natural viewing, a very small difference in image motions does occur due to parallax effects as the pupil translates with eye rotation. Near objects slip more rapidly across the retina than distant ones, but velocity ratios never approach those used in our experiment.

An object has to be touching the cornea for it to slip twice as fast as an object at infinity. It seems unlikely that the mechanism behind this illusion exists to deal with this very small parallax effect. Rather, we postulate that this illusion simply reveals the mechanism for motion compensation that is sufficient for the human visual system to properly sense world motion.

That is, the visual system senses and compensates for the direction of motion, rather than the actual position of objects on the retina. To our knowledge, the only other experimental encounter with one of the unnatural motion settings in our experiments was briefly mentioned in Riggs, Ratliff, Cornsweet, and Cornsweet in a study of the effects of retinal slip on acuity and persistence of visibility.

The experimental design ensured that the eye fixation behavior remained the same for all conditions. Second, the stimulus was only present when the eye was fixating on the fixation cross. As such, the subject could never make a motion assessment while looking directly at the AOSLO display. Finally, all conditions were presented in pseudorandom order, so the subject could never anticipate the next experimental condition.

To confirm that fixation behavior was not affected by the experimental condition, we had one subject perform the matching task for all settings of experimental condition C four times each and recorded s AOSLO videos for each trial. The subject was instructed to attend to the task for the entire period.

We used offline software to extract the eye motion for each trial. Figure 4 shows plots of each eye motion trace. These plots indicate normal fixation behavior with no systematic fixation differences between the experimental conditions. Typical fixation behavior during the matching tasks of experimental condition C. Plots show eye traces from four s trials for one subject.

Gain and angle settings are labeled on the left of each row. The standard deviation is typical for a fixating eye, and there are no apparent differences in fixation behavior for the different conditions. The delay between the capture of the eye motion and the execution of the stimulus must be considered in interpreting these results. In experimental condition B, in which the only moving stimulus is the dark square, this delay is 4 msec or less and, hence, shorter than the highest frequency components of the jitter and drift.

Consequently, we take the results of experimental condition B to be closest to describing the actual behavior. For experimental condition A, in which the entire field moves, the delay is one frame or 33 msec.

This interval is three times as long as the highest frequency components of jitter and drift and, consequently, is physiologically significant. Despite this difference in latencies, the perceived motion for the two conditions shown in Figure 2 is substantially the same. This indicates that the neuronal stabilization mechanism is robust enough to be unaffected by any direction noise introduced by the longer delay for experimental condition A.

For experimental condition C, both the field and stimulus used a prediction delay of 33 msec. Again, the results are insensitive to the extra delay. Non-retina—contingent motion was also caused by tracking errors, particularly under the most extreme gain conditions.

Eye Works 2: Image: Retina, Optic Nerve and Brain

As such, the motion estimates reported here should be considered as an upper bound on what might actually be perceived under ideal conditions. One of the goals of our study was to determine whether and the extent to which the visual system has knowledge of its motion during fixation.

Our results clearly rule out an overall insensitivity to motion, in which jittery retinal images are seen as stable regardless of direction. There must also be a signal indicating at least the direction of eye motion that is then used to perceptually stabilize individual targets based on their retinal slip direction. The experimental configuration imposed the necessity of presenting at least one spatially fixed stimulus: Other spatially fixed features were also visible across the field because we were unable to eliminate all light reflections within the system.

The movement of the world-referenced features across the retina could be the source of the eye motion signal used for computing the percept of motion for all parts of the retinal image. Alternatively, an efference copy signal of the eye drift might be available although these drifts are only a few arc minutes in extent. In either case, it appears to be sufficient that the eye motion signal have only enough precision to roughly specify direction of drift with much less precision for the speed of drift.

Speculation on the neural mechanisms that account for visual stability go back at least as far as Von Helmholtz. Until the incorporation of instruments capable of controlling the motion of a stimulus with respect to the motion of the eye, hence controlling the position of the stimulus on the retina in real time, experimental data was limited to clever manipulations of natural viewing conditions. Interpretations of the observed psychophysics led to support of both theoretical camps.

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The Purkinje eye tracker coupled with computer-generated displays and, most recently, the AOSLO equipped with a retina-contingent retinal display have allowed a more controlled approach to the question and, as can be seen from our results, broaden the question itself to include the detailed behavior of that stabilization.

Our results affect the interpretation of earlier results obtained with such devices. While the relevant literature is vast, we present as examples the implications of our results on several recently reported results by other investigators. Gur and Snodderly , using a dual Purkinje eye tracker and recording from V1, concluded that V1 receptive fields do not move to follow, and thereby stabilize, a moving stimulus. However, because the direction of motion of their stimulus is independent from the direction of eye motion, their results necessarily pool over all theta angles between stimulus motion and eye motion.

In a sense, Gur and Snodderly's results are consistent with ours but, in light of ours, do not support the neuroanatomical neurophysiological conclusion they assert. The only time the translation of RFs in V1 by whatever means would be distinguishable is when the stimulus motion is within the stabilized sector. It should be noted that this methodological issue raised by our results is not limited to Gur and Snodderly but would affect all methods that do not plot percept motion as a function of how the stimulus moves relative to eye motion.

For example, Tulunay-Keesey and VerHoeve examined natural and motion-controlled perception of motion near the threshold of detectability. They found that with background reference more relevant to conditions in our experiment , the threshold for motion visibility is somewhat higher for the stabilized condition than for the natural condition. The authors concluded that eye motion facilitates motion detection. Because such retinally stabilized stimuli are perceived as moving, it is not surprising that the threshold for detecting a random motion superimposed should be greater than when the eye-motion—induced component is erased perceptually by whatever mechanism is involved.

Their experiment was constructed as a two-alternative forced choice wherein subjects were asked to report whether they observed the target stimulus to be moving or still. The subjects' responses were presented as probability of correct categorizations. Their experiments included the additional dimension of referenced light versus unreferenced dark conditions.

Because the methodology here requires a fixation target, only Poletti et al. Within this small space, their results are consistent with ours, including the finding that stimuli that are retinally stabilized are perceived to move. Their methodology precluded the observation made here that the percept of motion is, in fact, a function of both eye motion and world reference—stimulus motion.

Our methodology precluded any conclusion as to whether the eye-motion input to this function is derived from optomotor sources or from analysis of the motion of the fixation cross or other inadvertent world-frame features on the retina. The combined results might be taken to suggest that the eye-motion input to the motion percept is, in fact, also retinally derived.

The experiments are sufficiently different that this conclusion should be taken, at best, tentatively. In some species, it has been suggested that the analysis of relative motion may begin in retinal ganglion cells Olveczky et al. Recordings from salamander and rabbit retinas showed a suppression of motion responses in ganglion cells unless the surround motion was different from the center motion. While this enhances relative motion over full-field motion, by itself it is not sufficient to produce our eye motion contingent percepts.

Although the stimulus motions we use are quite small, they are well above typical minimum motion thresholds McKee et al. Hence, the perception of relative stability for targets moving in directions opposite eye drift is likely the result of a neuronal mechanism for eye movement compensation rather than being a near-threshold artifact. The results of the experiment raise the question about the neuronal mechanisms that implement the observed behavior.

One such mechanism might involve a selective inhibition of motion mechanisms, analogous to the motion threshold increase proposed by Murakami but with selectivity for the moment-to-moment direction of overall retinal slip. A more sophisticated mechanism might involve an active stabilization of the targets based on individual motion estimates for each stimulus pattern.

A behavioral computational model of the latter kind of mechanism is provided in Appendix 2. While there are several long-standing neuronal models consistent to various degrees with the proposed model, we are not aware of any neuroanatomical or neurophysiological evidence supporting any particular circuitry or architecture. In summary, several surprising results and implications have arisen from these experiments. Apparently, perceptual stabilization is a subtle function of the moment-by-moment direction of the eye's inadvertent drifts in combination with the actual motion of a pattern on the retina.

Motions of an image on the retina that lie approximately in the direction opposite eye motion are perceptually stabilized, despite differences in the speed of that motion on the retina. As the motion of the image on the retina diverges from the direction that would be produced by eye motion, the percept of motion becomes increasingly veridical.