Sensitivity of smooth eye movement to small differences in target velocity

The precision of smooth pursuit eye movements was described by means of a new dependent measure, the "oculomotor difference threshold" (analogous to the perceptual difference threshold) which represents the smallest difference in target velocity that produces statistically distinguishable differences in eye velocity. Oculomotor difference thresholds for constant velocity motions were largest (greater than 50% of target velocity) during the initial 200 msec of target motion, despite fairly high average gains (0.7-1.4) during the same period. Oculomotor difference thresholds declined over time. By about 600-700 msec after the onset of target motion they reached values as low as the perceptual difference thresholds measured psychophysically with the same target velocities. The similarity of the difference thresholds suggests that equally precise sensory representations of target velocity influenced perception and smooth eye movements. Nonsensory influences on smooth eye movement were also found. Smooth pursuit velocity: (1) depended on the velocity of targets in preceding trials; (2) was decreased during the initial 200 msec of target motion when the duration of motion was reduced from 1 sec to 200 msec, a result which shows that high initial pursuit velocity depends on the expectation that pursuit will continue. These effects of context and expected duration allowed the eye to achieve quickly a velocity close to that of the target it was most likely to encounter. Study of the precision of pursuit may be valuable for characterizing its sensory input, but study of the effects of the context in which a stimulus appears and the effects of expectations about future target motion may be more valuable for understanding how smooth eye movements guarantee retinal image velocities optimal for vision.     

Contrast detection and direction discrimination of drifting gratings

Observers performed simple detection and left right discrimination of drifting sinusoidal gratings. Ratio of detection to discrimination sensitivities was measured under variations in several experimental parameters. In the first experiment, it was found that combinations of spatial and temporal frequency which resulted in the same velocity produced similar detection discrimination ratios. At an exposure duration of 800 msec, the relationship between the ratio and velocity described a power function with the intercept at 0.6 sec^?1. Decreasing duration shifted the curve to higher velocities. I examined the effect of grating orientation in a second experiment. Visual sensitivity was poorer for oblique than for vertical gratings with detection and discrimination exhibiting similar size anisotropies. In a third experiment, observers viewed gratings presented to different retinal loci, Visual performance in both detection and discrimination fell with greater eccentricity. However, motion discrimination fell more steeply resulting in an increase in the ratio. The results demonstrate that form and motion analyzing mechanisms cannot be distinguished by their response to changes of spatial frequency, orientation or retinal locus.     

Visually Elicited Head Rotation In Pigeons (Columba livia)

Horizontal rotational head movements were video-taped from pigeons standing freely in a rotating cylinder. The cylinder carried vertically striped patterns approximating a sinusoidally modulated horizontal intensity distribution. We altered systematically various stimulus parameters: spatial wavelength and contrast of the pattern, angular velocity of the pattern motion and mode of motion onset. We found: (1) both gradual acceleration of the patterned cylinder as well as immediate onset of pattern motion elicit the sequence of smooth following and saccadic resetting movement typical of the rotational “stare” head nystagmus; (2) in experiments with rapid onset of pattern motion, velocity of the smooth following response gradually increases to its steady-state level over a period of about 10 sec; (3) the saccadic head rotations are not stereotyped: larger and shorter saccades follow in an irregular sequence, saccadic velocity and average size varies with stimulus conditions; (4) in the range of 0.9–95 deg/sec, the velocity of the following phase increases in parallel with stimulus speed; (5) in the range of spatial wavelengths of the striped patterns from 6 to 45 deg, at a given drum velocity, patterns of short wavelengths elicit optokinetic head rotations with higher gain (head velocity/drum velocity) than patterns of long wavelengths; (6) response velocity increases with pattern contrast (Michaelson contrast 5, 32 and 75%), following approximately a logarithmic relation; (7) our results on rotational optokinetic head movements support the notion that the neural mechanism underlying motion detection operates like a correlation mechanism. Copyright © 1996 Elsevier Science Ltd.     

The time it takes to detect changes in speed and direction of visual motion

We studied the ability of human observers to detect abrupt changes in velocity of motion of a random dot pattern. The pattern moved horizontally for 0.9 s at velocity V0, then changed to V1 either in speed, or in direction for a time T and returned to the initial motion. The threshold duration for detection of the change was measured for initial speeds of 2, 4, 8 and 16 deg/s. The time to detect a velocity reversal was equal to that for detection of an increase in speed by a factor of three. The time to detect an abrupt cessation of motion was equal to the time for detection of an increase in speed by a factor of two. The time to detect a direction change, the speed being constant, decreased gradually with increasing angle between V0 and V1 from 12 to 180 degrees and with increasing V0; the detection time was a function of (V1-V0) almost independent of the value of V0. This finding supports the hypothesis of Dzhafarov et al. (Percept Psychophys 1993;54:373-750), that the visual system effectively reduces the detection of velocity changes (from V0 to V1) to the presumably more simple detection of a motion onset, from 0 to (V1-V0). The characteristics of the detection process in the cases of uni- and two-dimensional velocity changes are discussed.     

Attenuation of perceived motion smear during the vestibulo-ocular reflex

Previous studies indicated that less motion smear is perceived when a physically stationary target is presented during voluntary eye movements than when similar retinal-image motion occurs during steady fixation. In this study, we assessed whether the perception of motion smear is attenuated also during the involuntary vestibulo-ocular reflex (VOR). Normal observers matched the length of perceived smear in two experimental conditions that were designed to produce similar trajectories of retinal image motion. In the fixation condition, a small bright target was presented for a duration of 50-200 ms in rightward or leftward motion, while the observer remained stationary and maintained fixation. In the VOR condition, the target moved along with the observer, who underwent full-body rotation around a vertical axis in darkness. Horizontal eye movement recordings during VOR trials allowed us to calculate the velocity of retinal image motion on each VOR trial. The principal result was that the extent of perceived motion smear was significantly less during VOR than fixation trials, particularly for target durations of 100 ms or longer. These findings support the conclusion that extra-retinal signals during the involuntary VOR contribute to a reduction of perceived motion smear.     

Spatial distortions and processing latencies in the onset repulsion and Fröhlich effects

In the Fr?hlich illusion, the first position of a moving target is mis-localized in the direction of motion. In the onset repulsion effect, the opposite error occurs. To reconcile these conflicting error patterns, we improved previous methods by using natural pointing movements and a large range of target velocities. Displacement was found to increase in the direction of motion, but the linear function relating velocity and displacement was shifted opposite to the direction of target motion. The results suggest that onset localization may be determined by two independent factors: first, an (attentional) delay that accounts for the increase of displacement in the direction of motion with increasing velocity. This delay is visible in motor and probe judgments and explains the Fr?hlich illusion. Second, motor judgments are offset opposite to the direction of target motion. This bias is unique to motor judgments (pointing) and may be partially explained by attentional repulsion.     

Vertical and horizontal motion perception in congenital nystagmus

We investigated whether individuals with congenital nystagmus (CN) have abnormalities in motion perception and whether any such abnormality could be due to their nystagmus or to adaptive mechanisms to avoid oscillopsia. CN and control subjects performed motion detection and discrimination tasks. In the detection tasks, subjects reported the onset of motion and drift direction in either a vertical or horizontal direction. In the discrimination task, the stimulus was a high-contrast grating and moved vertically. Subjects judged whether successively presented reference and test velocities were the same or different, using a forced choice instruction. Vertical velocity detection was normal in the patient group. The vertical velocity discrimination task showed that the patients were less accurate than the controls, especially when velocities were slow. Horizontal velocity detection thresholds were raised in the patient group regardless of the direction of the slow phase velocity (spv) of the nystagmus. Evaluation of eye movement recordings performed during the task demonstrated that detection velocity was highest when stimulus motion and spv were in the same direction. When nystagmus was absent due to a prolonged neutral zone, thresholds did not reduce to normal values. The findings show that the image motion caused by the nystagmus cannot account for all the abnormalities found. Deficits occurred in the absence of nystagmus and when motion was orthogonal to the meridian of the nystagmus suggesting that the suppression of motion perception is, in part, due to adaptive mechanisms used to avoid oscillopsia.     

The detection of motion in the peripheral visual field

To assess the sensitivity of the periphery to motion, we measured differential motion detection and velocity discrimination as a function of eccentricity in the lower visual field. The differential motion threshold, a measure of the ability to detect relative motion (shear) between adjacent visual stimuli, is smaller than the minimum angle of resolution at all retinal loci tested. The target size required to produce the lowest differential motion threshold is surprisingly large, ranging from [ deg in the fovea to about 20 deg at 40° eccentricity. When the peripheral thresholds for differential motion and for resolution are normalized against the fovea and plotted on linear axes, the eccentricity functions are linear. Velocity discrimination (ΔV/V) is as precise in the periphery as it is in the fovea, amounting to about 6% for the optimum velocity range. In the fovea, the minimum Weber fraction is reached at velocities of 5deg/sec or faster. In the periphery this minimum is found for a faster range of velocities ( >30 deg/sec at 40° eccentricity). If target velocity is expressed in the resolution units/second appropriate to each tested eccentricity, the velocity discrimination functions coincide. Thus, while the spatial determinants of velocity discrimination follow the change in resolution found with eccentricity, peripheral temporal sensitivity must be nearly equal to foveal temporal sensitivity.     

The optimal displacement for the detection of motion

The optimal spatial displacement for the detection of motion by the human visual system was investigated using spatially narrow band stimuli. Direction discrimination was used for abruptly displaced stimuli. An optimal spatial displacement was found for the detection of motion and this bore a characteristic relationship to the spatial wavelength of the stimuli in motion; it was equivalent to 1/6 of the spatial wavelength of the stimulus for low contrast stimuli and 1/5 of the spatial wavelength for higher contrast stimuli. This finding, which in turn suggests that the spatial subunits of motion detectors may be separated by less than 1/4 spatial wavelength, receives some support from other psychophysical and neurophysiological studies.     

Oculomotor inhibition during smooth pursuit and its dependence on contrast sensitivity

Our eyes are never still, but tend to "freeze" in response to stimulus onset. This effect is termed “oculomotor inhibition” (OMI); its magnitude and time course depend on the stimulus parameters, attention, and expectation. We previously showed that the time course and duration of microsaccade and spontaneous eye-blink inhibition provide an involuntary measure of low-level visual properties such as contrast sensitivity during fixation. We investigated whether this stimulus-dependent inhibition also occurs during smooth pursuit, for both the catch-up saccades and the pursuit itself. Observers followed a target with continuous back-and-forth horizontal motion while a Gabor patch was briefly flashed centrally with varied spatial frequency and contrast. Catch-up saccades of the size of microsaccades had a similar pattern of inhibition as microsaccades during fixation, with stronger inhibition onset and faster inhibition release for more salient stimuli. Moreover, a similar stimulus dependency of inhibition was shown for pursuit latencies and peak velocity. Additionally, microsaccade latencies at inhibition release, peak pursuit velocities, and latencies at minimum pursuit velocity were correlated with contrast sensitivity. We demonstrated the generality of OMI to smooth pursuit for both microsaccades and the pursuit itself and its close relation to the low-level processes that define saliency, such as contrast sensitivity.     

Stimulus conditions that enhance anticipatory slow eye movements

Anticipatory slow eye movements are predictive responses that occur prior to both ramp and step target motions. These low velocity eye movements are enhanced and can be studied in isolation by transient target disappearance before ramp motion onset. Slow eye velocities also decrease prior to the termination of target motion. In experiments using a bistable apparent motion stimulus, it was found that perceived motion is a stimulus for anticipatory slow eye movements. This relationship between motion perception and anticipatory slow eye movements can explain previously noted differences between these predictive movements and the predictive component of smooth pursuit.     

Reaction time to motion onset and magnitude estimation of velocity in the presence of background motion

Reaction times (RT) to motion onset of a target grating moving at 0.4, 0.6, 0.8, 1.0 or 1.6°/s and magnitude estimation of the same velocities were studied in the presence of the surrounding background motion which was either in the same or opposite direction. Surprisingly, we found no relative motion effect: if the background motion, irrespective of its direction, affected the target, then it delayed the RTs and decreased velocity ratings. The background motion was effective on RTs to motion onset only when the target was relatively small and immediately surrounded by a moving background. Increases in RTs were mostly explained by an apparent slowdown of the target stimulus velocity which was caused by the interference from the moving background. The background motion also affected velocity ratings by decreasing them without systematic effect of the background motion direction.     

Stereothresholds for moving line stimuli for a range of velocities

This study examined the influence of lateral target motion on the stereothresholds for bright vertical lines at a range of velocities. Stimuli were presented for 200 ms with horizontal velocities from 0 to 12 deg/s. Observers' horizontal eye movements were recorded on additional trials, and confirmed that the velocity of retinal image motion closely matched the velocity of the stimulus. In three auxiliary experiments, stereothresholds were measured (1) after equating the detectability of targets that moved at different velocities, (2) for moving and stationary stimuli with durations between 20 and 200 ms, and (3) for stationary stimuli presented at eccentricities of 0.6 and 1.2 deg. The results indicate that stereothresholds are unaffected by velocities up to approximately 2 deg/s, but worsen in proportion to the velocity at higher speeds. The results of our auxiliary experiments demonstrate that the increase in stereothresholds during image motion cannot be attributed primarily to a reduction in the detectability of the stimulus, a decrease in the effective exposure duration, or non-foveal viewing. We conclude that the elevation of stereo thresholds during lateral motion is consistent with a shift in the sensitivity of the visual system toward lower spatial frequencies as a result of motion blur.     

Errors in direction-of-motion discrimination with complex stimuli

The direction of apparent motion in a complex pattern comprising a static 1-cycle/degree (c/deg) grating and a moving 3-c/deg grating changes with stimulus duration. At durations longer than about 150 msec, motion is seen almost veridically; the motion of the 3-c/deg grating, which is seen correctly, merely induces in the 1-c/deg grating a weak apparent motion in the opposite direction. At shorter durations, however, the only motion seen is in the opposite direction from that which, in fact, occurs. The reversed apparent motion is both compelling and consistent; it is reported both by naive and by experienced observers, and, although it only occurs for certain ranges of spatial frequency, contrast and duration, the ranges are substantial. The reversal appears to be almost independent of the temporal frequency and the spatial phase of the stimulus; it occurs both for discrete and for continuous motion. It seems likely that the apparent motion with short duration stimuli reveals properties of local visual movement detection previously unknown and difficult to account for within the framework of current models of motion perception.     

Reaction times to motion onset and motion detection thresholds reflect the properties of bilocal motion detectors

Several different psychophysical paradigms are used to study human motion perception. A unifying framework for the interpretation of all data is lacking. As a step towards a universal model for motion detection we show that previously published reaction times to motion onset and thresholds for the detection of periodic motion may be derived from the velocity dependent properties of bilocal motion detectors of the Reichardt correlator type. Thus, these data sets seem to support the concept of bilocal motion detectors.     

Effects of superior colliculus inhibition on visual motion processing in the lateral suprasylvian visual area of the cat

PURPOSE: To clarify whether visual inputs of the tectothalamocortical pathway influence motion processing within the lateral suprasylvian (LS) area of the cat. METHODS: This study was conducted in five cats. Tungsten microelectrodes were used for recording visual evoked potentials. The electrodes were introduced into the LS area. An array of 120 randomly located dots was projected onto the stimulus field (40 degrees x 40 degrees) in front of the animal by a slide projector. The dots were moved rightward and leftward alternatively with interstimulus intervals by a mirror attached to a galvanometer, the movements of which were controlled by a microcomputer. Each motion sequence consisted of an abrupt onset of motion that continued for 100 msec followed by an abrupt offset and a stationary phase of 900 msec; the total duration of each sequence was thus 1000 msec. The velocity of the motion was varied in 12 steps. The onset of motion was used as the trigger for recording evoked potentials. Single or multiple injections (two to three) of muscimol were made, mainly into the rostral superior colliculus (SC). The amplitudes of evoked potentials before and after the muscimol injection were compared. RESULTS: A large negative wave (N1) with the peak latency of 89.80+/-16.39 msec (mean +/- SD, n = 191) was recorded consistently. The amplitude of N1 was not altered by the muscimol injection into the SC when the velocity of motion was 50 deg/sec or less. When the velocity of motion was 75 deg/sec or more, however, the amplitude of N1 was reduced to 62% to 72% of that noted before the muscimol injection. CONCLUSIONS: These findings suggest that the LS area processes the visual motion inputs reaching through the two parallel pathways, the geniculostriate pathway and the tectothalamocortical pathway, when the velocity of visual motion is 75 deg/sec or more.     

Extrapolation of linear motion.

We investigated observers' ability to extrapolate a linear trajectory of a moving point, in order to determine how effectively the visual system can combine orientation and position information for moving stimuli. Observers saw a probe dot moving along a straight line toward a stationary target dot. The probe dot extinguished before reaching the target, and the observers' task was to judge whether an extrapolation of the trajectory of the probe would pass to the left or right of the target. Performance was measured as a function of probe velocity, length of the visible trajectory, and location of the target. The empirical results indicated that over a range of conditions, performance on this task is qualitatively similar to, but somewhat less accurate than, that on an analogous task with static stimuli. A four-component model is presented to account for the results. The model specifies an accurate extraction of probe motion parameters, extrapolation of the motion by an ideal observer, and limitations on the input to these processes in the form of visual field spatial inhomogeneity and temporal decay of position information.     

A comparison of oculomotor pursuit of a target in circular real, beta or sigma motion

Pursuit of a point target in real or apparent motion upon a dark, diffusely lighted or structured background was recorded with a scleral coil technique. Smooth and saccadic components were separated and analyzed with computer techniques. Sigma-pursuit was superior to pursuit of beta- or real motion: smooth pursuit gain was higher, saccadic rate was lower and the detrimental effect of a structured background was smaller. Due to directional errors, smooth pursuit velocity often exceeded target velocity when this was smaller than about 10 degrees/sec. However, the smooth component in the correct direction of the target motion had a gain less than or equal to 1.0 and decreasing at higher target velocities for all pursuit modes, inclusive sigma-pursuit.     

Texture segregation by motion under low luminance levels

We examined the effect of average luminance level on texture segregation by motion. We determined the minimum presentation duration required for subjects to detect a target defined by motion direction against a moving background. The average luminance level and retinal position of the target were systematically varied. We found that the minimum presentation duration needed for texture segregation depends significantly on the average luminance level and on retinal position. The minimum presentation duration increased as the mean luminance decreased. At a very low (presumably scotopic) luminance level, the motion-defined target was never detected rapidly. Under scotopic conditions, the minimum presentation duration was shorter in the periphery than in a near foveal region when the task was simple detection of the target. When the task included identifying the shape of the target patch, however, the target presented near the fovea was identified faster at all luminance levels. These results suggest that the performance of texture segregation is constrained by the spatiotemporal characteristics of the early visual system.     

Vernier in Motion: What Accounts for the Threshold Elevation?

Vernier acuity is susceptible to degradation by image motion. The purpose of this study was to determine to what extent vernier thresholds are elevated in the presence of image motion because of reduced stimulus visibility, due to contrast smearing, or to a shift in the spatial scale of analysis. To test the visibility hypothesis, we measured vernier thresholds as a function of stimulus velocity (0–6 deg/sec), for various levels of stimulus visibility, each normalized to the detection threshold at the respective velocity. Contrary to the prediction of the visibility hypothesis, vernier thresholds worsen as the velocity increases, even when the stimuli are equally visible. To test the shift in spatial scale hypothesis, we determined spatial frequency tuning functions for vernier discrimination and line detection tasks, using a masking paradigm. We measured vernier and line detection thresholds as a function of spatial frequency of a sine-wave mask (0.5-32 c/deg), and for stimulus and mask velocities ranging from 0 to 4 deg/sec. Peak masking for both vernier discrimination and line detection, which indicates the most sensitive band of spatial frequencies for each task, shifts systematically toward lower spatial frequencies as the velocity increases. The progressive increase in spatial scale largely accounts for the worsening of vernier thresholds for moving stimuli. Differences between peak masking for vernier discrimination and line detection were found at 0 and 1 deg/sec, suggesting that different mechanisms mediate the two tasks, at least at low velocities. The masking results are consistent with previous findings that directionally selective motion detectors mediate detection of moving stimuli, but suggest that these detectors do not analyze vernier offsets. We conclude that the elevation of vernier threshold for a moving stimulus is accounted for primarily by a shift of sensitivity to mechanisms of lower spatial frequency, and not by decreased stimulus visibility. Copyright © 1996 Elsevier Science Ltd.