Retinal Image Velocity Research Articles

Estimations of the Passing Height of Approaching Objects.

Limited optical cues associated with ball flight were inadequate to estimate the vertical passing distance of approaching balls. These results suggest that these optical cues either must be integrated with contextual and kinematic cues or must be of larger amplitude to contribute to estimates of vertical passing distance. To intercept or avoid approaching objects, individuals must estimate both when and where the object will arrive. The purpose of this experiment was to determine whether individuals could estimate the vertical passing height of a ball approaching at different linear speeds when vertical angular retinal image velocity and cues for time to contact were minimized. Twenty participants stood 40 feet from a pitching machine that projected tennis balls toward observers at six random speeds from 56 to 80 mph. The flight of the balls was stopped after 9 feet. The actual passing height ranged from about 35 (lowest speed) to 136 cm (highest speed). Observers indicated the height at which they expected the balls to arrive. Overall, the height estimates increased as ball speed increased (means, 121 ± 13 cm [lowest speed] and 131 ± 10 cm [highest speed]). However, only at the higher speeds were the absolute height estimates close to the actual height of the ball. At the higher ball speeds, estimates for participants with some experience in baseball or softball were more accurate (86.4% correct at the highest speed) than estimates for participants with no experience. Overall, estimates of vertical passing distance were inaccurate particularly at the lower speeds. Underestimates of vertical drop at lower speeds may have resulted from overestimates of ball speeds. At short exposure durations, optical cues associated with ball flight were inadequate for predictions of vertical passing distance at all speeds for the no-experience group and at lower speeds for the experienced group.

Absence of a vergence-mediated vestibulo-ocular reflex gain increase does not preclude adaptation

The gain (eye-velocity/head-velocity) of the angular vestibuloocular reflex (aVOR) during head impulses can be increased while viewing near-targets and when exposed to unilateral, incremental retinal image velocity error signals. It is not clear however, whether the tonic or phasic vestibular pathways mediate these gain increases. Determine whether a shared pathway is responsible for gain enhancement between vergence and adaptation of aVOR gain in patients with unilateral vestibular hypofunction (UVH). 20 patients with UVH were examined for change in aVOR gain during a vergence task and after 15-minutes of ipsilesional incremental VOR adaptation (uIVA) using StableEyes (a device that controls a laser target as a function of head velocity) during horizontal passive head impulses.A 5 % aVOR gain increase was defined as the threshold for significant change. 11/20 patients had >5% vergence-mediated gain increase during ipsi-lesional impulses. For uIVA, 10/20 patients had >5% ipsi-lesional gain increase. There was no correlation between the vergence-mediated gain increase and gain increase after uIVA training. Vergence-enhanced and uIVA training gain increases are mediated by separate mechanisms and/or vestibular pathways (tonic/phasic).The ability to increase the aVOR gain during vergence is not prognostic for successful adaptation training.

Understanding the cause of perceptual depth-sign ambiguity with dihedral angles created with motion parallax

Early research employing computer-generated dihedrals suggested that depth from motion parallax (MP) is unstable and depth-sign ambiguous. The pursuit theory of motion parallax suggests that perceptual instability and depth-sign reversals are the result of stimulus parameters exceeding the physically-possible conditions for a rigid stimulus as described by the Motion/Pursuit Law. When the Motion/Pursuit Ratio approaches 1, depth from MP approaches infinity. The visual system may recognize when stimulus retinal image velocity exceeds pursuit velocity (M/PR > 1,) representing an impossible rigid object. Such conditions are easily generated with computer-generated MP stimuli, producing depth-sign reversals and perhaps concomitant changes to perceived depth magnitude. To test this, we are measured perceived depth magnitude, in addition to depth-sign, with dihedral stimuli. Psychophysical stimuli included both physical and virtual dihedrals having two planes intersecting with a vertex on the horizontal meridian, facing or opposing the observer. With the 40 cm viewing distance, both stimulus types subtended 10.6 degrees. The virtual random-dot MP stimuli used M/PR gradients to depict the two slanted planes with varying relative depth magnitudes (60-110 mm) and therefore varying slants (19-30 degrees). Physical stimuli were 3D printed with varying slants (15-75 degrees). Observers indicated both slant of the front-facing plane using orientation of their palm, and perceived depth-sign of the vertex. With full-cue viewing of physical stimuli, observers underestimated slant by half (individuals varied between 0.3-0.7). Because an underestimate of slant signals an increase in perceived depth, the reported slants of MP stimuli were corrected with functions derived from the physical stimuli. Subsequently with MP stimuli, observers overestimated slopes indicating, similar to previous work, remarkable underestimation of perceived depth magnitude. In addition, increasing MP stimulus parameters to represent depth magnitudes that are impossible within the framework of the Motion/Pursuit Law produces both depth-sign reversals and reductions in perceived depth magnitude.

The effect of retinal image error update rate on human vestibulo-ocular reflex gain adaptation.

The primary function of the angular vestibulo-ocular reflex (VOR) is to stabilise images on the retina during head movements. Retinal image movement is the likely feedback signal that drives VOR modification/adaptation for different viewing contexts. However, it is not clear whether a retinal image position or velocity error is used primarily as the feedback signal. Recent studies examining this signal are limited because they used near viewing to modify the VOR. However, it is not known whether near viewing drives VOR adaptation or is a pre-programmed contextual cue that modifies the VOR. Our study is based on analysis of the VOR evoked by horizontal head impulses during an established adaptation task. Fourteen human subjects underwent incremental unilateral VOR adaptation training and were tested using the scleral search coil technique over three separate sessions. The update rate of the laser target position (source of the retinal image error signal) used to drive VOR adaptation was different for each session [50 (once every 20 ms), 20 and 15/35 Hz]. Our results show unilateral VOR adaptation occurred at 50 and 20 Hz for both the active (23.0 ± 9.6 and 11.9 ± 9.1% increase on adapting side, respectively) and passive VOR (13.5 ± 14.9, 10.4 ± 12.2%). At 15 Hz, unilateral adaptation no longer occurred in the subject group for both the active and passive VOR, whereas individually, 4/9 subjects tested at 15 Hz had significant adaptation. Our findings suggest that 1-2 retinal image position error signals every 100 ms (i.e. target position update rate 15-20 Hz) are sufficient to drive VOR adaptation.

Unilateral Adaptation of the Human Angular Vestibulo-Ocular Reflex

A recent study showed that the angular vestibulo-ocular reflex (VOR) can be better adaptively increased using an incremental retinal image velocity error signal compared with a conventional constant large velocity-gain demand (×2). This finding has important implications for vestibular rehabilitation that seeks to improve the VOR response after injury. However, a large portion of vestibular patients have unilateral vestibular hypofunction, and training that raises their VOR response during rotations to both the ipsilesional and contralesional side is not usually ideal. We sought to determine if the vestibular response to one side could selectively be increased without affecting the contralateral response. We tested nine subjects with normal vestibular function. Using the scleral search coil and head impulse techniques, we measured the active and passive VOR gain (eye velocity / head velocity) before and after unilateral incremental VOR adaptation training, consisting of self-generated (active) head impulses, which lasted ≈ 15 min. The head impulses consisted of rapid, horizontal head rotations with peak-amplitude 15°, peak-velocity 150°/s and peak-acceleration 3,000°/s(2). The VOR gain towards the adapting side increased after training from 0.92 ± 0.18 to 1.11 ± 0.22 (+22.7 ± 20.2 %) during active head impulses and from 0.91 ± 0.15 to 1.01 ± 0.17 (+11.3 ± 7.5 %) during passive head impulses. During active impulses, the VOR gain towards the non-adapting side also increased by ≈ 8 %, though this increase was ≈ 70 % less than to the adapting side. A similar increase did not occur during passive impulses. This study shows that unilateral vestibular adaptation is possible in humans with a normal VOR; unilateral incremental VOR adaptation may have a role in vestibular rehabilitation. The increase in passive VOR gain after active head impulse adaptation suggests that the training effect is robust.

Neuronal Variability of MSTd Neurons Changes Differentially With Eye Movement and Visually Related Variables

Neurons in macaque cortical area MSTd are driven by visual motion and eye movement related signals. This multimodal characteristic makes MSTd an ideal system for studying the dependence of neuronal activity on different variables. Here, we analyzed the temporal structure of spiking patterns during visual motion stimulation using 2 distinct behavioral paradigms: fixation (FIX) and optokinetic response. For the FIX condition, inter- and intra-trial variability of spiking activity decreased with increasing stimulus strength, complying with a recent neurophysiological study reporting stimulus-related decline of neuronal variability. In contrast, for the optokinetic condition variability increased together with increasing eye velocity while retinal image velocity remained low. Analysis of stimulus signal variability revealed a correlation between the normalized variance of image velocity and neuronal variability, but no correlation with normalized eye velocity variance. We further show that the observed difference in neuronal variability allows classifying spike trains according to the paradigm used, even when mean firing rates (FRs) were similar. The stimulus-dependence of neuronal variability may result from the local network structure and/or the variability characteristics of the input signals, but may also reflect additional timing-based mechanisms independent of the neuron's mean FR and related to the modality driving the neuron.

Compensation for equiluminant color motion during smooth pursuit eye movement

Motion perception is compromised at equiluminance. Because previous investigations have been primarily carried out under fixation conditions, it remains unknown whether and how equiluminant color motion comes into play in the velocity compensation for retinal image motion due to smooth pursuit eye movement. We measured the retinal image velocity required to reach subjective stationarity for a horizontally drifting sinusoidal grating in the presence of horizontal smooth pursuit. The grating was defined by luminance or chromatic modulation. When the subjective stationarity of the color motion was shifted toward environmental stationarity, compared with the subjective stationarity of luminance motion, that of color motion was farther from retinal stationarity, indicating that a slowing of color motion occurred before this factor contributed to the process by which retinal motion was integrated with a biological estimate of eye velocity during pursuit. The gain in the estimate of eye velocity per se was unchanged irrespective of whether the stimulus was defined by luminance or by color. Indeed, the subjective reduction in the speed of color motion during fixation was accounted for by the same amount of deterioration in speed. From these results, we conclude that the motion deterioration at equiluminance takes place prior to the velocity comparison.

The spiking irregularity of MSTd neurons depends on visual and oculomotor variables

Event Abstract Back to Event The spiking irregularity of MSTd neurons depends on visual and oculomotor variables Lukas Brostek1, 2*, Ulrich Büttner1, 3, Michael J. Mustari4 and Stefan Glasauer1, 2, 3 1 LMU, Clinical Neurosciences, Germany 2 BCCN Munich, Germany 3 LMU, Integrated Center for Research and Treatment of Vertigo, Germany 4 University of Washington, Washington National Primate Research Center, United States Neuronal activity in macaque cortical area MSTd is driven by retinal stimuli, as almost all neurons respond to moving large-field patterns. At the same time, the spiking activity in MSTd is modulated by an internally generated signal related to the monkey’s eye movements. This combination of both visual motion and eye movement related activity makes MSTd an ideal system for analyzing neuronal activity in dependence on different stimulus dimensions. In this work we analyzed the inter- and intra-trial variability of spiking activity by measuring Fano factor (FF), squared coefficient of variation (CV²) and local variation (LV). Two different paradigms were used: fixation with visual stimulation and optokinetic response to a moving large-field stimulus. For the first condition our results complied with a recent neurophysiological study reporting stimulus-related decline in neuronal variability. The second condition, however, revealed opposing behavior. We found that both stimulus variables, retinal image velocity and eye velocity, differentially affected neuronal variability. Spiking irregularity decreased when image velocity increased and eye velocity was kept low, but increased with increasing eye velocity and low image velocity. This finding might reflect a specificity of the local network structure or could be related to the variability characteristics of the input signals. Nevertheless, the observed behaviour may also have functional importance reflecting additional timing-based coding independent of the neuron's mean firing rate. Acknowledgements This work was supported by the German Federal Ministry of Education and Research Grants 01GQ0440 (BCCN), 01EO0901 (IFB), and National Institutes of Health Grants EY06069, RR000166. Keywords: eye movement control, Fano factor, MST, neuronal variability, spiking irregularity, Visual System Conference: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011, Freiburg, Germany, 4 Oct - 6 Oct, 2011. Presentation Type: Poster Topic: neural encoding and decoding (please use "neural coding and decoding" as keyword) Citation: Brostek L, Büttner U, Mustari MJ and Glasauer S (2011). The spiking irregularity of MSTd neurons depends on visual and oculomotor variables. Front. Comput. Neurosci. Conference Abstract: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011. doi: 10.3389/conf.fncom.2011.53.00228 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 04 Aug 2011; Published Online: 04 Oct 2011. * Correspondence: Mr. Lukas Brostek, LMU, Clinical Neurosciences, München, Germany, Lukas.Brostek@lrz.uni-muenchen.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Lukas Brostek Ulrich Büttner Michael J Mustari Stefan Glasauer Google Lukas Brostek Ulrich Büttner Michael J Mustari Stefan Glasauer Google Scholar Lukas Brostek Ulrich Büttner Michael J Mustari Stefan Glasauer PubMed Lukas Brostek Ulrich Büttner Michael J Mustari Stefan Glasauer Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Incorporating Prediction in Models for Two-Dimensional Smooth Pursuit

A predictive component can contribute to the command signal for smooth pursuit. This is readily demonstrated by the fact that low frequency sinusoidal target motion can be tracked with zero time delay or even with a small lead. The objective of this study was to characterize the predictive contributions to pursuit tracking more precisely by developing analytical models for predictive smooth pursuit. Subjects tracked a small target moving in two dimensions. In the simplest case, the periodic target motion was composed of the sums of two sinusoidal motions (SS), along both the horizontal and the vertical axes. Motions following the same or similar paths, but having a richer spectral composition, were produced by having the target follow the same path but at a constant speed (CS), and by combining the horizontal SS velocity with the vertical CS velocity and vice versa. Several different quantitative models were evaluated. The predictive contribution to the eye tracking command signal could be modeled as a low-pass filtered target acceleration signal with a time delay. This predictive signal, when combined with retinal image velocity at the same time delay, as in classical models for the initiation of pursuit, gave a good fit to the data. The weighting of the predictive acceleration component was different in different experimental conditions, being largest when target motion was simplest, following the SS velocity profiles.

Relationship between Body Size and Spatial Vision in the Praying Mantis - An Ontogenetic Study

Abstract Praying mantids determine prey-capture distances with the aid of binocular mechanisms and estimate distances to stationary objects such as jump targets with the aid of retinal image velocity. The latter mechanism makes use of the fact that the retinal image velocity resulting from translational self-motion (peering movements) is inversely proportional to the distance of the target object. The present study shows that for South African mantids Polyspilota sp., the interocular distance, an aspect of binocular vision, and the amplitude and velocity of peering movements, increase over the course of postembryonic development from 2nd instar to adult. In both cases there is a close positive correlation with body size. Calculated convergence angles and retinal image velocities indicate an improvement in absolute depth perception with increased body size. This is adaptive, because it allows the range of visual depth perception to increase with body size, strike distance and jumping capability, throughout...

The temporal impulse response function during smooth pursuit

Recent studies indicate that the extent of perceived motion smear is attenuated asymmetrically during smooth pursuit eye movements, based on the relative directions of the target and eye motion. We conducted two experiments to determine if the reduction of perceived smear during pursuit might be associated with an acceleration of the temporal impulse response function (TIRF). In Experiment 1, two-pulse increment sensitivity was determined during fixation and rightward pursuit for sequential flashes of a long horizontal line, presented with stimulus-onset asynchronies between 5.9 and 234 ms. In Experiment 2, temporal contrast sensitivity was measured during fixation and rightward pursuit for a vertical 1 cpd grating with retinal image velocities between 4 and 30 Hz. During pursuit, grating motion was either in the same or the opposite direction as the eye movement. TIRFs were modeled as the impulse responses of a second-order, low-pass linear system, fit to the two-pulse increment sensitivity data by an optimization procedure and to the temporal contrast sensitivity results by iterative Fourier synthesis. The results indicate that the natural temporal frequency of the fitted TIRFs was approximately 10% higher during pursuit than fixation. In Experiment 2, the increased natural frequency of the TIRF was restricted to the condition in which the grating moved spatially in the opposite direction of the pursuit eye movement. The results are consistent with the hypothesis that extra-retinal signals reduce the extent of perceived motion smear during pursuit, in part by increasing the speed of visual processing preferentially for one direction of image motion.

Signal Processing and Distribution in Cortical‐Brainstem Pathways for Smooth Pursuit Eye Movements

Smooth pursuit (SP) eye movements are used to maintain the image of a moving object relatively stable on the fovea. Even when tracking a single target over a dark background, multiple areas including frontal eye fields (FEF) and middle temporal (MT) and medial superior temporal (MST) cortex contribute to converting visual signals into initial commands for SP. Signals in the cortical pursuit system reach the oculomotor cerebellum through brainstem centers including the dorsolateral pontine nucleus (DLPN), nucleus reticularis tegmenti pontis (NRTP), and pretectal nucleus of the optic tract (NOT). The relative information carried in these parallel pathways remains to be fully defined. We used multiple linear-regression modeling to estimate the relative sensitivities of cortical (MST, FEF), pontine (NRTP, DLPN), and NOT neurons to eye- and retinal-error parameters (position, velocity, and acceleration) during step-ramp SP of macaques (Macaca mulatta). We found that a large proportion of pursuit-related MST and DLPN neurons were most sensitive to eye-velocity or retinal error velocity. In contrast, a large proportion of FEF and rostral NRTP neurons were most sensitive to eye acceleration. Visual neurons in MST, DLPN, and NOT were most sensitive to retinal image velocity.

Neural noise distorts perceived motion: the special case of the freezing illusion and the Pavard and Berthoz effect

When a slowly moving pattern is presented on a monitor which itself is moved, the pattern appears to freeze on the screen (Mesland and Wertheim in Vis Res 36(20):3325–3328, 1996) even if we move our head with the monitor, as with a head mounted display (Pavard and Berthoz in Perception 6:529–540, 1977). We present a simple model of these phenomena, which states that the perceived relative velocity between two stimuli (the pattern and the moving monitor) is proportional to the difference between the perceived velocities of these stimuli in space, minus a noise factor. The latter reflects the intrinsic noise in the neural signals that encode retinal image velocities. With noise levels derived from the literature the model fits empirical data well and also predicts strong distortions of visually perceived motion during vestibular stimulation, thus explaining both illusions as resulting from the same mechanism.

Correlation versus gradient type motion detectors: the pros and cons.

Visual motion contains a wealth of information about self-motion as well as the three-dimensional structure of the environment. Therefore, it is of utmost importance for any organism with eyes. However, visual motion information is not explicitly represented at the photoreceptor level, but rather has to be computed by the nervous system from the changing retinal images as one of the first processing steps. Two prominent models have been proposed to account for this neural computation: the Reichardt detector and the gradient detector. While the Reichardt detector correlates the luminance levels derived from two adjacent image points, the gradient detector provides an estimate of the local retinal image velocity by dividing the spatial and the temporal luminance gradient. As a consequence of their different internal processing structure, both the models differ in a number of functional aspects such as their dependence on the spatial-pattern structure as well as their sensitivity to photon noise. These different properties lead to the proposal that an ideal motion detector should be of Reichardt type at low luminance levels, but of gradient type at high luminance levels. However, experiments on the fly visual systems provided unambiguous evidence in favour of the Reichardt detector under all luminance conditions. Does this mean that the fly nervous system uses suboptimal computations, or is there a functional aspect missing in the optimality criterion? In the following, I will argue in favour of the latter, showing that Reichardt detectors have an automatic gain control allowing them to dynamically adjust their input-output relationships to the statistical range of velocities presented, while gradient detectors do not have this property. As a consequence, Reichardt detectors, but not gradient detectors, always provide a maximum amount of information about stimulus velocity over a large range of velocities. This important property might explain why Reichardt type of computations have been demonstrated to underlie the extraction of motion information in the fly visual system under all luminance levels.

Frame of reference transformations in motion perception during smooth pursuit eye movements

Smooth pursuit eye movements change the retinal image velocity of objects in the visual field. In order to change from a retinocentric frame of reference into a head-centric one, the visual system has to take the eye movements into account. Studies on motion perception during smooth pursuit eye movements have measured either perceived speed or perceived direction during smooth pursuit to investigate this frame of reference transformation, but never both at the same time. We devised a new velocity matching task, in which participants matched both perceived speed and direction during fixation to that during pursuit. In Experiment 1, the velocity matches were determined for a range of stimulus directions, with the head-centric stimulus speed kept constant. In Experiment 2, the retinal stimulus speed was kept approximately constant, with the same range of stimulus directions. In both experiments, the velocity matches for all directions were shifted against the pursuit direction, suggesting an incomplete transformation of the frame of reference. The degree of compensation was approximately constant across stimulus direction. We fitted the classical linear model, the model of Turano and Massof (2001) and that of Freeman (2001) to the velocity matches. The model of Turano and Massof fitted the velocity matches best, but the differences between de model fits were quite small. Evaluation of the models and comparison to a few alternatives suggests that further specification of the potential effect of retinal image characteristics on the eye movement signal is needed.

Perceived motion direction during smooth pursuit eye movements

Although many studies have been devoted to motion perception during smooth pursuit eye movements, relatively little attention has been paid to the question of whether the compensation for the effects of these eye movements is the same across different stimulus directions. The few studies that have addressed this issue provide conflicting conclusions. We measured the perceived motion direction of a stimulus dot during horizontal ocular pursuit for stimulus directions spanning the entire range of 360 degrees. The stimulus moved at either 3 or 8 degrees/s. Constancy of the degree of compensation was assessed by fitting the classical linear model of motion perception during pursuit. According to this model, the perceived velocity is the result of adding an eye movement signal that estimates the eye velocity to the retinal signal that estimates the retinal image velocity for a given stimulus object. The perceived direction depends on the gain ratio of the two signals, which is assumed to be constant across stimulus directions. The model provided a good fit to the data, suggesting that compensation is indeed constant across stimulus direction. Moreover, the gain ratio was lower for the higher stimulus speed, explaining differences in results in the literature.

Effects of Retinal Image Slip on Modulation of Visual Vestibulo‐Ocular Reflex During Near Viewing

During viewing of a near target, the gain of the visually modulated vestibulo-ocular reflex (vVOR) exceeds 1.0. We investigated whether retinal image slip contributes to this gain increase by measuring responses during strobe illumination. Subjects showed greater gain values during normal versus strobe illumination of the near target, even during rotation at frequencies above 1.0 Hz, indicating that retinal image velocity information is used to optimize vVOR.

Disparity-Based Coding of Three-Dimensional Surface Orientation by Macaque Middle Temporal Neurons

Gradients of binocular disparity across the visual field provide a potent cue to the three-dimensional (3-D) orientation of surfaces in a scene. Neurons selective for 3-D surface orientation defined by disparity gradients have recently been described in parietal cortex, but little is known about where and how this selectivity arises within the visual pathways. Because the middle temporal area (MT) has previously been implicated in depth perception, we tested whether MT neurons could signal the 3-D orientation (as parameterized by tilt and slant) of planar surfaces that were depicted by random-dot stereograms containing a linear gradient of horizontal disparities. We find that many MT neurons are tuned for 3-D surface orientation, and that tilt and slant generally have independent effects on MT responses. This separable coding of tilt and slant is reminiscent of the joint coding of variables in other areas (e.g., orientation and spatial frequency in V1). We show that tilt tuning remains unchanged when all coherent motion is removed from the visual stimuli, indicating that tilt selectivity is not a byproduct of 3-D velocity coding. Moreover, tilt tuning is typically insensitive to changes in the mean disparity (depth) of gradient stimuli, indicating that tilt tuning cannot be explained by conventional tuning for frontoparallel disparities. Finally, we explore the receptive field mechanisms underlying selectivity for 3-D surface orientation, and we show that tilt tuning arises through heterogeneous disparity tuning within the receptive fields of MT neurons. Our findings show that MT neurons carry high-level signals about 3-D surface structure, in addition to coding retinal image velocities.

Interaction of Retinal Image and Eye Velocity in Motion Perception

When we move our eyes, why does the world look stable even as its image flows across our retinas, and why do afterimages, which are stationary on the retinas, appear to move? Current theories say this is because we perceive motion by summation: if an object slips across the retina at r°/s while the eye turns at e°/s, the object's perceived velocity in space should be r + e. We show that activity in MT+, the visual-motion complex in human cortex, does reflect a mix of r and e rather than r alone. But we show also that, for optimal perception, r and e should not summate; rather, the signals coding e interact multiplicatively with the spatial gradient of illumination.

Version and vergence eye movements in humans: open-loop dynamics determined by monocular rather than binocular image speed

We examined the velocity dependence of the vergence and version eye movements elicited by motion stimuli that were symmetric or asymmetric at the two eyes. Movements of both eyes were recorded with the scleral search coil technique. Vergence was computed as the difference in the positions of the two eyes (left–right) and version was computed as the average position of the two eyes ((left+right)/2). Subjects faced a large tangent screen onto which two identical random-dot patterns were back-projected. Each pattern was viewed by one eye only using crossed-polarizers and its position was controlled by X/ Y mirror galvanometers. Viewing was always binocular and horizontal velocity steps (range, 5–240 deg/s) were applied to one (asymmetric stimulus) or both (symmetric stimulus) patterns ∼50 ms after a centering saccade. With the symmetric stimulus, the motion at the two eyes could be either in the opposite direction (eliciting vergence responses) or in the same direction (eliciting version responses). The asymmetric stimuli elicited both vergence and version. In all cases, minimum response latencies were very short (<90 ms). Velocity tuning curves (based on the changes in vergence and version over the time period, 90–140 ms) were all sigmoidal and peaked when the monocular (i.e., retinal) image velocities were 30–60 deg/s. The vergence (version) responses to symmetric stimuli were linearly related to the vergence (version) responses to asymmetric stimuli when expressed in terms of the monocular rather than the binocular image velocities. We conclude that the dynamical limits for both vergence and version are imposed in the monocular visual pathways, before the inputs from the two eyes are combined.