Rapid processing of retinal slip during saccades in macaque area MT

Perisaccadic encoding of temporal information in macaque area V4

The accurate processing of temporal information is of critical importance in everyday life. Yet, psychophysical studies in humans have shown that the perception of time is distorted around saccadic eye movements. The neural correlates of this misperception are still poorly understood. Behavioral and neural evidence suggest that it is tightly linked to other known perisaccadic modulations of visual perception. To further our understanding of how temporal processing is affected by saccades, we studied the representations of brief visual time intervals during fixation and saccades in area V4 of two awake macaques. We presented random sequences of vertical bar stimuli and extracted neural responses to double-pulse stimulation at varying interstimulus intervals. Our results show that temporal information about very brief intervals of as brief as 20 ms is reliably represented in the multiunit activity in area V4. Response latencies were not systematically modulated by the saccade. However, a general increase in perisaccadic activity altered the ratio of response amplitudes within stimulus pairs compared with fixation. In line with previous studies showing that the perception of brief time intervals is partly based on response levels, this may be seen as a possible correlate of the perisaccadic misperception of time.NEW & NOTEWORTHY We investigated for the first time how temporal information on very brief timescales is represented in area V4 around the time of saccadic eye movements. Overall, the responses showed an unexpectedly precise representation of time intervals. Our finding of a perisaccadic modulation of relative response amplitudes introduces a new possible correlate of saccade-related perceptual distortions of time.

Visual feature tuning of superior colliculus neural reafferent responses after fixational microsaccades

The primate superior colliculus (SC) is causally involved in microsaccade generation. Moreover, visually responsive SC neurons across this structure's topographic map, even at peripheral eccentricities much larger than the tiny microsaccade amplitudes, exhibit significant modulations of evoked response sensitivity when stimuli appear perimicrosaccadically. However, during natural viewing, visual stimuli are normally stably present in the environment and are only shifted on the retina by eye movements. Here we investigated this scenario for the case of microsaccades, asking whether and how SC neurons respond to microsaccade-induced image jitter. We recorded neural activity from two male rhesus macaque monkeys. Within the response field (RF) of a neuron, there was a stable stimulus consisting of a grating of one of three possible spatial frequencies. The grating was stable on the display, but microsaccades periodically jittered the retinotopic RF location over it. We observed clear short-latency visual reafferent responses after microsaccades. These responses were weaker, but earlier (relative to new fixation onset after microsaccade end), than responses to sudden stimulus onsets without microsaccades. The reafferent responses clearly depended on microsaccade amplitude as well as microsaccade direction relative to grating orientation. Our results indicate that one way for microsaccades to influence vision is through modulating how the spatio-temporal landscape of SC visual neural activity represents stable stimuli in the environment. Such representation depends on the specific pattern of temporal luminance modulations expected from the relative relationship between eye movement vector (size and direction) on one hand and spatial visual pattern layout on the other.NEW & NOTEWORTHY Despite being diminutive, microsaccades still jitter retinal images. We investigated how such jitter affects superior colliculus (SC) activity. We found that SC neurons exhibit short-latency visual reafferent bursts after microsaccades. These bursts reflect not only the spatial luminance profiles of visual patterns but also how such profiles are shifted by eye movement size and direction. These results indicate that the SC continuously represents visual patterns, even as they are jittered by the smallest possible saccades.

Saccade suppression depends on context

Although our eyes are in constant movement, we remain unaware of the high-speed stimulation produced by the retinal displacement. Vision is drastically reduced at the time of saccades. Here, I investigated whether the reduction of the unwanted disturbance could be established through a saccade-contingent habituation to intra-saccadic displacements. In more than 100 context trials, participants were exposed either to an intra-saccadic or to a post-saccadic disturbance or to no disturbance at all. After induction of a specific context, I measured peri-saccadic suppression. Displacement discrimination thresholds of observers were high after participants were exposed to an intra-saccadic disturbance. However, after exposure to a post-saccadic disturbance or a context without any intra-saccadic stimulation, displacement discrimination improved such that observers were able to see shifts as during fixation. Saccade-contingent habituation might explain why we do not perceive trans-saccadic retinal stimulation during saccades.

Vision During Saccadic Eye Movements

The perceptual consequences of eye movements are manifold: Each large saccade is accompanied by a drop of sensitivity to luminance-contrast, low-frequency stimuli, impacting both conscious vision and involuntary responses, including pupillary constrictions. They also produce transient distortions of space, time, and number, which cannot be attributed to the mere motion on the retinae. All these are signs that the visual system evokes active processes to predict and counteract the consequences of saccades. We propose that a key mechanism is the reorganization of spatiotemporal visual fields, which transiently increases the temporal and spatial uncertainty of visual representations just before and during saccades. On one hand, this accounts for the spatiotemporal distortions of visual perception; on the other hand, it implements a mechanism for fusing pre- and postsaccadic stimuli. This, together with the active suppression of motion signals, ensures the stability and continuity of our visual experience.

An Attention-Sensitive Memory Trace in Macaque MT Following Saccadic Eye Movements

We experience a visually stable world despite frequent retinal image displacements induced by eye, head, and body movements. The neural mechanisms underlying this remain unclear. One mechanism that may contribute is transsaccadic remapping, in which the responses of some neurons in various attentional, oculomotor, and visual brain areas appear to anticipate the consequences of saccades. The functional role of transsaccadic remapping is actively debated, and many of its key properties remain unknown. Here, recording from two monkeys trained to make a saccade while directing attention to one of two spatial locations, we show that neurons in the middle temporal area (MT), a key locus in the motion-processing pathway of humans and macaques, show a form of transsaccadic remapping called a memory trace. The memory trace in MT neurons is enhanced by the allocation of top-down spatial attention. Our data provide the first demonstration, to our knowledge, of the influence of top-down attention on the memory trace anywhere in the brain. We find evidence only for a small and transient effect of motion direction on the memory trace (and in only one of two monkeys), arguing against a role for MT in the theoretically critical yet empirically contentious phenomenon of spatiotopic feature-comparison and adaptation transfer across saccades. Our data support the hypothesis that transsaccadic remapping represents the shift of attentional pointers in a retinotopic map, so that relevant locations can be tracked and rapidly processed across saccades. Our results resolve important issues concerning the perisaccadic representation of visual stimuli in the dorsal stream and demonstrate a significant role for top-down attention in modulating this representation.

How does the brain keep track of specific attended features after eye movements? A new study of the macaque brain implicates the middle temporal (MT) area in the remapping of attentional pointers across saccades.

Humans experience a visually stable world despite the fact that eye, head, and body movements cause frequent shifts of the image on the retina. Humans and monkeys are also able to keep track of visual stimuli across such movements. One mechanism that may contribute to these abilities is “transsaccadic remapping,” in which the responses of some neurons in various attentional, oculomotor, and visual brain areas appear to anticipate the consequences of saccades. A current hypothesis proposes that the brain maintains “attentional pointers” to the locations of relevant stimuli and that, via transsaccadic remapping, it rapidly relocates these pointers to compensate for intervening eye movements. Whether stimulus features are also remapped across saccades (along with their location) remains unclear. Here, we show the presence of transsaccadic remapping in a macaque monkey brain area critical for visual motion processing, the middle temporal area (MT). This remapped response is stronger for an attended stimulus. We find only weak evidence for motion-direction information in the remapped response. These results support the attentional pointer hypothesis and demonstrate for the first time, to our knowledge, the impact of top-down attention on transsaccadic remapping in the brain.

Noisy decision thresholds can account for suboptimal detection of low coherence motion

Noise in sensory signals can vary over both space and time. Moving random dot stimuli are commonly used to quantify how the visual system accounts for spatial noise. In these stimuli, a fixed proportion of "signal" dots move in the same direction and the remaining "noise" dots are randomly replotted. The spatial coherence, or proportion of signal versus noise dots, is fixed across time; however, this means that little is known about how temporally-noisy signals are integrated. Here we use a stimulus with low temporal coherence; the signal direction is only presented on a fraction of frames. Human observers are able to reliably detect and discriminate the direction of a 200 ms motion pulse, even when just 25% of frames within the pulse move in the signal direction. Using psychophysical reverse-correlation analyses, we show that observers are strongly influenced by the number of near-target directions spread throughout the pulse, and that consecutive signal frames have only a small additional influence on perception. Finally, we develop a model inspired by the leaky integration of the responses of direction-selective neurons, which reliably represents motion direction, and which can account for observers' sub-optimal detection of motion pulses by incorporating a noisy decision threshold.

Saccade-induced image motion cannot account for post-saccadic enhancement of visual processing in primate MST

Primates use saccadic eye movements to make gaze changes. In many visual areas, including the dorsal medial superior temporal area (MSTd) of macaques, neural responses to visual stimuli are reduced during saccades but enhanced afterwards. How does this enhancement arise-from an internal mechanism associated with saccade generation or through visual mechanisms activated by the saccade sweeping the image of the visual scene across the retina? Spontaneous activity in MSTd is elevated even after saccades made in darkness, suggesting a central mechanism for post-saccadic enhancement. However, based on the timing of this effect, it may arise from a different mechanism than occurs in normal vision. Like neural responses in MSTd, initial ocular following eye speed is enhanced after saccades, with evidence suggesting both internal and visually mediated mechanisms. Here we recorded from visual neurons in MSTd and measured responses to motion stimuli presented soon after saccades and soon after simulated saccades-saccade-like displacements of the background image during fixation. We found that neural responses in MSTd were enhanced when preceded by real saccades but not when preceded by simulated saccades. Furthermore, we also observed enhancement following real saccades made across a blank screen that generated no motion signal within the recorded neurons' receptive fields. We conclude that in MSTd the mechanism leading to post-saccadic enhancement has internal origins.

Testing neuronal accounts of anisotropic motion perception with computational modelling

There is an over-representation of neurons in early visual cortical areas that respond most strongly to cardinal (horizontal and vertical) orientations and directions of visual stimuli, and cardinal- and oblique-preferring neurons are reported to have different tuning curves. Collectively, these neuronal anisotropies can explain two commonly-reported phenomena of motion perception – the oblique effect and reference repulsion – but it remains unclear whether neuronal anisotropies can simultaneously account for both perceptual effects. We show in psychophysical experiments that reference repulsion and the oblique effect do not depend on the duration of a moving stimulus, and that brief adaptation to a single direction simultaneously causes a reference repulsion in the orientation domain, and the inverse of the oblique effect in the direction domain. We attempted to link these results to underlying neuronal anisotropies by implementing a large family of neuronal decoding models with parametrically varied levels of anisotropy in neuronal direction-tuning preferences, tuning bandwidths and spiking rates. Surprisingly, no model instantiation was able to satisfactorily explain our perceptual data. We argue that the oblique effect arises from the anisotropic distribution of preferred directions evident in V1 and MT, but that reference repulsion occurs separately, perhaps reflecting a process of categorisation occurring in higher-order cortical areas.

Compression and suppression of shifting receptive field activity in frontal eye field neurons

Before each saccade, neurons in frontal eye field anticipate the impending eye movement by showing sensitivity to stimuli appearing where the neuron's receptive field will be at the end of the saccade, referred to as the future field (FF) of the neuron. We explored the time course of this anticipatory activity in monkeys by briefly flashing stimuli in the FF at different times before saccades. Different neurons showed substantial variation in FF time course, but two salient observations emerged. First, when we compared the time span of stimulus probes before the saccade to the time span of FF activity, we found a striking temporal compression of FF activity, similar to compression seen for perisaccadic stimuli in human psychophysics. Second, neurons with distinct FF activity also showed suppression at the time of the saccade. The increase in FF activity and the decrease with suppression were temporally independent, making the patterns of activity difficult to separate. We resolved this by constructing a simple model with values for the start, peak, and duration of FF activity and suppression for each neuron. The model revealed the different time courses of FF sensitivity and suppression, suggesting that information about the impending saccade triggering suppression reaches the frontal eye field through a different pathway, or a different mechanism, than that triggering FF activity. Recognition of the variations in the time course of anticipatory FF activity provides critical information on its function and its relation to human visual perception at the time of the saccade.

Visual perception and saccadic eye movements

We use saccades several times per second to move the fovea between points of interest and build an understanding of our visual environment. Recent behavioral experiments show evidence for the integration of pre- and postsaccadic information (even subliminally), the modulation of visual sensitivity, and the rapid reallocation of attention. The recent physiological literature has identified a characteristic modulation of neural responsiveness-perisaccadic reduction followed by a postsaccadic increase-that is found in many visual areas, but whose source is as yet unknown. This modulation seems optimal for reducing sensitivity during and boosting sensitivity between saccades, but no study has yet established a direct causal link between neural and behavioral changes.

Effects of fixational saccades on response timing in macaque lateral geniculate nucleus

Even during active fixation, small eye movements persist that might be expected to interfere with vision. Numerous brain mechanisms probably contribute to discounting this jitter. Changes in the timing of responses in the visual thalamus associated with fixational saccades are considered in this study. Activity of single neurons in alert monkey lateral geniculate nucleus (LGN) was recorded during fixation while pseudorandom visual noise stimuli were presented. The position of the stimulus on the display monitor was adjusted based on eye position measurements to control for changes in retinal locations due to eye movements. A method for extracting nonstationary first-order response mechanisms was applied, so that changes around the times of saccades could be observed. Saccade-related changes were seen in both amplitude and timing of geniculate responses. Amplitudes were greatly reduced around saccades. Timing was retarded slightly during a window of about 200 ms around saccades. That is, responses became more sustained. These effects were found in both parvocellular and magnocellular neurons. Timing changes in LGN might play a role in maintaining cortical responses to visual stimuli in the presence of eye movements, compensating for the spatial shifts caused by saccades via these shifts in timing.

Effects of saccades on visual processing in primate MSTd

In surveying their visual environment, primates, including humans make frequent rapid eye movements known as saccades. Saccades result in rapid motion of the retinal image and yet this motion is not perceived. We recorded saccade-related changes in neural activity in the dorsal medial superior temporal area (MSTd) of alert macaque monkeys. We show that the spontaneous activity of neurons in MSTd is modulated around the time of saccades. Some cells show considerable suppression of spontaneous activity, while most show early and significant enhancement. While this modulation of spontaneous activity is variable, the concomitant modulation of neural responses evoked by flashed visual stimuli is uniform and stereotypical - visual responses are suppressed for stimuli presented around the time of saccades and enhanced for stimuli presented afterwards. The combined modulation of spontaneous activity and evoked visual responses likely serves to reduce the detectability of peri-saccadic stimuli and promote the perceptual awareness of visual stimuli between saccades.

Applicability of White-Noise Techniques to Analyzing Motion Responses

Motion processing in visual neurons is often understood in terms of how they integrate light stimuli in space and time. These integrative properties, known as the spatiotemporal receptive fields (STRFs), are sometimes obtained using white-noise techniques where a continuous random contrast sequence is delivered to each spatial location within the cell's field of view. In contrast, motion stimuli such as moving bars are usually presented intermittently. Here we compare the STRF prediction of a neuron's response to a moving bar with the measured response in second-order interneurons (L-neurons) of dragonfly ocelli (simple eyes). These low-latency neurons transmit sudden changes in intensity and motion information to mediate flight and gaze stabilization reflexes. A white-noise analysis is made of the responses of L-neurons to random bar stimuli delivered either every frame (densely) or intermittently (sparsely) with a temporal sequence matched to the bar motion stimulus. Linear STRFs estimated using the sparse stimulus were significantly better at predicting the responses to moving bars than the STRFs estimated using a traditional dense white-noise stimulus, even when second-order nonlinear terms were added. Our results strongly suggest that visual adaptation significantly modifies the linear STRF properties of L-neurons in dragonfly ocelli during dense white-noise stimulation. We discuss the ability to predict the responses of visual neurons to arbitrary stimuli based on white-noise analysis. We also discuss the likely functional advantages that adaptive receptive field structures provide for stabilizing attitude during hover and forward flight in dragonflies.

Visual perception: saccadic omission--suppression or temporal masking?

Although we don't perceive visual stimuli during saccadic eye movements, new evidence shows that our brains do process these stimuli and they can influence our subsequent visual perception.

The relationship between saccadic suppression and perceptual stability

Introspection makes it clear that we do not see the visual motion generated by our saccadic eye movements. We refer to the lack of awareness of the motion across the retina that is generated by a saccade as saccadic omission [1]: the visual stimulus generated by the saccade is omitted from our subjective awareness. In the laboratory, saccadic omission is often studied by investigating saccadic suppression, the reduction in visual sensitivity before and during a saccade (see Ross et al. [2] and Wurtz [3] for reviews). We investigated whether perceptual stability requires that a mechanism like saccadic suppression removes perisaccadic stimuli from visual processing to prevent their presumed harmful effect on perceptual stability [4, 5]. Our results show that a stimulus that undergoes saccadic omission can nevertheless generate a shape contrast illusion. This illusion can be generated when the inducer and test stimulus are separated in space and is therefore thought to be generated at a later stage of visual processing [6]. This shows that perceptual stability is attained without removing stimuli from processing and suggests a conceptually new view of perceptual stability in which perisaccadic stimuli are processed by the early visual system, but these signals are prevented from reaching awareness at a later stage of processing.

Direction and contrast tuning of macaque MSTd neurons during saccades

Saccades are rapid eye movements that change the direction of gaze, although the full-field image motion associated with these movements is rarely perceived. The attenuation of visual perception during saccades is referred to as saccadic suppression. The mechanisms that produce saccadic suppression are not well understood. We recorded from neurons in the dorsal medial superior temporal area (MSTd) of alert macaque monkeys and compared the neural responses produced by the retinal slip associated with saccades (active motion) to responses evoked by identical motion presented during fixation (passive motion). We provide evidence for a neural correlate of saccadic suppression and expand on two contentious results from previous studies. First, we confirm the finding that some neurons in MSTd reverse their preferred direction during saccades. We quantify this effect by calculating changes in direction tuning index for a large cell population. Second, it has been noted that neural activity associated with saccades can arrive in the parietal cortex <or=30 ms earlier than activity produced by similar visual stimulation during fixation. This led to the question of whether the saccade-related responses were visual in origin or were motor signals arising from saccade-planning areas of the brain. By comparing the responses to saccades made over textured backgrounds of different contrasts, we provide strong evidence that saccade-related responses were visual in origin. Refinements of the possible models of saccadic suppression are discussed.

Saccadic modulation of neural responses: possible roles in saccadic suppression, enhancement, and time compression

Humans use saccadic eye movements to make frequent gaze changes, yet the associated full-field image motion is not perceived. The theory of saccadic suppression has been proposed to account for this phenomenon, but it is not clear whether suppression originates from a retinal signal at saccade onset or from the brain before saccade onset. Perceptually, visual sensitivity is reduced before saccades and enhanced afterward. Over the same time period, the perception of time is compressed and even inverted. We explore the origins and neural basis of these effects by recording from neurons in the dorsal medial superior temporal area (MSTd) of alert macaque monkeys. Neuronal responses to flashed presentations of a textured pattern presented at random times relative to saccades exhibit a stereotypical pattern of modulation. Response amplitudes are strongly suppressed for flashes presented up to 90 ms before saccades. Immediately after the suppression, there is a period of 200-450 ms in which flashes generate enhanced response amplitudes. Our results show that (1) MSTd is not directly suppressed, rather suppression is inherited from earlier visual areas; (2) early suppression of the visual system must be of extra-retinal origin; (3) postsaccadic enhancement of neural activity occurs in MSTd; and (4) the enhanced responses have reduced latencies. As a whole, these observations reveal response properties that could account for perceptual observations relating to presaccadic suppression, postsaccadic enhancement and time compression.

Dynamic contrast change produces rapid gain control in visual cortex

During normal vision, objects moving in the environment, our own body movements and our eye movements ensure that the receptive fields of visual neurons are being presented with continually changing contrasts. Thus, the visual input during normal behaviour differs from the type of stimuli traditionally used to study contrast coding, which are presented in a step-like manner with abrupt changes in contrast followed by prolonged exposure to a constant stimulus. The abrupt changes in contrast typically elicit brief periods of intense firing with low variability called onset transients. Onset transients provide the visual system with a powerful and reliable cue that the visual input has changed. In this paper we investigate visual processing in the primary visual cortex of cats in response to stimuli that change contrast dynamically. We show that 1-4 s presentations of dynamic increases and decreases in contrast can generate stronger contrast gain control than several minutes exposure to a stimulus of constant contrast. Thus, transient mechanisms of contrast coding are not only less variable than sustained responses but are also more rapid and flexible. Finally, we propose a quantitative model of contrast coding which accounts for changes in spike rate over time in response to dynamically changing image contrast.

Perception of direction is not compensated for neural latency

Neural activity in the middle temporal area (MT) is strongly correlated with motion perception. I analyzed the temporal relationship between the representation of direction in MT and the actual direction of a stimulus that continuously changed direction. The representation in MT lagged the stimulus by 45 msec. Hence, as far as the perception of direction is concerned, the hypothesis of lag compensation can be rejected.

Target Acceleration Can Be Extracted and Represented Within the Predictive Drive to Ocular Pursuit

Given sufficient exposure to stimulus presentation, the oculomotor system generates a representation of the stimulus characteristics, which is then used to predict the upcoming target motion. In addition to compensating for the perceptual-motor delay, these predictive processes perpetuate eye motion during a transient occlusion and compensate for the loss of visual input. At present, however, it is not well understood whether and how the oculomotor system extracts and represents target acceleration for subsequent predictive control. To this end, we used a target occlusion paradigm where both position and velocity of the target during the occlusion and at reappearance could not be predicted without extracting target acceleration before target disappearance. We found that the oculomotor response during the blanking period was not influenced by target acceleration when the initial exposure was 200 ms. However, smooth and saccadic eye movements did discriminate between the different levels of acceleration after an initial 500- or 800-ms exposure. In the event that the smooth response during the occlusion did not match well the target trajectory and thus eliminate a developing displacement error, there was an increased saccadic displacement. Still, the combined response during the blanking period did not eliminate retinal slip and position error at target reappearance. These results indicate that information on target acceleration can be extracted on-line, during pursuit of a visible ramp, and then used to drive a predictive oculomotor response in the absence of visual input.

Perception: Transient Disruptions to Neural Space–Time

How vision operates efficiently in the face of continuous shifts of gaze remains poorly understood. Recent studies show that saccades cause dramatic, but transient, changes in the spatial and also temporal tuning of cells in many visual areas, which may underly the perceptual compression of space and time, and serve to counteract the effects of the saccades and maintain visual stability.

Saccadic suppression of retinotopically localized blood oxygen level-dependent responses in human primary visual area V1

Saccadic eye movements are responsible for bringing relevant parts of the visual field onto the fovea for detailed analysis. Because the retina is physiologically unable to deliver sharp images at very high transsaccadic speeds, the visual system minimizes the repercussion of the blurry images we would otherwise perceive during transsaccadic vision by reducing general visual sensitivity and increasing the detection threshold for visual stimuli. Ruling out a pure retinal origin, the effects of saccadic suppression can be already observed some 75 ms before the onset of a saccadic eye movement and are maximal at the onset of motion. The perception of a briefly presented stimulus immediately before the onset of any retinal motion is thus impaired despite the fact that this stimulus is projected onto the stationary retina and is, therefore, physically identical to that presented when no saccadic programming is in course. In this functional magnetic resonance imaging event-related study, we flashed Gabor patches at different times before the onset of a horizontal saccade and measured blood oxygen level-dependent responses at their encoding regions in primary visual cortex (V1) while subjects judged the relative orientation of the stimuli. Closely matching the significant reduction in behavioral performance, the amplitude of the responses in V1 consistently decreased as the stimuli were presented closer to the saccadic onset. These results demonstrate that the neural processes underlying saccade programming transiently modulate cortical responses to briefly presented visual stimuli in areas as early a V1, providing additional evidence for the existence of an active saccadic suppression mechanism in humans.

Comparing Acceleration and Speed Tuning in Macaque MT: Physiology and Modeling

Studies of individual neurons in area MT have traditionally investigated their sensitivity to constant speeds. We investigated acceleration sensitivity in MT neurons by comparing their responses to constant steps and linear ramps in stimulus speed. Speed ramps constituted constant accelerations and decelerations between 0 and 240°/s. Our results suggest that MT neurons do not have explicit acceleration sensitivity, although speed changes affected their responses in three main ways. First, accelerations typically evoked higher responses than the corresponding deceleration rate at all rates tested. We show that this can be explained by adaptation mechanisms rather than differential processing of positive and negative speed gradients. Second, we inferred a cell's preferred speed from the responses to speed ramps by finding the stimulus speed at the latency-adjusted time when response amplitude peaked. In most cells, the preferred speeds inferred from deceleration were higher than those for accelerations of the same rate or from steps in stimulus speed. Third, neuron responses to speed ramps were not well predicted by the transient or sustained responses to steps in stimulus speed. Based on these findings, we developed a model incorporating adaptation and a neuron's speed tuning that predicted the higher inferred speeds and lower spike rates for deceleration responses compared with acceleration responses. This model did not predict acceleration-specific responses, in accordance with the lack of acceleration sensitivity in the neurons. The outputs of this single-cell model were passed to a population-vector–based model used to estimate stimulus speed and acceleration. We show that such a model can accurately estimate relative speed and acceleration using information from the population of neurons in area MT.