Subjects in a dark chamber exposed to angular acceleration while viewing a head-fixed target experience motion and displacement of the target relative to their body. Competing explanations of this phenomenon, known as the oculogyral illusion, have attributed it to the suppression of the vestibulo-ocular reflex (VOR) or to retinal slip. In the dark, the VOR evokes compensatory eye movements in the direction opposite to body acceleration. A head-fixed visual target will tend to suppress these eye movements. The VOR suppression hypothesis attributes the oculogyral illusion to the signals that prevent reflexive deviation of the eyes from the target thus resulting in apparent target displacement in the direction of acceleration. The retinal slip hypothesis attributes the illusion to inadequate fixation of the target with the eyes being involuntarily deviated in the direction opposite acceleration, the retinal slip being interpreted as target displacement in the direction of acceleration. Another possibility is that the illusion could arise from a change in the representation of the perceived head midline. To evaluate these three alternative hypotheses, we tested 8 subjects at 4 acceleration rates (2, 10, 20, 30°/s²) in each of three conditions:(a) fixate and point to a target light;(b) fixate to the target light and point to the head midline;(c) look straight ahead in the dark. The displacement magnitude of the oculogyral illusion was least at 2°/s² ≈ 2° and was ≈10° at the other acceleration rates. The presence of the target light significantly attenuated eye movements relative to the dark condition, but eye movements were still present at the 10, 20, and 30°/s² accelerations. The eye velocity profiles in the dark at different acceleration rates did not show a one-to-one inverse mapping to the magnitude of the oculogyral illusion at those rates. The perceived head midline was not significantly displaced at any of the acceleration rates. The oculogyral illusion thus has at least two contributing factors: the suppression of nystagmus at low acceleration rates and at higher acceleration rates, a partial suppression coupled with an integration of the drift of the eyes with respect to the fixation target.

Vibration of the dorsolateral neck stimulates proprioceptors that are normally active during head movement; this induces a visual illusion of contralateral motion and displacement of a stationary target seen against a homogenous background. The spatial constancy explanation of the illusion argues that it occurs because information about head movement is necessary for accurate egocentric localization of visual objects. Accurate egocentric localization, in turn, is necessary for the success of object-directed motor action, but previous studies failed to find evidence that vibration affects pointing toward visual targets in a normally illuminated, structured field. Our goal was to provide this evidence. Vibration lasting 12 s was applied to either side of the neck while observers (N = 11) pointed at the visual target with an unseen hand. Vibration of the right side of dorsal neck in the illuminated visual field induced a 26-mm lateral bias in pointing responses in comparison to the vibration of the left side. We conclude that the mechanism that takes into account neck proprioceptive signals also operates in full cues. The pointing bias in full cues generally co-occurred with reported stationariness of the visual target, suggesting a conflict between cues used in perception of body-centric position used to guide action, which include neck proprioception, and those used in perception of motion, for which object-relative retinal information is sufficient.

Vibratory stimulation of the neck muscles can elicit illusory drift of a visual target; after vibration stops, motion in the opposite direction is perceived. This motion aftereffect (MAE) could be due to adaptation of proprioceptive mechanisms that encode head orientation, or at a stage where visual and proprioceptive information are combined. To distinguish between these two possibilities, we applied vibratory stimulation to dorsolateral neck muscles for 15-s periods alternating with 15-s periods without vibration. Twenty-six observers used a hand-held tracker to indicate perceived motion of a stationary light-emitting diode (LED) in an otherwise dark room. In the critical condition, observers were in complete darkness during vibration, and the LED was only turned on in post-vibration periods. If adaptation was purely proprioceptive, a visual MAE should have occurred in this condition, but it did not. In a follow-up experiment (N = 9), the LED was presented intermittently to determine if there was a position aftereffect that might have been inhibited by processes signalling an absence of motion. No aftereffect occurred under these conditions either. In both experiments, a visual stimulus had to be present during the adaptation period in order to elicit an aftereffect. Results from our previous study ruled out an explanation based on suppression of eye movements. Thus, the most likely site responsible for the visual aftereffect lies with bimodal mechanisms combining proprioceptive and visual information. We conclude that the bimodal mechanisms adapted more quickly than the proprioceptive mechanisms from which they received input.

Vibratory stimulation of the neck muscles can elicit illusory drift of a visual target; after vibration stops, motion in the opposite direction is perceived. This motion aftereffect (MAE) could be due to adaptation of proprioceptive mechanisms that encode head orientation, or at a stage where visual and proprioceptive information are combined. To distinguish between these two possibilities, we applied vibratory stimulation to dorsolateral neck muscles for 15-s periods alternating with 15-s periods without vibration. Twenty-six observers used a hand-held tracker to indicate perceived motion of a stationary light-emitting diode (LED) in an otherwise dark room. In the critical condition, observers were in complete darkness during vibration, and the LED was only turned on in post-vibration periods. If adaptation was purely proprioceptive, a visual MAE should have occurred in this condition, but it did not. In a follow-up experiment (N = 9), the LED was presented intermittently to determine if there was a position aftereffect that might have been inhibited by processes signalling an absence of motion. No aftereffect occurred under these conditions either. In both experiments, a visual stimulus had to be present during the adaptation period in order to elicit an aftereffect. Results from our previous study ruled out an explanation based on suppression of eye movements. Thus, the most likely site responsible for the visual aftereffect lies with bimodal mechanisms combining proprioceptive and visual information. We conclude that the bimodal mechanisms adapted more quickly than the proprioceptive mechanisms from which they received input.

INTRODUCTION: While the directionality of tactile motion processing has been studied extensively, tactile speed processing and its relationship to direction is little-researched and poorly understood. We investigated this relationship in humans using the 'tactile speed aftereffect'(tSAE), in which the speed of motion appears slower following prolonged exposure to a moving surface. METHOD: We used psychophysical methods to test whether the tSAE is direction sensitive. After adapting to a ridged moving surface with one hand, participants compared the speed of test stimuli on the adapted and unadapted hands. We varied the direction of the adapting stimulus relative to the test stimulus. RESULTS: Perceived speed of the surface moving at 81 mms(-1) was reduced by about 30% regardless of the direction of the adapting stimulus (when adapted in the same direction, Mean reduction = 23 mms(-1), SD = 11; with opposite direction, Mean reduction = 26 mms(-1), SD = 9). In addition to a large reduction in perceived speed due to adaptation, we also report that this effect is not direction sensitive. CONCLUSIONS: Tactile motion is susceptible to speed adaptation. This result complements previous reports of reliable direction aftereffects when using a dynamic test stimulus as together they describe how perception of a moving stimulus in touch depends on the immediate history of stimulation. Given that the tSAE is not direction sensitive, we argue that peripheral adaptation does not explain it, because primary afferents are direction sensitive with friction-creating stimuli like ours (thus motion in their preferred direction should result in greater adaptation, and if perceived speed were critically dependent on these afferents' response intensity, the tSAE should be direction sensitive). The adaptation that reduces perceived speed therefore seems to be of central origin.

Muscle vibration excites muscle spindles and creates illusory movement of a body part in a blindfolded individual. It is followed by an aftereffect, an illusion of return movement when vibration stops. The aftereffect reflects adaptation in the proprioceptive system. This adaptation is susceptible to attentional manipulations (Seizova-Cajic and Azzi in Exp Brain Res 203(1):213-219, 2010), but it is not known whether it is open to cross-modal influences unaided by those manipulations. We attempted to answer this question by allowing vision of the vibrated, stationary arm. We asked our participants (n = 20) to retain focus on the feeling of movement. They reported any illusory movement during 60-s biceps vibration (at 90 Hz), as well as following its offset, when vision of the arm was removed. During vibration, the proprioceptive movement illusion persisted, although the stationary arm was visible, but its duration and strength were much reduced in comparison with the no-vision condition. The movement aftereffect, experienced in total darkness following vibration offset, was also substantially weaker. The results show that proprioceptive adaptation is strongly modulated by vision. We propose that two processes contribute: perceptual (cross-modal binding with conflicting vision reduces the proprioceptive movement signal) and attentional (view of a stationary arm distracts from the proprioceptive movement signal). Our finding that during vibration, participants felt movement in the arm they could see, which was stationary, shows that cross-modal binding partially failed. This happened because the two percepts were too discrepant. However, only one-the visual-appeared real, and we argue that such an outcome is consistent with general principles of intersensory integration.

Visual processing of basic perceptual attributes depends on attention. This has been well documented since the surprising initial report on attentional modulation of the visual motion aftereffect (Chaudhuri 1990). Here, we investigate proprioception and show for the first time that attention modulates adaptation to perceived limb movement. We used biceps vibration to induce illusory forearm extension in 10 participants and measured the aftereffect-perceived movement in the opposite direction. The aftereffect was largest when participants focused on the illusory extension during the adaptation period. To divert attention away from the illusory extension, a rapid serial visual presentation task was performed during the adaptation. The aftereffect was much smaller in this condition, indicating interference between the visual task and proprioceptive adaptation. In tests of an analogous interaction between audition and vision, earlier research found no effect. We suggest that conscious proprioception requires more attention than conscious processing of visual or auditory input.

We report an aftereffect in perception of the extent (or degree or range) of joint movement, showing for the first time that a prolonged exposure to a passive back-and-forth movement of a certain extent results in a change in judgment of the extent of a subsequently presented movement. The adapting stimulus, movement about the wrist, had an extent of either 30 degrees or 75 degrees , while the test stimulus was a 50 degrees movement. Following a 4-min adaptation period, the estimated magnitudes of the test stimuli were 61 degrees and 36 degrees in the 30 degrees and 75 degrees condition, respectively (t test(6)= 9.6; p < 0.001). The observed effect is an instance of repulsion or contrast commonly described in perception literature, with perceived value of the test stimulus pushed away from the adapting stimulus.

While viewing an unambiguously rotating circular array of bars for an extended period, most perceive the array to occasionally move in the direction opposite to its true motion. We find that this alternation in perception has similar dynamics to rivalry, including little correlation among the durations of successive percepts. We also describe analogous reversals in touch and in proprioception. In the proprioceptive case, biceps vibration induces illusory forearm extension. Occasionally, although the same stimulation continues, reversals occur-flexion is perceived rather than extension. Temporal sampling is often invoked to explain the visual reversals but it cannot explain these proprioceptive reversals. Instead, after initial adaptation to the stimulus, rivalry between signals indicating the opposing directions could potentially explain reversals in all three modalities.

Vibration of the dorsolateral neck stimulates proprioceptors that are normally active during head movement; this induces a visual illusion of contralateral motion and displacement of a stationary target seen against a homogenous background. The spatial constancy explanation of the illusion argues that it occurs because information about head movement is necessary for accurate egocentric localization of visual objects. Accurate egocentric localization, in turn, is necessary for the success of object-directed motor action, but previous studies failed to find evidence that vibration affects pointing toward visual targets in a normally illuminated, structured field. Our goal was to provide this evidence. Vibration lasting 12 s was applied to either side of the neck while observers (N = 11) pointed at the visual target with an unseen hand. Vibration of the right side of dorsal neck in the illuminated visual field induced a 26-mm lateral bias in pointing responses in comparison to the vibration of the left side. We conclude that the mechanism that takes into account neck proprioceptive signals also operates in full cues. The pointing bias in full cues generally co-occurred with reported stationariness of the visual target, suggesting a conflict between cues used in perception of body-centric position used to guide action, which include neck proprioception, and those used in perception of motion, for which object-relative retinal information is sufficient.

BACKGROUND Adaptation to constant stimulation has often been used to investigate the mechanisms of perceptual coding, but the adaptive processes within the proprioceptive channels that encode body movement have not been well described. We investigated them using vibration as a stimulus because vibration of muscle tendons results in a powerful illusion of movement. METHODOLOGY/PRINCIPAL FINDINGS We applied sustained 90 Hz vibratory stimulation to biceps brachii, an elbow flexor and induced the expected illusion of elbow extension (in 12 participants). There was clear evidence of adaptation to the movement signal both during the 6-min long vibration and on its cessation. During vibration, the strong initial illusion of extension waxed and waned, with diminishing duration of periods of illusory movement and occasional reversals in the direction of the illusion. After vibration there was an aftereffect in which the stationary elbow seemed to move into flexion. Muscle activity shows no consistent relationship with the variations in perceived movement. CONCLUSION We interpret the observed effects as adaptive changes in the central mechanisms that code movement in direction-selective opponent channels.

In this study locomotor and gaze dysfunction commonly observed in astronauts following spaceflight were modeled using two Galvanic vestibular stimulation (GVS) paradigms:(1) pseudorandom, and (2) head-coupled (proportional to the summed vertical linear acceleration and yaw angular velocity obtained from a head-mounted Inertial Measurement Unit). Locomotor and gaze function during GVS were assessed by tests previously used to evaluate post-flight astronaut performance; dynamic visual acuity (DVA) during treadmill locomotion at 80 m/min, and navigation of an obstacle course. During treadmill locomotion with pseudorandom GVS there was a 12% decrease in coherence between head pitch and vertical translation at the step frequency relative to the no GVS condition, which was not significantly different to the 15% decrease in coherence observed in astronauts following shuttle missions. This disruption in head stabilization likely resulted in a decrease in DVA equivalent to the reduction in acuity observed in astronauts 6 days after return from extended missions aboard the International Space Station (ISS). There were significant increases in time-to-completion of the obstacle course during both pseudorandom (21%) and head-coupled (14%) GVS, equivalent to an ISS astronaut 5 days post-landing. An attempt to suppress head movement was evident during both pseudorandom and head-coupled GVS while negotiating the obstacle course, with a 20 and 16%, decrease in head pitch and yaw velocity, respectively. The results of this study demonstrate that pseudorandom GVS generates many of the salient features of post-flight locomotor dysfunction observed in astronauts following short and long duration missions. An ambulatory GVS system may prove a useful adjunct to the current pre-flight astronaut training regimen.

The main objective of this study was to determine whether bone-conducted vibration (BCV) is equally effective in activating both semicircular canal and otolith afferents in the guinea pig or whether there is preferential activation of one of these classes of vestibular afferents. To answer this question a large number (346) of single primary vestibular neurons were recorded extracellularly in anesthetized guinea pigs and were identified by their location in the vestibular nerve and classed as regular or irregular on the basis of the variability of their spontaneous discharge. If a neuron responded to angular acceleration it was classed as a semicircular canal neuron, if it responded to maintained roll or pitch tilts it was classified as an otolith neuron. Each neuron was then tested by BCV stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction oscillator cemented to the guinea pig's skull. All stimulus intensities were referred to that animal's own auditory brainstem response (ABR) threshold to BCV clicks, and the maximum intensity used was within the animal's physiological range and was usually around 70 dB above BCV threshold. In addition two sensitive single axis linear accelerometers cemented to the skull gave absolute values of the stimulus acceleration in the rostro-caudal direction. The criterion for a neuron being classed as activated was an audible, stimulus-locked increase in firing rate (a 10% change was easily detectable) in response to the BCV stimulus. At the stimulus levels used in this study, semicircular canal neurons, both regular and irregular, were insensitive to BCV stimuli and very few responded: only nine of 189 semicircular canal neurons tested (4.7%) showed a detectable increase in firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level we delivered to the B-71 oscillator (which produced a peak-to-peak skull acceleration of around 6-8 g and was usually around 60-70 dB above the animal's own ABR threshold for BCV clicks). Regular otolithic afferents likewise had a poor response; only 14 of 99 tested (14.1%) showed any increase in firing rate up to the maximum BCV stimulus level. However, most irregular otolithic afferents (82.8%) showed a clear increase in firing rate in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48 were activated, with some being activated at very low intensities (only about 10 dB above the animal's ABR threshold to BCV clicks). Most of the activated otolith afferents were in the superior division of the vestibular nerve and were probably utricular afferents. That was confirmed by evidence using juxtacellular injection of neurobiotin near BCV activated neurons to trace their site of origin to the utricular macula. We conclude there is a very clear preference for irregular otolith afferents to be activated selectively by BCV stimuli at low stimulus levels and that BCV stimuli activate some utricular irregular afferent neurons. The BCV generates compressional and shear waves, which travel through the skull and constitute head accelerations, which are sufficient to stimulate the most sensitive otolithic receptor cells.

The line-motion illusion has been regarded as the result of attention. An alternative interpretation is that the illusion is related to apparent motion which would predict the stimuli to contain motion energy associated with the direction of the illusory motion. In order to examine this possibility Fourier transforms of x-t plots of line-motion stimuli were generated under a variety of conditions. The sums of amplitudes associated with movement in the directions away from the cue relative to that towards the cue were compared to previously published psychophysical observations. It was found that the amplitude sums are largely consistent with the psychophysical results. In the few cases where there were discrepancies between results based on amplitude spectra and psychophysical findings, these discrepancies could be accounted for by making relatively simple and plausible assumptions. The present observations suggest that motion energy may be sufficient to account for the line-motion illusion.

Many important results in visual neuroscience rely on the use of gaze-contingent retinal stabilization techniques. Our work focuses on the important fraction of these studies that is concerned with the retinal stabilization of visual filters that degrade some specific portions of the visual field. For instance, macular scotomas, often induced by age related macular degeneration, can be simulated by continuously displaying a gaze-contingent mask in the center of the visual field. The gaze-contingent rules used in most of these studies imply only a very minimal processing of ocular data. By analyzing the relationship between gaze and scotoma locations for different oculo-motor patterns, we show that such a minimal processing might have adverse perceptual and oculomotor consequences due mainly to two potential problems:(a) a transient blink-induced motion of the scotoma while gaze is static, and (b) the intrusion of post-saccadic slow eye movements. We have developed new gaze-contingent rules to solve these two problems. We have also suggested simple ways of tackling two unrecognized problems that are a potential source of mismatch between gaze and scotoma locations. Overall, the present work should help design, describe and test the paradigms used to simulate retinopathy with gaze-contingent displays.

Eye movements help capture optic-flow information necessary to perceive visually our self motion. Visual and vestibular systems control compensatory eye movements that serve to stabilize the retinal images we capture. We examined the role that these eye movements may play in generating visual illusions of self motion (or vection). Observers viewed radially expanding optic-flow displays while performing lateral translational head oscillations at 1 Hz. Simulated viewpoint changes in these displays were synchronized with head movements, either in an ipsilateral (minimal sensory conflict) or a contralateral (high sensory conflict) direction. In control conditions, the observer viewed purely radial displays. Vection-onset latency and overall vection strength ratings were recorded, as well as horizontal eye movements. Vection onsets and strength ratings were significantly greater when the observer's head movements were incorporated into the visual displays. However, vection strength ratings were very similar for both ipsilateral and contralateral active display oscillation. Surprisingly, the non-ecological contralateral viewpoint oscillation actually induced vection earlier, despite the relatively small eye-in-head rotations coordinating gaze in these conditions. Our results support the view that compensatory eye movements are controlled through cooperative visual and vestibular interactions, and show that linear vection is highly robust against large sensory conflicts.

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.

Vibration of the dorsolateral neck stimulates proprioceptors that are normally active during head movement; this induces a visual illusion of contralateral motion and displacement of a stationary target seen against a homogenous background. The spatial constancy explanation of the illusion argues that it occurs because information about head movement is necessary for accurate egocentric localization of visual objects. Accurate egocentric localization, in turn, is necessary for the success of object-directed motor action, but previous studies failed to find evidence that vibration affects pointing toward visual targets in a normally illuminated, structured field. Our goal was to provide this evidence. Vibration lasting 12 s was applied to either side of the neck while observers (N = 11) pointed at the visual target with an unseen hand. Vibration of the right side of dorsal neck in the illuminated visual field induced a 26-mm lateral bias in pointing responses in comparison to the vibration of the left side. We conclude that the mechanism that takes into account neck proprioceptive signals also operates in full cues. The pointing bias in full cues generally co-occurred with reported stationariness of the visual target, suggesting a conflict between cues used in perception of body-centric position used to guide action, which include neck proprioception, and those used in perception of motion, for which object-relative retinal information is sufficient.

Vibratory stimulation of the neck muscles can elicit illusory drift of a visual target; after vibration stops, motion in the opposite direction is perceived. This motion aftereffect (MAE) could be due to adaptation of proprioceptive mechanisms that encode head orientation, or at a stage where visual and proprioceptive information are combined. To distinguish between these two possibilities, we applied vibratory stimulation to dorsolateral neck muscles for 15-s periods alternating with 15-s periods without vibration. Twenty-six observers used a hand-held tracker to indicate perceived motion of a stationary light-emitting diode (LED) in an otherwise dark room. In the critical condition, observers were in complete darkness during vibration, and the LED was only turned on in post-vibration periods. If adaptation was purely proprioceptive, a visual MAE should have occurred in this condition, but it did not. In a follow-up experiment (N = 9), the LED was presented intermittently to determine if there was a position aftereffect that might have been inhibited by processes signalling an absence of motion. No aftereffect occurred under these conditions either. In both experiments, a visual stimulus had to be present during the adaptation period in order to elicit an aftereffect. Results from our previous study ruled out an explanation based on suppression of eye movements. Thus, the most likely site responsible for the visual aftereffect lies with bimodal mechanisms combining proprioceptive and visual information. We conclude that the bimodal mechanisms adapted more quickly than the proprioceptive mechanisms from which they received input.

A stationary pattern with asymmetrical luminance gradients can appear to move. We hypothesized that the source signal of this illusion originates in retinal image motions due to fixational eye movements. We investigated the inter-subject correlation between fixation instability and illusion strength. First, we demonstrated that the strength of the illusion can be quantified by the nulling technique. Second, we concurrently measured cancellation velocity and fixation instability for each subject, and found a positive correlation between them. The same relationship was also found within a single observer when the visual stimulus was artificially moved in the simulation of fixation instability. Third, we confirmed the same correlation with eye movements for a wider variety of illusory displays. These results suggest that fixational eye movements indeed play a relevant role in generating this motion illusion.

Movement of the body, head, or eyes with respect to the world creates one of the most common yet complex situations in which the visuomotor system must localize objects. In this situation, vestibular, proprioceptive, and extra-retinal information contribute to accurate visuomotor control. The utility of retinal motion information, on the other hand, is questionable, since a single pattern of retinal motion can be produced by any number of head or eye movements. Here we investigated whether retinal motion during a smooth pursuit eye movement contributes to visuomotor control. When subjects pursued a moving object with their eyes and reached to the remembered location of a separate stationary target, the presence of a moving background significantly altered the endpoints of their reaching movements. A background that moved with the pursuit, creating a retinally stationary image (no retinal slip), caused the endpoints of the reaching movements to deviate in the direction of pursuit, overshooting the target. A physically stationary background pattern, however, producing retinal image motion opposite to the direction of pursuit, caused reaching movements to become more accurate. The results indicate that background retinal motion is used by the visuomotor system in the control of visually guided action.

Smooth pursuit eye movements change the retinal image motion of objects in the visual field. To enable an observer to perceive the motion of these objects veridically, the visual system has to compensate for the effects of the eye movements. The occurrence of the Filehne-illusion (illusory motion of a stationary object during smooth pursuit) shows that this compensation is not always perfect. The amplitude of the illusion appears to decrease with increasing presentation durations of the stationary object. In this study we investigated whether presentation duration has the same effect when an observer views a vertically moving object during horizontal pursuit. In this case, the pursuit eye movements cause the perceived motion path to be oblique instead of vertical; this error in perceived motion direction should decrease with higher presentation durations. In Experiment 1, we found that the error in perceived motion direction indeed decreased with increasing presentation duration, especially for higher pursuit velocities. The results of Experiment 2 showed that the error in perceived motion direction did not depend on the moment during pursuit at which the stimulus was presented, suggesting that the degree of compensation for eye movements is constant throughout pursuit. The results suggest that longer presentation durations cause the eye movement signal that is used by the visual system to increase more than the retinal signal.

Both perceptual and motor systems must decode visual information from the distributed activity of large populations of cortical neurons. We have sought a common framework for understanding decoding strategies for visually guided movement and perception by asking whether the strong motion aftereffects seen in the perceptual domain lead to similar expressions in motor output. We found that motion adaptation indeed has strong sequelae in the direction and speed of smooth pursuit eye movements. After adaptation with a stimulus that moves in a given direction for 7 sec, the direction of pursuit is repelled from the direction of pursuit targets that move within 90 degrees of the adapting direction. The speed of pursuit decreases for targets that move at the direction and speed of the adapting stimulus and is repelled from the adapting speed in the sense that the decrease either becomes greater or smaller (eventually turning to an increase) when tracking targets move slower or faster than the adapting speed. The effects of adaptation are spatially specific and fixed to the retinal location of the adapting stimulus. The magnitude of adaptation of pursuit speed and direction is uncorrelated, suggesting that the two parameters are decoded independently. Computer simulation of motion adaptation in the middle temporal visual area (MT) shows that vector-averaging decoding of the population response in MT can account for the effects of adaptation on the direction of pursuit. Our results suggest a unified framework for thinking, in terms of population decoding, about motion adaptation for both perception and action.