Velocity after-effects: the effects of adaptation to moving stimuli on the perception of subsequently seen moving stimuli

Continuous psychophysics for two-variable experiments; A new "Bayesian participant" approach

Interest in continuous psychophysical approaches as a means of collecting data quickly under natural conditions is growing. Such approaches require stimuli to be changed randomly on a continuous basis so that participants can not guess future stimulus states. Participants are generally tasked with responding continuously using a continuum of response options. These features introduce variability in the data that is not present in traditional trial-based experiments. Given the unique weaknesses and strengths of continuous psychophysical approaches, we propose that they are well suited to quickly mapping out relationships between above-threshold stimulus variables such as the perceived direction of a moving target as a function of the direction of the background against which the target is moving. We show that modelling the participant in such a two-variable experiment using a novel "Bayesian Participant" model facilitates the conversion of the noisy continuous data into a less-noisy form that resembles data from an equivalent trial-based experiment. We also show that adaptation can result from longer-than-usual stimulus exposure times during continuous experiments, even to features that the participant is not aware of. Methods for mitigating the effects of adaptation are discussed.

Reduced direction discrimination sensitivity in visual motion adaptation, and the role of perceptual learning

We investigated visual direction discrimination under the influence of motion aftereffect (MAE). Participants in each experiment first adapted to a horizontally drifting grating before deciding whether a drifting test grating moved to the left or right. A psychometric function was obtained as a function of the velocity of the test. Interestingly, in addition to the horizontal shift of the psychometric function that typified the MAE, the slope of the psychometric function became shallower after adaptation, indicating decreased discrimination sensitivity. However, this decrease was only observed in psychophysically experienced participants. Motivated, but psychophysically inexperienced participants only showed this effect after weeks of perceptual learning. This shallowing effect transferred to the untrained adaptation direction (e.g., from leftward adaptation to rightward), although perceptual learning of improved discrimination could not transfer. When the test duration was lengthened to reduce task difficulty, less training was needed to produce the same effect. These results indicate that, post-adaptation and when steady measurements could be obtained, left-right motion direction discrimination sensitivity was reduced.

Adaptation to one perceived motion direction can generate multiple velocity aftereffects

Sensory adaptation is a useful tool to identify the links between perceptual effects and neural mechanisms. Even though motion adaptation is one of the earliest and most documented aftereffects, few studies have investigated the perception of direction and speed of the aftereffect at the same time, that is the perceived velocity. Using a novel experimental paradigm, we simultaneously recorded the perceived direction and speed of leftward or rightward moving random dots before and after adaptation. For the adapting stimulus, we chose a horizontally-oriented broadband grating moving upward behind a circular aperture. Because of the aperture problem, the interpretation of this stimulus is ambiguous, being consistent with multiple velocities, and yet it is systematically perceived as moving at a single direction and speed. Here we ask whether the visual system adapts to the multiple velocities of the adaptor or to just the single perceived velocity. Our results show a strong repulsion aftereffect, away from the adapting velocity (downward and slower), that increases gradually for faster test stimuli as long as these stimuli include some velocities that match some of the ambiguous ones of the adaptor. In summary, the visual system seems to adapt to the multiple velocities of an ambiguous stimulus even though a single velocity is perceived. Our findings can be well described by a computational model that assumes a joint encoding of direction and speed and that includes an extended adaptation component that can represent all the possible velocities of the ambiguous stimulus.

A visual search asymmetry for relative novelty in the visual field based on sensory adaptation

The ability to detect sudden changes in the environment is important for survival. However, studies of “change blindness” have shown that image differences are hard to detect when a time delay or a mask is imposed between the different images. However, when sensory adaptation is permitted by accurate fixation, we find that change detection is not only possible but asymmetrical: a single changed target amongst 15 unchanging distractors is much easier to detect than a target defined by its lack of change. Although adaptation may selectively reduce the apparent contrast of unchanged objects, the asymmetry in “change salience” cannot be attributed to any such reduction because genuine reductions in target contrast increase, rather than decrease, target detectability. Analogous results preclude attribution to apparent differences between (a) target onset and distractor onset and (b) their temporal frequencies (both flickered at 7.5 Hz, minimizing afterimages). Our results demonstrate a hitherto underappreciated (or unappreciated) advantage conferred by low-level sensory adaptation: it automatically elevates the salience of previously absent objects.

Attention and the motion aftereffect

We measured the effects of attentional distraction on the time course and asymptote of motion adaptation strength, using visual search performance (percent correct and reaction time). In the first two experiments, participants adapted to a spatial array of moving Gabor patches, either all vertically oriented (Experiment 1) or randomly oriented (Experiment 2). On each trial, the adapting array was followed by a test array in which all of the test patches except one were identical in orientation and movement direction to their retinotopically corresponding adaptors, but the target moved in the opposite direction to its adaptor. Participants were required to identify the location of the changed target with a mouse click. The ability to do so increased with the number of adapting trials. Neither search speed nor accuracy was affected by an attentionally demanding conjunction task at the fixation point during adaptation, suggesting low-level (preattentive) sites in the visual pathway for the adaptation. In Experiment 3, the same participants were required to identify the one element in the test array that was slowly moving. Reaction times in this case were elevated following adaptation, but once again there was no significant effect of the distracting task upon performance. In Experiment 4, participants were required to make eye movements, so that retinotopically corresponding adaptors could be distinguished from spatiotopically corresponding adaptors. Performance in Experiments 1 and 2 correlated positively with reaction times in Experiment 3, suggesting a general trait for adaptation strength.

Reference effects on decision-making elicited by previous rewards

Substantial evidence has highlighted reference effects occurring during decision-making, whereby subjective value is not calculated in absolute terms but relative to the distribution of rewards characterizing a context. Among these, within-choice effects are exerted by options simultaneously available during choice. These should be distinguished from between-choice effects, which depend on the distribution of options presented in the past. Influential theories on between-choice effects include Decision-by-Sampling, Expectation-as-Reference and Divisive Normalization. Surprisingly, previous literature has focused on each theory individually disregarding the others. Thus, similarities and differences among theories remain to be systematically examined. Here we fill this gap by offering an overview of the state-of-the-art of research about between-choice reference effects. Our comparison of alternative theories shows that, at present, none of them is able to account for the full range of empirical data. To address this, we propose a model inspired by previous perspectives and based on a logistic framework, hence called logistic model of subjective value. Predictions of the model are analysed in detail about reference effects and risky decision-making. We conclude that our proposal offers a compelling framework for interpreting the multifaceted manifestations of between-choice reference effects.

Low-level mediation of directionally specific motion aftereffects: Motion perception is not necessary

Previous psychophysical experiments with normal human observers have shown that adaptation to a moving dot stream causes directionally specific repulsion in the perceived angle of a subsequently viewed moving probe. In this study, we used a two-alternative forced choice task with roving pedestals to determine the conditions that are necessary and sufficient for producing directionally specific repulsion with compound adaptors, each of which contains two oppositely moving, differently colored component streams. Experiment 1 provided a demonstration of repulsion between single-component adaptors and probes moving at approximately 90° or 270°. In Experiment 2, oppositely moving dots in the adaptor were paired to preclude the appearance of motion. Nonetheless, repulsion remained strong when the angle between each probe stream and one component was approximately 30°. In Experiment 3, adapting dot pairs were kept stationary during their limited lifetimes. Their orientation content alone proved insufficient for producing repulsion. In Experiments 4–6, the angle between the probe and both adapting components was approximately 90° or 270°. Directional repulsion was found when observers were asked to visually track one of the adapting components (Exp. 6), but not when they were asked to attentionally track it (Exp. 5), nor while they passively viewed the adaptor (Exp. 4). Our results are consistent with a low-level mechanism for motion adaptation. This mechanism is not selective for stimulus color and is not susceptible to attentional modulation. The most likely cortical locus of adaptation is area V1.

Adaptation-Induced Blindness Is Orientation-Tuned and Monocular

We examined the recently discovered phenomenon of Adaptation-Induced Blindness (AIB), in which highly visible gratings with gradual onset profiles become invisible after exposure to a rapidly flickering grating, even at very high contrasts. Using very similar stimuli to those in the original AIB experiment, we replicated the original effect across multiple contrast levels, with observers at chance in detecting the gradual onset stimuli at all contrasts. Then, using full-contrast target stimuli with either abrupt or gradual onsets, we tested both the orientation tuning and interocular transfer of AIB. If, as the original authors suggested, AIB were a high-level (perhaps parietally mediated) effect resulting from the ‘gating’ of awareness, we would not expect the effects of AIB to be tuned to the adapting orientation, and the effect should transfer interocularly. Instead, we find that AIB (which was present only for the gradual onset target stimuli) is both tightly orientation-tuned and shows absolutely no interocular transfer, consistent with a very early cortical locus.

Perceived duration of brief visual events is mediated by timing mechanisms at the global stages of visual processing

There is a growing body of evidence pointing to the existence of modality-specific timing mechanisms for encoding sub-second durations. For example, the duration compression effect describes how prior adaptation to a dynamic visual stimulus results in participants underestimating the duration of a sub-second test stimulus when it is presented at the adapted location. There is substantial evidence for the existence of both cortical and pre-cortical visual timing mechanisms; however, little is known about where in the processing hierarchy the cortical mechanisms are likely to be located. We carried out a series of experiments to determine whether or not timing mechanisms are to be found at the global processing level. We had participants adapt to random dot patterns that varied in their motion coherence, thus allowing us to probe the visual system at the level of motion integration. Our first experiment revealed a positive linear relationship between the motion coherence level of the adaptor stimulus and duration compression magnitude. However, increasing the motion coherence level in a stimulus also results in an increase in global speed. To test whether duration compression effects were driven by global speed or global motion, we repeated the experiment, but kept global speed fixed while varying motion coherence levels. The duration compression persisted, but the linear relationship with motion coherence was absent, suggesting that the effect was driven by adapting global speed mechanisms. Our results support previous claims that visual timing mechanisms persist at the level of global processing.

Spatiotopic coding during dynamic head tilt

Humans maintain a stable representation of the visual world effortlessly, despite constant movements of the eyes, head, and body, across multiple planes. Whereas visual stability in the face of saccadic eye movements has been intensely researched, fewer studies have investigated retinal image transformations induced by head movements, especially in the frontal plane. Unlike head rotations in the horizontal and sagittal planes, tilting the head in the frontal plane is only partially counteracted by torsional eye movements and consequently induces a distortion of the retinal image to which we seem to be completely oblivious. One possible mechanism aiding perceptual stability is an active reconstruction of a spatiotopic map of the visual world, anchored in allocentric coordinates. To explore this possibility, we measured the positional motion aftereffect (PMAE; the apparent change in position after adaptation to motion) with head tilts of ∼42° between adaptation and test (to dissociate retinal from allocentric coordinates). The aftereffect was shown to have both a retinotopic and spatiotopic component. When tested with unpatterned Gaussian blobs rather than sinusoidal grating stimuli, the retinotopic component was greatly reduced, whereas the spatiotopic component remained. The results suggest that perceptual stability may be maintained at least partially through mechanisms involving spatiotopic coding.NEW & NOTEWORTHY Given that spatiotopic coding could play a key role in maintaining visual stability, we look for evidence of spatiotopic coding after retinal image transformations caused by head tilt. To this end, we measure the strength of the positional motion aftereffect (PMAE; previously shown to be largely spatiotopic after saccades) after large head tilts. We find that, as with eye movements, the spatial selectivity of the PMAE has a large spatiotopic component after head rotation.

The duration compression effect is mediated by adaptation of both retinotopic and spatiotopic mechanisms

The duration compression effect is a phenomenon in which prior adaptation to a spatially circumscribed dynamic stimulus results in the duration of subsequent subsecond stimuli presented in the adapted region being underestimated. There is disagreement over the frame of reference within which the duration compression phenomenon occurs. One view holds that the effect is driven by retinotopic-tuned mechanisms located at early stages of visual processing, and an alternate position is that the mechanisms are spatiotopic and occur at later stages of visual processing (MT+). We addressed the retinotopic-spatiotopic question by using adapting stimuli - drifting plaids - that are known to activate global-motion mechanisms in area MT. If spatiotopic mechanisms contribute to the duration compression effect, drifting plaid adaptors should be well suited to revealing them. Following adaptation participants were tasked with estimating the duration of a 600ms random dot stimulus, whose direction was identical to the pattern direction of the adapting plaid, presented at either the same retinotopic or the same spatiotopic location as the adaptor. Our results reveal significant duration compression in both conditions, pointing to the involvement of both retinotopic-tuned and spatiotopic-tuned mechanisms in the duration compression effect.

The relative contributions of global and local acceleration components on speed perception and discriminability following adaptation

The perception of speed is dependent on the history of previously presented speeds. Adaptation to a given speed regularly results in a reduction of perceived speed and an increase in speed discriminability and in certain circumstances can result in an increase in perceived speed. In order to determine the relative contributions of the local and global speed components on perceived speed, this experiment used expanding dot flow fields with accelerating (global), decelerating (global) and mixed accelerating/decelerating (local) speed patterns. Profound decreases in perceived speed are found when viewing low test speeds after adaptation to high speeds. Small increases in the perceived speed of high test speeds occur following adaptation to low speeds. There were small but significant differences in perceived stimulus speed after adaptation due to different acceleration profiles. No evidence for global modulation of speed discriminability following adaptation was found.

Unbiased Measures of Interocular Transfer of Motion Adaptation

Numerous studies have measured the extent to which motion aftereffects transfer interocularly. However, many have done so using bias-prone methods, and studies rarely compare different types of motion directly. Here, we use a technique designed to reduce bias (Morgan, 2013, Journal of Vision, 13(8):26, 1–11) to estimate interocular transfer (IOT) for five types of motion: simple translational motion, expansion/contraction, rotation, spiral, and complex translational motion. We used both static and dynamic targets with subjects making binary judgments of perceived speed. Overall, the average IOT was 65%, consistent with previous studies (mean over 17 studies of 67% transfer). There was a main effect of motion type, with translational motion producing stronger IOT (mean: 86%) overall than any of the more complex varieties of motion (mean: 51%). This is inconsistent with the notion that IOT should be strongest for motion processed in extrastriate regions that are fully binocular. We conclude that adaptation is a complex phenomenon too poorly understood to make firm inferences about the binocular structure of motion systems.

Direction-contingent duration compression is primarily retinotopic

Previous research has shown that prior adaptation to a spatially circumscribed, oscillating grating results in the duration of a subsequent stimulus briefly presented within the adapted region being underestimated. There is an on-going debate about where in the motion processing pathway the adaptation underlying this distortion of sub-second duration perception occurs. One position is that the LGN and, perhaps, early cortical processing areas are likely sites for the adaptation; an alternative suggestion is that visual area MT+ contains the neural mechanisms for sub-second timing; and a third position proposes that the effect is driven by adaptation at multiple levels of the motion processing pathway. A related issue is in what frame of reference - retinotopic or spatiotopic - does adaptation induced duration distortion occur. We addressed these questions by having participants adapt to a unidirectional random dot kinematogram (RDK), and then measuring perceived duration of a 600 ms test RDK positioned in either the same retinotopic or the same spatiotopic location as the adaptor. We found that, when it did occur, duration distortion of the test stimulus was direction contingent; that is it occurred when the adaptor and test stimuli drifted in the same direction, but not when they drifted in opposite directions. Furthermore the duration compression was evident primarily under retinotopic viewing conditions, with little evidence of duration distortion under spatiotopic viewing conditions. Our results support previous research implicating cortical mechanisms in the duration encoding of sub-second visual events, and reveal that these mechanisms encode duration within a retinotopic frame of reference.

Adaptation disrupts motion integration in the primate dorsal stream

Sensory systems adjust continuously to the environment. The effects of recent sensory experience-or adaptation-are typically assayed by recording in a relevant subcortical or cortical network. However, adaptation effects cannot be localized to a single, local network. Adjustments in one circuit or area will alter the input provided to others, with unclear consequences for computations implemented in downstream circuits. Here, we show that prolonged adaptation with drifting gratings, which alters responses in the early visual system, impedes the ability of area MT neurons to integrate motion signals in plaid stimuli. Perceptual experiments reveal a corresponding loss of plaid coherence. A simple computational model shows how the altered representation of motion signals in early cortex can derail integration in MT. Our results suggest that the effects of adaptation cascade through the visual system, derailing the downstream representation of distinct stimulus attributes.

Implied dynamics biases the visual perception of velocity

We expand the anecdotic report by Johansson that back-and-forth linear harmonic motions appear uniform. Six experiments explore the role of shape and spatial orientation of the trajectory of a point-light target in the perceptual judgment of uniform motion. In Experiment 1, the target oscillated back-and-forth along a circular arc around an invisible pivot. The imaginary segment from the pivot to the midpoint of the trajectory could be oriented vertically downward (consistent with an upright pendulum), horizontally leftward, or vertically upward (upside-down). In Experiments 2 to 5, the target moved uni-directionally. The effect of suppressing the alternation of movement directions was tested with curvilinear (Experiment 2 and 3) or rectilinear (Experiment 4 and 5) paths. Experiment 6 replicated the upright condition of Experiment 1, but participants were asked to hold the gaze on a fixation point. When some features of the trajectory evoked the motion of either a simple pendulum or a mass-spring system, observers identified as uniform the kinematic profiles close to harmonic motion. The bias towards harmonic motion was most consistent in the upright orientation of Experiment 1 and 6. The bias disappeared when the stimuli were incompatible with both pendulum and mass-spring models (Experiments 3 to 5). The results are compatible with the hypothesis that the perception of dynamic stimuli is biased by the laws of motion obeyed by natural events, so that only natural motions appear uniform.

How long did it last? You would better ask a human

In the future, human-like robots will live among people to provide company and help carrying out tasks in cooperation with humans. These interactions require that robots understand not only human actions, but also the way in which we perceive the world. Human perception heavily relies on the time dimension, especially when it comes to processing visual motion. Critically, human time perception for dynamic events is often inaccurate. Robots interacting with humans may want to see the world and tell time the way humans do: if so, they must incorporate human-like fallacy. Observers asked to judge the duration of brief scenes are prone to errors: perceived duration often does not match the physical duration of the event. Several kinds of temporal distortions have been described in the specialized literature. Here we review the topic with a special emphasis on our work dealing with time perception of animate actors versus inanimate actors. This work shows the existence of specialized time bases for different categories of targets. The time base used by the human brain to process visual motion appears to be calibrated against the specific predictions regarding the motion of human figures in case of animate motion, while it can be calibrated against the predictions of motion of passive objects in case of inanimate motion. Human perception of time appears to be strictly linked with the mechanisms used to control movements. Thus, neural time can be entrained by external cues in a similar manner for both perceptual judgments of elapsed time and in motor control tasks. One possible strategy could be to implement in humanoids a unique architecture for dealing with time, which would apply the same specialized mechanisms to both perception and action, similarly to humans. This shared implementation might render the humanoids more acceptable to humans, thus facilitating reciprocal interactions.

Sustained attention is not necessary for velocity adaptation

Prolonged adaptation to a stimulus, such as a drifting grating, lowers sensitivity for detecting similar stimuli, and changes their appearance, for example, making gratings of the same orientation appear of lower contrast and move more slowly. It has been suggested that adaptation is increased by sustained attention to the adapting stimulus and is decreased by distracting attention with a competing task. This paper describes a novel 2AFC (spatial) measure of adaptation in which adaptation and bias are carefully distinguished by the random interleaving of different test conditions. The experiment revealed significant adaptation of perceived velocity, but no effect of attentional distraction.

Independent effects of adaptation and attention on perceived speed

Adaptation and attention are two mechanisms by which sensory systems manage limited bioenergetic resources: Whereas adaptation decreases sensitivity to stimuli just encountered, attention increases sensitivity to behaviorally relevant stimuli. In the visual system, these changes in sensitivity are accompanied by a change in the appearance of different stimulus dimensions, such as speed. Adaptation causes an underestimation of speed, whereas attention leads to an overestimation of speed. In the two experiments reported here, we investigated whether the effects of these mechanisms interact and how they affect the appearance of stimulus features. We tested the effects of adaptation and the subsequent allocation of attention on perceived speed. A quickly moving adaptor decreased the perceived speed of subsequent stimuli, whereas a slow adaptor did not alter perceived speed. Attention increased perceived speed regardless of the adaptation effect, which indicates that adaptation and attention affect perceived speed independently. Moreover, the finding that attention can alter perceived speed after adaptation indicates that adaptation is not merely a by-product of neuronal fatigue.

Effects of speeding up or slowing down animate or inanimate motions on timing

It has recently been suggested that time perception and motor timing are influenced by the presence of biological movements and animacy in the visual scene. Here, we investigated the interactions among timing, speed and animacy in two experiments. In Experiment 1, observers had to press a button in synchrony with the landing of a falling ball while a dancer or a whirligig moved in the background of the scene. The speed of these two characters was artificially changed across sessions. We found striking differences in the timing of button-press responses as a function of the condition. Responses were delayed considerably with increasing speed of the whirligig. By contrast, the effect of the dancer's speed was weaker and in the opposite direction. In Experiment 2, we assessed the perceived animacy of these characters and found that the dancer was rated as much more animate than the whirligig, irrespective of the character speed. The results are consistent with the hypothesis that event timers are selectively biased as a function of perceived animacy, implicating high-level mechanisms for time modulation. However, response timing interacts with perceived animacy and speed in a complex manner.

Active movement restores veridical event-timing after tactile adaptation

Growing evidence suggests that time in the subsecond range is tightly linked to sensory processing. Event-time can be distorted by sensory adaptation, and many temporal illusions can accompany action execution. In this study, we show that adaptation to tactile motion causes a strong contraction of the apparent duration of tactile stimuli. However, when subjects make a voluntary motor act before judging the duration, it annuls the adaptation-induced temporal distortion, reestablishing veridical event-time. The movement needs to be performed actively by the subject: passive movement of similar magnitude and dynamics has no effect on adaptation, showing that it is the motor commands themselves, rather than reafferent signals from body movement, which reset the adaptation for tactile duration. No other concomitant perceptual changes were reported (such as apparent speed or enhanced temporal discrimination), ruling out a generalized effect of body movement on somatosensory processing. We suggest that active movement resets timing mechanisms in preparation for the new scenario that the movement will cause, eliminating inappropriate biases in perceived time. Our brain seems to utilize the intention-to-move signals to retune its perceptual machinery appropriately, to prepare to extract new temporal information.

Hierarchy of direction-tuned motion adaptation in human visual cortex

Prolonged exposure to a single direction of motion alters perception of subsequent static or dynamic stimuli and induces substantial changes in behaviors of motion-sensitive neurons, but the origin of neural adaptation and neural correlates of perceptual consequences of motion adaptation in human brain remain unclear. Using functional magnetic resonance imaging, we measured motion adaptation tuning curves in a fine scale by probing changes in cortical activity after adaptation for a range of directions relative to the adapted direction. We found a clear dichotomy in tuning curve shape: cortical responses in early-tier visual areas reduced at around both the adapted and opposite direction, resulting in a bidirectional tuning curve, whereas response reduction in high-tier areas occurred only at around the adapted direction, resulting in a unidirectional tuning curve. We also found that the psychophysically measured adaptation tuning curves were unidirectional and best matched the cortical adaptation tuning curves in the middle temporal area (MT) and the medial superior temporal area (MST). Our findings are compatible with, but not limited to, an interpretation in which direct impacts of motion adaptation occur in both unidirectional and bidirectional units in early visual areas, but the perceptual consequences of motion adaptation are manifested in the population activity in MT and MST, which may inherit those direct impacts of adaptation from the directionally selective units.

The Hierarchical Order of Processes Underlying the Direction Illusion and the Direction Aftereffect

Motion perception involves the processing of velocity signals through several hierarchical stages of the visual cortex. To better understand this process, a number of studies have sought to localise the neural substrates of two misperceptions of motion direction, the direction illusion (DI) and the direction aftereffect (DAE). These studies have produced contradictory evidence as to the hierarchical order of the processing stages from which the respective phenomena arise. We have used a simple stimulus configuration to further investigate the sequential order of processes giving rise to the DI and DAE. To this end, we measured the two phenomena invoked in combination, and also manually parsed this combined effect into its two constituents by measuring the two phenomena individually in both possible sequential orders. Comparing the predictions made from each order to the outcome from the combined effect allowed us to test the tenability of two models: the DAE-first model and the DI-first model. Our results indicate that DAE-invoking activity does not occur earlier in the motion processing hierarchy than DI-invoking activity. Although the DI-first model is not inconsistent with our data, the possible involvement of non-sequential processing may be better able to reconcile these results with those of previous studies.

The effect of luminance on simulated driving speed

Perceived speed is modulated by a range of stimulus attributes such as contrast, luminance and adaptation duration. It has been suggested that such changes in perceived speed may influence driving behaviour. In order to evaluate the effect of luminance on driving speed we have measured subjects' driving speed in a driving simulator for a range of luminance and speed over time. The results indicate that reducing luminance results in a decrease in driving speed for all speeds measured. This reduction in driving speed at low luminance is consistent with previous findings that perceived speed increases at low luminance. However, the results also indicated that driving speed remained stable over a 30s period. The stability of driving speed over time is inconsistent with previous findings that perceived speed reduces exponentially as a function of adaptation duration. The results are suggestive of a scheme whereby driving speed is consistent with the known effects of luminance upon perceived speed but may also be modulated by higher order processes that serve to maintain a constant speed over time.

The many directions of time

Event duration perception is fundamental to cognitive functioning. Recent research has shown that localized sensory adaptation compresses perceived duration of brief visual events in the adapted location; however, there is disagreement on whether the source of these temporal distortions is cortical or pre-cortical. The current study reveals that spatially localized duration compression can also be direction contingent, in that duration compression is induced when adapting and test stimuli move in the same direction but not when they move in opposite directions. Because of its direction-contingent nature, the induced duration compression reported here is likely to be cortical in origin. A second experiment shows that the adaptation processes driving duration compression can occur at or beyond human cortical area MT+, a specialized motion center located upstream from primary visual cortex. The direction-specificity of these temporal mechanisms, in conjunction with earlier reports of pre-cortical temporal mechanisms driving duration perception, suggests that our encoding of subsecond event duration is driven by activity at multiple levels of processing.

Motion perception: behavior and neural substrate

Visual motion perception is vital for survival. Single-unit recordings in primate primary visual cortex (V1) have revealed the existence of specialized motion sensing neurons; perceptual effects such as the motion after-effect demonstrate their importance for motion perception. Human psychophysical data on motion detection can be explained by a computational model of cortical motion sensors. Both psychophysical and physiological data reveal at least two classes of motion sensor capable of sensing motion in luminance-defined and texture-defined patterns, respectively. Psychophysical experiments also reveal that motion can be seen independently of motion sensor output, based on attentive tracking of visual features. Sensor outputs are inherently ambiguous, due to the problem of univariance in neural responses. In order to compute stimulus direction and speed, the visual system must compare the responses of many different sensors sensitive to different directions and speeds. Physiological data show that this computation occurs in the visual middle temporal (MT) area. Recent psychophysical studies indicate that information about spatial form may also play a role in motion computations. Adaptation studies show that the human visual system is selectively sensitive to large-scale optic flow patterns, and physiological studies indicate that cells in the middle superior temporal (MST) area derive this sensitivity from the combined responses of many MT cells. Extraretinal signals used to control eye movements are an important source of signals to cancel out the retinal motion responses generated by eye movements, though visual information also plays a role. A number of issues remain to be resolved at all levels of the motion-processing hierarchy. WIREs Cogni Sci 2011 2 305-314 DOI: 10.1002/wcs.110 For further resources related to this article, please visit the WIREs website Additional Supporting Information may be found in http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/index.html.

Spatially localized time shifts of the perceptual stream

Visual events trigger representations in different locations and times in the brain. In experience, however, these various neural responses refer to a single unified cause. To investigate how representations might be brought into temporal alignment, we attempted to locally manipulate neural processing in such a way that identical, simultaneous sequences would appear temporally misaligned. After adaptation to a 20 Hz sequentially expanding and contracting concentric grating, a running clock presented in the adapted region of the visual field appeared advanced relative to an identical clock presented simultaneously in an unadapted region. No such effect was observed following 5-Hz adaptation. Clock time reports following an exogenous cue showed the same effect of adaptation on perceived time, demonstrating that the apparent temporal misalignment was not mediated by differences in target selection or allocation of attention. This effect was not mediated by the apparent speed of the adapted clock: a clock in a 20-Hz-adapted spatial location appeared slower than a clock in a 5-Hz-adapted location, rather than faster. Furthermore, reaction times for a clock-hand orientation discrimination task were the same following 5- and 20-Hz adaptation, indicating that neural processing latencies were not differentially affected. Altogether, these findings suggest that the fragmented perceptual stream might be actively brought into temporal alignment through adaptive local mechanisms operating in spatially segregated regions of the visual field.

Spatial maps for time and motion

In this article, we review recent research studying the mechanisms for transforming coordinate systems to encode space, time and motion. A range of studies using functional imaging and psychophysical techniques reveals mechanisms in the human brain for encoding information in external rather than retinal coordinates. This reinforces the idea of a tight relationship between space and time, in the parietal cortex of primates.

Visual motion aftereffects arise from a cascade of two isomorphic adaptation mechanisms

Prolonged exposure to a moving stimulus can substantially alter the perceived velocity (both speed and direction) of subsequently presented stimuli. Here, we show that these changes can be parsimoniously explained with a model that combines the effects of two isomorphic adaptation mechanisms, one nondirectional and one directional. Each produces a pattern of velocity biases that serves as an observable "signature" of the corresponding mechanism. The net effect on perceived velocity is a superposition of these two signatures. By examining human velocity judgments in the context of different adaptor velocities, we are able to separate these two signatures. The model fits the data well, successfully predicts subjects' behavior in an additional experiment using a nondirectional adaptor, and is in agreement with a variety of previous experimental results. As such, the model provides a unifying explanation for the diversity of motion aftereffects.

Motion fading and the motion aftereffect share a common process of neural adaptation

After prolonged viewing of a slowly drifting or rotating pattern under strict fixation, the pattern appears to slow down and then momentarily stop. Here, we show that this motion fading occurs not only for slowly moving stimuli, but also for stimuli moving at high speed; after prolonged viewing of high-speed stimuli, the stimuli appear to slow down but not to stop. We report psychophysical evidence that the same neural adaptation process likely gives rise to motion fading and to the motion aftereffect.

Perception of movement extent depends on the extent of previous movements

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.

Simultaneous adaptation of retinal and extra-retinal motion signals

A number of models of motion perception include estimates of eye velocity to help compensate for the incidental retinal motion produced by smooth pursuit. The 'classical' model uses extra-retinal motor command signals to obtain the estimate. More recent 'reference-signal' models use retinal motion information to enhance the extra-retinal signal. The consequence of simultaneously adapting to pursuit and retinal motion is thought to favour the reference-signal model, largely because the perception of motion during pursuit ('perceived stability') changes despite the absence of a standard motion aftereffect. The current experiments investigated whether the classical model could also account for these findings. Experiment 1 replicated the changes to perceived stability and then showed how simultaneous motion adaptation changes perceived retinal speed (a velocity aftereffect). Contrary to claims made by proponents of the reference-signal model, adapting simultaneously to pursuit and retinal motion therefore alters the retinal motion inputs to the stability computation. Experiment 2 tested the idea that simultaneous motion adaptation sets up a competitive interaction between two types of velocity aftereffect, one retinal and one extra-retinal. The results showed that pursuit adaptation by itself drove perceived stability in one direction and that adding adapting retinal motion drove perceived stability in the other. Moreover, perceived stability changed in conditions that contained no mismatch between adapting pursuit and adapting retinal motion, contrary to the reference-signal account. Experiment 3 investigated whether the effects of simultaneous motion adaptation were directionally tuned. Surprisingly no tuning was found, but this was true for both perceived stability and retinal velocity aftereffect. The three experiments suggest that simultaneous motion adaptation alters perceived stability based on separable changes to retinal and extra-retinal inputs. Possible mechanisms underlying the extra-retinal velocity aftereffect are discussed.

Influence of adapting speed on speed and contrast coding in the primary visual cortex of the cat

Adaptation is a ubiquitous property of the visual system. Adaptation often improves the ability to discriminate between stimuli and increases the operating range of the system, but is also associated with a reduced ability to veridically code stimulus attributes. Adaptation to luminance levels, contrast, orientation, direction and spatial frequency has been studied extensively, but knowledge about adaptation to image speed is less well understood. Here we examined how the speed tuning of neurons in cat primary visual cortex was altered after adaptation to speeds that were slow, optimal, or fast relative to each neuron's speed response function. We found that the preferred speed (defined as the speed eliciting the peak firing rate) of the neurons following adaptation was dependent on the speed at which they were adapted. At the population level cells showed decreases in preferred speed following adaptation to speeds at or above the non-adapted speed, but the preferred speed did not change following adaptation to speeds lower than the non-adapted peak. Almost all cells showed response gain control (reductions in absolute firing capacity) following speed adaptation. We also investigated the speed dependence of contrast adaptation and found that most cells showed contrast gain control (rightward shifts of their contrast response functions) and response gain control following adaptation at any speed. We conclude that contrast adaptation may produce the response gain control associated with speed adaptation, but shifts in preferred speed require an additional level of processing beyond contrast adaptation. A simple model is presented that is able to capture most of the findings.

Neural mechanisms for timing visual events are spatially selective in real-world coordinates

It is generally assumed that perceptual events are timed by a centralized supramodal clock. This study challenges this notion in humans by providing clear evidence that visual events of subsecond duration are timed by visual neural mechanisms with spatially circumscribed receptive fields, localized in real-world, rather than retinal, coordinates.

Subtractive and divisive adaptation in visual motion computations

Models of visual motion processing that introduce priors for low speed through Bayesian computations are sometimes treated with scepticism by empirical researchers because of the convenient way in which parameters of the Bayesian priors have been chosen. Using the effects of motion adaptation on motion perception to illustrate, we show that the Bayesian prior, far from being convenient, may be estimated on-line and therefore represents a useful tool by which visual motion processes may be optimized in order to extract the motion signals commonly encountered in every day experience. The prescription for optimization, when combined with system constraints on the transmission of visual information, may lead to an exaggeration of perceptual bias through the process of adaptation. Our approach extends the Bayesian model of visual motion proposed byWeiss et al. [Weiss Y., Simoncelli, E., & Adelson, E. (2002). Motion illusions as optimal perception Nature Neuroscience, 5:598-604.], in suggesting that perceptual bias reflects a compromise taken by a rational system in the face of uncertain signals and system constraints.

The segregation and integration of colour in motion processing revealed by motion after-effects

Analysis of the colour and motion of objects is widely believed to take place within segregated processing pathways in the primate visual system. However, it is apparent that this segregation cannot remain absolute and that there must be some capacity for integration across these sub-modalities. In this study, we have assessed the extent to which colour constitutes a separable entity in human motion processing by measuring the chromatic selectivity of two kinds of after-effect resulting from motion adaptation. First, the traditional motion after-effect, where prolonged inspection of a unidirectional moving stimulus results in illusory motion in the opposite direction, was found to exhibit a high degree of chromatic selectivity. The second type of after-effect, in which motion adaptation induces misperceptions in the spatial position of stationary objects, was completely insensitive to chromatic composition. This dissociation between the chromatic selectivities of these after-effects shows that chromatic inputs remain segregated at early stages of motion analysis, while at higher levels of cortical processing there is integration across chromatic, as well as achromatic inputs, to produce a unified perceptual output.

MEG responses correlated with the visual perception of velocity change

Magnetoencephalography (MEG) was used to find neural activities, in the human brain, involved in perception of velocity changes in visual motion. We recorded MEG responses evoked by the stimuli whose velocity increased by 40% or 80% of baseline velocities of 1.0, 2.0, 3.0, and 4.0 deg/s. The velocity increment threshold and the manual reaction time (RT) were also measured under similar stimulus conditions. To manipulate observer's sensitivity to velocity increments, the MEG responses and the psychophysical performances were measured after adaptation to motion in one direction (adapted condition) or alternating directions (control condition). MEG responses evoked by velocity increments peaked at 200-290 ms (M1), and the M1 amplitudes, especially those obtained for 40% increments, were correlated with the sensitivities, which are the reciprocal of velocity increment thresholds. Furthermore, motion adaptation enhanced sensitivity to velocity increments and increased the M1 amplitudes. These results suggest a close correlation between the perceptual velocity increment and the evoked MEG response. In other words, the results suggest that velocity increments are detectable when there is a constant increment in magnetic neural response. As for latencies, nearly constant value of M1 latency did not quantitatively match a large decrease in manual RT with the increase in the baseline velocity. Motion adaptation reduced neither the peak MEG latency nor the manual RT.

Adaptation in Macaque MT Reduces Perceived Speed and Improves Speed Discrimination

The visual system adapts to its environment. Some adaptive changes are detrimental-perception is no longer veridical. Others are beneficial-the ability to discriminate two stimuli improves. The latter may reflect the visual system's ability to zoom-in on the currently relevant properties of the environment. We studied the neural basis of adaptive changes in the middle temporal area (MT) of macaque monkey visual cortex. Our data show that brief adaptation to a moving stimulus reduces the magnitude of neural responses and reduces the width of speed tuning curves. Comparable with what has recently been reported in the direction domain, the response reduction was largest when the test speed was different from the adaptation speed. Using an ideal observer analysis, we show that these response changes in MT are consistent with a reduction in perceived speed as well as an improvement in speed discrimination. This supports the view that adaptive response changes in MT are not just a consequence of neural fatigue, but an active process that enhances the discrimination of speed.

A ratio model of perceived speed in the human visual system

The perceived speed of moving images changes over time. Prolonged viewing of a pattern (adaptation) leads to an exponential decrease in its perceived speed. Similarly, responses of neurones tuned to motion reduce exponentially over time. It is tempting to link these phenomena. However, under certain conditions, perceived speed increases after adaptation and the time course of these perceptual effects varies widely. We propose a model that comprises two temporally tuned mechanisms whose sensitivities reduce exponentially over time. Perceived speed is taken as the ratio of these filters' outputs. The model captures increases and decreases in perceived speed following adaptation and describes our data well with just four free parameters. Whilst the model captures perceptual time courses that vary widely, parameter estimates for the time constants of the underlying filters are in good agreement with estimates of the time course of adaptation of direction selective neurones in the mammalian visual system.

Direction-specific adaptation of magnetic responses to motion onset

We investigated the direction-specificity of motion adaptation, by recording magnetic responses evoked by motion onsets under both adapted and control conditions. The inter-stimulus interval was equated between the conditions to precisely evaluate the effect of motion adaptation itself. The onset stimuli at 1.5, 3.0 or 6.0 deg/s moved in the same direction or in the opposite direction to an adaptation stimulus at 3.0 deg/s. The perceived velocity of each test stimulus was measured in separate sessions. The most prominent peak (M2) of evoked responses appeared around 200-300 ms after motion onsets, and the dipoles were mainly estimated in the temporo-occipital area. Adaptation largely affected both perceived velocities and the M2 amplitudes. The M2 amplitudes were decreased by adaptation for both directions of test stimuli, and the decreases were significantly larger for the test stimuli in the adapted direction (49-63% of control condition) than for the test stimuli in the opposite direction (17-27% of control condition). The present study, for the first time, found that magnetic responses evoked by motion onsets reflect the activities of neurons that have direction-specificity.

The Effect of Central and Peripheral Field Stimulation on the Rise Time and Gain of Human Optokinetic Nystagmus

We wished to examine the spatial (gain) and temporal (rise time) properties of human optokinetic nystagmus (OKN) as a function of stimulus velocity and field location. Stimuli were either M-scaled random dots or vertical stripes that moved at velocities between 20–80 deg s−1. Three field conditions were examined: full field; a 20 deg central field; and a 12.5 deg central-field mask. OKN gain was found to be significantly affected by stimulus velocity and stimulus location, with the higher stimulus velocities and the 12.5 deg central-field mask giving lower gains. Steady-state gains for all three field conditions were not found to be affected by prior adaptation to stationary or moving stimuli. The 63% rise time was found to be significantly affected by the stimulus velocity, whereas this was not the case for the 90% rise time. Neither rise time was found to be significantly affected by the field location. These results indicate that, although the effectiveness (gain) of peripheral retina is lower than that of the central retina during optokinetic stimulation, the peripheral retina has access to common mechanisms responsible for the fast component of OKN.

A population decoding framework for motion aftereffects on smooth pursuit eye movements

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.

Effect of adaptation direction on the motion VEP and perceived speed of drifting gratings

The N200 amplitude of the motion-onset VEP evoked by a parafoveal grating of variable contrast (0.5-64%), constant speed (2 degrees/s), direction (horizontally rightward), and spatial frequency (2 cpd) was studied before and after adaptation to a stationary or drifting grating (1, 2, or 4 degrees/s rightward or leftward). These results are compared to those for the pattern-appearance VEP. Psychophysical measurements were made simultaneously of the perceived speed. While iso-directional (rightward) adaptation leads to a mean amplitude reduction of 39%, the decrease after counter-directional adaptation has a size of 20%. The post-adaptation matches of perceived speed differ in dependence on the iso-directional adapting speed and decrease on average to 98%, 85%, and 69% of the pre-adapt perceived speed after 1, 2, and 4 degrees/s adapting speeds, respectively. The perceived speed is moderately reduced (83% of the pre-adapt value) after counter-directional adaptation nearly independently of the adapting speed. A model of velocity processing is presented, which enables us to predict the trends of the experimental motion VEP and perceived speed data.

A visual motion sensor based on the properties of V1 and MT neurons

The motion response properties of neurons increase in complexity as one moves from primary visual cortex (V1), up to higher cortical areas such as the middle temporal (MT) and the medial superior temporal area (MST). Many of the features of V1 neurons can now be replicated using computational models based on spatiotemporal filters. However until recently, relatively little was known about how the motion analysing properties of MT neurons could originate from the V1 neurons that provide their inputs. This has constrained the development of models of the MT-MST stages which have been linked to higher level motion processing tasks such as self-motion perception and depth estimation. I describe the construction of a motion sensor built up in stages from two spatiotemporal filters with properties based on V1 neurons. The resulting composite sensor is shown to have spatiotemporal frequency response profiles, speed and direction tuning responses that are comparable to MT neurons. The sensor is designed to work with digital images and can therefore be used as a realistic front-end to models of MT and MST neuron processing; it can be probed with the same two-dimensional motion stimuli used to test the neurons and has the potential to act as a building block for more complex models of motion processing.