The perception and discrimination of speed in complex motion

Perceptual Thresholds for Radial Optic Flow Distortion in Near-Eye Stereoscopic Displays

We provide the first perceptual quantification of user's sensitivity to radial optic flow artifacts and demonstrate a promising approach for masking this optic flow artifact via blink suppression. Near-eye HMOs allow users to feel immersed in virtual environments by providing visual cues, like motion parallax and stereoscopy, that mimic how we view the physical world. However, these systems exhibit a variety of perceptual artifacts that can limit their usability and the user's sense of presence in VR. One well-known artifact is the vergence-accommodation conflict (VAC). Varifocal displays can mitigate VAC, but bring with them other artifacts such as a change in virtual image size (radial optic flow) when the focal plane changes. We conducted a set of psychophysical studies to measure users' ability to perceive this radial flow artifact before, during, and after self-initiated blinks. Our results showed that visual sensitivity was reduced by a factor of 10 at the start and for ~70 ms after a blink was detected. Pre- and post-blink sensitivity was, on average, ~O.15% image size change during normal viewing and increased to ~1.5- 2.0% during blinks. Our results imply that a rapid (under 70 ms) radial optic flow distortion can go unnoticed during a blink. Furthermore, our results provide empirical data that can be used to inform engineering requirements for both hardware design and software-based graphical correction algorithms for future varifocal near-eye displays. Our project website is available at https://gamma.umd.edu/ROF/.

The effect of eccentricity on the linear-radial speed bias: Testing the motion-in-depth model

Radial motion is perceived as faster than linear motion when local spatiotemporal properties are matched. This radial speed bias (RSB) is thought to occur because radial motion is partly interpreted as motion-in-depth. Geometry dictates that a fixed amount of radial expansion at increasing eccentricities is consistent with smaller motion in depth, so it is perhaps surprising that the impact of eccentricity on RSB has not been examined. With this issue in mind, across 3 experiments we investigated the RSB as a function of eccentricity. In a 2IFC task, participants judged which of a linear (test - variable speed) or radial (reference - 2 or 4°/s) stimulus appeared to move faster. Linear and radial stimuli comprised 4 Gabor patches arranged left, right, above and below fixation at varying eccentricities (3.5°-14°). For linear stimuli, Gabors all drifted left or right, whereas for radial stimuli Gabors drifted towards or away from the centre. The RSB (difference in perceived speeds between matched linear and radial stimuli) was recovered from fitted psychometric functions. Across all 3 experiments we found that the RSB decreased with eccentricity but this tendency was less marked beyond 7° - i.e. at odds with the geometry, the effect did not continue to decrease as a function of eccentricity. This was true irrespective of whether stimuli were fixed in size (Experiment 1) or varied in size to account for changes in spatial scale across the retina (Experiment 2). It was also true when we removed conflicting stereo cues via monocular viewing (Experiment 3). To further investigate our data, we extended a previous model of speed perception, which suggests perceived motion for such stimuli reflects a balance between two opposing perceptual interpretations, one for motion in depth and the other for object deformation. We propose, in the context of this model, that our data are consistent with placing greater weight on the motion in depth interpretation with increasing eccentricity and this is why the RSB does not continue to reduce in line with purely geometric constraints.

Double dissociation in radial and rotational motion sensitivity

Neurophysiological experiments have shown that a shared region of the primate visual system registers both radial and rotational motion. Radial and rotational motion also share computational features. Despite these neural and computational similarities, prior experiments have disrupted radial, but not rotational, motion sensitivity -a single dissociation. Here we report stimulus manipulations that extend the single dissociation to a double dissociation, thereby showing further separability between radial and rotational motion sensitivity. In Exp 1 bilateral plaid stimuli with or without phase-noise either radiated or rotated before changing direction. College students reported whether the direction changed first on the left or right–a temporal order judgment (TOJ). Phase noise generated significantly larger disruptions to rotational TOJs than to radial TOJs, thereby completing the double dissociation. In Exp 2 we conceptually replicated this double dissociation by switching the task from TOJs to simultaneity judgments (SJs). Phase noise generated significantly larger disruptions to rotational SJs than to radial SJs. This disruption pattern reversed after changing the plaids’ motion from same- to opposite-initial directions. The double dissociations reported here revealed distinct dependencies for radial and rotational motion sensitivity. Radial motion sensitivity depended strongly on information about global depth. Rotational motion sensitivity depended strongly on positional information about local luminance gradients. These distinct dependencies arose downstream from the neural mechanisms that detect local linear components within radial and rotational motion. Overall, the differential impairments generated by our psychophysical experiments demonstrate independence between radial and rotational motion sensitivity, despite their neural and computational similarities.

Selectivity to approaching motion in retinal inputs to the dorsal visual pathway

To efficiently navigate through the environment and avoid potential threats, an animal must quickly detect the motion of approaching objects. Current models of primate vision place the origins of this complex computation in the visual cortex. Here, we report that detection of approaching motion begins in the retina. Several ganglion cell types, the retinal output neurons, show selectivity to approaching motion. Synaptic current recordings from these cells further reveal that this preference for approaching motion arises in the interplay between presynaptic excitatory and inhibitory circuit elements. These findings demonstrate how excitatory and inhibitory circuits interact to mediate an ethologically relevant neural function. Moreover, the elementary computations that detect approaching motion begin early in the visual stream of primates.

Gloss and Speed Judgments Yield Different Fine Tuning of Saccadic Sampling in Dynamic Scenes

Image motion contains potential cues about the material properties of objects. In earlier work, we proposed motion cues that could predict whether a moving object would be perceived as shiny or matte. However, whether the visual system uses these cues is still uncertain. Herein, we use the tracking of eye movements as a tool to understand what visual information observers use when engaged in material perception. Observers judged either the gloss or the speed of moving blobby shapes in an eye tracking experiment. Results indicate that during glossiness judgments, participants tend to look at gloss-diagnostic dynamic features more than during speed judgments. This suggests a fine tuning of the visual system to properties of moving stimuli: Task relevant information is actively singled out and processed in a dynamically changing environment.

Superior Visual Timing Sensitivity in Auditory But Not Visual World Class Drum Corps Experts

World class drum corps require cooperation among performance artists to render precisely synchronized and asynchronized events. For example, drum corps visual aesthetics often feature salient radial and rotational motion displays from the color guard. Accordingly, extensive color guard training might predict superior visual timing sensitivity to asynchronies in radial and rotational motion displays. Less intuitively, one might instead predict superior visual timing sensitivity among world class drum corps musicians, who regularly subdivide musical tempos into brief time units. This prediction arises from the possibility that auditory training transfers cross-modally. Here, we investigated whether precise visual temporal order judgments (TOJs) more strongly align with color guard’s visual training or musicians’ auditory training. To mimic color guard visual displays, stimuli comprised bilateral plaid patterns that radiated or rotated before changing direction asynchronously. Human participants indicated whether the direction changed first on the left or right, called a TOJ. Twenty-five percussionists, 67 brass players, and 29 color guard members from a world class drum corps collectively completed 67,760 visual TOJ trials. Percussionists exhibited significantly lower TOJ thresholds than did brass players, who exhibited significantly lower TOJ thresholds than did the color guard. Group median thresholds spanned an order of magnitude, ranging between 29 ms (percussionists judging rotational asynchronies) and 290 ms (color guard judging radial asynchronies). The results suggest that visual timing can improve more by training cross-modally than intramodally, even when intramodal training and testing stimuli closely match. More broadly, pre-existing training histories can provide a unique window into the timing sensitivity of the nervous system.

Computer-animated stimuli to measure motion sensitivity: constraints on signal design in the Jacky dragon

Identifying perceptual thresholds is critical for understanding the mechanisms that underlie signal evolution. Using computer-animated stimuli, we examined visual speed sensitivity in the Jacky dragon Amphibolurus muricatus, a species that makes extensive use of rapid motor patterns in social communication. First, focal lizards were tested in discrimination trials using random-dot kinematograms displaying combinations of speed, coherence, and direction. Second, we measured subject lizards’ ability to predict the appearance of a secondary reinforcer (1 of 3 different computer-generated animations of invertebrates: cricket, spider, and mite) based on the direction of movement of a field of drifting dots by following a set of behavioural responses (e.g., orienting response, latency to respond) to our virtual stimuli. We found an effect of both speed and coherence, as well as an interaction between these 2 factors on the perception of moving stimuli. Overall, our results showed that Jacky dragons have acute sensitivity to high speeds. We then employed an optic flow analysis to match the performance to ecologically relevant motion. Our results suggest that the Jacky dragon visual system may have been shaped to detect fast motion. This pre-existing sensitivity may have constrained the evolution of conspecific displays. In contrast, Jacky dragons may have difficulty in detecting the movement of ambush predators, such as snakes and of some invertebrate prey. Our study also demonstrates the potential of the computer-animated stimuli technique for conducting nonintrusive tests to explore motion range and sensitivity in a visually mediated species.

3D Visual Response Properties of MSTd Emerge from an Efficient, Sparse Population Code

Neurons in the dorsal subregion of the medial superior temporal (MSTd) area of the macaque respond to large, complex patterns of retinal flow, implying a role in the analysis of self-motion. Some neurons are selective for the expanding radial motion that occurs as an observer moves through the environment ("heading"), and computational models can account for this finding. However, ample evidence suggests that MSTd neurons exhibit a continuum of visual response selectivity to large-field motion stimuli. Furthermore, the underlying computational principles by which these response properties are derived remain poorly understood. Here we describe a computational model of macaque MSTd based on the hypothesis that neurons in MSTd efficiently encode the continuum of large-field retinal flow patterns on the basis of inputs received from neurons in MT with receptive fields that resemble basis vectors recovered with non-negative matrix factorization. These assumptions are sufficient to quantitatively simulate neurophysiological response properties of MSTd cells, such as 3D translation and rotation selectivity, suggesting that these properties might simply be a byproduct of MSTd neurons performing dimensionality reduction on their inputs. At the population level, model MSTd accurately predicts eye velocity and heading using a sparse distributed code, consistent with the idea that biological MSTd might be well equipped to efficiently encode various self-motion variables. The present work aims to add some structure to the often contradictory findings about macaque MSTd, and offers a biologically plausible account of a wide range of visual response properties ranging from single-unit selectivity to population statistics.

SIGNIFICANCE STATEMENT

Using a dimensionality reduction technique known as non-negative matrix factorization, we found that a variety of medial superior temporal (MSTd) neural response properties could be derived from MT-like input features. The responses that emerge from this technique, such as 3D translation and rotation selectivity, spiral tuning, and heading selectivity, can account for a number of empirical results. These findings (1) provide a further step toward a scientific understanding of the often nonintuitive response properties of MSTd neurons; (2) suggest that response properties, such as complex motion tuning and heading selectivity, might simply be a byproduct of MSTd neurons performing dimensionality reduction on their inputs; and (3) imply that motion perception in the cortex is consistent with ideas from the efficient-coding and free-energy principles.

Visual Acceleration Perception for Simple and Complex Motion Patterns

Humans are able to judge whether a target is accelerating in many viewing contexts, but it is an open question how the motion pattern per se affects visual acceleration perception. We measured acceleration and deceleration detection using patterns of random dots with horizontal (simpler) or radial motion (more visually complex). The results suggest that we detect acceleration better when viewing radial optic flow than horizontal translation. However, the direction within each type of pattern has no effect on performance and observers detect acceleration and deceleration similarly within each condition. We conclude that sensitivity to the presence of acceleration is generally higher for more complex patterns, regardless of the direction within each type of pattern or the sign of acceleration.

Effects of Vertical Direction and Aperture Size on the Perception of Visual Acceleration

It is not well understood whether the distance over which moving stimuli are visible affects our sensitivity to the presence of acceleration or our ability to track such stimuli. It is also uncertain whether our experience with gravity creates anisotropies in how we detect vertical acceleration and deceleration. To address these questions, we varied the vertical extent of the aperture through which we presented vertically accelerating and decelerating random dot arrays. We hypothesized that observers would better detect and pursue accelerating and decelerating stimuli that extend over larger than smaller distances. In Experiment 1, we tested the effects of vertical direction and aperture size on acceleration and deceleration detection accuracy. Results indicated that detection is better for downward motion and for large apertures, but there is no difference between vertical acceleration and deceleration detection. A control experiment revealed that our manipulation of vertical aperture size affects the ability to track vertical motion. Smooth pursuit is better (i.e., with higher peak velocities) for large apertures than for small apertures. Our findings suggest that the ability to detect vertical acceleration and deceleration varies as a function of the direction and vertical extent over which an observer can track the moving stimulus.

Visual directional anisotropy does not mirror the directional anisotropy apparent in postural sway

Presenting a large optic flow pattern to observers is likely to cause postural sway. However, directional anisotropies have been reported, in that contracting optic flow induces more postural sway than expanding optic flow. Recently, we showed that the biomechanics of the lower leg cannot account for this anisotropy (Holten, Donker, Verstraten, & van der Smagt, 2013, Experimental Brain Research, 228, 117-129). The question we address in the current study is whether differences in visual processing of optic flow directions, in particular the perceptual strength of these directions, mirrors the anisotropy apparent in postural sway. That is, can contracting optic flow be considered to be a perceptually stronger visual stimulus than expanding optic flow? In the current study we use a breaking continuous flash suppression paradigm where we assume that perceptually stronger visual stimuli will break the flash suppression earlier, making the suppressed optic flow stimulus visible sooner. Surprisingly, our results show the opposite, in that expanding optic flow is detected earlier than contracting optic flow.

Effects of radial direction and eccentricity on acceleration perception

Radial optic flow can elicit impressions of self-motion--vection--or of objects moving relative to the observer, but there is disagreement as to whether humans have greater sensitivity to expanding or to contracting optic flow. Although most studies agree there is an anisotropy in sensitivity to radial optic flow, it is unclear whether this asymmetry is a function of eccentricity. The issue is further complicated by the fact that few studies have examined how acceleration sensitivity is affected, even though observers and objects in the environment seldom move at a constant speed. To address these issues, we investigated the effects of direction and eccentricity on the ability to detect acceleration in radial optic flow. Our results indicate that observers are better at detecting acceleration when viewing contraction compared with expansion and that eccentricity has no effect on the ability to detect accelerating radial optic flow. Ecological interpretations are discussed.

How active perception and attractor dynamics shape perceptual categorization: a computational model

We propose a computational model of perceptual categorization that fuses elements of grounded and sensorimotor theories of cognition with dynamic models of decision-making. We assume that category information consists in anticipated patterns of agent-environment interactions that can be elicited through overt or covert (simulated) eye movements, object manipulation, etc. This information is firstly encoded when category information is acquired, and then re-enacted during perceptual categorization. The perceptual categorization consists in a dynamic competition between attractors that encode the sensorimotor patterns typical of each category; action prediction success counts as "evidence" for a given category and contributes to falling into the corresponding attractor. The evidence accumulation process is guided by an active perception loop, and the active exploration of objects (e.g., visual exploration) aims at eliciting expected sensorimotor patterns that count as evidence for the object category. We present a computational model incorporating these elements and describing action prediction, active perception, and attractor dynamics as key elements of perceptual categorizations. We test the model in three simulated perceptual categorization tasks, and we discuss its relevance for grounded and sensorimotor theories of cognition.

The global slowdown effect: why does perceptual grouping reduce perceived speed?

The percept of four rotating dot pairs is bistable. The "local percept" is of four pairs of dots rotating independently. The "global percept" is of two large squares translating over one another (Anstis & Kim 2011). We have previously demonstrated (Kohler, Caplovitz, & Tse 2009) that the global percept appears to move more slowly than the local percept. Here, we investigate and rule out several hypotheses for why this may be the case. First, we demonstrate that the global slowdown effect does not occur because the global percept is of larger objects than the local percept. Second, we show that the global slowdown effect is not related to rotation-specific detectors that may be more active in the local than in the global percept. Third, we find that the effect is also not due to a reduction of image elements during grouping and can occur with a stimulus very different from the one used previously. This suggests that the effect may reflect a general property of perceptual grouping. Having ruled out these possibilities, we suggest that the global slowdown effect may arise from emergent motion signals that are generated by the moving dots, which are interpreted as the ends of "barbell bars" in the local percept or the corners of the illusory squares in the global percept. Alternatively, the effect could be the result of noisy sources of motion information that arise from perceptual grouping that, in turn, increase the influence of Bayesian priors toward slow motion (Weiss, Simoncelli, & Adelson 2002).

Human cortical and behavioral sensitivity to patterns of complex motion at eccentricity

Complex patterns of image motion (contracting, expanding, rotating, and spiraling fields) are important in the coordination of visually guided behaviors. Whereas specialized detectors in monkey visual cortex show selectivity for particular patterns of complex motion, their representation in human visual cortex remains unclear. In the present study, functional magnetic resonance imaging (fMRI) was used to investigate the sensitivity of functionally defined regions of human visual cortex to parametrically modulated complex motion trajectories, coupled with complementary psychophysical testing. A unique stimulus design made it possible to disambiguate the neural responses and psychophysical sensitivity to complex motions per se from the distribution of local motions relative to the fovea, which are known to enhance cortical activity when presented radial to fixation. This involved presenting several small, separate motion fields in the periphery in a manner that distinguished them from global optic flow patterns. The patterns were morphed through complex motion space in a systematic time-locked fashion when presented in the scanner. Anisotropies were observed in the fMRI signal, marked by an enhanced response to expanding vs. contracting fields, even in early visual cortex. Anisotropies in the psychophysical sensitivity measures followed a similar pattern that was correlated with activity in areas hV4, V5/MT, and MST. This represents the first systematic examination of complex motion perception at both a behavioral and neural level in human observers. The characteristic processing anisotropy revealed in both data sets can inform models of complex motion processing, particularly with respect to computations performed in early visual cortex.

Skill-Related Differences between Athletes and Nonathletes in Speed Discrimination

This study examined differences in decision-making time and accuracy as attributes of speed discrimination between participants skilled and less skilled in ball games. A total of 130 men, ages 18 to 28 years ( M = 21.2, SD = 2.6), participated. The athlete sample (skilled group) comprised Estonian National League volleyball ( n = 26) and basketball players ( n = 27). The nonathlete sample (less skilled group) included 77 soldiers of the Estonian Defence Force with no reported top level experience in ball games. Speed-discrimination stimuli were images of red square shapes presented moving along the sagittal axis at four different virtual velocities on a computer (PC) screen which represented the frontal plane. Analysis indicated that only decision-making time was significantly different between the elite athlete and nonathlete groups. This finding suggests a possible effect of ball-game skills for decision-making time in speed discrimination.

Sensitivity to rotational motion in early infancy

Sensitivity to rotational motion, one of the fundamental components of optic flow, was tested in infants aged 2 and 3 months. The infants in both groups showed significant sensitivity to rotational motion only in the high-speed condition (10.62 degrees/s). There was no significant increase in motion sensitivity between 2 and 3 months of age, indicating that there is not a significant developmental change during this period. A comparison of our results with previous findings that showed a significant increase in radial motion sensitivity between 2 and 3 months suggests that different motion sensitivities have different developmental time courses.

Enhanced temporal but not attentional processing in expert tennis players

In tennis, as in many disciplines of sport, fine spatio-temporal resolution is required to reach optimal performance. While many studies on tennis have focused on anticipatory skills or decision making, fewer have investigated the underlying visual perception abilities. In this study, we used a battery of seven visual tests that allowed us to assess which kind of visual information processing is performed better by tennis players than other athletes (triathletes) and non-athletes. We found that certain time-related skills, such as speed discrimination, are superior in tennis players compared to non-athletes and triathletes. Such tasks might be used to improve tennis performance in the future.

An effect of relative motion on trajectory discrimination

Psychophysical studies point to the existence of specialized mechanisms sensitive to the relative motion between an object and its background. Such mechanisms would seem ideal for the motion-based segmentation of objects; however, their properties and role in processing the visual scene remain unclear. Here we examine the contribution of relative motion mechanisms to the processing of object trajectory. In a series of four psychophysical experiments we examine systematically the effects of relative direction and speed differences on the perceived trajectory of an object against a moving background. We show that background motion systematically influences the discrimination of object direction. Subjects' ability to discriminate direction was consistently better for objects moving opposite a translating background than for objects moving in the same direction as the background. This effect was limited to the case of a translating background and did not affect perceived trajectory for more complex background motions associated with self-motion. We interpret these differences as providing support for the role of relative motion mechanisms in the segmentation and representation of object motions that do not occlude the path of an observer's self-motion.

Effect of signal intensity on perceived speed

The effect of signal intensity (proportion of dots moving in the same direction compared to noise dots that move in random directions) on perceived speed was investigated. It was found that increasing signal level decreased the perceived speed of the stimulus. This finding indicates that global-motion pooling processes play a role in the extraction of speed information. It is suggested that the amount of relative motion in the stimulus influences perceived speed, with perceived speed increasing with increasing relative motion. The results are discussed in relation to the notion that speed and direction are processed, at least in part, differently.

Global motion mechanisms compensate local motion deficits in a patient with a bilateral occipital lobe lesion

Successive stages of cortical processing encode increasingly more complex types of information. In the visual motion system this increasing complexity, complemented by an increase in spatial summation, has proven effective in characterizing the mechanisms mediating visual perception. Here we report psychophysical results from a motion-impaired stroke patient, WB, whose pattern of deficits over time reveals a systematic shift in spatial scale for processing speed. We show that following loss in sensitivity to low-level motion direction WB's representation of speed shifts to larger spatial scales, consistent with recruitment of intact high-level mechanisms. With the recovery of low-level motion processing WB's representation of speed shifts back to small spatial scales. These results support the recruitment of high-level visual mechanisms in cases where lower-level function is impaired and suggest that, as an experimental paradigm, spatial summation may provide an important avenue for investigating functional recovery in patients following damage to visually responsive cortex.

Radial motion bias in macaque frontal eye field

The visual responsiveness and spatial tuning of frontal eye field (FEF) neurons were determined using a delayed memory saccade task. Neurons with visual responses were then tested for direction selectivity using moving random dot patterns centered in the visual receptive field. The preferred axis of motion showed a significant tendency to be aligned with the receptive-field location so as to favor motion toward or away from the center of gaze. Centrifugal (outward) motion was preferred over centripetal motion. Motion-sensitive neurons in FEF thus appear to have a direction bias at the population level. This bias may facilitate the detection or discrimination of expanding optic flow patterns. The direction bias is similar to that seen in visual area MT and in posterior parietal cortex, from which FEF receives afferent projections. The outward motion bias may explain asymmetries in saccades made to moving targets. A representation of optic flow in FEF might be useful for planning eye movements during navigation.

Psychophysical evidence for a radial motion bias in complex motion discrimination

In a graded motion pattern task we measured observers' ability to discriminate small changes in the global direction of complex motion patterns. Performance varied systematically as a function of the test motion (radial, circular, or spiral) with thresholds for radial motions significantly lower than for circular motions. Thresholds for spiral motions were intermediate. In all cases thresholds were lower than for direction discrimination using planar motions and increased with removal of the radial speed gradient, consistent with the use of motion pattern specific mechanisms that integrate motion along complex trajectories. The radial motion bias and preference for speed gradients observed here is similar to the preference for expanding motions and speed gradients reported in cortical area MSTd, and may suggest the presence of comparable neural mechanisms in the human visual motion system.

Torsional eye movements during psychophysical testing with rotating patterns

Torsional eye movements were measured while subjects viewed a large, high contrast windmill pattern rotating at 53 degrees /s or a small (5 degrees diameter) dot pattern rotating at 115 degrees /s. Both stimuli generated rotational eye movements consisting of torsional optokinetic nystagmus (tOKN) superimposed on a slow torsional drift in the direction of pattern rotation. With the wide-field windmill stimulus, torsional drifts of up to 7 degrees over 20 s were found. The dot pattern produced drifts of up to 2 degrees over 5-20 s. In both cases, the slow-phase speeds during tOKN were low (0.5-1 degrees /s). We conclude that reductions in slip speed are minimal with rotating stimuli, so torsional eye speeds will have a minimal effect on investigations of rotational motion aftereffect strength and perceived speed. While the slow-phase tOKN gain is low, the slow drift in torsional eye position will have significant effects on psychophysical results when the tests rely on keeping selected regions of the stimulus confined to specific areas of the retina, as is the case for phantom or remote motion aftereffects.

Local computation of angular velocity in rotational visual motion

Retinal images evolve continuously over time owing to self-motions and to movements in the world. Such an evolving image, also known as optic flow, if arising from natural scenes can be locally decomposed in a Bayesian manner into several elementary components, including translation, expansion, and rotation. To take advantage of this decomposition, the brain has neurons tuned to these types of motions. However, these neurons typically have large receptive fields, often spanning tens of degrees of visual angle. Can neurons such as these compute elementary optic-flow components sufficiently locally to achieve a reasonable decomposition? We show that human discrimination of angular velocity is local. Local discrimination of angular velocity requires an accurate estimation of the center of rotation within the optic-flow field. Inaccuracies in estimating the center of rotation result in a predictable systematic error when one is estimating local angular velocity. Our results show that humans make the predicted errors. We discuss how the brain might estimate the elementary components of the optic flow locally by using large receptive fields.

Measurement of angular velocity in the perception of rotation

Humans are sensitive to the parameters of translational motion, namely, direction and speed. At the same time, people have special mechanisms to deal with more complex motions, such as rotations and expansions. One wonders whether people may also be sensitive to the parameters of these complex motions. Here, we report on a series of experiments that explore whether human subjects can use angular velocity to evaluate how fast a rotational motion is. In four experiments, subjects were required to perform a task of speed-of-rotation discrimination by comparing two annuli of different radii in a temporal 2AFC paradigm. Results showed that humans could rely on a sensitive measurement of angular velocity to perform this discrimination task. This was especially true when the quality of the rotational signal was high (given by the number of dots composing the annulus). When the signal quality decreased, a bias towards linear velocity of 5-80% appeared, suggesting the existence of separate mechanisms for angular and linear velocity. This bias was independent from the reference radius. Finally, we asked whether the measurement of angular velocity required a rigid rotation, that is, whether the visual system makes only one global estimate of angular velocity. For this purpose, a random-dot disk was built such that all the dots were rotating with the same tangential speed, irrespectively of radius. Results showed that subjects do not estimate a unique global angular velocity, but that they perceive a non-rigid disk, with angular velocity falling inversely proportionally with radius.

Asymmetrical masking between radial and parallel motion flow in transparent displays

Human spatial behavior remains surprisingly accurate and stable, despite the complexities of the concurrent retinal image flow. How does visual motion analysis contribute to behavioral stability? To approach this question, we rely on current knowledge about heading perception and coherent motion extraction from animated random dot displays. This chapter presents psychophysical evidence for an asymmetry in the global visual decomposition of the retinal motion flow. We examined the ability of human observers to identify the radial and the parallel flow component in displays where the two motions were transparently superimposed. Whereas sensitivity to the radial motion remained unaffected by the presence of parallel flow, sensitivity to the latter was greatly degraded in the presence of radial flow. This imbalance was observed within the first 200 ms of motion exposure, prior to the perception of heading (430 ms; Hooge et al. (1999) Exp. Brain Res., 129: 615-628). Although such an asymmetrical masking is well suited to minimize heading errors, whether it really contributes to the extraction of behavioral direction remains an open question.

Perceiving visual expansion without optic flow

When an observer moves forward in the environment, the image on his or her retina expands. The rate of this expansion conveys information about the observer's speed and the time to collision. Psychophysical and physiological studies have provided abundant evidence that these expansionary motions are processed by specialized mechanisms in mammalian visual systems. It is commonly assumed that the rate of expansion is estimated from the divergence of the optic-flow field (the two-dimensional field of local translational velocities). But this rate might also be estimated from changes in the size (or scale) of image features. To determine whether human vision uses such scale-change information, we have synthesized stochastic texture stimuli in which the scale of image elements increases gradually over time, while the optic-flow pattern is random. Here we show, using these stimuli, that observers can estimate expansion rates from scale-change information alone, and that pure scale changes can produce motion after-effects. These two findings suggest that the visual system contains mechanisms that are explicitly sensitive to changes in scale.

Dissociable factors affect speed perception and discrimination

Factors affecting our judgement of the speed of visual motion were investigated. Two types of judgement were made: perceived speed relative to a standard comparison stimulus, and discrimination between the speeds of similar stimuli. The factors affect ng these two judgements were found to be doubly dissociable, suggesting that they may be constrained by processing at different levels of the visual hierarchy. The results are discussed in terms of the 3-D interpretation of visual image motion, and related to possible neural substrates.