Perception of three-dimensional structure from motion

Single point motion kinematics convey emotional signals in children and adults

This study investigates whether humans recognize different emotions conveyed only by the kinematics of a single moving geometrical shape and how this competence unfolds during development, from childhood to adulthood. To this aim, animations in which a shape moved according to happy, fearful, or neutral cartoons were shown, in a forced-choice paradigm, to 7- and 10-year-old children and adults. Accuracy and response times were recorded, and the movement of the mouse while the participants selected a response was tracked. Results showed that 10-year-old children and adults recognize happiness and fear when conveyed solely by different kinematics, with an advantage for fearful stimuli. Fearful stimuli were also accurately identified at 7-year-olds, together with neutral stimuli, while, at this age, the accuracy for happiness was not significantly different than chance. Overall, results demonstrates that emotions can be identified by a single point motion alone during both childhood and adulthood. Moreover, motion contributes in various measures to the comprehension of emotions, with fear recognized earlier in development and more readily even later on, when all emotions are accurately labeled.

The Motion-Silencing Illusion Depends on Object-Centered Representation

Motion silencing is a striking and unexplained visual illusion wherein changes that are otherwise salient become difficult to perceive when the changing elements also move. We develop a new method for quantifying illusion strength (Experiments 1a and 1b), and we demonstrate a privileged role for rotational motion on illusion strength compared with highly controlled stimuli that lack rotation (Experiments 2a to 3b). These contrasts make it difficult to explain the illusion in terms of lower-level detection limits. Instead, we explain the illusion as a failure to attribute changes to locations. Rotation exacerbates the illusion because its perception relies upon structured object representations. This aggravates the difficulty of attributing changes by demanding that locations are referenced relative to both an object-internal frame and an external frame. Two final experiments (4a and 4b) add support to this account by employing a synchronously rotating external frame of reference that diminishes otherwise strong motion silencing. All participants were Johns Hopkins University undergraduates.

Perceptual transitions between object rigidity and non-rigidity: Competition and cooperation among motion energy, feature tracking, and shape-based priors

Why do moving objects appear rigid when projected retinal images are deformed non-rigidly? We used rotating rigid objects that can appear rigid or non-rigid to test whether shape features contribute to rigidity perception. When two circular rings were rigidly linked at an angle and jointly rotated at moderate speeds, observers reported that the rings wobbled and were not linked rigidly, but rigid rotation was reported at slow speeds. When gaps, paint, or vertices were added, the rings appeared rigidly rotating even at moderate speeds. At high speeds, all configurations appeared non-rigid. Salient features thus contribute to rigidity at slow and moderate speeds but not at high speeds. Simulated responses of arrays of motion-energy cells showed that motion flow vectors are predominantly orthogonal to the contours of the rings, not parallel to the rotation direction. A convolutional neural network trained to distinguish flow patterns for wobbling versus rotation gave a high probability of wobbling for the motion-energy flows. However, the convolutional neural network gave high probabilities of rotation for motion flows generated by tracking features with arrays of MT pattern-motion cells and corner detectors. In addition, circular rings can appear to spin and roll despite the absence of any sensory evidence, and this illusion is prevented by vertices, gaps, and painted segments, showing the effects of rotational symmetry and shape. Combining convolutional neural network outputs that give greater weight to motion energy at fast speeds and to feature tracking at slow speeds, with the shape-based priors for wobbling and rolling, explained rigid and non-rigid percepts across shapes and speeds (R2 = 0.95). The results demonstrate how cooperation and competition between different neuronal classes lead to specific states of visual perception and to transitions between the states.

Perceptual Transitions between Object Rigidity & Non-rigidity: Competition and cooperation between motion-energy, feature-tracking and shape-based priors

Why do moving objects appear rigid when projected retinal images are deformed non-rigidly? We used rotating rigid objects that can appear rigid or non-rigid to test whether shape features contribute to rigidity perception. When two circular rings were rigidly linked at an angle and jointly rotated at moderate speeds, observers reported that the rings wobbled and were not linked rigidly but rigid rotation was reported at slow speeds. When gaps, paint or vertices were added, the rings appeared rigidly rotating even at moderate speeds. At high speeds, all configurations appeared non-rigid. Salient features thus contribute to rigidity at slow and moderate speeds, but not at high speeds. Simulated responses of arrays of motion-energy cells showed that motion flow vectors are predominantly orthogonal to the contours of the rings, not parallel to the rotation direction. A convolutional neural network trained to distinguish flow patterns for wobbling versus rotation, gave a high probability of wobbling for the motion-energy flows. However, the CNN gave high probabilities of rotation for motion flows generated by tracking features with arrays of MT pattern-motion cells and corner detectors. In addition, circular rings can appear to spin and roll despite the absence of any sensory evidence, and this illusion is prevented by vertices, gaps, and painted segments, showing the effects of rotational symmetry and shape. Combining CNN outputs that give greater weight to motion energy at fast speeds and to feature tracking at slow, with the shape-based priors for wobbling and rolling, explained rigid and nonrigid percepts across shapes and speeds (R2=0.95). The results demonstrate how cooperation and competition between different neuronal classes leads to specific states of visual perception and to transitions between the states.

Delay and Speed of Visual Feedback of a Keystroke Cause Illusory Heaviness and Stiffness

Imposing a delay between an action (e.g., a limb movement) and its related visual feedback (e.g., a cursor movement on the display) induces a peculiar sensation of heaviness or stiffness. Earlier studies have examined this delay-induced heaviness or stiffness sensation in relation to the non-arbitrary causal relationship between an action and its effect. Here, “non-arbitrary causal relationship” means that an action produces a specific and deterministic pattern of visual feedback; for example, a leftward limb movement consistently and deterministically causes a leftward visual motion. In modern graphical user interfaces, on the other hand, users often control visual information by pressing keys, wherein the relationship between the keystroke and the change in visual information is arbitrary. The present study examined whether the sensation of heaviness, stiffness and bumpiness could be caused when participants' keystroke produced a delayed arbitrary visual feedback. Participants were asked to press and hold down an assigned key to cause temporal luminance changes in a square centered on the display, an arbitrary visual feedback of their keystroke. Not only the onset delay of the temporal luminance change from the participant's keystroke but also the speed of the temporal luminance change were examined as a visual cue to heaviness, stiffness, or bumpiness. In Experiment 1, the participants' task was to give a rating for the strength of the heaviness, stiffness, or bumpiness perceived when they pressed the key. Our results showed that the heaviness and stiffness ratings increased as the delay increased and decreased as the speed increased. To check whether the manipulation of the delay and speed of the visual feedback caused changes in the subjective evaluation of sensorimotor incongruence, in Experiment 2, we asked the participants to give a rating for the sense of agency. The rating scores decreased as the delay increased and increased as the speed increased. The delay and speed influenced the rating scores for the sense of agency in the opposite direction to those for heaviness/stiffness. We discuss that the brain determines the heaviness and stiffness during a keystroke based on internalized statistics relating to the delay and speed of the action feedback.

The Z-Box illusion: dominance of motion perception among multiple 3D objects

In the present article, we examine a novel illusion of motion-the Z-Box illusion-in which the presence of a bounding object influences the perception of motion of an ambiguous stimulus that appears within. Specifically, the stimuli are a structure-from-motion (SFM) particle orb and a wireframe cube. The orb could be perceived as rotating clockwise or counterclockwise while the cube could only be perceived as moving in one direction. Both stimuli were presented on a two-dimensional (2D) display with inferred three-dimensional (3D) properties. In a single experiment, we examine motion perception of a particle orb, both in isolation and when it appears within a rotating cube. Participants indicated the orb's direction of motion and whether the direction changed at any point during the trial. Accuracy was the critical measure while motion direction, the number of particles in the orb and presence of the wireframe cube were all manipulated. The results suggest that participants could perceive the orb's true rotation in the absence of the cube so long as it was made up of at least ten particles. The presence of the cube dominated perception as participants consistently perceived congruent motion of the orb and cube, even when they moved in objectively different directions. These findings are considered as they relate to prior research on motion perception, computational modelling of motion perception, structure from motion and 3D object perception.

Visualization as a stimulus domain for vision science

Traditionally, vision science and information/data visualization have interacted by using knowledge of human vision to help design effective displays. It is argued here, however, that this interaction can also go in the opposite direction: the investigation of successful visualizations can lead to the discovery of interesting new issues and phenomena in visual perception. Various studies are reviewed showing how this has been done for two areas of visualization, namely, graphical representations and interaction, which lend themselves to work on visual processing and the control of visual operations, respectively. The results of these studies have provided new insights into aspects of vision such as grouping, attentional selection and the sequencing of visual operations. More generally yet, such results support the view that the perception of visualizations can be a useful domain for exploring the nature of visual cognition, inspiring new kinds of questions as well as casting new light on the limits to which information can be conveyed visually.

Objective pupillometry shows that perceptual styles covary with autistic-like personality traits

We measured the modulation of pupil size (in constant lighting) elicited by observing transparent surfaces of black and white moving dots, perceived as a cylinder rotating about its vertical axis. The direction of rotation was swapped periodically by flipping stereo-depth of the two surfaces. Pupil size modulated in synchrony with the changes in front-surface color (dilating when black). The magnitude of pupillary modulation was larger for human participants with higher Autism-Spectrum Quotient (AQ), consistent with a local perceptual style, with attention focused on the front surface. The modulation with surface color, and its correlation with AQ, was equally strong when participants passively viewed the stimulus. No other indicator, including involuntary pursuit eye movements, covaried with AQ. These results reinforce our previous report with a similar bistable stimulus (Turi, Burr, & Binda, 2018), and go on to show that bistable illusory motion is not necessary for the effect, or its dependence on AQ.

Idiosyncratic preferences in transparent motion and binocular rivalry are dissociable

Previous studies revealed that there are idiosyncratic preferences to perceive certain motion directions in front during motion transparency depth rivalry (Mamassian & Wallace, 2010; Schütz, 2014). Meanwhile, other studies reported idiosyncratic preferences in binocular rivalry during the onset stage (Carter & Cavanagh, 2007; Stanley, Carter, & Forte, 2011). Here we investigated the relationship of idiosyncratic preferences in transparent motion and binocular rivalry. We presented two dot clouds that were moving in opposite directions. In the transparent motion condition, both dot clouds were presented to both eyes and participants had to report the dot cloud they perceived in front. In the binocular rivalry condition, the dot clouds were presented to different eyes and participants had to report the dominant dot cloud. There were strong idiosyncratic directional preferences in transparent motion and rather weak directional preferences in binocular rivalry. In general, binocular rivalry was dominated by biases in contrast polarity, whereas transparent motion was dominated by biases in motion direction. A circular correlation analysis showed no correlation between directional preferences in transparent motion and binocular rivalry. These findings show that idiosyncratic preferences in a visual feature can be dissociated at different stages of processing.

Perceptual Coupling Based on Depth and Motion Cues in Stereovision-Impaired Subjects

When an object is partially occluded, the different parts of the object have to be perceptually coupled. Cues that can be used for perceptual coupling are, for instance, depth ordering and visual motion information. In subjects with impaired stereovision, the brain is less able to use stereoscopic depth cues, making them more reliant on other cues. Therefore, our hypothesis is that stereovision-impaired subjects have stronger motion coupling than stereoscopic subjects. We compared perceptual coupling in 8 stereoscopic and 10 stereovision-impaired subjects, using random moving dot patterns that defined an ambiguous rotating cylinder and a coaxially presented nonambiguous half cylinder. Our results show that, whereas stereoscopic subjects exhibit significant coupling in the far plane, stereovision-impaired subjects show no coupling and under our conditions also no stronger motion coupling than stereoscopic subjects.

Changes in low-level neural properties underlie age-dependent visual decision making

Aging typically slows down cognitive processes, specifically those related to perceptual decisions. However, the neurobiological mechanisms underlying these age-associated changes are still elusive. To address this, we studied the effect of aging on both perceptual and binocular rivalry in various presentation conditions. Two age groups of participants reported their spontaneous percept switches during continuous presentation and percept choices during intermittent presentation. We find no significant age effect on the mean and cumulative frequencies of percept switch durations under continuous presentation. However, the data show a significant age effect on coefficient of variation, ratio of standard deviation to mean of percept durations. Our results also reveal that the alternation rate for percept choices significantly declines at an older age under intermittent presentation. The latter effect is even more pronounced at shorter inter-stimulus durations. These results together with the predictions of existing neural models for bistable perception imply that age-dependency of visual perceptual decisions is caused by shifts in neural adaptation and noise, not by a change in inhibition strength. Thus, variation in the low-level neural properties, adaptation and noise, cause age-dependent properties in visual perceptual decisions.

A recursive Bayesian updating model of haptic stiffness perception

Stiffness of many materials follows Hooke's Law, but the mechanism underlying the haptic perception of stiffness is not as simple as it seems in the physical definition. The present experiments support a model by which stiffness perception is adaptively updated during dynamic interaction. Participants actively explored virtual springs and estimated their stiffness relative to a reference. The stimuli were simulations of linear springs or nonlinear springs created by modulating a linear counterpart with low-amplitude, half-cycle (Experiment 1) or full-cycle (Experiment 2) sinusoidal force. Experiment 1 showed that subjective stiffness increased (decreased) as a linear spring was positively (negatively) modulated by a half-sinewave force. In Experiment 2, an opposite pattern was observed for full-sinewave modulations. Modeling showed that the results were best described by an adaptive process that sequentially and recursively updated an estimate of stiffness using the force and displacement information sampled over trajectory and time. (PsycINFO Database Record

The influence of action-effect anticipation on bistable perception: differences between onset rivalry and ambiguous motion

Perception is strongly shaped by the actions we perform. According to the theory of event coding, and forward models of motor control, goal-directed action preparation activates representations of desired effects. These expectations about the precise stimulus identity of one's action-outcomes (i.e. identity predictions) are thought to selectively influence perceptual processing of action-contingent effects. However, the existing evidence for such identity-prediction effects is scarce and mixed. Here, we developed a new paradigm to capture such effects and examined whether action-outcome predictions can bias the perception of binocular onset rivalry (Experiments 1a and 1b) and bistable motion (Experiment 2). Participants performed learning tasks in which they were exposed to action-outcome associations. On test trials, actions were followed by bistable stimuli that could be perceived as being either congruent or incongruent with the aforementioned associations (i.e. rivalrous oriented gratings in Experiments 1a and 1b and spheres with ambiguous rotation directions in Experiment 2). Across three experiments, we show that, whilst exposure to action-effect associations can bias the apparent motion direction of ambiguous spheres, it fails to influence perceptual selection of grating orientations in binocular onset rivalry. This pattern of results extends previous work on ambiguous motion by demonstrating that action-induced modulations do not generalize to all types of bistable percepts.

Consciousness reflected in the eyes

People with higher autistic traits display stronger fluctuations in pupil size when presented with an optical illusion.

Aging does not affect integration times for the perception of depth from motion parallax

To successfully navigate throughout the world, observers must rapidly recover depth information. One depth cue that is especially important for a moving observer is motion parallax. To perceive unambiguous depth from motion parallax, the visual system must integrate information from two different proximal signals, retinal image motion and a pursuit eye movement. Previous research has shown that aging affects both of these necessary components for motion parallax depth perception, but no research has yet investigated how aging affects the mechanism for integrating motion and pursuit information to recover depth from motion parallax. The goal of the current experiment was to assess the integration time required by older adults to process depth information. In four psychophysical conditions, younger and older observers made motion and depth judgments about stationary or translating random-dot stimuli. Stimulus presentations in all four psychophysical conditions were followed by a high-contrast pattern mask, and minimum stimulus presentation durations (stimulus-to-mask onset asynchrony, or SOA) were measured. These SOAs reflect the minimum neural processing time required to make motion and motion parallax depth judgments. Pursuit latency was also measured. The results revealed that, after accounting for age-related delays in motion processing and pursuit onset, older and younger adults required similar temporal intervals to combine retinal image motion with an internal pursuit signal for the perception of depth. These results suggest that the mechanism for motion and pursuit integration is not affected by age.

Motion of glossy objects does not promote separation of lighting and surface colour

The surface properties of an object, such as texture, glossiness or colour, provide important cues to its identity. However, the actual visual stimulus received by the eye is determined by both the properties of the object and the illumination. We tested whether operational colour constancy for glossy objects (the ability to distinguish changes in spectral reflectance of the object, from changes in the spectrum of the illumination) was affected by rotational motion of either the object or the light source. The different chromatic and geometric properties of the specular and diffuse reflections provide the basis for this discrimination, and we systematically varied specularity to control the available information. Observers viewed animations of isolated objects undergoing either lighting or surface-based spectral transformations accompanied by motion. By varying the axis of rotation, and surface patterning or geometry, we manipulated: (i) motion-related information about the scene, (ii) relative motion between the surface patterning and the specular reflection of the lighting, and (iii) image disruption caused by this motion. Despite large individual differences in performance with static stimuli, motion manipulations neither improved nor degraded performance. As motion significantly disrupts frame-by-frame low-level image statistics, we infer that operational constancy depends on a high-level scene interpretation, which is maintained in all conditions.

Bistability and Biasing Effects in the Perception of Ambiguous Point-Light Walkers

The perceptually bistable character of point-light walkers has been examined in three experiments. A point-light figure without explicit depth cues constitutes a perfectly ambiguous stimulus: from all viewpoints, multiple interpretations are possible concerning the depth orientation of the figure. In the first experiment, it is shown that non-lateral views of the walker are indeed interpreted in two orientations, either as facing towards the viewer or as facing away from the viewer, but that the interpretation in which the walker is oriented towards the viewer is reported more frequently. In the second experiment the point-light figure was walking backwards, making the global orientation of the point-light figure opposite to the direction of global motion. The interpretation in which the walker was facing the viewer was again reported more frequently. The robustness of these findings was examined in the final experiment, in which the effects of disambiguating the stimulus by introducing a local depth cue (occlusion) or a more global depth cue (applying perspective projection) were explored.

Dorsal and ventral stream contributions to form-from-motion perception in a patient with form-from motion deficit: a case report

The main model of visual processing in primates proposes an anatomo-functional distinction between the dorsal stream, specialized in spatio-temporal information, and the ventral stream, processing essentially form information. However, these two pathways also communicate to share much visual information. These dorso-ventral interactions have been studied using form-from-motion (FfM) stimuli, revealing that FfM perception first activates dorsal regions (e.g., MT+/V5), followed by successive activations of ventral regions (e.g., LOC). However, relatively little is known about the implications of focal brain damage of visual areas on these dorso-ventral interactions. In the present case report, we investigated the dynamics of dorsal and ventral activations related to FfM perception (using topographical ERP analysis and electrical source imaging) in a patient suffering from a deficit in FfM perception due to right extrastriate brain damage in the ventral stream. Despite the patient's FfM impairment, both successful (observed for the highest level of FfM signal) and absent/failed FfM perception evoked the same temporal sequence of three processing states observed previously in healthy subjects. During the first period, brain source localization revealed cortical activations along the dorsal stream, currently associated with preserved elementary motion processing. During the latter two periods, the patterns of activity differed from normal subjects: activations were observed in the ventral stream (as reported for normal subjects), but also in the dorsal pathway, with the strongest and most sustained activity localized in the parieto-occipital regions. On the other hand, absent/failed FfM perception was characterized by weaker brain activity, restricted to the more lateral regions. This study shows that in the present case report, successful FfM perception, while following the same temporal sequence of processing steps as in normal subjects, evoked different patterns of brain activity. By revealing a brain circuit involving the most rostral part of the dorsal pathway, this study provides further support for neuro-imaging studies and brain lesion investigations that have suggested the existence of different brain circuits associated with different profiles of interaction between the dorsal and the ventral streams.

The contribution of dynamic visual cues to audiovisual speech perception

Seeing a speaker's facial gestures can significantly improve speech comprehension, especially in noisy environments. However, the nature of the visual information from the speaker's facial movements that is relevant for this enhancement is still unclear. Like auditory speech signals, visual speech signals unfold over time and contain both dynamic configural information and luminance-defined local motion cues; two information sources that are thought to engage anatomically and functionally separate visual systems. Whereas, some past studies have highlighted the importance of local, luminance-defined motion cues in audiovisual speech perception, the contribution of dynamic configural information signalling changes in form over time has not yet been assessed. We therefore attempted to single out the contribution of dynamic configural information to audiovisual speech processing. To this aim, we measured word identification performance in noise using unimodal auditory stimuli, and with audiovisual stimuli. In the audiovisual condition, speaking faces were presented as point light displays achieved via motion capture of the original talker. Point light displays could be isoluminant, to minimise the contribution of effective luminance-defined local motion information, or with added luminance contrast, allowing the combined effect of dynamic configural cues and local motion cues. Audiovisual enhancement was found in both the isoluminant and contrast-based luminance conditions compared to an auditory-only condition, demonstrating, for the first time the specific contribution of dynamic configural cues to audiovisual speech improvement. These findings imply that globally processed changes in a speaker's facial shape contribute significantly towards the perception of articulatory gestures and the analysis of audiovisual speech.

Feature integration and object representations along the dorsal stream visual hierarchy

The visual system is split into two processing streams: a ventral stream that receives color and form information and a dorsal stream that receives motion information. Each stream processes that information hierarchically, with each stage building upon the previous. In the ventral stream this leads to the formation of object representations that ultimately allow for object recognition regardless of changes in the surrounding environment. In the dorsal stream, this hierarchical processing has classically been thought to lead to the computation of complex motion in three dimensions. However, there is evidence to suggest that there is integration of both dorsal and ventral stream information into motion computation processes, giving rise to intermediate object representations, which facilitate object selection and decision making mechanisms in the dorsal stream. First we review the hierarchical processing of motion along the dorsal stream and the building up of object representations along the ventral stream. Then we discuss recent work on the integration of ventral and dorsal stream features that lead to intermediate object representations in the dorsal stream. Finally we propose a framework describing how and at what stage different features are integrated into dorsal visual stream object representations. Determining the integration of features along the dorsal stream is necessary to understand not only how the dorsal stream builds up an object representation but also which computations are performed on object representations instead of local features.

Contributions of magno- and parvocellular channels to conscious and non-conscious vision

The dorsal and ventral cortical pathways, driven predominantly by magnocellular (M) and parvocellular (P) inputs, respectively, assume leading roles in models of visual information processing. Although in prior proposals, the dorsal and ventral pathways support non-conscious and conscious vision, respectively, recent modelling and empirical developments indicate that each pathway plays important roles in both non-conscious and conscious vision. In these models, the ventral P-pathway consists of one subpathway processing an object's contour features, e.g. curvature, the other processing its surface attributes, e.g. colour. Masked priming studies have shown that feed-forward activity in the ventral P-pathway on its own supports non-conscious processing of contour and surface features. The dorsal M-pathway activity contributes directly to conscious vision of motion and indirectly to object vision by projecting to prefrontal cortex, which in turn injects top-down neural activity into the ventral P-pathway and there 'ignites' feed-forward-re-entrant loops deemed necessary for conscious vision. Moreover, an object's shape or contour remains invisible without the prior conscious registration of its surface properties, which for that reason are taken to comprise fundamental visual qualia. Besides suggesting avenues for future research, these developments bear on several recent and past philosophical issues.

Rotating columns: relating structure-from-motion, accretion/deletion, and figure/ground

We present a novel phenomenon involving an interaction between accretion deletion, figure-ground interpretation, and structure-from-motion. Our displays contain alternating light and dark vertical regions in which random-dot textures moved horizontally at constant speed but in opposite directions in alternating regions. This motion is consistent with all the light regions in front, with the dark regions completing amodally into a single large surface moving in the background, or vice versa. Surprisingly, the regions that are perceived as figural are also perceived as 3-D volumes rotating in depth (like rotating columns)-despite the fact that dot motion is not consistent with 3-D rotation. In a series of experiments, we found we could manipulate which set of regions is perceived as rotating volumes simply by varying known geometric cues to figure ground, including convexity, parallelism, symmetry, and relative area. Subjects indicated which colored regions they perceived as rotating. For our displays we found convexity to be a stronger cue than either symmetry or parallelism. We furthermore found a smooth monotonic decay of the proportion by which subjects perceive symmetric regions as figural, as a function of their relative area. Our results reveal an intriguing new interaction between accretion-deletion, figure-ground, and 3-D motion that is not captured by existing models. They also provide an effective tool for measuring figure-ground perception.

A role of 3-D surface-from-motion cues in motion-induced blindness

Motion-induced blindness (MIB), the illusory disappearance of local targets against a moving mask, has been attributed to both low-level stimulus-based effects and high-level processes, involving selection between local and more global stimulus contexts. Prior work shows that MIB is modulated by binocular disparity-based depth-ordering cues. We assessed whether the depth effect is specific to disparity by studying how monocular 3-D surface from motion affects MIB. Monocular kinetic depth cues were used to create a global 3-D hourglass with concave and convex surfaces. MIB increased for stationary targets on the convex relative to the concave area, extending the role of 3-D cues. Interestingly, this convexity effect was limited to the left visual field--replicating spatial anisotropies in MIB. The data indicate a causal role of general 3-D surface coding in MIB, consistent with MIB being affected by high-level, visual representations.

Impaired processing of 3D motion-defined faces in mild cognitive impairment and healthy aging: an fMRI study

Mild cognitive impairment (MCI), which shows high risk for conversion to Alzheimer's disease (AD), is accompanied by progressive visual deteriorations that so far are poorly understood. Here, we compared dorsal and ventral visual stream functional magnetic resonance imaging (fMRI) activity among amnestic MCI, healthy elderly, and young participants during structure-from-motion (SFM) face categorization performance. Task performance varied with stimulus depth and duration levels and differences among groups were highly correlated with face-related fMRI activation patterns. Young participants showed larger activation to faces than scrambled faces (face sensitivity) in the right fusiform face area (FFA) and right occipital face area (OFA) whereas in elderly, this difference was reduced. Surprisingly, in MCI, scrambled faces elicited larger activation in right FFA/OFA than faces. The latter observation may be related to the additional finding of elevated depth sensitivity in left FFA/OFA of MCI, suggesting that an increased representation of low-level stimulus aspects may impair face perception in MCI. Discriminant function analysis using face and depth sensitivity indices in FFA/OFA classified MCI and healthy elderly with 88.2% accuracy, marking a fundamental distinction between groups. Potentially related findings include altered activation patterns in dorsal-ventral stream integration regions and attention-related networks of MCI patients. Our results highlight aberrant visual and additional potentially compensatory processes that identify dispositions of (preclinical) AD.

Integration time for the perception of depth from motion parallax

The perception of depth from relative motion is believed to be a slow process that "builds-up" over a period of observation. However, in the case of motion parallax, the potential accuracy of the depth estimate suffers as the observer translates during the viewing period. Our recent quantitative model for the perception of depth from motion parallax proposes that relative object depth (d) can be determined from retinal image motion (dθ/dt), pursuit eye movement (dα/dt), and fixation distance (f) by the formula: d/f≈dθ/dα. Given the model's dynamics, it is important to know the integration time required by the visual system to recover dα and dθ, and then estimate d. Knowing the minimum integration time reveals the incumbent error in this process. A depth-phase discrimination task was used to determine the time necessary to perceive depth-sign from motion parallax. Observers remained stationary and viewed a briefly translating random-dot motion parallax stimulus. Stimulus duration varied between trials. Fixation on the translating stimulus was monitored and enforced with an eye-tracker. The study found that relative depth discrimination can be performed with presentations as brief as 16.6 ms, with only two stimulus frames providing both retinal image motion and the stimulus window motion for pursuit (mean range=16.6-33.2 ms). This was found for conditions in which, prior to stimulus presentation, the eye was engaged in ongoing pursuit or the eye was stationary. A large high-contrast masking stimulus disrupted depth-discrimination for stimulus presentations less than 70-75 ms in both pursuit and stationary conditions. This interval might be linked to ocular-following response eye-movement latencies. We conclude that neural mechanisms serving depth from motion parallax generate a depth estimate much more quickly than previously believed. We propose that additional sluggishness might be due to the visual system's attempt to determine the maximum dθ/dα ratio for a selection of points on a complicated stimulus.

United we sense, divided we fail: context-driven perception of ambiguous visual stimuli

Ambiguous visual stimuli provide the brain with sensory information that contains conflicting evidence for multiple mutually exclusive interpretations. Two distinct aspects of the phenomenological experience associated with viewing ambiguous visual stimuli are the apparent stability of perception whenever one perceptual interpretation is dominant, and the instability of perception that causes perceptual dominance to alternate between perceptual interpretations upon extended viewing. This review summarizes several ways in which contextual information can help the brain resolve visual ambiguities and construct temporarily stable perceptual experiences. Temporal context through prior stimulation or internal brain states brought about by feedback from higher cortical processing levels may alter the response characteristics of specific neurons involved in rivalry resolution. Furthermore, spatial or crossmodal context may strengthen the neuronal representation of one of the possible perceptual interpretations and consequently bias the rivalry process towards it. We suggest that contextual influences on perceptual choices with ambiguous visual stimuli can be highly informative about the neuronal mechanisms of context-driven inference in the general processes of perceptual decision-making.

Dorsal-ventral integration in the recognition of motion-defined unfamiliar faces

The primate visual system is organized into two parallel anatomical pathways, both originating in early visual areas but terminating in posterior parietal or inferior temporal regions. Classically, these two pathways have been thought to subserve spatial vision and visual guided actions (dorsal pathway) and object identification (ventral pathway). However, evidence is accumulating that dorsal visual areas may also represent many aspects of object shape in absence of demands for attention or action. Dorsal visual areas exhibit selectivity for three-dimensional cues of depth and are considered necessary for the extraction of surfaces from depth cues and can carry out cognitive functions with such cues as well. These results suggest that dorsal visual areas may participate in object recognition, but it is unclear to what capacity. Here, we tested whether three-dimensional structure-from-motion (SFM) cues, thought to be computed exclusively by dorsal stream mechanisms, are sufficient to drive complex object recognition. We then tested whether recognition of such stimuli relies on dorsal stream mechanisms alone, or whether dorsal-ventral integration is invoked. Results suggest that such cues are sufficient to drive unfamiliar face recognition in normal participants and that ventral stream areas are necessary for both identification and learning of unfamiliar faces from SFM cues.

Stimulus-contrast-induced biases in activation order reveal interaction between V1/V2 and human MT+

The luminance contrast of a visual stimulus is known to modulate the response properties of areas V1 and the human MT complex (hMT+), but has not been shown to modulate interactions between these two areas. We examined the direction of information transfer between V1/V2 and hMT+ at different stimulus contrasts by measuring magnetoencephalographic (MEG) responses to moving and stationary stimuli presented centrally or peripherally. To determine the direction of information flow, the different response latencies among stimuli and hemispheres in V1/V2 was compared with those of hMT+. At high contrast, responses to stimulus motion and position began in V1/V2, and were followed in hMT+ with a delay between 34 and 55 ms. However, at low contrast, lateralized responses in hMT+ came first, with those in V1/V2 lagging with a delay of 27 ms. Also, at high contrast, stationary stimuli produced greater responses than motion stimuli in V1/V2, while the reverse was true in hMT+, whose response lagged behind the initial response in V1/V2. The same activation order was found using Mutual Information Analysis of the response variances for each condition. Here, the response variances in hMT+ mimicked and trailed those of V1/V2 at high contrast, whereas the reverse was true at low contrast. Such consistent interactions found using two different methodologies strongly supports a processing link between these two areas. The results also suggest that feedback from hMT+ for low-contrast stimuli compensates for unresolved processing in V1/V2 when the input of a visual image is weak.

The hierarchy of directional interactions in visual motion processing

It is well known that context influences our perception of visual motion direction. For example, spatial and temporal context manipulations can be used to induce two well-known motion illusions: direction repulsion and the direction after-effect (DAE). Both result in inaccurate perception of direction when a moving pattern is either superimposed on (direction repulsion), or presented following adaptation to (DAE), another pattern moving in a different direction. Remarkable similarities in tuning characteristics suggest that common processes underlie the two illusions. What is not clear, however, is whether the processes driving the two illusions are expressions of the same or different neural substrates. Here we report two experiments demonstrating that direction repulsion and the DAE are, in fact, expressions of different neural substrates. Our strategy was to use each of the illusions to create a distorted perceptual representation upon which the mechanisms generating the other illusion could potentially operate. We found that the processes mediating direction repulsion did indeed access the distorted perceptual representation induced by the DAE. Conversely, the DAE was unaffected by direction repulsion. Thus parallels in perceptual phenomenology do not necessarily imply common neural substrates. Our results also demonstrate that the neural processes driving the DAE occur at an earlier stage of motion processing than those underlying direction repulsion.

General validity of Levelt's propositions reveals common computational mechanisms for visual rivalry

The mechanisms underlying conscious visual perception are often studied with either binocular rivalry or perceptual rivalry stimuli. Despite existing research into both types of rivalry, it remains unclear to what extent their underlying mechanisms involve common computational rules. Computational models of binocular rivalry mechanisms are generally tested against Levelt's four propositions, describing the psychophysical relation between stimulus strength and alternation dynamics in binocular rivalry. Here we use a bistable rotating structure-from-motion sphere, a generally studied form of perceptual rivalry, to demonstrate that Levelt's propositions also apply to the alternation dynamics of perceptual rivalry. Importantly, these findings suggest that bistability in structure-from-motion results from active cross-inhibition between neural populations with computational principles similar to those present in binocular rivalry. Thus, although the neural input to the computational mechanism of rivalry may stem from different cortical neurons and different cognitive levels the computational principles just prior to the production of visual awareness appear to be common to the two types of rivalry.

Early interactions between neuronal adaptation and voluntary control determine perceptual choices in bistable vision

At the onset of bistable stimuli, the brain needs to choose which of the competing perceptual interpretations will first reach awareness. Stimulus manipulations and cognitive control both influence this choice process, but the underlying mechanisms and interactions remain poorly understood. Using intermittent presentation of bistable visual stimuli, we demonstrate that short interruptions cause perceptual reversals upon the next presentation, whereas longer interstimulus intervals stabilize the percept. Top-down voluntary control biases this process but does not override the timing dependencies. Extending a recently introduced low-level neural model, we demonstrate that percept-choice dynamics in bistable vision can be fully understood with interactions in early neural processing stages. Our model includes adaptive neural processing preceding a rivalry resolution stage with cross-inhibition, adaptation, and an interaction of the adaptation levels with a neural baseline. Most importantly, our findings suggest that top-down attentional control over bistable stimuli interacts with low-level mechanisms at early levels of sensory processing before perceptual conflicts are resolved and perceptual choices about bistable stimuli are made.

Visual cortex allows prediction of perceptual states during ambiguous structure-from-motion

We investigated the role of retinotopic visual cortex and motion-sensitive areas in representing the content of visual awareness during ambiguous structure-from-motion (SFM), using functional magnetic resonance imaging (fMRI) and multivariate statistics (support vector machines). Our results indicate that prediction of perceptual states can be very accurate for data taken from dorsal visual areas V3A, V4D, V7, and MT+ and for parietal areas responsive to SFM, but to a lesser extent for other visual areas. Generalization of prediction was possible, because prediction accuracy was significantly better than chance for both an unambiguous stimulus and a different experimental design. Detailed analysis of eye movements revealed that strategic and even encouraged beneficial eye movements were not the cause of the prediction accuracy based on cortical activation. We conclude that during perceptual rivalry, neural correlates of visual awareness can be found in retinotopic visual cortex, MT+, and parietal cortex. We argue that the organization of specific motion-sensitive neurons creates detectable biases in the preferred direction selectivity of voxels, allowing prediction of perceptual states. During perceptual rivalry, retinotopic visual cortex, in particular higher-tier dorsal areas like V3A and V7, actively represents the content the visual awareness.

Endogenous influences on perceptual bistability depend on exogenous stimulus characteristics

We investigated the influence of changing physical parameters and task on bistable perception of an ambiguously rotating sphere (SFM). Increasing dot-density and velocity decreased the duration of perceptual phases during both passive viewing and voluntary control exertion. Our main finding is that voluntary control of perception depends on the physical parameters constituting the stimulus. This dependency places important constraints on the mechanisms mediating voluntary control as these mechanisms cannot operate independently of stimulus characteristics. In addition, local asymmetries in dot-densities can trigger alternations towards the most salient direction, which is not necessarily associated with largest number of dots: competition between perceptual interpretations during SFM appears to occur between surface-based representations rather than between individual elements. Finally, we show that voluntary control remains effective, even when attentive tracking of individual stimulus elements is no longer possible.

Subjective appearance of ambiguous structure-from-motion can be driven by objective switches of a separate less ambiguous context

Two ambiguous transparent structure-from-motion (SFM) stimuli often appear to co-rotate. Grossmann & Dobbins (2003) reported breakdown of such perceptual coupling when one stimulus was made unambiguous (by rendering it opaque), leading them to propose that coupling depends generally on differential stimulus ambiguity. In contrast, we demonstrate robust stimulus-driven coupling even when one SFM stimulus is relatively disambiguated, by using relative-luminance and/or binocular-disparity cues. Such context stimuli could induce stimulus-driven coupling by disambiguating the transparent stimulus, though critically only when the context was clearly non-opaque and coaxial with the ambiguous stimulus. This demonstrates long-range information-sharing between separate stimulus representations, subject to specific constraints.

Perceiving depth in point-light actions

The present study investigates how observers assign depth in point-light figures, by manipulating spatiotemporal characteristics of the stimuli. Previous research on the perception of point-light walkers revealed bistability (i.e., that a point-light walker is perceived as either facing the viewer or facing away from the viewer) and the presence of a perceptual bias (i.e., a tendency to perceive the figure as facing the viewer). Here, we study the generality of these phenomena by having observers indicate the global depth orientation of different ambiguous point-light actions. Results demonstrate bistability for all actions, but the presence of a preferred interpretation depends strongly on the performed action, showing that the process of depth assignment takes into account the movements the point-light figure performs. Two additional experiments, using unfamiliar movement patterns without strong semantic correlates, show that purely kinematic aspects of a naction also strongly affect d epth assignment. Together, the results reveal the perception of depth in point-light figures to be a flexible processinvolving both bottom-up and top-down components.

Local factors determine the stabilization of monocular ambiguous and binocular rivalry stimuli

Perceptual alternation in viewing bistable stimuli can be slowed or halted if the stimuli are presented intermittently. Memory of the recent perceptual experience has been proposed to explain this stabilization effect. But the nature of this "perceptual memory" remains unclear. By using a bistable rotating cylinder and two dichoptically presented orthogonal gratings, we explored the features that are important for the stabilization by changing a particular feature of the stimuli between alternate presentations. For the rotating cylinder, changing its color, rotating speed, size, or its stereo depth had no or minimal effect on the stabilization of its perceived rotation direction. For binocular rivalry, when the two gratings were matched in strength and then swapped between the two eyes synchronously with the intermittent presentation, the percepts were usually stabilized to one eye. In both cases, perceptual stabilization occurred only if the stimuli were presented to the same retinal location. These results suggest that the stabilization of monocular bistable stimuli is likely due to the removal of local adaptation, insensitive to the features that define the object identity. For binocular rivalry, preservation of the direction of interocular suppression rather than memory of the stimulus identity accounts for the stabilization effect.

Stabilized Structure from Motion without Disparity Induces Disparity Adaptation

3D structures can be perceived based on the patterns of 2D motion signals. With orthographic projection of a 3D stimulus onto a 2D plane, the kinetic information can give a vivid impression of depth, but the depth order is intrinsically ambiguous, resulting in bistable or even multistable interpretations. For example, an orthographic projection of dots on the surface of a rotating cylinder is perceived as a rotating cylinder with ambiguous direction of rotation. We show that the bistable rotation can be stabilized by adding information, not to the dots themselves, but to their spatial context. More interestingly, the stabilized bistable motion can generate consistent rotation aftereffects. The rotation aftereffect can only be observed when the adapting and test stimuli are presented at the same stereo depth and the same retinal location, and it is not due to attentional tracking. The observed rotation aftereffect is likely due to direction-contingent disparity adaptation, implying that stimuli with kinetic depth may have activated neurons sensitive to different disparities, even though the stimuli have zero relative disparity. Stereo depth and kinetic depth may be supported by a common neural mechanism at an early stage in the visual system.

The role of occlusion in the perception of depth, lightness, and opacity

A theory is presented that explains how the visual system infers the lightness, opacity, and depth of surfaces from stereoscopic images. It is shown that the polarity and magnitude of image contrast play distinct roles in surface perception, which can be captured by 2 principles of perceptual inference. First, a contrast depth asymmetry principle articulates how the visual system computes the ordinal depth and lightness relationships from the polarity of local, binocularly matched image contrast. Second, a global transmittance anchoring principle expresses how variations in contrast magnitudes are used to infer the presence of transparent surfaces. It is argued that these principles provide a unified explanation of how the visual system computes the 3-D surface structure of opaque and transparent surfaces.

Apparent motion cues distort object localisation in egocentric space

The visual localisation of objects in space is thought to rely on retinal information defining the environmental context and non-retinal cues from proprioception and motor commands. Here, the influence of dynamic contextual cues on the perception of egocentric space in a reaching task was investigated. Compared to performances with realistic motion or static cues, target localisation was less accurate when apparent motion was used to provide contextual information about space between the hand and the target. This effect could not be explained by the 'presence' of motion, or a bias in depth perception. Since the distortion was connected with the reaching area it was concluded that cognitive factors can unconsciously influence the perception of egocentric space, in particular distance estimation. We propose a mechanism for this whereby signals from areas MT/MST (middle temporal/medial superior temporal) create a perceptual bias through cortico-cortical connections with posterior parietal cortex.

Human cortical object recognition from a visual motion flowfield

Moving dots can evoke a percept of the spatial structure of a three-dimensional object in the absence of other visual cues. This phenomenon, called structure from motion (SFM), suggests that the motion flowfield represented in the dorsal stream can form the basis of object recognition performed in the ventral stream. SFM processing is likely to contribute to object perception whenever there is relative motion between the observer and the object viewed. Here we investigate the motion flowfield component of object recognition with functional magnetic resonance imaging. Our SFM stimuli encoded face surfaces and random three-dimensional control shapes with matched curvature properties. We used two different types of an SFM stimulus with the dots either fixed to the surface of the object or moving on it. Despite the radically different encoding of surface structure in the two types of SFM, both elicited strong surface percepts and involved the same network of cortical regions. From early visual areas, this network extends dorsally into the human motion complex and parietal regions and ventrally into object-related cortex. The SFM stimuli elicited a face-selective response in the fusiform face area. The human motion complex appears to have a central role in SFM object recognition, not merely representing the motion flowfield but also the surface structure of the motion-defined object. The motion complex and a region in the intraparietal sulcus reflected the motion state of the SFM-implicit object, responding more strongly when the implicit object was in motion than when it was stationary.

Does perception of biological motion rely on specific brain regions?

Perception of biological motions plays a major adaptive role in identifying, interpreting, and predicting the actions of others. It may therefore be hypothesized that the perception of biological motions is subserved by a specific neural network. Here we used fMRI to verify this hypothesis. In a group of 10 healthy volunteers, we explored the hemodynamic responses to seven types of visual motion displays: drifting random dots, random dot cube, random dot cube with masking elements, upright point-light walker, inverted point-light walker, upright point-light walker display with masking elements, and inverted point-light walker display with masking elements. A gradient in activation was observed in the occipitotemporal junction. The responses to rigid motion were localized posteriorly to those responses elicited by nonrigid motions. Our results demonstrate that in addition to the posterior portion of superior temporal sulcus, the left intraparietal cortex is involved in the perception of nonrigid biological motions.

Attentional modulation of visual motion perception

How is the perception and processing of visual motion affected by attention? This review examines recent research in cognition, perception and neurophysiology that explores how ongoing behavioural tasks (and the attentional states they impose) modulate the processing of visual motion. Although traditional views hold that motion is processed in an obligatory, 'pre-attentive' manner, evidence for processing in a task-independent manner is scant. Recent studies of human perception that have measured motion priming, motion aftereffects, uncertainty effects, and motion-interaction effects indicate instead that even simple aspects of motion processing may be substantially affected by whether motion information in a task is used or ignored by the perceiver. Single-unit studies in brain areas sensitive to visual motion in monkeys, and functional imaging studies on humans, also indicate that task and attentional state affect activity levels in brain regions thought to be important in motion perception. This review brings together these converging findings of attentional modulation of motion perception and considers them in light of object-oriented theories of attention.