Human MT+ mediates perceptual filling-in during apparent motion

Gestalt neurons and emergent properties in visual perception: A novel concept for the transformation from local to global processing

Gestalten in visual perception are defined by emergent properties of the whole, which cannot be predicted from the sum of its parts; rather, they arise by virtue of inherent principles, the Laws of Seeing. This review attempts to assign neurophysiological correlates to select emergent properties in motion and contour perception and proposes parallels to the processing of local versus global attributes by classical versus contextual receptive fields. The aim is to identify Gestalt neurons in the visual system to account for the Laws of Seeing in causal terms and to explain "Why do things look as they do" (Koffka, 1935, p. 76).

Radial bias alters high-level motion perception

The visual system involves various orientation and visual field anisotropies, one of which is a preference for radial orientations and motion directions. By radial, we mean those directions coursing symmetrically outward from the fovea into the periphery. This bias stems from anatomical and physiological substrates in the early visual system. We recently reported that this low-level visual anisotropy can alter perceived object orientation. Here, we report that radial bias can also alter another higher-level system, the perceived direction of apparent motion. We presented a bistable apparent motion quartet in the center of the screen while participants fixated on various locations around the quartet. Participants (N = 22) were strongly biased to see the motion direction that was radial with respect to their fixation, controlling for any biases with center fixation. This was observed using a vertical-horizontal quartet as well as an oblique quartet (45° rotated quartet). The latter allowed us to rule out the contribution of the hemisphere effect where motion across the midline is perceived less often. These results extend our earlier findings on perceived object orientation, showing that low-level structural aspects of the visual system alter yet another higher-level visual process, that of apparent motion perception.

Local motion signals silence the perceptual solution of global apparent motion

Stimuli for apparent motion can have ambiguity in frame-to-frame correspondences among visual elements. This occurs when visual inputs cause a correspondence problem that allows multiple alternatives of perceptual solutions. Herein we examined the influence of local visual motions on a perceptual solution under such a multistable situation. We repeatedly alternated two frames of stimuli in a circular configuration in which discrete elements in two different colors alternated in space and switched their colors frame by frame. These stimuli were compatible with three perceptual solutions: globally consistent clockwise and counterclockwise rotations and color flickers at the same locations without such global apparent motion. We added a sinusoidal grating continuously drifting within each element to examine whether the perceptual solution for the global apparent motion was affected by the local continuous motions. We found that the local motions suppressed global apparent motion and promoted another perceptual solution that the local elements were only flickering between the two colors and drifting within static windows. It was concluded that local continuous motions as counterevidence against global apparent motion contributed to individuating visual objects and integrating visual features for maintaining object identity at the same location.

Do you hear what I see? How do early blind individuals experience object motion?

One of the most important tasks for 3D vision is tracking the movement of objects in space. The ability of early blind individuals to understand motion in the environment from noisy and unreliable auditory information is an impressive example of cortical adaptation that is only just beginning to be understood. Here, we compare visual and auditory motion processing, and discuss the effect of early blindness on the perception of auditory motion. Blindness leads to cross-modal recruitment of the visual motion area hMT+ for auditory motion processing. Meanwhile, the planum temporale, associated with auditory motion in sighted individuals, shows reduced selectivity for auditory motion. We discuss how this dramatic shift in the cortical basis of motion processing might influence the perceptual experience of motion in early blind individuals. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

V5/MT+ modulates spatio-temporal integration differently across and within hemifields: Causal evidence from TMS

It is unclear how the brain reaches the correct balance between temporal and spatial processing necessary to perceive motion across space. Here, we tested whether visual motion area V5/MT + plays a causal role in Ternus illusion. Ternus displays can be perceived as showing either group motion or element motion and are empirically useful for dissociating temporal and spatial grouping across visual fields. Online single-pulse TMS was applied to observers during the presentation of Ternus displays, either within or across hemifields, over left V5/MT + or, respectively, a control site in the left somatosensory cortex, or an additional 'Sham' control condition. In the cross-hemifields condition, observers perceived more element motion with TMS over left V5/MT + than in either control condition. By contrast, in the within-hemifield condition, observers reported more group motion after left V5/MT + TMS. Our findings demonstrate a causal role of left V5/MT+ in the spatio-temporal grouping of Ternus apparent motion, and in maintaining the balance of spatio-temporal processing both within and across individual hemifields.

Neural Basis of Biased Competition in Development: Sensory Competition in Visual Cortex of School-Aged Children

The fundamental receptive field (RF) architecture in human visual cortex becomes adult-like by age 5. However, visuo-spatial functions continue to develop until teenage years. This suggests that, despite the early maturation of the RF structure, functional interactions within and across RFs may mature slowly. Here, we used fMRI to investigate functional interactions among multiple stimuli in the visual cortex of school children (ages 8 to 12) in the context of biased competition theory. In the adult visual system, multiple objects presented in the same visual field compete for neural representation. These competitive interactions occur at the level of the RF and are therefore closely linked to the RF architecture. Like in adults, we found suppression of evoked responses in children's visual cortex when multiple stimuli were presented simultaneously. Such suppression effects were modulated by the spatial distance between the stimuli as a function of RF size across the visual system. Our findings suggest that basic competitive interactions in the visual cortex of children above age 8 operate in an adult-like manner, with subtle differences in early visual areas and area MT. Our study establishes a paradigm and provides baseline data to investigate the neural basis of visuo-spatial processing in typical and atypical development.

Apparent Motion Induces Activity Suppression in Early Visual Cortex and Impairs Visual Detection

Apparent motion (AM) is induced when two stationary visual stimuli are presented in alternating sequence. Intriguingly, AM leads to an impaired detectability of stimuli along the AM path (i.e., AM-induced masking). It has been hypothesized that AM triggers an internal representation of a moving object in early visual cortex, which competes with stimulus-evoked representations of visual stimuli on the motion path in early visual cortex of 25 human adults (16 female). We tested this hypothesis by measuring BOLD responses in early visual cortex during the process of AM-induced masking, using fMRI and population receptive field methods. Surprisingly, and counter to our hypothesis, we showed that AM suppressed, rather than increased, BOLD responses along early visual (V1 and V2) representations of the AM path, including regions that were not directly activated by the AM inducer stimuli. This activity suppression of the visual response predicted the subsequent reduction in detectability of the target that appeared in the middle of the AM path. Our data thereby provide direct empirical evidence for suppressive neural mechanisms underlying AM and suggest that illusory motion can render us blind to objects on the motion path by suppressing neural activity at the earliest cortical stages of visual perception.SIGNIFICANCE STATEMENT When two spatially distinct visual objects are presented in alternating sequence, apparent motion (AM) occurs and impairs detectability of stimuli along its path. The underlying mechanism is thought to be that increased activation in human early visual cortex evoked by AM interferes with the representation of the stimulus. Strikingly, however, we show that AM suppresses neural activity along the motion path, and the strength of activity suppression predicts the subsequent behavioral performance decrement in terms of detecting a stimulus along the AM path. Our findings provide empirical evidence for a suppressive, rather than faciliatory, mechanism underlying AM.

Neural responses to apparent motion can be predicted by responses to non-moving stimuli

When two objects are presented in alternation at two locations, they are seen as a single object moving from one location to the other. This apparent motion (AM) percept is experienced for objects located at short and also at long distances. However, current models cannot explain how the brain integrates information over large distances to create such long-range AM. This study investigates the neural markers of AM by parcelling out the contribution of spatial and temporal interactions not specific to motion. In two experiments, participants’ EEG was recorded while they viewed two stimuli inducing AM. Different combinations of these stimuli were also shown in a static context to predict an AM neural response where no motion is perceived. We compared the goodness of fit between these different predictions and found consistent results in both experiments. At short-range, the addition of the inhibitory spatial and temporal interactions not specific to motion improved the AM prediction. However, there was no indication that spatial or temporal non-linear interactions were present at long-range. This suggests that short- and long-range AM rely on different neural mechanisms. Importantly, our results also show that at both short- and long-range, responses generated by a moving stimulus could be well predicted from conditions in which no motion is perceived. That is, the EEG response to a moving stimulus is simply a combination of individual responses to non-moving stimuli. This demonstrates a dissociation between the brain response and the subjective percept of motion.

•

EEG responses are inhibited by spatial and temporal stimulus interactions.

•

These interactions are important for motion at short but not at long distances.

•

We find no trace of a specific neural signature of motion perception.

•

Neural responses to motion are well predicted by responses to non-moving stimuli.

Neural mechanisms underlying sound-induced visual motion perception: An fMRI study

Studies of crossmodal interactions in motion perception have reported activation in several brain areas, including those related to motion processing and/or sensory association, in response to multimodal (e.g., visual and auditory) stimuli that were both in motion. Recent studies have demonstrated that sounds can trigger illusory visual apparent motion to static visual stimuli (sound-induced visual motion: SIVM): A visual stimulus blinking at a fixed location is perceived to be moving laterally when an alternating left-right sound is also present. Here, we investigated brain activity related to the perception of SIVM using a 7T functional magnetic resonance imaging technique. Specifically, we focused on the patterns of neural activities in SIVM and visually induced visual apparent motion (VIVM). We observed shared activations in the middle occipital area (V5/hMT), which is thought to be involved in visual motion processing, for SIVM and VIVM. Moreover, as compared to VIVM, SIVM resulted in greater activation in the superior temporal area and dominant functional connectivity between the V5/hMT area and the areas related to auditory and crossmodal motion processing. These findings indicate that similar but partially different neural mechanisms could be involved in auditory-induced and visually-induced motion perception, and neural signals in auditory, visual, and, crossmodal motion processing areas closely and directly interact in the perception of SIVM.

Stationary Objects Flashed Periodically Appear to Move During Smooth Pursuit Eye Movement

We discovered that a white disc flashed twice at the same location appears to move during smooth pursuit eye tracking in the direction opposite to that of the eye movement. We called this novel phenomenon movement-induced apparent motion (MIAM). Using the method of constant stimuli, we measured the required displacement of the second appearance of the disc in the pursuit direction to null the effect during the closed-loop stage of smooth pursuit eye tracking. We observed a strong linear relationship between the points of subjective stationarity and the inter-stimuli intervals for four smooth pursuit eye movement speeds. The slopes and y-intercepts of these linear fits were well predicted by the hypothesis according to which subjects saw illusory motion from the first to the second retinal projections of the flashed disc during smooth pursuit eye movement, with no extra-retinal signal compensation.

Pivotal role of hMT+ in long-range disambiguation of interhemispheric bistable surface motion

It remains an open question whether long-range disambiguation of ambiguous surface motion can be achieved in early visual cortex or instead in higher level regions, which concerns object/surface segmentation/integration mechanisms. We used a bistable moving stimulus that can be perceived as a pattern comprehending both visual hemi-fields moving coherently downward or as two widely segregated nonoverlapping component objects (in each visual hemi-field) moving separately inward. This paradigm requires long-range integration across the vertical meridian leading to interhemispheric binding. Our fMRI study (n = 30) revealed a close relation between activity in hMT+ and perceptual switches involving interhemispheric segregation/integration of motion signals, crucially under nonlocal conditions where components do not overlap and belong to distinct hemispheres. Higher signal changes were found in hMT+ in response to spatially segregated component (incoherent) percepts than to pattern (coherent) percepts. This did not occur in early visual cortex, unlike apparent motion, which does not entail surface segmentation. We also identified a role for top-down mechanisms in state transitions. Deconvolution analysis of switch-related changes revealed prefrontal, insula, and cingulate areas, with the right superior parietal lobule (SPL) being particularly involved. We observed that directed influences could emerge either from left or right hMT+ during bistable motion integration/segregation. SPL also exhibited significant directed functional connectivity with hMT+, during perceptual state maintenance (Granger causality analysis). Our results suggest that long-range interhemispheric binding of ambiguous motion representations mainly reflect bottom-up processes from hMT+ during perceptual state maintenance. In contrast, state transitions maybe influenced by high-level regions such as the SPL. Hum Brain Mapp 38:4882-4897, 2017. © 2017 Wiley Periodicals, Inc.

Time-compressed preplay of anticipated events in human primary visual cortex

Perception is guided by the anticipation of future events. It has been hypothesized that this process may be implemented by pattern completion in early visual cortex, in which a stimulus sequence is recreated after only a subset of the visual input is provided. Here we test this hypothesis using ultra-fast functional magnetic resonance imaging to measure BOLD activity at precisely defined receptive field locations in visual cortex (V1) of human volunteers. We find that after familiarizing subjects with a spatial sequence, flashing only the starting point of the sequence triggers an activity wave in V1 that resembles the full stimulus sequence. This preplay activity is temporally compressed compared to the actual stimulus sequence and remains present even when attention is diverted from the stimulus sequence. Preplay might therefore constitute an automatic prediction mechanism for temporal sequences in V1.

Perception is guided by anticipating future events, but it is not clear how this is computed neurally. Here, the authors use ultra-fast fMRI to show that humans preplay anticipated visual sequences in the primary visual cortex and that this preplay correlates with faster detection of the stimuli.

Apparent Motion Suppresses Responses in Early Visual Cortex: A Population Code Model

Two stimuli alternately presented at different locations can evoke a percept of a stimulus continuously moving between the two locations. The neural mechanism underlying this apparent motion (AM) is thought to be increased activation of primary visual cortex (V1) neurons tuned to locations along the AM path, although evidence remains inconclusive. AM masking, which refers to the reduced detectability of stimuli along the AM path, has been taken as evidence for AM-related V1 activation. AM-induced neural responses are thought to interfere with responses to physical stimuli along the path and as such impair the perception of these stimuli. However, AM masking can also be explained by predictive coding models, predicting that responses to stimuli presented on the AM path are suppressed when they match the spatio-temporal prediction of a stimulus moving along the path. In the present study, we find that AM has a distinct effect on the detection of target gratings, limiting the maximum performance at high contrast levels. This masking is strongest when the target orientation is identical to the orientation of the inducers. We developed a V1-like population code model of early visual processing, based on a standard contrast normalization model. We find that AM-related activation in early visual cortex is too small to either cause masking or to be perceived as motion. Our model instead predicts strong suppression of early sensory responses during AM, consistent with the theoretical framework of predictive coding.

Two spatially separate stimuli presented in rapid succession often induce the illusory perception of a moving stimulus (apparent motion or AM). Its underlying mechanism is thought to be increased activation in primary visual cortex representing the motion path. Indirect evidence for this account comes from the reduced detectability of stimuli presented along the motion path (AM masking). Here, we developed a computational model of AM-related effects on visual processing in early visual cortex, which predicted a neural activation that is too small to either account for the observed masking or the perception of motion. Instead, our model predicts strong suppression of neural responses to stimuli presented along the motion path, especially when they match the spatio-temporal prediction of a stimulus moving along the path. Our findings support predictive coding models of visual processing, in which higher-level predictions about motion explain away lower-level responses to expected sensory input.

Decoding information about dynamically occluded objects in visual cortex

During dynamic occlusion, an object passes behind an occluding surface and then later reappears. Even when completely occluded from view, such objects are experienced as continuing to exist or persist behind the occluder even though they are no longer visible. The contents and neural basis of this persistent representation remain poorly understood. Questions remain as to whether there is information maintained about the object itself (i.e. its shape or identity) or non-object-specific information such as its position or velocity as it is tracked behind an occluder, as well as which areas of visual cortex represent such information. Recent studies have found that early visual cortex is activated by "invisible" objects during visual imagery and by unstimulated regions along the path of apparent motion, suggesting that some properties of dynamically occluded objects may also be neurally represented in early visual cortex. We applied functional magnetic resonance imaging in human subjects to examine representations within visual cortex during dynamic occlusion. For gradually occluded, but not for instantly disappearing objects, there was an increase in activity in early visual cortex (V1, V2, and V3). This activity was spatially-specific, corresponding to the occluded location in the visual field. However, the activity did not encode enough information about object identity to discriminate between different kinds of occluded objects (circles vs. stars) using MVPA. In contrast, object identity could be decoded in spatially-specific subregions of higher-order, topographically organized areas such as ventral, lateral, and temporal occipital areas (VO, LO, and TO) as well as the functionally defined LOC and hMT+. These results suggest that early visual cortex may only represent the dynamically occluded object's position or motion path, while later visual areas represent object-specific information.

Retinotopic encoding of the Ternus-Pikler display reflected in the early visual areas

The visual representation of the world is often assumed to be retinotopic, and many visual brain areas are indeed organized retinotopically. Visual perception, however, is not based on a reference frame anchored in retinotopic coordinates. For example, when an object moves, motion of its constituent parts is perceived relative to the object rather than in retinotopic coordinates. The moving object thus serves as a nonretinotopic reference system for computing the properties of its parts. It is largely unknown how the brain accomplishes this feat. Here, we used the Ternus-Pikler display to pit retinotopic processing in a stationary reference system against nonretinotopic processing in a moving one. Using 7T fMRI, we found that the average blood-oxygen-level dependent activations in V1, V2, and V3 reflected the retinotopic properties, but not the nonretinotopic percepts, of the Ternus-Pikler display. In the human motion processing complex (hMT+), activations were compatible with both retinotopic and nonretinotopic encoding. Thus, hMT+ may be the first visual area encoding the nonretinotopic percepts of the Ternus-Pikler display.

Perceptual Experience of Visual Motion Activates hMT+ Independently From the Physical Reality: fMRI Insights From the Looming Pinna Figure

The human motion processing area, hMT+, has been labeled the critical neural area for processing of real and illusory visual motion in radial 2D patterns. However, the activation in hMT+ during perception of illusory rotation in the looming double-circular Pinna Figure (PF) generated in 3D space has not been observed yet. To do so, an optic-flow like motion of rings (looming) in PF was generated on a computer screen. A psychophysically precise nulling procedure allowed quantifying the individual amount of the perceived illusory rotation in PF (PI) for each participant. The interpolation of the individual illusory motion parameters created a subjectively non-rotating PF and a physically rotating control stimulus of identical rotary strength as the PI. The physically rotating control was a double-circular figure which diverged from PF only in its arrangement of luminance gradients. In a 3-Tesla scanner, participants were presented with a random order of rotating and non-rotating figures (illusory, real, no rotation, and nulled PI). Both types, illusory and real rotation, when equal in perceptual strength for the observer, were found to be processed by hMT+.

Reconstructing representations of dynamic visual objects in early visual cortex

As raw sensory data are partial, our visual system extensively fills in missing details, creating enriched percepts based on incomplete bottom-up information. Despite evidence for internally generated representations at early stages of cortical processing, it is not known whether these representations include missing information of dynamically transforming objects. Long-range apparent motion (AM) provides a unique test case because objects in AM can undergo changes both in position and in features. Using fMRI and encoding methods, we found that the "intermediate" orientation of an apparently rotating grating, never presented in the retinal input but interpolated during AM, is reconstructed in population-level, feature-selective tuning responses in the region of early visual cortex (V1) that corresponds to the retinotopic location of the AM path. This neural representation is absent when AM inducers are presented simultaneously and when AM is visually imagined. Our results demonstrate dynamic filling-in in V1 for object features that are interpolated during kinetic transformations.

EEG Correlates of Relative Motion Encoding

A large portion of the visual cortex is organized retinotopically, but perception is usually non-retinotopic. For example, a reflector on the spoke of a bicycle wheel appears to move on a circular or prolate cycloidal orbit as the bicycle moves forward, while in fact it traces out a curtate cycloidal trajectory. The moving bicycle serves as a non-retinotopic reference system to which the motion of the reflector is anchored. To study the neural correlates of non-retinotopic motion processing, we used the Ternus-Pikler display, where retinotopic processing in a stationary reference system is contrasted against non-retinotopic processing in a moving one. Using high-density EEG, we found similar brain responses for both retinotopic and non-retinotopic rotational apparent motion from the earliest evoked peak (around 120 ms) and throughout the rest of the visual processing, but only minor correlates of the motion of the reference system itself (mainly around 100-120 ms). We suggest that the visual system efficiently discounts the motion of the reference system from early on, allowing a largely reference system independent encoding of the motion of object parts.

Memory for shape reactivates the lateral occipital complex

Memory is thought to be a constructive process in which the cortical regions associated with processing event features are reactivated during retrieval. Although there is evidence for non-detailed cortical reactivation during retrieval (e.g., memory for visual or auditory information reactivates the visual or auditory processing regions, respectively), there is limited evidence that memory can reactivate cortical regions associated with processing detailed, feature-specific information. Such evidence is critical to our understanding of the mechanisms of episodic retrieval. The present functional magnetic resonance imaging (fMRI) study assessed whether the lateral occipital complex (LOC), a region that preferentially processes shape, is associated with retrieval of shape information. During encoding, participants were presented with colored abstract shapes that were either intact or scrambled. During retrieval, colored disks were presented and participants indicated whether the corresponding shape was previously "intact" or "scrambled". To assess whether conscious retrieval of intact shapes reactivated LOC, we conducted a conjunction of shape perception/encoding and accurate versus inaccurate retrieval of intact shapes, which produced many activations in LOC. To determine whether activity in LOC was specific to intact shapes, we conducted a conjunction of shape perception/encoding and intact versus scrambled shapes, which also produced many activations in LOC. Furthermore, memory for intact shapes in each hemifield produced contralateral activity in LOC (e.g., memory for left visual field intact shapes activated right LOC), which reflects the specific reinstatement of perception/encoding activity. The present results extend previous feature-specific memory reactivation evidence and support the view that memory is a constructive process.

Neural correlates of illusory line motion

Illusory line motion (ILM) refers to a motion illusion in which a flash at one end of a bar prior to the bar's instantaneous presentation or removal results in the percept of motion. While some theories attribute the origin of ILM to attention or early perceptual mechanisms, others have proposed that ILM results from impletion mechanisms that reinterpret the static bar as one in motion. The current functional magnetic resonance imaging study examined participants while they made decisions about the direction of motion in which a bar appeared to be removed. Preceding the instantaneous removal of the bar with a flash at one end resulted in a motion percept away from the flash. If this flash and the bar's removal overlapped in time, it appeared that the bar was removed towards the flash (reverse ILM). Independent of the motion type, brain responses indicated activations in areas associated with motion (MT+), endogenous and exogenous attention (intraparietal sulcus, frontal eye fields, and ventral frontal cortex), and response selection (ACC). ILM was associated with lower percept scores and higher activations in ACC relative to real motion, but no differences in shape-selective areas emerged. This pattern of brain activation is consistent with the attentional gradient model or bottom-up accounts of ILM in preference to impletion.

Apparent motion can impair and enhance target visibility: the role of shape in predicting and postdicting object continuity

Some previous studies have reported that the visibility of a target in the path of an apparent motion sequence is impaired; other studies have reported that it is facilitated. Here we test whether the relation of shape similarity between the inducing and target stimuli has an influence on visibility. Reasoning from a theoretical framework in which there are both predictive and postdictive influences on shape perception, we report experiments involving three-frame apparent motion sequences. In these experiments, we systematically varied the congruence between target shapes and contextual shapes (preceding and following). Experiment 1 established the baseline visibility of the target, when it was presented in isolation and when it was preceded or followed by a single contextual shape. This set the stage for Experiment 2, where the shape congruence between the target and both contextual shapes was varied orthogonally. The results showed a remarkable degree of synergy between predictive and postdictive influences, allowing a backward-masked shape that was almost invisible when presented in isolation to be discriminated with a d′ of 2 when either of the contextual shapes are congruent. In Experiment 3 participants performed a shape-feature detection task with the same stimuli, with the results indicating that the predictive and postdictive effects were now absent. This finding confirms that shape congruence effects on visibility are specific to shape perception and are not due to either general alerting effects for objects in the path of a motion signal nor to low-level perceptual filling-in.

Transfer of predictive signals across saccades

Predicting visual information facilitates efficient processing of visual signals. Higher visual areas can support the processing of incoming visual information by generating predictive models that are fed back to lower visual areas. Functional brain imaging has previously shown that predictions interact with visual input already at the level of the primary visual cortex (V1; Harrison et al., 2007; Alink et al., 2010). Given that fixation changes up to four times a second in natural viewing conditions, cortical predictions are effective in V1 only if they are fed back in time for the processing of the next stimulus and at the corresponding new retinotopic position. Here, we tested whether spatio-temporal predictions are updated before, during, or shortly after an inter-hemifield saccade is executed, and thus, whether the predictive signal is transferred swiftly across hemifields. Using an apparent motion illusion, we induced an internal motion model that is known to produce a spatio-temporal prediction signal along the apparent motion trace in V1 (Muckli et al., 2005; Alink et al., 2010). We presented participants with both visually predictable and unpredictable targets on the apparent motion trace. During the task, participants saccaded across the illusion whilst detecting the target. As found previously, predictable stimuli were detected more frequently than unpredictable stimuli. Furthermore, we found that the detection advantage of predictable targets is detectable as early as 50-100 ms after saccade offset. This result demonstrates the rapid nature of the transfer of a spatio-temporally precise predictive signal across hemifields, in a paradigm previously shown to modulate V1.

Motion streaks do not influence the perceived position of stationary flashed objects

In the present study, we investigated whether motion streaks, produced by fast moving dots Geisler 1999, distort the positional map of stationary flashed objects producing the well-known motion-induced position shift illusion (MIPS). The illusion relies on motion-processing mechanisms that induce local distortions in the positional map of the stimulus which is derived by shape-processing mechanisms. To measure the MIPS, two horizontally offset Gaussian blobs, placed above and below a central fixation point, were flashed over two fields of dots moving in opposite directions. Subjects judged the position of the top Gaussian blob relative to the bottom one. The results showed that neither fast (motion streaks) nor slow moving dots influenced the perceived spatial position of the stationary flashed objects, suggesting that background motion does not interact with the shape-processing mechanisms involved in MIPS.

Sound can enhance the suppression of visual target detection in apparent motion trajectory

Detection performance is impaired for a visual target presented in an apparent motion (AM) trajectory, and this AM interference weakens when orientation information is inconsistent between the target and AM stimuli. These indicate that the target is perceptually suppressed by internal object representations of AM stimuli established along the AM trajectory. Here, we showed that transient sounds presented together with AM stimuli could enhance the magnitude of AM interference. Furthermore, this auditory effect attenuated when frequencies of the sounds were inconsistent during AM. We also confirmed that the sounds wholly elevated the magnitude of AM interference irrespective of the inconsistency in orientation information between the target and AM stimuli when the saliency of the sounds was maintained. These results suggest that sounds can contribute to the robust establishment and spatiotemporal maintenance of the internal object representation of an AM stimulus.

An intracranial event-related potential study on transformational apparent motion. Does its neural processing differ from real motion?

How the brain processes visual stimuli has been extensively studied using scalp surface electrodes and magnetic resonance imaging. Using these and other methods, complex gratings have been shown to activate the ventral visual stream, whereas moving stimuli preferentially activate the dorsal stream. In the current study, a first experiment assessed brain activations evoked by complex gratings using intracranial electroencephalography in 10 epileptic patients implanted with subdural electrodes. These stimuli of intermediate levels of complexity were presented in such a way that transformational apparent motion (TAM) was perceived. Responses from both the ventral and the dorsal pathways were obtained. The response characteristics of visual area 4 and the fusiform cortex were of similar amplitudes, suggesting that both ventral areas are recruited for the processing of complex gratings. On the other hand, TAM-induced responses of dorsal pathway areas were relatively noisier and of lower amplitudes, suggesting that TAM does not activate motion-specific structures to the same extent as does real motion. To test this hypothesis, we examined the activity evoked by TAM in comparison to the one produced by real motion in a patient implanted with the same subdural electrodes. Findings demonstrated that neural response to real motion was much stronger than that evoked by TAM, in both the primary visual cortex (V1) and other motion-sensitive areas within the dorsal pathway. These results support the conclusion that apparent motion, even if perceptually similar to real motion, is not processed in a similar manner.

The Role of Mask Coherence in Motion-Induced Blindness

Motion-induced blindness (MIB) is the perceived disappearance of a salient target when surrounded by a moving mask. Much research has focused on the role of target characteristics on perceived disappearance by a coherently moving mask. However, we asked a different question: mainly, are there certain characteristics about the mask that can impact disappearance? To address this, we behaviorally tested whether MIB is enhanced or reduced by the property of common fate. In experiments 1, 2, and 3, we systematically manipulated the motion coherence of the mask and measured the amount of target disappearance. Results showed that, as mask coherence increased, perceived target disappearance decreased. This pattern was unaffected by the lifetime of the moving dots, the dot density of the motion stimulus, or the target eccentricity. In experiment 4, we investigated whether the number of motion directions contained in an incoherent mask could account for our findings. Using masks containing 1, 3, and 5 motion directions, we found that disappearance did not increase proportionally to the number of motion directions. We discuss our findings in line with current proposed mechanisms of MIB.

A proposed neural mechanism underlying auditory continuity illusions

A numerical thought experiment was conducted to assess whether stimulus-specific, short-term changes in auditory neural responsiveness could explain the formation of auditory objects underlying the auditory continuity illusion. A tonotopic, two-layer feedforward network model with one time constant for synaptic weight augmentation based on firing rate, and an independent time constant for synaptic weight decay was presented with classical continuity illusion stimuli. The results suggest that the continuity illusion could, in principle, be explained by basic, duration-dependent auditory circuit behavior, which could emerge at either early or later stages of processing.

fMRI–Adaptation Studies of Viewpoint Tuning in the Extrastriate and Fusiform Body Areas

People are easily able to perceive the human body across different viewpoints, but the neural mechanisms underpinning this ability are currently unclear. In three experiments, we used functional MRI (fMRI) adaptation to study the view-invariance of representations in two cortical regions that have previously been shown to be sensitive to visual depictions of the human body—the extrastriate and fusiform body areas (EBA and FBA). The BOLD response to sequentially presented pairs of bodies was treated as an index of view invariance. Specifically, we compared trials in which the bodies in each image held identical poses (seen from different views) to trials containing different poses. EBA and FBA adapted to identical views of the same pose, and both showed a progressive rebound from adaptation as a function of the angular difference between views, up to ∼30°. However, these adaptation effects were eliminated when the body stimuli were followed by a pattern mask. Delaying the mask onset increased the response (but not the adaptation effect) in EBA, leaving FBA unaffected. We interpret these masking effects as evidence that view-dependent fMRI adaptation is driven by later waves of neuronal responses in the regions of interest. Finally, in a whole brain analysis, we identified an anterior region of the left inferior temporal sulcus (l-aITS) that responded linearly to stimulus rotation, but showed no selectivity for bodies. Our results show that body-selective cortical areas exhibit a similar degree of view-invariance as other object selective areas—such as the lateral occipitotemporal area (LO) and posterior fusiform gyrus (pFs).

Processing of Targets in Smooth or Apparent Motion Along the Vertical in the Human Brain: An fMRI Study

Neural substrates for processing constant speed visual motion have been extensively studied. Less is known about the brain activity patterns when the target speed changes continuously, for instance under the influence of gravity. Using functional MRI (fMRI), here we compared brain responses to accelerating/decelerating targets with the responses to constant speed targets. The target could move along the vertical under gravity (1 g), under reversed gravity (−1 g), or at constant speed (0 g). In the first experiment, subjects observed targets moving in smooth motion and responded to a GO signal delivered at a random time after target arrival. As expected, we found that the timing of the motor responses did not depend significantly on the specific motion law. Therefore brain activity in the contrast between different motion laws was not related to motor timing responses. Average BOLD signals were significantly greater for 1 g targets than either 0 g or −1 g targets in a distributed network including bilateral insulae, left lingual gyrus, and brain stem. Moreover, in these regions, the mean activity decreased monotonically from 1 g to 0 g and to −1 g. In the second experiment, subjects intercepted 1 g, 0 g, and −1 g targets either in smooth motion (RM) or in long-range apparent motion (LAM). We found that the sites in the right insula and left lingual gyrus, which were selectively engaged by 1 g targets in the first experiment, were also significantly more active during 1 g trials than during −1 g trials both in RM and LAM. The activity in 0 g trials was again intermediate between that in 1 g trials and that in −1 g trials. Therefore in these regions the global activity modulation with the law of vertical motion appears to hold for both RM and LAM. Instead, a region in the inferior parietal lobule showed a preference for visual gravitational motion only in LAM but not RM.

Filling-in, spatial summation, and radiation of pain: evidence for a neural population code in the nociceptive system

The receptive field organization of nociceptive neurons suggests that noxious information may be encoded by population-based mechanisms. Electrophysiological evidence of population coding mechanisms has remained limited. However, psychophysical studies examining interactions between multiple noxious stimuli can provide indirect evidence that neuron population recruitment can contribute to both spatial and intensity-related percepts of pain. In the present study, pairs of thermal stimuli (35 degrees C/49 degrees C or 49 degrees C/49 degrees C) were delivered at different distances on the leg (0, 5, 10, 20, 40 cm) and abdomen (within and across dermatomes) and subjects evaluated pain intensity and perceived spatial attributes of stimuli. Reports of perceived pain spreading to involve areas that were not stimulated (radiation of pain) were most frequent at 5- and 10-cm distances (chi(2) = 34.107, P < 0.0001). Perceived connectivity between two noxious stimuli (filling-in) was influenced by the distance between stimuli (chi(2) = 16.756, P < 0.01), with the greatest connectivity reported at 5- and 10-cm separation distances. Spatial summation of pain occurred over probe separation distances as large as 40 cm and six dermatomes (P < 0.05), but was maximal at 5- and 10-cm separation distances. Taken together, all three of these phenomena suggest that interactions between recruited populations of neurons may support both spatial and intensity-related dimensions of the pain experience.

Type of featural attention differentially modulates hMT+ responses to illusory motion aftereffects

Activity in the human motion complex (hMT(+)/V5) is related to the perception of motion, be it either real surface motion or an illusion of motion such as apparent motion (AM) or motion aftereffect (MAE). It is a long-lasting debate whether illusory motion-related activations in hMT(+) represent the motion itself or attention to it. We have asked whether hMT(+) responses to MAEs are present when shifts in arousal are suppressed and attention is focused on concurrent motion versus nonmotion features. Significant enhancement of hMT(+) activity was observed during MAEs when attention was focused either on concurrent spatial angle or color features. This observation was confirmed by direct comparison of adapting (MAE inducing) versus nonadapting conditions. In contrast, this effect was diminished when subjects had to report on concomitant speed changes of superimposed AM. The same finding was observed for concomitant orthogonal real motion (RM), suggesting that selective attention to concurrent illusory or real motion was interfering with the saliency of MAE signals in hMT(+). We conclude that MAE-related changes in the global activity of hMT(+) are present provided selective attention is not focused on an interfering feature such as concurrent motion. Accordingly, there is a genuine MAE-related motion signal in hMT(+) that is neither explained by shifts in arousal nor by selective attention.

Microsaccade rate varies with subjective visibility during motion-induced blindness

Motion-induced blindness (MIB) occurs when a dot embedded in a motion field subjectively vanishes. Here we report the first psychophysical data concerning effects of microsaccade/eyeblink rate upon perceptual switches during MIB. We find that the rate of microsaccades/eyeblink rises before and after perceptual transitions from not seeing to seeing the dot, and decreases before perceptual transitions from seeing it to not seeing it. In addition, event-related fMRI data reveal that, when a dot subjectively reappears during MIB, the blood oxygen-level dependent (BOLD) signal increases in V1v and V2v and decreases in contralateral hMT+. These BOLD signal changes observed upon perceptual state changes in MIB could be driven by the change of perceptual states and/or a confounding factor, such as the microsaccade/eyeblink rate.

A spatio-temporal interaction on the apparent motion trace

During the perception of apparent motion, activity along the apparent motion trace has been found in the primary visual cortex. It has been hypothesized that this activity interferes with stimuli presented on the apparent motion trace ("motion masking"). We investigated whether this perceptual interference varies with regard to the trajectory of a moving object token in a detection task. We found a general decrease of detectability of targets presented on the trace. Surprisingly, targets presented in time with the trajectory were detected significantly more often than targets which appeared out of time. We relate this finding to a spatio-temporally specific prediction of visual events along the apparent motion trace.

Depth Representation of Moving 3-D Objects in Apparent-Motion Path

Apparent motion is perceived when two objects are presented alternately at different positions. The internal representations of apparently moving objects are formed in an apparent-motion path which lacks physical inputs. We investigated the depth information contained in the representation of 3-D moving objects in an apparent-motion path. We examined how probe objects—briefly placed in the motion path—affected the perceived smoothness of apparent motion. The probe objects comprised 3-D objects which were defined by being shaded or by disparity (convex/concave) or 2-D (flat) objects, while the moving objects were convex/concave objects. We found that flat probe objects induced a significantly smoother motion perception than concave probe objects only in the case of the convex moving objects. However, convex probe objects did not lead to smoother motion as the flat objects did, although the convex probe objects contained the same depth information for the moving objects. Moreover, the difference between probe objects was reduced when the moving objects were concave. These counterintuitive results were consistent in conditions when both depth cues were used. The results suggest that internal representations contain incomplete depth information that is intermediate between that of 2-D and 3-D objects.

Exaggerated color perception in a patient with visual form agnosia

Previous studies on visual form agnosic patients have shown that their color perception is relatively preserved when monochromatic figures are used. However, it is unclear whether their color perception remains normal when figures are composed of two parts in different colors. The results showed that patient X.F. had difficulty in naming both colors when the two colors were placed next to each other, and in discriminating the two-color figure from the figure presented in its larger color. In contrast, X.F. could name the two colors when they were physically separated. These data suggest that X.F. manifests exaggerated color perception, producing a color filling-in effect that may be mediated by her spared early visual area.

The temporal sequence of evoked and induced cortical responses to implied-motion processing in human motion area V5/MT+

Previous functional magnetic resonance imaging (fMRI) studies have demonstrated that the human visual motion area V5/MT+ is differentially activated by stimuli in which the presence of motion is implied by the content of static photographs, compared with similar static scenes in which no motion is implied. Here, using a group magnetoencephalography study, we confirm the role of V5/MT+ in the perception of implied motion (IM) by the measurement and localization of task-related evoked and induced oscillatory responses, and demonstrate the temporal sequence of these responses. Within the lateral occipital complex, including V5/MT+, statistically significant differential oscillatory responses to IM and implied-static (IS) stimuli were only found in the beta band (15-20 Hz). An early, evoked, beta power increase (IM>IS) occurred at about 150 ms, whilst a power decrease (IM<IS) occurred at approximately 700 ms. We additionally show that the difference between the early evoked beta responses for the IM and IS conditions only occurs when participants are able to ascertain whether an IM or IS photograph is expected. We therefore hypothesize that the enhanced early response represents covert priming of V5/MT+ for the arrival of a motion stimuli, whilst the late-induced beta power increase for IS stimuli indicates suppression of activity within V5/MT+.

Spatial versus temporal grouping in a modified Ternus display

The Ternus display can induce a percept of 'element motion' or 'group motion'. Conventionally, this has been attributed to two different motion processes, with different spatial and temporal ranges. In contrast, recent studies have emphasised spatial and temporal grouping principles as underlying the apparent motion percepts in the Ternus display. The present study explored effects of spatial and temporal grouping on the apparent motion percept in a novel Ternus display of oriented Gabor elements with no inter-frame interval. Each frame of this stimulus could be further divided into 'sub-frames', and the orientation of the carriers was changed across these sub-frames. In four experiments transitions were found between the motion percepts with changes in orientation across time (Experiment 1) and space (Experiment 2), and with a temporal offset in the orientation change of the outer element (Experiment 3) to the extent that group motion was not perceived even with large orientation changes over time that previously led to group motion (Experiment 4). Collectively, these results indicate that while spatial properties have an influence in determining the percept of the Ternus display, temporal properties also have a strong influence, and can override spatial grouping. However, these temporal effects cannot be attributed to spatio-temporal limits of low-level motion processes. Some aspects of the observed spatial grouping effects can be accounted for in terms of a modified association field, which may occur through connectivity of orientation selective units in V1. The temporal effects observed are considered in terms of temporal integration, the transitional value at a temporal offset of 40ms being remarkably similar to psychophysical and neurophysiological estimates of the peak temporal impulse response. These temporal responses could be detected at a higher level in the system, providing a basis for apparent motion perception.

Contour discontinuities subserve two types of form analysis that underlie motion processing

Form analysis subserves motion processing in at least two ways: first, in terms of figural segmentation dedicated to solving the problem of figure-to-figure matching over time, and second, in terms of defining trackable features whose unambiguous motion signals can be generalized to ambiguously moving portions of an object. The former is a primarily ventral process involving the lateral occipital complex and also retinotopic areas such as V2 and V4, and the latter is a dorsal process involving V3A. Contour discontinuities, such as corners, deep concavities, maxima of positive curvature, junctions, and terminators, play a central role in both types of form analysis. Transformational apparent motion will be discussed in the context of figural segmentation and matching, and rotational motion in the context of trackable features. In both cases the analysis of form must proceed in parallel with the analysis of motion, in order to constrain the ongoing analysis of motion.

The nature of memory related activity in early visual areas

Memory for visual items can evoke activity in visual processing regions, which is typically assumed to reflect conscious remembering. However, based on previous findings, we hypothesized that such activity in early visual areas (BA17, BA18) may reflect priming, a form of nonconscious memory. We tested this hypothesis in two fMRI experiments with similar stimulus protocols, but explicit or implicit task instructions. During initial runs, abstract shapes were presented to either side of fixation, filled with parallel lines of random orientation and color. In subsequent runs, old and new shapes (plus related shapes in Experiment 2) were presented at fixation. In Experiment 1, participants were instructed to remember each shape and its spatial location during initial runs; during subsequent runs they classified each shape as old and on the "left", old and on the "right", or "new". A right fusiform gyrus region (BA18) and a left lingual gyrus region (BA18) were preferentially associated with shapes previously presented on the left and right, respectively. In support of our hypothesis, this early visual area activity was independent of response accuracy for spatial location. In Experiment 2, for each shape, participants identified parallel line orientation relative to horizontal. Consistent with our hypothesis, specific neural activity was observed in early visual regions (BA17, BA18, extending into BA19), with old activity greater than related and new activity (likely reflecting priming). The results of these experiments provide convergent evidence that memory related early visual area activity (BA17, BA18) can reflect nonconscious processing.

Bistable illusory rebound motion: Event-related functional magnetic resonance imaging of perceptual states and switches

The neural correlates of a recently discovered visual illusion that we call 'illusory rebound motion' (IRM) are described. This illusion is remarkable because motion is perceived in the absence of any net motion energy in the stimulus. When viewing bars alternating between white and black on a gray background, the percept alternates between one of flashing bars (veridical) and the IRM illusion, where the bars appear to shoot back and forth rather like the opening and closing of a zipper. The event-related functional magnetic resonance imaging (fMRI) data reported here reveal that (1) the blood-oxygen-level-dependent (BOLD) signal in the human analog of macaque motion processing area MT (hMT+) increases when there is a perceptual change from "no-IRM" to "see-IRM" and decreases when there is a perceptual change from "see-IRM" to "no-IRM," although the stimulus remains constant; and (2) the BOLD signal in early retinotopic areas (V1, V2, and V3d) shows switch-related activation whenever there is a perceptual change, regardless whether from IRM to no-IRM or vice versa. We conclude that hMT+ is a neural correlate of this novel illusory motion percept because BOLD signal in hMT+ modulates with the perception of IRM.

V3A processes contour curvature as a trackable feature for the perception of rotational motion

Contour curvature (CC) is a vital cue for the analysis of both form and motion. Using functional magnetic resonance imaging, we localized the neural correlates of CC for the processing and perception of rotational motion. We found that the blood oxygen level-dependent signal in retinotopic area V3A and possibly also lateral occipital cortex (LOC) varied parametrically with the degree of CC. Control experiments ruled out the possibility that these modulations resulted from either changes in the area of the stimuli, the velocity with which contour elements were actually translating, or perceived angular velocity. We conclude that neurons within V3A and perhaps also LOC process continuously moving CC as a trackable feature. These data are consistent with the hypothesis that V3A contains neural populations that process trackable form features such as CC, not to solve the "ventral problem" of determining object shape but in order to solve the "dorsal problem" of what is going where.

Primary visual cortex activation on the path of apparent motion is mediated by feedback from hMT+/V5

Apparent motion (AM) is the illusory perception of real motion created when two spatially distinct stationary visual objects are presented in alternating sequence. In common with many other illusory percepts, activation during AM can be identified in unstimulated regions of V1 representing the illusory motion path. However, little is known about the mechanisms underlying such activation and its relationship with motion-sensitive area hMT+/V5. Using fMRI and a novel AM stimulus, we replicated previous findings showing a correlate of the perceived AM path in V1. To more closely characterize the mechanisms underlying these activations, we performed analyses of effective connectivity and found that the AM-induced activations on the illusory AM path were associated with enhanced feedback (but not feedforward) connectivity from hMT+/V5. These findings provide for the first time evidence for the involvement of cortico-cortical coupling in generating an illusory percept of AM. They therefore emphasize the role of recurrent processing between visual cortical areas in human perceptual awareness.

Images of Illusory Motion in Primary Visual Cortex

Illusory motion can be generated by successively flashing a stationary visual stimulus in two spatial locations separated by several degrees of visual angle. In appropriate conditions, the apparent motion is indistinguishable from real motion: The observer experiences a luminous object traversing a continuous path from one stimulus location to the other through intervening positions where no physical stimuli exist. The phenomenon has been extensively investigated for nearly a century but little is known about its neurophysiological foundation. Here we present images of activations in the primary visual cortex in response to real and apparent motion. The images show that during apparent motion, a path connecting the cortical representations of the stimulus locations is filled in by activation. The activation along the path of apparent motion is similar to the activation found when a stimulus is presented in real motion between the two locations.

An MEG study into the visual perception of apparent motion in depth

This study investigated (1) the responses associated with the perception of motion in depth induced by a series of scale-changing paradigms, and (2) the locations of the dipoles in the brain elicited by stimuli of motion in depth and in-plane motion. The former was determined using two types of stimulation: real motion and apparent motion in scale (AMS; where two frames with size-changed patterns were presented alternately); and the latter was determined by moving a full-field checkerboard pattern forward or rightward smoothly with the same time course. The results from analyzing magnetoencephalography (MEG) component M160 to differentiate the signals of perception can be summarized as follows: (1) the neurons stimulated by apparent motion (AM) might be similar to those stimulated by a real motion, since there was no statistical difference associated with the signals at M160 and the dipole locations; (2) the perceptional signal of motion in depth seems to be more sensitive when scale-changing information is present; and (3) asymmetrical responses are present in the visual system, with responses being more sensitive to expanding stimuli than to contracting ones, and with the activity being more prominent in the right occipitotemporal area. Overall, this study indicates that the responses evoked by the stimuli causing motion in depth are allocated more to area V3a rather to area V5.

Neural correlates of transformational apparent motion

When a figure discretely and instantaneously changes its shape, observers typically do not perceive the abrupt transition between shapes that in fact occurs. Rather, a continuous shape change is perceived. Although this illusory "transformational apparent motion" (TAM) is a faulty construction of the visual system, it is not arbitrary. From the many possible shape changes that could have been inferred, usually just one is perceived because only one is consistent with the shape-based rules that the visual system uses to (1) segment figures from one another within a scene and (2) match figures to themselves across successive scenes. TAM requires an interaction between neuronal circuits that process form relationships with circuits that compute motion trajectories. In particular, this form-motion interaction must happen before TAM is perceived because the direction of perceived motion is dictated by form relationships among figures in successive images. The present fMRI study (n = 19) provides the first evidence that both form (LOC, posterior fusiform gyrus) and motion (hMT+) processing areas are more active when TAM is perceived than in a control stimulus where it is not. Retinotopic areas (n = 10), hMT+ (n = 7), and LOC (n = 7) were mapped in a subset of subjects.

RESULTS

There is greater BOLD response to TAM than to the control condition in V1 and all subsequent retinotopic areas, as well as in hMT+ and the LOC, suggesting that areas that process form interact with hMT+ to construct the perception of moving figures.

Orientation-tuned FMRI adaptation in human visual cortex

Adaptation is a general property of almost all neural systems and has been a longstanding tool of psychophysics because of its power to isolate and temporarily reduce the contribution of specific neural populations. Recently, adaptation designs have been extensively applied in functional MRI (fMRI) studies to infer neural selectivity in specific cortical areas. However, there has been considerable variability in the duration of adaptation used in these experiments. In particular, although long-term adaptation has been solidly established in psychophysical and neurophysiological studies, it has been incorporated into few fMRI studies. Furthermore, there has been little validation of fMRI adaptation using stimulus dimensions with well-known adaptive properties (e.g., orientation) and in better understood regions of cortex (e.g., primary visual cortex, V1). We used an event-related fMRI experiment to study long-term orientation adaptation in the human visual cortex. After long-term adaptation to an oriented pattern, the fMRI response in V1, V2, V3/VP, V3A, and V4 to a test stimulus was proportional to the angular difference between the adapting and test stimuli. However, only V3A and V4 showed this response pattern with short-term adaptation. In a separate experiment, we measured behavioral contrast detection thresholds after adaptation and found that the fMRI signal in V1 closely matched the psychophysically derived contrast detection thresholds. Similar to the fMRI results, adaptation induced threshold changes strongly depended on the duration of adaptation. In addition to supporting the existence of adaptable orientation-tuned neurons in human visual cortex, our results show the importance of considering timing parameters in fMRI adaptation experiments.