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

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.

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EEG responses are inhibited by spatial and temporal stimulus interactions.

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

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

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

Towards a unified perspective of object shape and motion processing in human dorsal cortex

Although object-related areas were discovered in human parietal cortex a decade ago, surprisingly little is known about the nature and purpose of these representations, and how they differ from those in the ventral processing stream. In this article, we review evidence for the unique contribution of object areas of dorsal cortex to three-dimensional (3-D) shape representation, the localization of objects in space, and in guiding reaching and grasping actions. We also highlight the role of dorsal cortex in form-motion interaction and spatiotemporal integration, possible functional relationships between 3-D shape and motion processing, and how these processes operate together in the service of supporting goal-directed actions with objects. Fundamental differences between the nature of object representations in the dorsal versus ventral processing streams are considered, with an emphasis on how and why dorsal cortex supports veridical (rather than invariant) representations of objects to guide goal-directed hand actions in dynamic visual environments.

Back from the future: Volitional postdiction of perceived apparent motion direction

Among physical events, it is impossible that an event could alter its own past for the simple reason that past events precede future events, and not vice versa. Moreover, to do so would invoke impossible self-causation. However, mental events are constructed by physical neuronal processes that take a finite duration to execute. Given this fact, it is conceivable that later brain events could alter the ongoing interpretation of previous brain events if they arrive within this finite duration of interpretive processing, before a commitment is made to what happened. In the current study, we show that humans can volitionally influence how they perceive an ambiguous apparent motion sequence, as long as the top-down command occurs up to 300ms after the occurrence of the actual motion event in the world. This finding supports the view that there is a temporal integration period over which perception is constructed on the basis of both bottom-up and top-down inputs.

The lemon illusion: seeing curvature where there is none

Curvature is a highly informative visual cue for shape perception and object recognition. We introduce a novel illusion—the Lemon Illusion—in which subtle illusory curvature is perceived along contour regions that are devoid of physical curvature. We offer several perceptual demonstrations and observations that lead us to conclude that the Lemon Illusion is an instance of a more general illusory curvature phenomenon, one in which the presence of contour curvature discontinuities lead to the erroneous extension of perceived curvature. We propose that this erroneous extension of perceived curvature results from the interaction of neural mechanisms that operate on spatially local contour curvature signals with higher-tier mechanisms that serve to establish more global representations of object shape. Our observations suggest that the Lemon Illusion stems from discontinuous curvature transitions between rectilinear and curved contour segments. However, the presence of curvature discontinuities is not sufficient to produce the Lemon Illusion, and the minimal conditions necessary to elicit this subtle and insidious illusion are difficult to pin down.

Extrastriate cortical activity reflects segmentation of motion into independent sources

Identical local image motion signals can arise from countless object motions in the world. In order to resolve this ambiguity, the visual system must somehow integrate motion signals arising from different locations along an object's contour. Difficulties arise, however, because image contours can derive from multiple objects and from occlusion. Thus, correctly integrating respective objects' motion signals presupposes the specification of what counts as an object. Depending on how this form analysis problem is solved, dramatically different object motion percepts can be constructed from the same set of local image motions. Here we apply fMRI to investigate the mechanisms underlying the segmentation and integration of motion signals that are critical to motion perception in general. We hold the number of image objects constant, but vary whether these objects are perceived to move independently or not. We find that BOLD signal in V3v, V4v, V3A, V3B and MT varies with the number of distinct sources of motion information in the visual scene. These data support the hypothesis that these areas integrate form and motion information in order to segment motion into independent sources (i.e. objects) thereby overcoming ambiguities that arise at the earliest stages of motion processing.