Extrastriate Visual Areas Integrate Form Features over Space and Time to Construct Representations of Stationary and Rigidly Rotating Objects.

When an object moves behind a bush, for example, its visible fragments are revealed at different times and locations across the visual field. Nonetheless, a whole moving object is perceived. Unlike traditional modal and amodal completion mechanisms known to support spatial form integration when all parts of a stimulus are simultaneously visible, relatively little is known about the neural substrates of the spatiotemporal form integration (STFI) processes involved in generating coherent object representations from a succession visible fragments. We used fMRI to identify brain regions involved in two mechanisms supporting the representation of stationary and rigidly rotating objects whose form features are shown in succession: STFI and position updating. STFI allows past and present form cues to be integrated over space and time into a coherent object even when the object is not visible in any given frame. STFI can occur whether or not the object is moving. Position updating allows us to perceive a moving object, whether rigidly rotating or translating, even when its form features are revealed at different times and locations in space. Our results suggest that STFI is mediated by visual regions beyond V1 and V2. Moreover, although widespread cortical activation has been observed for other motion percepts derived solely from form-based analyses [Tse, P. U. Neural correlates of transformational apparent motion. Neuroimage, 31, 766-773, 2006; Krekelberg, B., Vatakis, A., & Kourtzi, Z. Implied motion from form in the human visual cortex. Journal of Neurophysiology, 94, 4373-4386, 2005], increased responses for the position updating that lead to rigidly rotating object representations were only observed in visual areas KO and possibly hMT+, indicating that this is a distinct and highly specialized type of processing.

Two plus blue equals green: grapheme-color synesthesia allows cognitive access to numerical information via color.

In grapheme-color synesthesia, graphemes (e.g., numbers or letters) evoke color experiences. It is generally reported that the opposite is not true: colors will not generate experiences of graphemes or their associated information. However, recent research has provided evidence that colors can implicitly elicit symbolic representations of associated graphemes. Here, we examine if these representations can be cognitively accessed. Using a mathematical verification task replacing graphemes with color patches, we find that synesthetes can verify such problems with colors as accurately as with graphemes. Doing so, however, takes time: ~250 ms per color. Moreover, we find minimal reaction time switch-costs for switching between computing with graphemes and colors. This demonstrates that given specific task demands, synesthetes can cognitively access numerical information elicited by physical colors, and they do so as accurately as with graphemes. We discuss these results in the context of possible cognitive strategies used to access the information.

On the Spatiotemporal Nature of Vision, as Revealed by Covered Bridges and Puddles: A Dispatch from Vermont.

Retinal painting, anorthoscopic perception and amodal completion are terms to describe visual phenomena that highlight the spatiotemporal integrative mechanisms that underlie primate vision. Although commonly studied using simplified lab-friendly stimuli presented on a computer screen, this is a report of observations made in a novel real-world context that highlight the rich contributions the mechanisms underlying these phenomena make to naturalistic vision.

The maintenance and updating of representations of no longer visible objects and their parts.

When an object partially or completely disappears behind an occluding surface, a representation of that object persists. For example, fragments of no longer visible objects can serve as an input into mid-level constructive visual processes, interacting and integrating with currently visible portions to form perceptual units and global motion signals. Remarkably, these persistent representations need not be static and can have their positions and orientations updated postdictively as new information becomes visible. In this chapter, we highlight historical considerations, behavioral evidence, and neural correlates of this type of representational updating of no longer visible information at three distinct levels of visual processing. At the lowest level, we discuss spatiotemporal boundary formation in which visual transients can be integrated over space and time to construct local illusory edges, global form, and global motion percepts. At an intermediate level, we review how the visual system updates form information seen at one moment in time and integrates it with subsequently available information to generate global shape and motion representations (e.g., spatiotemporal form integration and anorthoscopic perception). At a higher level, when an entire object completely disappears behind an occluder, the object's identity and predicted position can be maintained in the absence of visual information.

Spatiotemporal Form Integration: sequentially presented inducers can lead to representations of stationary and rigidly rotating objects.

Objects in the world often are occluded and in motion. The visible fragments of such objects are revealed at different times and locations in space. To form coherent representations of the surfaces of these objects, the visual system must integrate local form information over space and time. We introduce a new illusion in which a rigidly rotating square is perceived on the basis of sequentially presented Pacman inducers. The illusion highlights two fundamental processes that allow us to perceive objects whose form features are revealed over time: Spatiotemporal Form Integration (STFI) and Position Updating. STFI refers to the spatial integration of persistent representations of local form features across time. Position updating of these persistent form representations allows them to be integrated into a rigid global motion percept. We describe three psychophysical experiments designed to identify spatial and temporal constraints that underlie these two processes and a fourth experiment that extends these findings to more ecologically valid stimuli. Our results indicate that although STFI can occur across relatively long delays between successive inducers (i.e., greater than 500 ms), position updating is limited to a more restricted temporal window (i.e., ~300 ms or less), and to a confined range of spatial (mis)alignment. These findings lend insight into the limits of mechanisms underlying the visual system's capacity to integrate transient, piecemeal form information, and support coherent object representations in the ever-changing environment.

Color synesthesia improves color but impairs motion perception.

A recent study showed that color synesthetes have increased color sensitivity but impaired motion perception. This is exciting because little research has examined how synesthesia affects basic perceptual processes outside the context of synesthetic experiences. The results suggest that synesthesia broadly impacts perception with greater neural implications than previously considered.

Form features provide a cue to the angular velocity of rotating objects.

As an object rotates, each location on the object moves with an instantaneous linear velocity, dependent upon its distance from the center of rotation, whereas the object as a whole rotates with a fixed angular velocity. Does the perceived rotational speed of an object correspond to its angular velocity, linear velocities, or some combination of the two? We had observers perform relative speed judgments of different-sized objects, as changing the size of an object changes the linear velocity of each location on the object's surface, while maintaining the object's angular velocity. We found that the larger a given object is, the faster it is perceived to rotate. However, the observed relationships between size and perceived speed cannot be accounted for simply by size-related changes in linear velocity. Further, the degree to which size influences perceived rotational speed depends on the shape of the object. Specifically, perceived rotational speeds of objects with corners or regions of high-contour curvature were less affected by size. The results suggest distinct contour features, such as corners or regions of high or discontinuous contour curvature, provide cues to the angular velocity of a rotating object.

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

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

The effect of attention on context dependent synesthetic experiences.

Here we report the results of a brief experiment investigating the role of attention in mediating contextual effects on synesthetic experiences. Specifically, we examine a grapheme-color synesthete for whom the grapheme letter 'O' and number '0' are associated with two very different colors. We presented the grapheme '0' in an array of graphemes that provided ambiguous contextual cues, such that the same grapheme could be perceived either as the number '0' or as the letter 'O'. We find that an attentional cue that draws attention to one or the other of the contexts biases the perceived synesthetic color of the '0' grapheme to that associated with the cued context. This is true even when the physical color of the grapheme corresponds to the un-cued context.

Attentive and pre-attentive processes in change detection and identification.

In studies of change blindness, observers often have the phenomenological impression that the blindness is overcome all at once, so that change detection, localization and identification apparently occur together. Three experiments are described that explore dissociations between these processes using a discrete trial procedure in which 2 visual frames are presented sequentially with no intervening inter-frame-interval. The results reveal that change detection and localization are essentially perfect under these conditions regardless of the number of elements in the display, which is consistent with the idea that change detection and localization are mediated by pre-attentive parallel processes.In contrast, identification accuracy for an item before it changes is generally poor, and is heavily dependent on the number of items displayed. Identification accuracy after a change is substantially better, but depends on the new item's duration. This suggests that the change captures attention, which substantially enhances the likelihood of correctly identifying the new item. However, the results also reveal a limited capacity to identify unattended items. Specifically, we provide evidence that strongly suggests that, at least under these conditions, observers were able to identify two items without focused attention. Our results further suggest that spatial pre-cues that attract attention to an item before the change occurs simply ensure that the cued item is one of the two whose identity is encoded.