Readiness potentials driven by non-motoric processes.

An increase in brain activity known as the "readiness potential" (RP) can be seen over central scalp locations in the seconds leading up to a volitionally timed movement. This activity precedes awareness of the ensuing movement by as much as two seconds and has been hypothesized to reflect preconscious planning and/or preparation of the movement. Using a novel experimental design, we teased apart the relative contribution of motor-related and non-motor-related processes to the RP. The results of our experiment reveal that robust RPs occured in the absence of movement and that motor-related processes did not significantly modulate the RP. This suggests that the RP measured here is unlikely to reflect preconscious motor planning or preparation of an ensuing movement, and instead may reflect decision-related or anticipatory processes that are non-motoric in nature.

Network structure and dynamics of the mental workspace.

The conscious manipulation of mental representations is central to many creative and uniquely human abilities. How does the human brain mediate such flexible mental operations? Here, multivariate pattern analysis of functional MRI data reveals a widespread neural network that performs specific mental manipulations on the contents of visual imagery. Evolving patterns of neural activity within this mental workspace track the sequence of informational transformations carried out by these manipulations. The network switches between distinct connectivity profiles as representations are maintained or manipulated.

Hypnotizing Libet: Readiness potentials with non-conscious volition.

The readiness potential (RP) is one of the most controversial topics in neuroscience and philosophy due to its perceived relevance to the role of conscious willing in action. Libet and colleagues reported that RP onset precedes both volitional movement and conscious awareness of willing that movement, suggesting that the experience of conscious will may not cause volitional movement (Libet, Gleason, Wright, & Pearl, 1983). Rather, they suggested that the RP indexes unconscious processes that may actually cause both volitional movement and the accompanying conscious feeling of will (Libet et al., 1983; pg. 640). Here, we demonstrate that volitional movement can occur without an accompanying feeling of will. We additionally show that the neural processes indexed by RPs are insufficient to cause the experience of conscious willing. Specifically, RPs still occur when subjects make self-timed, endogenously-initiated movements due to a post-hypnotic suggestion, without a conscious feeling of having willed those movements.

Dynamic Volume Completion and Deformation.

A new class of dynamic volume completion is introduced, where image elements (e.g., occluding semi-ellipses placed at the edge of an object) can link across a gap between two or more objects, leading to the perception of illusory volumes that deform as those image elements are set into relative motion. These new demonstrations provide further evidence that volume completion is not dictated solely by contour relatability constraints, but is instead a dynamic process of 3D shape construction that also takes into account dynamic cues to object shape, even in the absence of any contour relatability whatsoever.

Volume Completion Between Contour Fragments at Discrete Depths.

Building on the modal and amodal completion work of Kanizsa, Carman and Welch showed that binocular stereo viewing of two disparate images can give rise to a percept of 3D curved, nonclosed illusory contours and surfaces. Here, it is shown that binocular presentation can also give rise to the percept of closed curved surfaces or volumes that appear to vary smoothly across discrete depths in binocularly fused images, although in fact only two binocular disparities are discretely defined between corresponding contour elements of the inducing elements. Surfaces are filled in from one depth layer's visible contours to another layer's visible contours within virtual contours that are interpolated on the basis of good contour continuation between the visible portions of contour. These single depth contour segments are taken not to arise from surface edges, as in Kanizsa's or Carman and Welch's examples, but from segments of "rim" where the line of sight just grazes a surface that continues behind and beyond the rim smoothly. When there are two or more surface-propagating contour segments, the propagated surfaces can continue away from the inferred rim, merge, and then close behind the self-occluding visible surface into an everywhere differentiable closed surface or volume. Illusory surfaces can possess a depth and perceived surface curvature that is consistent with all visible contour segments, despite the absence of local disparity cues at interpolated 3D surface locations far from any visible contour. These demonstrations cannot be easily explained by existing models of visual processing. They place constraints on the surface and volume generation processes that construct our 3D world under normal viewing conditions.

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.

A contour propagation approach to surface filling-in and volume formation.

A new approach to surface and volume formation is introduced in response to the question, "Why do some silhouettes look 3 dimensional (3D) and others look 2D?" The central idea is that form information can propagate away from a "propagable segment" (PS) of occluding contour that could have projected onto the image from the visible portion of a cross-section of a surface. A key property of a PS is that it exhibits abrupt curvature changes where it meets the rest of the occluding contour. An algorithm is described for filling in curved surfaces from a PS: When copies of a PS are propagated into the interior, they act as cross-sectional surface contours that also exhibit abrupt curvature changes with the rest of the occluding contour. The result is a nonmetric coding of 3D-shape in terms of local ordinal surface curvature and orientation relationships that is scale, translation, and rotation invariant.

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 whole moves less than the spin of its parts.

When individually moving elements in the visual scene are perceptually grouped together into a coherently moving object, they can appear to slow down. In the present article, we show that the perceived speed of a particular global-motion percept is not dictated completely by the speed of the local moving elements. We investigated a stimulus that leads to bistable percepts, in which local and global motion may be perceived in an alternating fashion. Four rotating dot pairs, when arranged into a square-like configuration, may be perceived either locally, as independently rotating dot pairs, or globally, as two large squares translating along overlapping circular trajectories. Using a modified version of this stimulus, we found that the perceptually grouped squares appeared to move more slowly than the locally perceived rotating dot pairs, suggesting that perceived motion magnitude is computed following a global analysis of form. Supplemental demos related to this article can be downloaded from app.psychonomic-journals.org/content/supplemental.

The duration of 3-d form analysis in transformational apparent motion.

Transformational apparent motion (TAM) occurs when a figure changes discretely from one configuration to another overlapping configuration. Rather than an abrupt shape change, the initial shape is perceived to transform smoothly into the final shape as if animated by a series of intermediate shapes. We find that TAM follows an analysis of form that takes 80-140 msec. Form analysis can function both at and away from equiluminance and can occur over contours defined by uniform regions as well as outlines. Moreover, the forms analyzed can be 3-D, resulting in motion paths that appear to smoothly project out from or into the stimulus plane. The perceived transformation is generally the one that involves the least change in the shape or location of the initial figure in a 3-D sense. We conclude that perception of TAM follows an analysis of 3-D form that takes approximately 100 msec. This stage of form analysis may be common to both TAM and second-order motion.

Attention and the subjective expansion of time.

During brief, dangerous events, such as car accidents and robberies, many people report that events seem to pass in slow motion, as if time had slowed down. We have measured a similar, although less dramatic, effect in response to unexpected, nonthreatening events. We attribute the subjective expansion of time to the engagement of attention and its influence on the amount of perceptual information processed. We term the effect time's subjective expansion (TSE) and examine here the objective temporal dynamics of these distortions. When a series of stimuli are shown in succession, the low-probability oddball stimulus in the series tends to last subjectively longer than the high-probability stimulus even when they last the same objective duration. In particular, (1) there is a latency of at least 120 msec between stimulus onset and the onset of TSE, which may be preceded by subjective temporal contraction; (2) there is a peak in TSE at which subjective time is particularly distorted at a latency of 225 msec after stimulus onset; and (3) the temporal dynamics of TSE are approximately the same in the visual and the auditory domains. Two control experiments (in which the methods of magnitude estimation and stimulus reproduction were used) replicated the temporal dynamics of TSE revealed by the method of constant stimuli, although the initial peak was not apparent with these methods. In addition, a third, control experiment (in which the method of single stimuli was used) showed that TSE in the visual domain can occur because of semantic novelty, rather than image novelty per se. Overall, the results support the view that attentional orienting underlies distortions in perceived duration.