Perceptual organization and visual awareness: the case of amodal completion.

We investigated the involvement of visual awareness in amodal completion, and specifically, whether visual awareness plays a differential role in local versus global completion, using a primed shape discrimination paradigm and the color-opponent flicker technique to render the prime invisible. In four experiments, participants discriminated the shape of a target preceded by a partly occluded or a neutral prime. All primes were divergent occlusion patterns in which the local completion is based on good continuation of the contours at the point of occlusion and the global completion is based on maximum symmetry. The target corresponded to the shape that could arise as a result of local or global completion of the occluded prime. For each experiment with an invisible prime we conducted a version with a visible prime. Our results suggest that local completion, but not global completion, of a partly occluded shape can take place in the absence of visual awareness, but apparently only when the visible occluded shape generates a single, local completion. No completion, either local or global, appears to take place in the absence of visual awareness when the visible occluded shape generates multiple completions. The implications of these results to the differential role of visual awareness in local and global completions and to the relationship between multiple completions and unconscious amodal completions are discussed.

The role of visual awareness in processing of global structure: Evidence from the perceptual organization of hierarchical patterns.

We examined whether the perceptual organization of hierarchical structure, and specifically, the integration of local elements into a global shape, requires visual awareness, using a masked priming paradigm, sandwich masking to render the prime invisible, and two types of primes, many-element hierarchical patterns composed of many relatively small elements (Experiment 1) and few-element hierarchical patterns composed of a few relatively large elements (Experiment 2). A significant response priming of the local elements of many-element patterns was observed for invisible primes, whereas a significant response priming of the global shape was found only for the visible primes. For the few-element patterns, only significant response priming of the local elements was observed, as expected, for both visible and invisible primes. Our results suggest that local elements are represented in the absence of visual awareness, regardless of their number and relative size. Visual awareness, however, can be essential for grouping local elements into a global shape. The critical role of awareness in deriving global structure implies that global perception, which characterizes much of our early perception of objects and scenes, is likely to depend on visual awareness.

Perceptual organization of line configurations: Is visual awareness necessary?

We examined whether configuring, which determines the appearance of grouped elements as a global shape, requires visual awareness, using a priming paradigm and two invisibility-inducing methods, CFS and sandwich masking. The primes were organized into configurations based on closure, collinearity, and symmetry (collinear primes), or on closure and symmetry (noncollinear primes). The prime-target congruency could be in configuration or in elements. During CFS, no significant response-priming was observed for invisible primes. When masking induced invisibility, a significant configuration response-priming was found for collinear and noncollinear primes, visible and invisible, with larger magnitude for the former. An element response-priming of equal magnitude was evident for visible and invisible noncollinear primes. Our results suggest that configuring can be accomplished in the absence of visual awareness when stimuli are rendered invisible by sandwich masking, but it benefits from visual awareness. Our results also suggest sensitivity to the available grouping cues in unconscious processing.

Can perceptual grouping unfold in the absence of awareness? Comparing grouping during continuous flash suppression and sandwich masking.

In this study we examined whether grouping by luminance similarity and grouping by connectedness can occur in the absence of visual awareness, using a priming paradigm and two methods to render the prime invisible, CFS and sandwich masking under matched conditions. For both groupings, significant response priming effects were observed when the prime was reported invisible under sandwich masking, but none were obtained under CFS. These results provide evidence for unconscious grouping, converging with previous findings showing that visual awareness is not essential for certain perceptual organization processes to occur. They are also consistent with findings indicating that processing during CFS is limited, and suggest the involvement of higher visual areas in perceptual organization. Moreover, these results demonstrate that whether a process can occur without awareness is dependent on the level at which the suppression induced by the method used for rendering the stimulus inaccessible to awareness takes place.

Extrinsic grouping factors in motion-induced blindness.

We investigated how various grouping factors altered subjective disappearances of the individual targets in the motion-induced blindness display. The latter relies on a moving mask to render highly salient static targets temporarily subjectively invisible. Specifically, we employed two extrinsic grouping factors, the connectedness and the common region, and examined whether their presence would make targets more resilient against the suppression. In addition, we investigated whether the presence of an illusory Kanizsa triangle would affect the suppression of the inducing Pac-Man elements. We quantified the perceptual dynamics using the proportion of the disappearance time (this indicates whether targets became more resilient against the suppression), and the proportion of simultaneous disappearance and reappearance events (characterizes the tendency for the targets to disappear or reappear as a group). We report that a single mask that encompassed all targets (a common region grouping) significantly increased the proportion of simultaneous disappearance and reappearance events, but had no effect on the proportion of the disappearance time. In contrast, a line that connected two targets significantly decreased the total invisibility time, but had no impact on the simultaneity of the disappearance and reappearance events. We found no statistically significant effect of the presence of the illusory Kanizsa triangle on either measure. Finally, we found no interaction either between the common region and the connectedness or between the common region and the presence of the illusory Kanizsa triangle. Our results indicate that extrinsic grouping factors might influence the perception differently than the intrinsic ones and highlight the importance of using several measures to characterize the perceptual dynamics, as various grouping factors might affect it differentially.

A Common Mechanism for Perceptual Reversals in Motion-Induced Blindness, the Troxler Effect, and Perceptual Filling-In.

Several striking visual phenomena involve a physically present stimulus that alternates between being perceived and being "invisible." For example, motion-induced blindness, the Troxler effect, and perceptual filling-in all consist of subjective alternations where an item repeatedly changes from being seen to unseen. In the present study, we explored whether these three specific visual phenomena share any commonalities in their alternation rates and patterns to better understand the mechanisms of each. Data from 69 individuals revealed moderate to strong correlations across the three phenomena for the number of perceptual disappearances and the accumulated duration of the disappearances. Importantly, these effects were not correlated with eye movement patterns (saccades) assessed through eye tracking, differences in motion sensitivity as indexed by dot coherence and speed perception thresholds, or simple reaction time abilities. Principal component analyses revealed a single component that explained 67% of the variance for the number of perceptual reversals and 60% for the accumulated duration of the disappearances. The temporal dynamics of illusory disappearances was also compared for each phenomenon, and normalized durations of disappearances were well fit by a gamma distribution with similar shape parameters for each phenomenon, suggesting that they may be driven by a single oscillatory mechanism.