Amodal completion across the brain: The impact of structure and knowledge.

This study investigates the phenomenon of amodal completion within the context of naturalistic objects, employing a repetition suppression paradigm to disentangle the influence of structure and knowledge cues on how objects are completed. The research focuses on early visual cortex (EVC) and lateral occipital complex (LOC), shedding light on how these brain regions respond to different completion scenarios. In LOC, we observed suppressed responses to structure and knowledge-compatible stimuli, providing evidence that both cues influence neural processing in higher-level visual areas. However, in EVC, we did not find evidence for differential responses to completions compatible or incompatible with either structural or knowledge-based expectations. Together, our findings suggest that the interplay between structure and knowledge cues in amodal completion predominantly impacts higher-level visual processing, with less pronounced effects on the early visual cortex. This study contributes to our understanding of the complex mechanisms underlying visual perception and highlights the distinct roles played by different brain regions in amodal completion.

Temporal properties of amodal completion: Influences of knowledge.

We studied the influence of knowledge in the interpretation of partly occluded objects. In the past decades, amodal completion has often been studied by using abstract, meaningless outlines of rather stylistic, geometric shapes. It has been recognized that smooth continuation of partly occluded contours behind an occluding surface is a strong completion tendency. In the current study we contrast this structurally driven completion tendency with knowledge driven tendencies. We used a set of partly occluded well-known objects for which structure-based completions and knowledge-based completions resulted in either the same or different interpretations. We adopted the behavioural primed matching paradigm to measure differential priming effects due to these completion tendencies. Our results implied differential temporal properties for structure-based and knowledge-based effects during perception of partly occluded objects. Interestingly, knowledge has an influence as early as 150 ms after the onset of the prime.

Touching and Hearing Unseen Objects: Multisensory Effects on Scene Recognition.

In three experiments, we investigated the influence of object-specific sounds on haptic scene recognition without vision. Blindfolded participants had to recognize, through touch, spatial scenes comprising six objects that were placed on a round platform. Critically, in half of the trials, object-specific sounds were played when objects were touched (bimodal condition), while sounds were turned off in the other half of the trials (unimodal condition). After first exploring the scene, two objects were swapped and the task was to report, which of the objects swapped positions. In Experiment 1, geometrical objects and simple sounds were used, while in Experiment 2, the objects comprised toy animals that were matched with semantically compatible animal sounds. In Experiment 3, we replicated Experiment 1, but now a tactile-auditory object identification task preceded the experiment in which the participants learned to identify the objects based on tactile and auditory input. For each experiment, the results revealed a significant performance increase only after the switch from bimodal to unimodal. Thus, it appears that the release of bimodal identification, from audio-tactile to tactile-only produces a benefit that is not achieved when having the reversed order in which sound was added after having experience with haptic-only. We conclude that task-related factors other than mere bimodal identification cause the facilitation when switching from bimodal to unimodal conditions.

Disentangling effects of structure and knowledge in perceiving partly occluded shapes: An ERP study.

Using event-related potentials (ERPs) we investigated the influence of object knowledge on perceiving partly occluded shapes. We created stimuli based on well-known objects of which the middle part was occluded. Object completions that were compatible with structure consisted of the connection of the visible fragments by smoothly extending their contours. In contrast, object completions that were incompatible with structure consisted of separate disconnected completions of the fragments. Furthermore, object completions could be in line with, or conflict with expectations based on knowledge. We measured ERPs when hidden parts were revealed by removing the occluder, and observed an early positive ERP peaking around 115-140ms at occipital sites, presumably triggered by physical differences. Most importantly, we observed a positive ERP peaking around 300-400ms at parieto-occipital sites that could be related to influences of both structure and knowledge. An additional analysis controlling for differential stimulus characteristics revealed similar conclusions. All in all, we demonstrate that the interpretation of partly occluded shapes is not solely driven by stimulus structure, but that it can also be influenced by knowledge of objects.

The neural signature of the Fraser illusion: an explorative EEG study on Fraser-like displays.

We studied neural correlates accompanying the Fraser spiral illusion. The Fraser spiral illusion consists of twisted cords superimposed on a patchwork background arranged in concentric circles, which is typically perceived as a spiral. We tested four displays: the Fraser spiral illusion and three variants derived from it by orthogonally combining featural properties. In our stimuli, the shape of the cords comprised either concentric circles or a single spiral. The cords themselves consisted of black and white lines in parallel to the contour of the cords (i.e., parallel cords), or oblique line elements (i.e., twisted cords). The displays with twisted cords successfully induced illusory percepts, i.e., circles looked like spirals (the Fraser spiral illusion) and spirals looked like circles (i.e., a "reverse Fraser illusion"). We compared the event-related potentials in a Stimulus (Circle, Spiral) × Percept (Circle, Spiral) design. A significant main effect of Stimulus was found at the posterior scalp in an early component (P220-280) and a significant main effect of Percept was found over the anterior scalp in a later component (P350-450). Although the EEG data suggest stimulus-based processing in the posterior area in an early time window and percept-based processing in the later time window, an overall clear-cut stimulus-percept segregation was not found due to additional interaction effects. Instead, the data, especially in the later time window in the anterior area, point at differential processing for the condition comprising circle shapes but spiral percepts (i.e., the Fraser illusion).

Differential familiarity effects in amodal completion: support from behavioral and electrophysiological measurements.

We studied the effects of learning on amodal completion of partly occluded shapes. Amodal completion may originate from local characteristics of the partly occluded contours, resulting in local completions, or from global characteristics, resulting in global completions. Two classes of occlusion patterns were constructed: convergent occlusion patterns, in which global and local completions resulted in the same shape, and the much more ambiguous divergent occlusion patterns, in which these completions resulted in different shapes. We used a sequential matching paradigm and obtained behavioral responses (Experiment 1s and 2) and electroencephalogram recordings (Experiment 3) to investigate whether previously learned shapes influenced completions of partly occluded shapes. Experiment 1 revealed the preference for different completions of both occlusion patterns. In Experiment 2, learning effects were found only for test shapes following divergent occlusion patterns. Experiment 3 showed differential effects with regard to convergent and divergent occlusion patterns on a positive event-related potential in the 150- to 300-ms range, before learning. After learning, modulation of this effect was only found for the divergent occlusion patterns. The results show that amodal completion of shapes can be influenced by a simple learning task when multiple completions of partly occluded shapes are perceptually plausible.

Afterimage watercolors: an exploration of contour-based afterimage filling-in.

We investigated filling-in of colored afterimages and compared them with filling-in of "real" colors in the watercolor illusion. We used shapes comprising two thin adjacent undulating outlines of which the inner or the outer outline was chromatic, while the other was achromatic. The outlines could be presented simultaneously, inducing the original watercolor effect, or in an alternating fashion, inducing colored afterimages of the chromatic outlines. In Experiment 1, using only alternating outlines, these afterimages triggered filling-in, revealing an "afterimage watercolor" effect. Depending on whether the inner or the outer outline was chromatic, filling-in of a complementary or a similarly colored afterimage was perceived. In Experiment 2, simultaneous and alternating presentations were compared. Additionally, gray and black achromatic contours were tested, having an increased luminance contrast with the background for the black contours. Compared to "real" color filling-in, afterimage filling-in was more easily affected by different luminance settings. More in particular, afterimage filling-in was diminished when high-contrast contours were used. In the discussion we use additional demonstrations in which we further explore the "watercolor afterimage." All in all, comparisons between both types of illusions show similarities and differences with regard to color filling-in. Caution, however, is warranted in attributing these effects to different underlying processing differences.