Perceptual filling-in and the resonant binding of distributed cortical representations

Pessoa et al. (1998a) underexposed the broad and rich variety of stimuli in the amodal completion domain. The disambiguation of occluded shapes depends on very specific figural properties. Elaborations on such disambiguations of rich and complex stimuli, tied up with a visuocognitive origin of amodal completion, further position Pessoa et al.’s considerations on neural filling-in and the personal-subpersonal distinction. Pessoa et al. (1998a) have written an impressive paper on perceptual completion, comprising a wide scope of phenomena with diverse phenomenological qualities. Amodal completion certainly belongs to the weakest of the discussed completion phenomena. Every observer can witness that the phenomenological presence of amodal contour completions is not as compelling as, for example, blind spot filling-in, neon color spreading, or even the perception of illusory contours. Nevertheless, convincing psychophysical data exist on the relevance of amodal contour completions as well. Much of the research in the domain is concerned with the disambiguation of occluded shapes. That is, while in fact an infinitive number of different completions are possible for each and every visual pattern, only a few of them are plausible. Although Pessoa et al. briefly mentioned that local and global figural properties may influence completion, the richness of the domain, the diversity of completions, and the vision-versus-cognition dilemma (typical of this domain), are underexposed and not related to their own concepts (e.g., the personal-subpersonal distinction). This is a missed opportunity. Whereas in local approaches, completion depends on specific local configurations and proceeds by way of curved or linear interpolation between contour ends, in global approaches specific overall shape regularities (like bilateral symmetries) determine completion.1 It is important to note here that local and global strategies may converge to the same shape but may also diverge to different shapes. The relevance of and competition between, both types of completions have been investigated by, for example, Sekuler (1994), Sekuler et al. (1994), and Van Lier et al. (1994; 1995a; 1995b). 2 The stimulus-dependent plausibility of a small set of completions is not as self-evident as it might seem to be. For example, with his “ignorance-of-absence” assumption Dennett (1991) also disregards the specific influence of figural properties on completion by stating that the brain jumps to a conclusion (the issue here would be: what conclusion? ). The unique status of a small set of completions also holds if the stimulus domain is further extended, for example, to a completion of the unseen back of a nonfamiliar object (Van Lier & Wagemens 1999) or so-called fuzzy completions of quasi-irregular shapes (Van Lier 1999). The more enriched the stimuli (see also Fig.1), the more compelling the question of whether we are (still) dealing with visual completions. Would there be a fundamental difference between the visual and/or cognitive processing of, say, a partly occluded square and the back of a tree trunk (Van Lier 1999)? Is a simple “vision/cognition” verdict possible anyway? As briefly indicated by Pessoa et al., the vision-versus-cognition issue has been part of the debate in the literature. In particular, global completions are often thought to be cognitive (e.g., Kanizsa 1985). So far, however, psychophysical evidence for the relevance of local and global amodal completions does not rigorously separate visual processing from cognitive processing. In addition, investigating this issue in terms of neural activation in the visual cortical area will turn out to be a hazardous enterprise – not only because the presumed mapping between the responsible neural substrate and the specific completion is unclear (see also Pessoa et al.), but also because there is simply no clear-cut border where the visual system ends and the cognitive system starts.3 So, even if there were evidence on the neural filling-in of the BEHAVIORAL AND BRAIN SCIENCES (2001) 24, 1135–1148 Printed in the United States of America © 2001 Cambridge University Press 0140-525X/01 $12.5