Iterative Models in Figure-Ground Perception: Tests and Challenges

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Visual perception seems trivially easy. Nevertheless, it has long withstood the attempts of both computer scientists and vision scientists to crack its code, most likely because past theories were based predominantly on conscious vision. In order for perceivers to experience a coherent visual world, perceptual input must be organized into separate objects, or "figures." As part of this process, the brain decides where a figure lies with respect to every border between two contiguous regions of space. Consider a vertical border, for instance. The brain decides whether that border is a boundary for a figure lying on the left or the right. When the border is perceived as a boundary of a figure lying on the left, the region on the right seems simply to continue behind the figure at the border; no shape is perceived there. These "figure-ground" decisions are made outside of conscious awareness. Because of this, it is extremely difficult to investigate the mechanisms that produce figure-ground perception. An important, unanswered, question is whether figure-ground perception is accomplished via fast, feed-forward mechanisms or whether iterative mechanisms involving feedback from higher processing levels are involved. In feed-forward models, the input is processed in successive stages until a coherent percept emerges. In iterative models, feed-forward processing is not sufficient; feedback from higher to lower levels is necessary to create the percept.Mary Peterson and her colleagues at the University of Arizona have designed visual displays that allow them to investigate this question. One display type is a small symmetric, bounded silhouette lying on a larger ground. Portions of familiar shapes are hidden along the groundside of the silhouette's border; these shapes are not perceived consciously, only the shape of the enclosed silhouette is perceived consciously. (The classic "face-vase illusion" is an example of this.) Previous experiments have shown that the shape of the hidden object is suppressed when it is not perceived, supporting the view that figure-ground perception results from inhibitory competition between shapes that might be seen on opposite sides of a border; the winner is perceived as the shaped figure, whereas the loser is suppressed and the portion of space where it might have been seen is perceived as a shapeless ground. These displays are designed to isolate competition at the shape processing stage. Dr. Peterson and colleagues will also conduct a series of experiments which test whether suppression can be observed at lower levels where individual parts are represented and at higher levels where shape descriptions ("semantics") are represented. According to a feed-forward account, suppression of the losing shape would prevent access to its semantics; hence, no effects should be evident at higher levels. Without feedback, no effects should be evident at lower, part-processing, levels either. An iterative view could account for suppression at lower and/or higher levels by assuming that the outcome of the between-shape competition was relayed to higher and lower levels. Thus the proposed experiments will adjudicate between these competing views of how perception occurs. The researchers will also attempt to identify the processes involved in segregating figures from grounds in crowded real-world scenes. The perception of these more complex displays constitutes a challenge to current iterative models of figure-ground perception. The planned research will provide a foundation for neurophysiological experiments and formal computational models of vision and will contribute to our understanding of the temporal and spatial dynamics of shape perception.