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Enumeration of Illusory Contours

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Title: Enumeration of Illusory Contours
Author: Dienes, Natasha; Trick, Lana
Department: Department of Psychology
Program: Psychology
Advisor: Trick, Lana
Abstract: Visual search and enumeration tasks were used to assess the attentional demands of processing illusory contour figures (figures composed of contours that are perceived despite there being no physical changes in the scene). In all tasks, participants were required to find or enumerate a vertical rectangle(s) defined by either line-endings or “pacmen” figures (Kanizsa class). Search time for line-end class figures did not increase markedly as the number of horizontal distractors increased (efficient search). In contrast, when searching for Kanizsa targets amid horizontal distractors, search was inefficient. Simple enumeration (enumeration of targets) and selective enumeration (enumeration of targets in distractors) tasks were used to disambiguate two stages of figure processing: individuation and shape discrimination. Specifically, simple enumeration was used to assess the attentional demands of unit formation and individuation (defining a unit as unique and separate from everything else) of the objects and selective enumeration measured the attentional demands required to discriminate target shapes from distractor shapes. Participants enumerated 1-9 vertical real or illusory contour rectangles either without distractors or with 4 or 8 horizontal distractors. Subitizing, a fast (40-100 ms/target) process specialized for small numbers of items (e.g. 1-3), is only evident when enumeration makes low attentional demands; when attentional demands are greater, the same slow process is used throughout the number range (e.g. 200+ ms/target in the 1-8 range, Trick & Pylyshyn, 1994). Both line-end and Kanizsa class real and illusory contour figures were subitized in simple enumeration, indicating that individuation of these figures is not attentionally demanding. However, subitizing was only evident for line-end real-contour figures in selective enumeration. This suggests that attention plays a role in defining shapes formed by illusory contours. These findings contribute to our understanding of both illusory contours and enumeration.
Date: 2014-08
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