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Abstract:
Purpose: To determine spatial and temporal characteristics of mechanisms that bridge gaps between line segments. Methods: The presentation time was measured which was necessary for localization and identification of a triangular shape made up of (a) pacmen, (b) pacmen with extended lines, (c) lines, (d) corners or (e) pacmen with circles (see Figure). The triangle was embedded in a field of distractors of the same components at random orientations. Subjects had to indicate whether the triangle was left or right of the midline (localization) and pointing up or down (identification). Poststimulus masks consisted of pinwheels (a, b, e) or random lines (c, d). Stimuli were presented on a gray background and defined by luminance or isoluminant contrast. Results: Thresholds for identification were fastest when the triangle was defined by real contours (b: 98 msec; c: 106 msec), slightly slower for corners and pacmen (d: 129 msec; a: 157 msec), and much slower for the amodally completed pacmen (e: 359 msec). For all pattern types localization was about 20 msec faster than identification. Compared to low contrast luminance stimuli, processing of isoluminant stimuli was equally fast for targets defined by real contours (c), but much slower for illusory contours (a). Conclusions: Since processing is faster for corners than for pacmen, bridging line gaps is not dependent on the formation of illusory contours.
