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Abstract:
We measured spatial and temporal characteristics of mechanisms that
bridge gaps between line segments. The presentation time was measured
which was necessary for localization and identification of a triangular
shape made up of (a) pacmen, (b) pacmen with lines, (c) lines, (d) line
segments (corners) or (e) pacmen with circles (amodal completion). The
triangle was embedded in a field of distractors of the same components
at random orientations. Subjects had to indicate whether the triangle
was to the left or to the right of the midline (localization) and
whether it was pointing upward or downward (identification).
Poststimulus masks consisted of pinwheels (a, b, e) or randomly
oriented lines (c, d). Stimuli were presented on a gray background and
defined by luminance or isoluminant contrast. Thresholds 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: 355 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). We conclude
that speed of contour integration depends greatly on the spatial
configuration of the stimulus, but not directly on the formation of
illusory contours. The contour integration process is impaired under
conditions of isoluminance, but not the perception of contours per se.
