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Recently, after two decades of research in visual texture discrimination and in the discrimination of briefly presented patterns, a new structure of human vision has emerged. Accordingly, human vision is mediated by two separate visual systems: a preattentive and an attentive one. The parallel preattentive visual system cannot process complex forms, yet can, almost instantaneously, without effort or scrutiny, detect differences in a few local conspicuous features, regardless of the number of elements or where they occur. These "textons" (Julesz, 1981) are elongated blobs (e.g. rectangles, ellipses, or line segments) of specific color, angular orientation, width, length, binocular and movement disparity, and flicker rate. For line segments, their terminators (ends-of-lines) and crossings often behave as textons. The preattentive system can count only the number of textons and detect their locale, but cannot perceive the positional relations between them (Julesz, 1981, 1981a). However, the preattentive system can direct focal attention to the areas where differences in texton density occur in about 50 msec steps (four times faster than eye-movements). Only in this narrow aperture of focal attention (which can be as small as a few minutes of arc) can positional differences be perceived, permitting the prodigious feats of form recognition (Bergen and Julesz, 1981). Since disparity differences in random-dot stereograms can be perceived in a 50 msec presentation (followed by masking) provided the binocular disparity is beyond a critical limit, disparity is also a texton. It can be shown (for review, see Julesz and Schumer, 1981) that the largest disparity that can yield stereopsis in random-dot stereograms is a monotonic function of the cyclopean target area. Furthermore, the cyclopean modulation function (for corrugated sinusoidal depth gratings) is very "myopic" with a cit-off around 3 c/deg. Since stereopsis is important in quality inspection, an objective determination of stereoscopic abilities using evoked potential techniques to dynamic random-dot correlograms and stereograms is also discussed (Julesz, Kropfl and Petrig, 1980; Julesz and Kropfl, 1982). These techniques also permit the early diagnosis of stereo-blindness that afflicts 2% of the human population and might lead to its prevention.
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