Dependence of Contour Detection Thresholds on the Pattern of Element Spacing

John S. Nafziger¹, Shih-Cheng Yen², and Leif H. Finkel^1,2

¹Institute of Neurological Sciences
²Department of Bioengineering
3320 Smith Walk, 301 Hayden Hall
University of Pennsylvania
Philadelphia, PA 19104, U. S. A.

Abstract

Purpose: Contour detection thresholds have previously been measured using evenly spaced elements (Field et al., 1993; Kovacs et al., 1996). We investigated the effect of non-even element spacings on detection thresholds.

Methods: Subjects performed a 2AFC task to detect contours embedded in stimuli composed of 625 randomly oriented gabor patches. Stimuli extended over a viewing area of 17ƒx17ƒ, individual gabor patches were 0.3ƒx0.3ƒ. Stimuli were presented for 160 ms with an ISI of 500-1000 ms. Thresholds were determined for contours with evenly spaced elements, contours in which the inter-element spacing increased proportionally along the contour, and contours with random inter-element spacings.

Results: For straight line contours, the detectability of proportionally-spaced contours exceeds that of evenly spaced contours with the same average element spacing. At detection threshold, the average spacing of proportionally-spaced contours, , exceeds the average spacing of evenly-spaced contours, . Proportionally-spaced contours are detectable under conditions in which the initial, smallest inter-element interval is at and all successive spacings on the contour exceed . Under this condition, detection thresholds for contours with fewer elements are significantly lower, indicating that contour elements at spacings exceeding improve detection.

Conclusions: Detection thresholds measured with evenly spaced contours may not reflect the maximum possible inter-element separation. Given an initial region of a contour with threshold spacing, adjoining contour elements can be detected at greater inter-element spacings. These results are consistent with models of contour salience in which facilitation of the initial elements improves detection of subsequent elements (Pettet et al., 1996; Yen and Finkel, 1996).