A contour formed of aligned Gabor patches is easily detected amidst a dense background of randomly oriented patches; the longer the string of aligned patches, the more easily the contour is detected (D. J. Field, A. Hayes, & R. F. Hess, 1993). Here we show that a short string of collinear elements acts as a cue pointing to other patches of similar orientation that lie along the path defined by the string. Cueing might increase the gain of similarly oriented elements in the vicinity and/or reduce the number of potential locations (uncertainty) that the observer monitors. To assess the strength of the contour cue, I measured sensitivity to contrast increments on a test patch placed at various offsets relative to the cueing contour. Noise density and the length of cueing contour were also manipulated. Signal detection theory analysis of the psychometric function provided estimates of the gain and uncertainty parameters associated with each condition. In the presence of noise, observers were best at detecting a contrast increment on a patch that was aligned with the cueing string. Gain estimates were high for the aligned condition but decreased with increasing offset from this position. Uncertainty estimates were invariant with offset at a given noise level but increased with increasing noise. Uncertainty also decreased significantly when the length of the cueing string was increased. The high gain for collinear test patches and the ability of the contour to reduce uncertainty at high noise densities is indeed similar to the effect of an explicit cue (e.g., Z.-L. Lu & B. A. Dosher, 1998), suggesting that one part of the contour cues other parts and contributes to the detectability of contours in noise.
Publication Type: Journal Article
Authors: Preeti Verghese
Publication: Journal of vision, Volume 9, p.2 1-16 (2009)