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Abstract

Contextual effects of collinear lines and features on the processing of line targets, finding expression in detection facilitation, have been described recently in the literature (Dresp and Bonnet, 1995 Vision Research 35 1071 -- 1078; Kapadia, Ito, Gilbert, and Westheimer, 1995 Neuron 15 843 -- 856). We are reporting a study of the effects of contrast intensity and contrast polarity on these contextual effects. Thin subthreshold lines, not detectable on a plain background, were flashed at a position collinear with a context line of varying luminance and polarity. A temporal 2AFC procedure was used to determine detection thresholds. The mere presence of a collinear context line of weak contrast, regardless of its polarity, was found to make the subthreshold lines detectable. For target lines and context lines of the same contrast polarity, increasing context contrast first enhances, then diminishes, target detectability. For target lines and context lines of opposite contrast polarity, increasing context contrast enhances target detectability. These preliminary results indicate that contrast intensity and relative contrast polarity interact in the genesis of grouping effects or contextual effects at early stages of visual processing.