Bright lines and edges facilitate the detection of small light targets

Thresholds for the detection of a small light target (increment thresholds), measured at the ends of white lines and small luminance edges, are lower than when the target is presented on a plain field. This facilitation effect disappears when: (1) the line-end is 'stopped' by another line with perpendicular orientation; (2) the inducing line is black instead of white; and (3) when the inducer does not carry information about orientation (e.g., a small dot). These observations suggest that polarity specific and orientation selective neural activation, extending collinearly from the inducing lines and edges, produces a local increase in visual sensitivity. The possible role of such a mechanism in contour completion and the formation of illusory contours is discussed.