Detection facilitation by collinear stimuli in humans: dependence on strength and sign of contrast

We measured detection of a thin vertical line (target) in the presence of a slightly thicker collinear, adjacent line (inducer). Sign and strength of contrast of the inducer were varied. Test lines could be either bright or dark. Detection thresholds were obtained through a temporal two-alternative forced-choice (2AFC) procedure with the method of constant stimuli. When target and inducer had equal contrast polarity, low thresholds of target lines were observed for low inducer contrasts and increased with increasing inducer contrast. With opposite contrast polarity of target and inducer, thresholds were high for low inducer contrasts and decreased for increasing contrast thereof. Our results support the hypothesis that cortical mechanisms with different sensitivity to the sign and strength of contrast participate in the detection facilitation of line contours.