Lateral interactions across space reveal links between processing streams for luminance-modulated and contrast-modulated stimuli

Abstract

Foveal detection thresholds for luminance-modulated (LM) and contrast-modulated (CM) blobs in the presence of fixed modulation, laterally placed noise blobs (separations of 0–6°) were measured in four observers with normal vision. Detection thresholds measured for LM blobs placed between highly visible LM flankers (1 1 1) and for CM blobs placed between highly visible CM flankers (2 2 2) produces a similar pattern of lateral interaction effects, i.e. masking where the stimuli overlap and facilitation for separations of 4–8× blob sd units. The region of facilitation is not matched by shallow psychometric function slopes. Detection thresholds measured for LM blobs placed between highly visible CM flankers (2 1 2) are generally facilitatory but relatively raised for separations of 0.5–2°. For CM blobs placed between highly visible LM flankers (1 2 1), facilitation is stronger in the 0.5–2° region. A significant correlation between thresholds and psychometric function slopes is found only for the 2 1 2 condition. We propose a model with two separate but interacting processing streams for the detection of LM and CM targets that may engage different cortical loci.