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

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 degrees ) were measured in four observers with normal vision. Detection thresholds measured for LM blobs placed between highly visible LM flankers (111) and for CM blobs placed between highly visible CM flankers (222) produces a similar pattern of lateral interaction effects, i.e. masking where the stimuli overlap and facilitation for separations of 4-8x 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 (212) are generally facilitatory but relatively raised for separations of 0.5-2 degrees . For CM blobs placed between highly visible LM flankers (121), facilitation is stronger in the 0.5-2 degrees region. A significant correlation between thresholds and psychometric function slopes is found only for the 212 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.