Human Search Performance is a Threshold Function of Cortical Image Separation

Abstract

We have previously reported that detection performance of monkey subjects during active visual search is constrained by object density. Here we report similar results for human subjects. Methods. Three subjects searched for a trial-by-trial cued T or L target in arrays of rotated (60 deg increments) T's & L's. Array set sizes of 6, 12, 24, or 48 items were randomly placed within a 35.5 × 26.5 deg display area. Each stimulus segment was 1.0 × 0.25 deg. Stimuli were either all green or all red, varied by trial. The task was to find and fixate the target for 600 ms. Target detection was defined as having occurred when the next saccade captured the target by landing within 1.1 deg of the target center. Eye position measurement was made using an SMI Eyelink system. Detection probability was measured as a function of the V1 cortical image separation of the target and its nearest neighbor. Separations were measured on a 3D model of the curved V1 surface constructed using current estimates of a human cortical magnification factor. Results. In humans, as well as in monkeys, active search proceeded via sequential fixations of stimuli (75% of fixations within 2 deg of nearest stimulus). Target detection probability can be described as a threshold function of the mm separation of the V1 cortical image representations of the target and its nearest distractor. To achieve comparable performance humans require about twice (2X) the cortical separation as monkeys. The similarity of this result to the known doubling of the size of the ocular dominance column widths in humans versus monkeys, suggests that the scale of spatial interactions between objects is linked to the scale of hypercolumns in V1.