Collinear contour integration impairs visual search before binocular fusion

Abstract

Perceptual grouping plays an indispensable role on figure/ground segregation and attention distribution. However, its relation with visual attention is not well understood yet. Jingling & Tseng (2012) reported that visual search was harder when the target overlapped with a global distractor constituted by collinear elements but not by non-collinear elements. The present study aims at investigating how early this collinear grouping affects selective search in visual pathway. Our observers viewed a 9x9 search display containing identical vertical (or horizontal) bars except a randomly selected distractor column consisting of orthogonal bars. This distractor was grouped into a collinear (snake-like) or non-collinear (ladder-like) organization. Participants judged orientation of a target located either on the distractor column or the other columns. In Experiment 1, we varied the distractor length (= 1, 5, or 9 bars) and found search impairment occurred only at the visual display when the collinear distractor reached a critical length (= 5 bars), not under or when distractor is non-collinear. In Experiment 2, we used a stereoscope to split the distractor column into two eyes: one eye saw a distractor column with varied length (distractor_length_mono = 1, 5, or 9 bars) while the other eye saw the rest parts of the distractor. When both eyes were properly fused, observers saw a search display identical to Experiment 1 with the longest distractor length (distractor_length_bino = 9 bars). If collinear contour affected visual search after binocular fusion, the search impairment effect should be observable in all three conditions. However, we found that observers’ RTs were identical to that in Experiment 1, suggesting monocular collinear contour information dictated selective attention. Our results imply that the effect of collinear grouping on attention is likely to be driven by bottom-up processes.