Two-photon imaging of V1 responses to “pop-out” texture patterns in awake macaques

Abstract

A line segment among an array of uniformly differently oriented line segments can be detected effortlessly. Various experimental and modeling studies have linked this ‘pop-out’ phenomenon to V1 processing. Here we studied the V1 neuronal activities associated with visual “pop-out” in V1 superficial- layer neurons in awake macaques with two-photon calcium imaging (GCaMP5). The target stimulus was a drifting target Gabor (2-cycles/sec speed, 90%-contrast, 6-cpd SF, 2o- eccentricity) at various orientations. The surrounding stimuli were 1-2 rings of Gabors with uniform or random orientations around the center stimulus. Neuronal responses from 1119 orientation-tuned cells 150 μm deep from the cortical surface were recorded. When the target was surrounded by one ring of uniform Gabors, the cells’ optimal responses were suppressed, with stronger suppression at collinear orientation, and weaker suppression as surround orientation deviated from collinear, which floored at 45o deviation. Adding one or two more rings of Gabors at collinear orientation increased surround suppression, reducing the median R_sur/R_base ratio from 0.578 to 0.496 and 0.462. This trend persisted with surround stimulus orientation up to 22.5o deviation from collinear. In contrast, at 45o or larger deviation, increasing the number of surround rings reduced surround suppression. At orthogonal surround orientation, the R_sur/R_base ratio increased from 0.658 to 0.736 and 0.758. In addition, surround suppression by randomly oriented increased with more rings of surround stimuli, as in the case of collinear surround stimuli. These results demonstrate significantly lower surround suppression when a target is imbedded in orthogonal than in collinear surround stimuli, especially with the surround stimulus set is large and resembles “pop-out” textures (R_sur/R_base ratio = 0.758. Vs. 0.462 with 3 rings of orthogonal vs. collinear stimuli). In “pop-out” textures, far orthogonal surround stimuli may disinhibit near surround stimuli to relieve surround suppression and achieve a pop out percept.