Abstract
A line target surrounded by orthogonal lines can be detected effortlessly. This perceptual pop-out phenomenon has been linked to weakened suppression of V1 neuronal responses by cross-orientation surrounds than by iso-orientation surrounds when compared to the target-only condition. Here we studied the “neuronal” pop-out effect with two-photon calcium imaging in V1 of awake fixating macaques, investigating its response properties and mechanisms from a population perspective. The stimulus was a central Gabor target centered on the classical RFs with iso- or cross-orientation surrounds of various sizes placed outside the classical RFs. Data collected from four macaques show that besides weakening iso-surround suppression, cross surrounds also reduce trial-by-trial variability (Fano factor). Further, response enhancement by cross surrounds is accompanied by suppression of responses to cross surrounds per se, suggesting a push-pull effect once reported in contour integration. Comparing population orientation tuning functions indicates that iso surrounds mostly produce orientation-untuned suppression, reducing responses of neurons tuned to all orientations, plus some target orientation-tuned suppression. Meanwhile, cross surrounds mainly reduce orientation-untuned suppression by scaling up all neurons’ responses, with little impact on orientation-tuned suppression. These changes can be nicely described by a simplified divisive gain control model R_s (θ)=R_t (θ)^n/k, in which R_t (θ) is the measure target-only population orientation tuning function, and changes of n and k represent orientation-tuned and untuned surround modulation. While iso surrounds induce n and more substantial k changes, cross surrounds mainly reduce k. These population results suggest mostly orientation-untuned iso surround suppression, inconsistent with extant models of surround modulation, but in line with some recent evidence for V1 horizontal connections targeting heterogeneous orientation domains, especially at longer distances (Chavene et al., 2022). Meanwhile, cross surrounds create orientation discontinuity, which may boost all neurons’ responses to de-suppress the orientation-untuned suppression and produce neuronal popout, likely through separate feedback modulation.