A two-photon imaging study on the roles of macaque V1 and V4 in texture segregation

Abstract

The segregation of visual scenes on the basis of texture differences is a fundamental process in early/middle vision. Previous studies are inconsistent on whether V1 neurons could signal texture based figures from grounds (Lamme, 1995), or simply detect local feature discontinuities through surround modulation (Rossi_et_al., 2001). Here we used two-photon calcium imaging to study neuronal activities associated with texture-based figure-ground stimuli in V1, as well as in V4, in awake fixating macaques. Responses to parafoveal texture stimuli were obtained from 1237 orientation-tuned V1 neurons in two macaques, and from 1049 orientation-tuned V4 neurons in two additional macaques. The figure-ground texture stimuli consisted of a 32x32 arcdeg array of oriented lines in 4 orientations (ground), within which a 4x4 arcdeg square patch (figure) was formed by orthogonally orientated line segments. The center position of the figure patch was varied relative to the pRF of the recording site, so that the pRF fell within the figure, on the figure-ground boundary, or within the ground. Responses to a single oriented bar (to identify orientation-tuned neurons and measure tuning functions) and the uniform textures were also recorded. V1 neurons’ responses to texture stimuli were suppressed compared to neurons’ peak responses to an oriented bar. Slightly less suppression was observed when the figure-ground boundary fell on the pRF, regardless of neurons’ orientation preferences. Similar effects were not observed in V4 neurons, likely because larger V4 RFs made it difficult to align with a specific part of the texture stimuli. However, V4 neurons that preferred higher SFs (≥4 cpd) responded strongly to the figure-ground boundary oriented in the neurons’ preferred orientation. These results suggest that V1 neurons may weakly signal the figure-ground boundary by detecting the local feature discontinuities, while V4 neurons participate in texture segregation by detecting the second-order figure-ground boundary.