Plaid Detectors in Macaque V1 Revealed by Two-Photon Calcium Imaging

Neuronal responses to one-dimensional orientations are combined to represent two-dimensional composite patterns; this plays a key role in intermediate-level vision such as texture segmentation. However, where and how the visual cortex starts to represent composite patterns, such as a plaid consisting of two superimposing gratings of different orientations, remains neurophysiologically elusive. Psychophysical and modeling evidence has suggested the existence of early neural mechanisms specialized in plaid detection [1-6], but the responses of V1 neurons to an optimally orientated grating are actually suppressed by a superimposing grating of different orientation (i.e., cross-orientation inhibition) [7, 8]. Would some other V1 neurons be plaid detectors? Here, we used two-photon calcium imaging [9] to compare the responses of V1 superficial-layer neurons to gratings and plaids in awake macaques. We found that many non-orientation-tuned neurons responded weakly to gratings but strongly to plaids, often with plaid orientation selectivity and cross-angle selectivity. In comparison, most (∼94%) orientation-tuned neurons showed more or less cross-orientation inhibition, regardless of the relative stimulus contrasts. Only a small portion (∼8%) of them showed plaid facilitation at off-peak orientations. These results suggest separate subpopulations of plaid and grating responding neurons. Because most of these plaid neurons (∼95%) were insensitive to motion direction, they were plaid pattern detectors, not plaid motion detectors.