Abstract
Orientation and color tuning are basic response properties of visual cortical neurons. Recent developments of monkey two-photon calcium imaging allow simultaneous recordings of large number of neurons, providing a powerful tool to study functional organizations of orientation and color tuning at single-neuron resolution. The current work studies orientation and color functional maps in macaque V4, and contrasts them to V1 maps. Neuronal responses to a b/w or equiluminant color grating (contrast = 90%, SF = 1.6 cpd) were recorded. V1 neurons in inter-blob areas show higher orientation selectivity, significant orientation clustering, and lower color selectivity, and in the blob area show opposite results. Those showing significant color selectivity are tuned to limited colors. Overall the orientation selectivity of neurons increases as color selectivity decreases. Over 16k V4 neurons in thirteen FOVs of four monkeys were identified. About 21% of them were orientation selective, much less than around 90% in V1. Unlike V1 orientation clustering, orientation preferences in V4 neurons showed no clear or weak orientation clustering. In FOVs with stronger color selectivity, color tuning was clustered, and color selectivity decreases as orientation selectivity increases. When much larger cortical surfaces of V4 were studied with single-photon imaging at a coarser resolution, the vector sums of responses to various orientations tended to have very large circular variances (>0.85), indicating often similar responses to different orientations and thus weak orientation clustering on a more global scale. In summary, V4 neurons are at best weakly clustered in the orientation domain but more clustered in the color domain. More evenly distributed orientation preferences may facilitate the emergence of responses to complex patterns involving multiple orientations. The broader range of color tuning and color clustering confirm that V4 plays a pivotal role in color perception (color center).