Representation of Surface Luminance and Brightness in Macaque Visual Cortex
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Perceived brightness of a surface is determined not only by the luminance of the surface (local information), but also by the luminance of its surround (global information). Recent studies have indicated that activities of neurons in the early visual areas are affected not only by stimuli within their receptive fields (RFs), but also by stimuli outside the RE This suggests that the interaction between local and global information in brightness perception may be accounted for in terms of contextual modulation of the activities of luminance-sensitive neurons in the early visual area. We investigated the effects of local and global luminance on the activity of neurons in the primary visual cortex (VI) of awake macaque monkeys performing a visual fixation task. Neural responses to homogeneous surface stimuli at least three times as large as the receptive field were examined. We first tested the sensitivity of neurons to variation in local surface luminance while the luminance of the surround was kept constant. A large majority of surface-responsive neurons (106/115) changed their activity depending on the surface luminance in monotonic fashion; this was more evident in the later response period. Then, the effect of the global luminance was examined in 81 neurons by varying the luminance of the surround while keeping the surface luminance constant. The responses of some neurons (25/81) were not affected by the surround luminance. In other neurons, responses were affected by the luminance of the surround. In some of these neurons, the effects of the luminance of the surface and the surround were in the same direction, while in the others, the directions were the opposite. The former seem to encode the level of illumination, while the responses of the latter parallel the perceived brightness of the surface. These findings suggest that significant interaction between local and global luminance information takes place in VI and perceived brightness of a homogeneous surface is represented at the level of VI.