Surface representation in the visual system

Perception of surface accompanies the impression that a certain area of the visual field is occupied by some quality, such as color, brightness and transparency. This does not mean, however, that information about surface quality must be obtained throughout the area. It has been shown in many situations that our visual system has ability to interpolate information obtained at the border of the surface and to perceive homogeneous surfaces. The most dramatic demonstration of this is the perceptual filling-in at the blind spot. In order to understand the neural representation of surface in the visual system, we conducted a series of experiments using macaque monkeys. First, we examined if neurons in the primary visual cortex (V1) respond when a homogeneous surface is presented on the receptive field. Neurons representing the parafoveal visual field were tested and it was found that about one third of neurons showed significant responses when the cell's receptive field was contained in a homogeneous surface. Then we examined neuron activities in the retinotopic representation of the blind spot in V1. Although there is no retinal input in the blind spot, a homogeneous surface is perceived within the blind spot as a result of filling-in. We tested whether neurons in this region were activated when a homogeneous surface was perceived in the blind spot as a result of filling-in. We found some neurons in V1 were activated by stimuli which lead to the filling-in. These results indicate that when a surface area is perceived, neurons are activated throughout the region in V1 topographically corresponding to the perceived surface and not restricted to the region representing the border of the surface.