Dynamics of spatial summation in primary visual cortex of alert monkeys

One of the fundamental tasks of the visual cortex is to integrate input fro m different parts of the retina, parsing an image into contours and surface s, and then assembling these features into coherent representations of obje cts. To examine the role of the primary visual cortex in the integration of visual information, we measured the response properties of neurons under d ifferent stimulus conditions. surprisingly, we found that even the most con ventional measures of receptive field (RF) size were not fixed, but could v ary depending on stimulus contrast and foreground-background relationships. On average, the length of the excitatory RF was 4-fold greater for a low-c ontrast stimulus than for a stimulus at high contrast. Embedding a high-con trast stimulus in a textured background tended to suppress neuronal respons es and produced an enlargement in RF size similar to that observed by decre asing the contrast of an isolated stimulus. The results show that RF dimens ions are regulated in a dynamic manner that depends both on local stimulus characteristics, such as contrast, and on global relationships between a st imulus and its surroundings.