Spatial sensitivity, responsivity, and surround suppression of LGN cell responses in the macaque

Responses of cells in the lateral geniculate nucleus (LGN) of the macaque monkey have been measured for different sizes of chromatic and achromatic stimuli, with relative luminance spanning a range of 3-6 log units. Homogeneous illuminated test fields, centered on the receptive field, were used. Responses to these stimuli deviated from what is expected for the grating stimuli used to study the contrast-sensitive mechanisms in the visual pathway. For test fields smaller than the center of the receptive field, both the excitatory and the inhibitory cone-opponent components were present in the response, and the sensitivity to both components increased with the same factor when the test field increased in size (area summation). For test field areas extending into the suppressive surround of the extraclassical receptive field, the excitatory and the inhibitory cone opponents were both suppressed, again by the same factor. This suppression of the cell's responsiveness, as a function of test spot area, was described by a logarithmic function, and the spatial sensitivity of attenuation could therefore be described by a power function of radius. The logarithmic suppression was clear for parvocellular and koniocellular cells but was more prominent for magnocellular cells. The surround field suppression was also found for the prepotential inputs to LGN cells, indicating a retinal origin. The difference of Gaussian (DOG) model has been used successfully to describe the cells' contrast behavior for grating stimuli. However, this model fails to describe the constant excitatory/inhibitory response balance needed to obtain color (hue) stability for light stimuli of different sizes but with the same Commission Internationale de l'Eclairage (CIE) chromaticity and luminance factor. Neither the constant responsiveness found in the center of the receptive field nor the suppressive response in the surround can be described by the DOG model.