A detailed analysis was made of the response characteristics of single cells in the lateral geniculate nucleus of the macaque monkey. The goal was to understand how these cells contribute to the processing of visual information. Data were analyzed from a representative sample of 147 cells, whose responses to equal-energy spectra (presented as diffuse flashes of monochromatic light) were recorded at three radiance levels. On the basis of their responses, the cells were divided into two general classes: (a) spectrally nonopponent cells which respond to all wavelengths with either an increase or decrease in firing rate, (b) spectrally opponent cells (about two-thirds of the sample) which respond with an increase in firing rate to some parts of the spectrum and a decrease to other parts. Four types of opponent cells were found: (i) red excitatory and green inhibitory (+R−G), (ii) green excitatory and red inhibitory (+G−R), (iii) yellow excitatory and blue inhibitory (+Y−B), (iv) blue excitatory and yellow inhibitory (+B−Y). Comparisons with psychophysical data indicated that nonopponent cells transmit brightness information; opponent cells, however, carry information about color, the hue of a light being determined by the relative responses of the four types. The saturation of spectral lights appears to be related to the differences in responses of opponent and non-opponent cells.
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