Spectral Response Curves and Receptive Fields of Pre- and Postgeniculate Fibres of the Cat

Abstract

Unit discharges were recorded from the optic tract, optic radiation and visual cortex of the cat immobilized with D-tubocurarine.
1. Three response types, “on” “on-off” and “off”, were distinguished at pre- and postgeniculate levels. The discharge rate was found to be decidedly low at the postgeniculate level as compared with that at the pregeniculate.
2. The relation between the average frequency of impulses and log intensity of illumination was generally sigmoid in shape, but the “on” component of most “on-off” units did not show any remarkable dependence on the intensity. This component was peculiar also in that it showed a frequency minimum at a certain intensity.
3. Scotopic spectral sensitivity curves obtained at three levels of the visual pathway were almost identical, having a maximum at about 500mμ.
4. Average-frequencies of impulses plotted against wave-lengths or spectral response curves were found different from the spectral sensitivity curves, especially at higher levels of intensity of illumination. The difference was greater at the postgeniculate level, where the curve consisted of two elevations extending over 420-480mμ on the one hand and over 520-600mμ on the other.
5. The receptive fields of pre- and postgeniculate single fibres were mapped by means of an automatic device. The receptive fields of most units, pre- or postgeniculate, contracted as the intensity of background illumination was raised, but in some “on-off” units an expansion of the receptive field was observed above a certain level of adaptation.