Abstract
Extracellular responses were recorded from single neurons in the lateral geniculate nucleus (LGN) of the cat during presentation of pairs of brief visual stimuli identical to those that produce orientation-selective paired-pulsed suppression in the visual cortex. LGN neurons also show paired-pulse suppression, but the suppression is not orientation selective, and it occurs only for short interstimulus intervals (ISIs; usually less than 200 msec). At longer ISIs, most LGN neurons show a period of facilitation. Thus, the paired-pulse suppression in the LGN cannot account for that seen in the visual cortex. Paired-pulse suppression in the LGN was found to be enhanced by stimulation of the receptive field surround. LGN neurons also showed a second type of suppression, termed “offset suppression,” which consisted of a more long-lasting suppression of spontaneous activity following the offset of an excitatory visual stimulus. The suppression of spontaneous activity was accompanied by a reduction of the antidromic excitability, assessed by stimulating LGN axons within the cortex or optic radiation. Unlike paired-pulsed suppression, offset suppression was not enhanced by increased stimulation of the receptive field surround. Paired-pulse suppression and offset suppression are most likely due to different mechanisms because they have different time courses and depend differently on the spatial properties of the stimuli. Functionally, paired-pulse suppression may be related to the reduced visual sensitivity that accompanies eye movements, while offset suppression may serve to enhance temporal contrast.
