Activity in the optic tract and lateral geniculate nucleus of th cat during the first moments of light adaptation in the scotopic region.

Abstract

1. Short duration background luminances at scotopic levels suppress the spontaneous activity of off-center neurones of the cat's optic tract and activate on-center neurones, up to a maximal intensity limit after which the ongoing discharge in off-center neurones disappears and in on-center neurones is no longer influenced. Response patterns in the lateral geniculate nucleus (LGN) in off-center neurones are similar to the tract; activity in on-center neurones is poorly correlated with background luminance. 2. Transient or phasic response was measured for all types of neurones to a short, constant intensity stimulus given in the dark and at various adaptation luminances. For on-center neurones of both tract and LGN the phasic response was attenuated regularly over several log units of increasing background luminance. Off-center neurones are far weaker in phasic response rate than on-center neurones and are easily suppressed by background light. 3. Curves describing phasic response as a function of contrast between test light and adaptation luminance very probably reflect changes in the retinal “gain control”. Ongoing activity, while showing essential differences between the two antagonistic neuronal subsystems (off-center neurones or D-system and on-center or B-system), is clearly related to neither phasic response nor the retinal gain control. 4. Human subjects could easily identify differences in contrast between test light and background luminance. They are less successful in identifying background alone. 5. The findings are discussed regarding psychophysical results and some theories relating perception and nervous activity, and to the role of activation and inhibition in optic tract and geniculate.