Neuronal plasticity in the afferent visual system - I. Effect of flicker stimulation on responses of retinal and geniculate units in the cat

The effect of long lasting flicker stimulation on the excitatory responses of single units in the optic tract (OT), and the lateral geniculate body (LGB) of cat, was analysed with respect to changes in response strength. ON- and OFF-center units were stimulated with periodic light increments and decrements, respectively, in the receptive field center (RFC). OT units were classified into phasic and tonic, LGB units into phasic, tonic and intermediate, according to their response to a 500 msec stimulus. During the first 30 seconds of 1-Hz flicker stimulation, all OT units and over 90% of the LGB units exhibited a progressive decrease of their responses to roughly half the initial magnitude. The decay was significantly steeper in LGB units than in OT units. Trend analysis of the later responses (after 30 sec) revealed a further slow decrease in tonic LGB units, amounting to 20% in 10 minutes, but not in OT units. The recovery after 10 min of flicker stimulation took about 2-5 min. No systematic difference in the time course of responses was found between ON- and OFF-center units and between the phasic and tonic OT units. Part of the phasic and intermediate LGB units (about 20% of all LGB units) behaved differently in terms of fast habituation of a response component. The influence of intensity, frequency, and mode (increment or decrement) of stimulation was studied, leading to the conclusion that changes in light adaptation alone cannot account for the findings. The stronger decay of LGB units relative to the OT output is discussed either on the basis of passive, intrinsic geniculate processes (tonic units), or in terms of active, acculumating hyperpolarizing activity (phasic and intermediate units).