Monocular activation of visual cortex in normal and monocularly deprived cats: an analysis of evoked potentials.

Abstract

The unilaterally induced patterns of prominent excitatory postsynaptic activity within areas 17 and 18 were investigated in normal and monocularly deprived cats. They were elicited by optic nerve electrical stimulation and evaluated with the 1-dimensional current source-density method. In area 18 of normal cats the unilaterally and bilaterally induced current source-density patterns closely resemble each other. None of the mono-, di- or trisynaptic activities is potentiated by binocular convergence. In area 18 of monocularly deprived cats the synaptic currents elicited by stimulating the nerve on the deprived side lead to approximately the same spatial and temporal distribution of sinks and sources as those induced from the normal eye; the amplitudes are considerably smaller. This reduction is similar for mono-, di- and trisynaptic responses, which indicates that the imbalance between activity from the deprived and non-deprived eye is mainly due to reduced input to the cortical target cells from the deprived eye, and that the activity from the deprived eye still relayed to these cells is passed on to supra- and infragranular layers without diminution and in the same way as activity from the normal eye. The imbalance of afferent activity from the deprived and non-deprived eye is apparent in the evoked potentials recorded from the white matter. Activity from the deprived eye may already be strongly reduced in the thalamo-cortical fibers. In monocularly deprived cats, the monosynaptic activities from the 2 eyes are often segregated in depth within layer IV; this did not occur in normal cats. In area 17 of normal and deprived cats, only a small fraction of the potential monosynaptic activity can be elicited by electrical stimulation of the optic nerves because of transmission failure in the lateral geniculate nucleus. Comparison of the current source-density patterns elicited from the normal and deprived nerve in monocularly deprived cats indicates that activity produced by fast-conducting afferents is more affected (reduced) by deprivation than that conveyed by slower afferents.