Models of synaptic plasticity in the nervous system have conventionally assumed a mechanism in which spike activity of a postsynaptic cell enhances the efficacy of recently active presynaptic inputs. Making use of the prompt and dramatic response of the visual cortex to occlusion of vision in one eye during the critical period, we tested the role of postsynaptic activity in ocular dominance plasticity. To do so, we selectively blocked cortical cell discharges with a continuous intracortical infusion of the inhibitory neurotransmitter agonist muscimol during a period of monocular deprivation. This drug inhibits cortical cell discharges with no apparent effect on the activity of their presynaptic geniculocortical inputs. Recording from single cortical cells after they had recovered from the muscimol-induced blockade, we found a consistent shift in the responsiveness of the visual cortex in favor of the less-active, closed eye, while the normal shift in favor of the more-active, open eye was evident in regions not affected by the treatment. Such an inhibition-coupled expression of plasticity in favor of the less-active, closed eye cannot be explained by a nonspecific disruption of cortical function. We interpret these results to indicate (i) that the postsynaptic cell is crucially involved in plasticity of the visual cortex, (ii) that the direction of cortical plasticity depends on postsynaptic membrane conductance or polarization, and (iii) that plasticity can occur in the absence of postsynaptic spike activity.
