Pattern adaptation in cat visual cortex is a co-operative phenomenon.

The effects of microiontophoretic application of glutamate, GABA and the GABA antagonist, bicuculline methiodide were tested on the degree of adaptation exhibited by striate cortical cells to moving sin wave grating patterns. Application of GABA, which prevents firing of the cell and thereby any fatigue of the cell, did not reduce the degree of adaptation. Administration of either glutamate or GABA, without simultaneous exposure to the adapting high-contrast gratings did not reduce the sensitivity of the cell to subsequent exposure of a low-contrast grating, showing that adaptation is not caused by the excitatory or inhibitory activity of the cell itself. Application of the GABA antagonist, bicuculline did not prevent pattern adaptation, indicating that the lowered sensitivity of the cell is not mediated by a GABAergic inhibition acting on the cell. Thus adaptation of a striate neuron is not due to altered sensitivity of the cell to a constant input but depends upon changes in the input itself. It is most likely that these changes occur in a co-operative cortical network, whose effect on individual cortical cells is mediated by intracortical excitatory connections.