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1. The iontophoretic application of bicuculline, an antagonist of GABA, the putative inhibitory transmitter in the visual cortex, has been used to examine the contribution of post-synaptic inhibitory processes to the directional selectivity of simple, complex and hypercomplex cells in the cat's striate cortex.
2. The directional selectivity of simple cells was significantly reduced or eliminated during the iontophoretic application of bicuculline. This supports the view that the selectivity is derived from the action of a GABA-mediated post-synaptic inhibitory input modifying their response to a non-directionally specific excitatory input.
3. Complex cells were subdivided into three categories on the basis of the action of iontophoretically applied bicuculline on their directional selectivity, receptive field characteristics and distribution in terms of cortical layer. They are referred to as type `1', `2' and `3' complex cells.
4. The directional specificity of type `1' complex cells was eliminated during the iontophoretic application of bicuculline. It seems likely, therefore, that they receive a non-directionally specific excitatory input and that, as for simple cells, the directional specificity derives from the action of a GABA-mediated post-synaptic inhibitory input. No type `1' complex cells were recorded below layer IV.
5. The directional specificity of type `2' complex cells was unaffected by the iontophoretic application of bicuculline, despite increases in response magnitude, a block of the action of iontophoretically applied GABA and, in some cases, changes in other receptive field properties. It is suggested that these cells receive a directionally specific excitatory input. The type `2' complex cells were found both superficial and deep to layer IV with the majority in layer V.
6. Type `3' complex cells appear to have very similar receptive field properties to those of the cells described by other workers as projecting to the superior colliculus. They were found predominantly in layer V. Their directional specificity was not eliminated by the iontophoretic application of bicuculline. However, they exhibited a powerful suppression of the resting discharge in response to stimulus motion in the non-preferred direction. Iontophoretic application of ammonium ions revealed a small excitatory response in place of the suppression. It appears from these observations that the directional specificity of the type `3' complex cells could be determined, at least in part, by an inhibitory process which is not GABA-mediated.
7. The directional specificity of hypercomplex cells found in layers II and III was unaffected by the iontophoretic application of bicuculline, and they showed no suppression of their background discharge level in response to stimulus motion in the non-preferred direction. This evidence is consistent with the view that they receive a directionally specific excitatory input.
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