An intracellular study of neuronal organization in the visual cortex.

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Neuronal connections in the visual cortex of cat (areas 17 and 18) were studied with intracellular recording and electrical stimulation techniques under Nembutal anaesthesia. Four types of axonal projection were seen; 1. association efferent cells projecting to adjacent cerebral cortex on the ipsilateral side, 2. commissural efferent cells to visual cortex on the contralateral side, 3. corticofugal efferent cells to the ipsilateral lateral geniculate body and superior colliculus, and 4. non-efferent cells whose projection is confined within the visual cortex. Both association and commissural efferent cells were located in layer III, corticofugal efferent cells in layer V and non-efferent cells in layers II–VI. Upon these cells two types of synaptic actions were exerted by the specific visual afferents that originate from the lateral geniculate body; 1. type I, monosynaptic excitation plus disynaptic inhibition and 2. type II, disynaptic excitation plus trisynaptic inhibition. Type I effects were found in layers III–V, and type II in layers II and VI. In the border region between areas 17 and 18 monosynaptic excitation and disynaptic inhibition were produced also by the commissural efferents originating from the contralateral visual cortex. On the basis of these results, a possible neuronal circuitry in the visual cortex is postulated.