MECHANISMS UNDERLYING ORIENTATION SELECTIVITY OF NEURONS IN THE PRIMARY VISUAL-CORTEX OF THE MACAQUE

1. Effects of blocking intracortical inhibition by microiontophoretic administration of bicuculline methiodide (BMI), a selective antagonist for GABA(A) receptors, on orientation selectivity of 109 neurones wer e studied in the primary visual cortex (V1) of anaesthetized and paral ysed monkeys. 2. The averaged orientation tuning of visual responses o f cells was poor in cytochrome oxidase-rich blobs of lager II/III and in layer IVc beta, moderate in layers IVb, IVc alpha and V, and sharp in the interblob region of layer II/III and in layers IVa and VI. 3. I ontophoretic administration of BMI reduced the sharpness of orientatio n tuning of cells to a varying extent in each layer. In most cells, fu rthermore, the originally ineffective stimuli induced visual responses during the BMI administration, suggesting that excitatory inputs evok ed by the non-optimally oriented stimuli were masked by GABAergic inhi bition. Nevertheless, the maximal facilitation was observed in the res ponse to the optimally or near-optimally oriented stimuli. 4. There wa s a difference in such an effect of BMI among layers. Orientation sele ctivity of cells in interblobs in layer II/III and in layer IVb was se nsitive to BMI whereas that of cells in layer VI Mras relatively insen sitive to BMI, suggesting a larger contribution of excitatory mechanis ms to the orientation selectivity in this layer. 5. In the orientation -selective cells, an analysis of the magnitude of excitation and inhib ition evoked by stimuli at various orientations suggests that both inp uts tune around the optimal orientation and their magnitudes are almos t proportional to each other except at the optimal orientation. This a nalysis also indicates that the orientation tuning of inhibition had a less prominent peak around the optimal orientation than that of excit ation. This dominance of excitation over inhibition around the optimal orientation may function to accentuate the response to the optimally oriented stimulus. 6. These results suggest that, in the monkey V1, th e orientation selectivity of cells is largely dependent on the orienta tion-biased excitatory and inhibitory inputs which have a broader tuni ng profile, covering from the optimal to null-orientation, than that o bserved in extracellularly recorded responses at the control level.