H. Sato et al., MECHANISMS UNDERLYING ORIENTATION SELECTIVITY OF NEURONS IN THE PRIMARY VISUAL-CORTEX OF THE MACAQUE, Journal of physiology, 494(3), 1996, pp. 757-771
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.