1. Effects of blocking intracortical inhibition by microiontophoretic administration of bicuculline methiodide (BMI), a selective antagonist for GABAA receptors, on orientation selectivity of 109 neurones were studied in the primary visual cortex (V1) of anaesthetized and paralysed monkeys. 2. The averaged orientation tuning of visual responses of cells was poor in cytochrome oxidaserich blobs of layer II/III and in layer IVc/J, moderate in layers IVb, IVca and V, and sharp in the interblob region of layer II/III and in layers IVa and VI. 3. Iontophoretic administration of BMI reduced the sharpness of orientation tuning of cells to a varying extent in each layer. In most cells, furthermore, the originally ineffective stimuli induced visual responses during the BMI administration, suggesting that excitatory inputs evoked by the non-optimally oriented stimuli were masked by GABAergic inhibition. Nevertheless, the maximal facilitation was observed in the response to the optimally or near-optimally oriented stimuli. 4. There was a difference in such an effect of BMI among layers. Orientation selectivity of cells in interblobs in layer II/III and in layer IVb was sensitive to BMI whereas that of cells in layer VI was relatively insensitive to BMI, suggesting a larger contribution of excitatory mechanisms to the orientation selectivity in this layer. 5. In the orientation-selective cells, an analysis of the magnitude of excitation and inhibition evoked by stimuli at various orientations suggests that both inputs tune around the optimal orientation and their magnitudes are almost proportional to each other except at the optimal orientation. This analysis also indicates that the orientation tuning of inhibition had a less prominent peak around the optimal orientation than that of excitation. 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 Vi, the orientation selectivity of cells is largely dependent on the orientation-biased excitatory and inhibitory inputs which have a broader tuning profile, covering from the optimal to null-orientation, than that observed in extracellularly recorded responses at the control level.
To test the possibility of cross-talk between parallel pathways dealing with different aspects of visual information such as orientation, direction of motion and colour in cortical area V2, we quantitatively analysed visual responses of 121 V2 cells recorded from anaesthetized and paralysed macaques and compared them with those of 147 V1 cells. A selectivity index of visual responses was calculated for each neuron, which was then classified as selective or not to a particular attribute of visual stimuli. Twenty-one percent of the V2 neurons had dual selectivity to both colour and direction of stimulus motion (C&D cells). In V1, only 5% of the cells were C&D cells. Thus, the proportion of C&D cells significantly increased from V1 to V2. We also carried out cross-correlation analysis of spike trains recorded simultaneously from pairs of V2 neurons or pairs of V1 neurons. In V2, correlated firings could be observed between cells with completely different optimal orientation, such as orthogonal, while it was never observed in V1. The cross-correlation analysis further indicted that functional interactions in V2 were more widespread than those in V1. These results suggest that neurons which have different functional properties become less segregated, and that functional interactions become more widespread in V2 than in V1.
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