1. Cats were monocularly deprived for 3 mo starting at 8-9 mo, 12 mo, 15 mo, and several years of age. Single cells were recorded in both visual cortexes of each cat, and the ocular dominance and layer determined for each cell. Ocular dominance histograms were then constructed for layers II/III, IV, and V/VI for each group of animals. 2. There was a statistically significant shift in the ocular dominance for cells in layers II/III and V/VI for the animals deprived between 8-9 and 11-12 mo of age. There was a small but not statistically significant shift for cells in layer IV from the animals deprived between 8-9 and 11-12 mo of age, and for cells in layers V/VI from the animals deprived between 15 and 18 mo of age. There was no noticeable shift in ocular dominance for any other layers in any other group of animals. 3. We conclude that the critical period for monocular deprivation is finally over at approximately 1 yr of age for extragranular layers (layers II, III, V, and VI) in visual cortex of the cat.

In the primary visual cortex (V1), the single-neuron response to a grating stimulus placed in the classical receptive field (CRF) is suppressed by a similar stimulus presented in the CRF surround. To assess the input mechanism underlying the surround suppression, we tested the effects of iontophoretically administered GABA A -receptor antagonist, bicuculline methiodide (BMI), for the 46 V1 neurons in anesthetized cats. First, the stimulus-size tuning curves were studied, with or without BMI administration, for each neuron by changing the size of the grating patch. During the BMI administration, the shape of the normalized size tuning curve did not change considerably. Second, the dependency of surround suppression on the orientation of the surround grating was examined. In the control, the surround suppression showed the clear orientation tuning that peaked at an orientation the same as the optimal orientation of the CRF response. The BMI administration did not change the orientation dependency of surround suppression. We also estimated the relative contribution of excitation and inhibition to the size and orientation tuning of surround suppression. It was concluded that cortical excitation and inhibition were well balanced, having similar tuning profiles for both stimulus size and orientation of the surround grating. Furthermore, surround stimuli used for V1 neurons suppressed the CRF response of neurons in the lateral geniculate nucleus. These results suggest that surround suppression is not primarily attributable to the intracortical inhibition, but because of a reduction of thalamocortical inputs, which drive the cortical excitation and inhibition, and a subsequent decrease in the cortical excitatory interactions.

1. Effects of microionophoretic application of acetylcholine (ACh) and its antagonists on neuronal responses to visual stimuli and to electrical stimulation of the lateral geniculate nucleus were studied in the cat striate cortex. 2. Responses elicited visually and electrically were facilitated by ACh in 74% of the cells tested, whereas the responses were suppressed in 16%. These ACh effects were blocked by a muscarinic antagonist, atropine, but not by a nicotinic antagonist, hexamethonium, indicating that the ACh effects are mediated through muscarinic receptors. A single application of atropine suppressed visual responses of cells facilitated by ACh, whereas it enhanced those of cells inhibited by ACh, suggesting that endogenous ACh may tonically modulate visual responsivity of cortical neurons. 3. In most cells with the facilitatory ACh effect, responses with single spikes to the electrical stimulation became more consistent, often with double spikes, during the ACh application. The suppressive effects of ACh were noted most often in cells with a longer response latency to electrical stimulation of lateral geniculate nucleus. 4. In most of the facilitated cells the spontaneous activity remained null or very low during ACh application, in spite of marked enhancement of visual responses, suggesting that ACh may improve the signal-to-noise ratio (S/N) of cortical neuron activity. To confirm this suggestion, we calculated a S/S + N index by counting the total number of spikes in the responses (S) and that in peristimulus time histogram (S + N) and found that it was improved during the ACh application in about a half of the cells, whereas it became worse in about one-fifth. 5. In most of the facilitated cells, ACh enhanced visual responses not only to optimal but also to nonoptimal stimuli, resulting in no improvement or even worsening of the orientation selectivity. This was also the case in the selectivity of direction of stimulus movement. 6. The laminar location of the facilitated cells was biased toward layers V and VI of the cortex, although they also made up the majority in layers II + III and about half the tested cells in layers IVab and IVc. 7. In the light of recent understanding of cortical circuitry, these results suggest that the cholinergic innervation to cortical neurons may play a role in improvement of the S/N ratio of information processing in the striate cortex and in facilitation of sending processed informations to other visual centers.

The role of N-methyl-D-aspartate (NMDA) receptors in cat visual cortex was studied as a function of both layer and age by iontophoresis of the NMDA antagonist (D)-2-amino-5-phosphonovaleric acid (APV). Effects on both visual responses and spontaneous activity were observed. In superficial layers (II and III), D-APV reduced visual responses substantially at all ages. Iontophoresis of D-APV with 10 nA of ejecting current for 2-3 min was sufficient to reduce the response to approximately one third of control levels. The magnitude of the reduction did not vary with age. In granular and deep layers (IV, V, and VI), D-APV affected the visual response in young animals but only spontaneous activity in older animals. On average, visual responses were reduced to about half at 20-23 days of age and to about 75% at 4 weeks of age but in most cases were not significantly affected in adults. The rapid change in the functional effect of NMDA receptors over the third and fourth week in granular and deep layers suggests a role in development. There was a reasonable age correlation between the change in effect and the period of geniculocortical afferent segregation. Further experiments will be necessary to determine whether NMDA receptors are necessary for segregation to occur. The presence of an NMDA component to the visual response in the adult in layers II and III argues either that these layers retain some form of plasticity in the adult or that NMDA receptors play a role in the transmission of normal visual input to these layers.

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.