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Biomedical Engineering Department, Northwestern University, Evanston, IL 60208.

1. Changes in different components of the cone-driven centre responses of cat retinal ganglion cells were studied before and during local application of 2-amino-4-phosphonobutyric acid (APB). Responses were elicited with bar stimuli whose luminance was above ('brighter') or below ('dimmer') the photopic background luminance. The bars were centrally located, and were similar in width to the receptive field centre. 2. APB acted differently on the on- and off-centre cells. For on-centre X and Y cells, all components of the responses to bright and dim bars were diminished by APB. For the off-centre X and Y cells. APB reduced all components except the transient increase in firing rate when the bright bar was turned off or when the dim bar was turned on. 3. These results suggest that the centre response mechanism of off-centre X and Y cells comprises APB-sensitive and APB-resistant components. The APB-sensitive component is more sustained and responds to both brightening and dimming stimuli. The APB-resistant component is more transient and responds primarily to dimming stimuli. For on-centre X and Y cells only APB-sensitive components could be demonstrated. 4. Experiments with stationary sinusoidal gratings modulated at 0.5-10 Hz showed that responses of off-centre cells were more affected by APB at low than at high temporal frequencies, confirming that the APB-sensitive pathway is responsible for more of the low temporal frequency responses. As expected from the responses to bar stimuli, APB had a uniform effect at all temporal frequencies in on-centre cells. 5. For off-centre cells, the APB-sensitive component is probably derived from input from depolarizing bipolar cells, and the APB-resistant component is derived from hyperpolarizing bipolar input, although one or both pathways could also involve amacrine cells. The combination of these pathways increases the range of temporal frequencies to which the cell can respond and also increases the range of response amplitudes. 6. The lack of differential effects on on-centre cells may have several explanations. The most likely explanations are that only depolarizing bipolars contribute significantly to the centre responses of these cells under the conditions of these experiments, or that there is an APB-sensitive synapse somewhere in the retina besides the one from cones to depolarizing bipolars.

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