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Biomedical Engineering Division, Northwestern University, Evanston, IL 60201.

1. Y-type ganglion cells in the cat's retina were stimulated with bars of light and grating patterns at photopic luminances. Stimuli were stationary, and luminance at each point was varied sinusoidally in time at 2 Hz. Impulse rates were recorded from single cells. 2. When the stimulus was a narrow bar of light, the impulse rate approached a sinusoidal function of time as contrast was reduced. The linear behaviour of each cell was therefore characterized by taking the limit of response parameters as contrast approached zero. 3. The ratio of surround strength to centre strength varied widely between cells but the two strengths were approximately equal on average. The difference between surround phase and centre phase averaged 168 deg. 4. As contrast increased, responses became rectified. Rectifier output was well described by a power law of stimulus amplitude, where the power was usually 1.4 or 1.5. 5. Response phase advanced with increasing contrast, and at high response amplitudes grew less than proportionally with contrast. These effects were assumed due to the contrast gain control described by Shapley & Victor (1978). 6. Gratings in which luminance varied sinusoidally with distance were used to determine Y cell spatial resolution. The second-harmonic amplitude of the response diminished rapidly with increasing spatial frequency: the radius of the best-fitting Gaussian mechanism was about 0.25 deg for a cell at 10 deg eccentricity. 7. This spatial resolution is close to the linear resolution of X cells as determined by Linsenmeier, Frishman, Jakiela & Enroth-Cugell (1982). 8. A receptive field model incorporating both linear and non-linear elements is described. The model consists of an array of subunit pathways, each of which has a centre-surround organization followed by a rectifier; a pool weights and sums subunit outputs, and signals are then passed through a contrast gain control. 9. The model accounts qualitatively for the over-all centre-surround organization of Y cell linear responses, the dependence of frequency-doubled responses on spatial frequency, and impulse rate as a function of time for a variety of bar and grating stimuli.

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