Light responses of ganglion cells in the retina of the turtle

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Abstract
Recordings were made from single axons of 218 ganglion cells in the optic nerve of the turtle, P. scripta elegans. Responses to light stimuli were used to classify 182 of the cells into 4 functional groups. Movement-sensitive cells (113 units) responded best to moving stimuli independent of the stimulus color or direction of motion. Their receptive fields were organized into antagonistic centers and surrounds with the same spectral sensitivity. Based on their patterns of response they were of 3 types: ON-center, OFF-center and ON-OFF. Directionally selective cells (65 units) responded best to stimuli that moved in 1 preferred direction and not to stimuli in the opposite direction. Their receptive fields had antagonistic centers and surrounds with the same spectral sensitivity. An area of silent inhibition occurred on 1 side of the receptive fields. The preferred directions appeared to fall into 3 groups separated by about 120.degree.. Response patterns were of 2 types; OFF-center and ON-OFF. Sensitizable color cells (2 units) had center-surround organization with red-green opponent responses from the center and the surround (double opponent cells). The sensitivity to either of the opponent colors was effectively turned on or off by a steady background of the other opponent color. Orientation cells (2 units) responded best to moving bars of specific orientations in the visual field. Their receptive fields consisted of adjacent mutually antagonistic areas with linear boundaries. Under photopic conditions all of the ganglion cells were most sensitive to light near 630 nm wave-length, the spectral maximum of the red-sensitive single cones in the turtle retina. About 75% of the cells had a secondary spectral peak near 560 nm, the spectral maximum of the green-sensitive single cones. Three cells had unusually broad sensitivity extending into the blue protion of the spectrum. Over half of the cells received additional input from rods as evidenced by a shift in spectral sensitivity and increases in latencies and receptive field sizes with dark-adaptation. Measurements of the absolute sensitivity of the ganglion cells showed that a threshold response required absorption of several hundred photons in the red-sensitive cones of the receptive field center. After 10 min dark adaptation the rods required absorption of about 20 photons.