Adaptation pools and excitation receptive fields of rat retinal ganglion cells.

Electrophysiological experiments on a variety of animals and psychophysical studies on man indicate that light adaptation spreads laterally across the retina. The evidence suggests that the magnitude of this spread is greater than can be attributed to scattered light (Rushton, 1965a, b; Easter, 1968; Cleland and Enroth-Cugell, 1968; Barlow and Andrews, 1967; Burkhardt and Berntson, 1972; Enroth-Cugell and Shapley, 1973; Green, Tong and Cicerone, 1977), implying that adaptive signals are neurally pooled. There is not, however, general agreement about whether the adaptive summation area is the same as or smaller than the size of the ordinary receptive field. The studies on goldfish (Easter, 1968) and frogs (Burkhardt and Berntson, 1972) show that the adaptation pool is narrower than the response receptive field. The work of Cleland and Enroth-Cugell (1968) and Enroth-Cugell and Shapley (1973) on the cat show that response summation areas and adaptation summation areas are the same size. These conclusions are based on experiments using different methods. Easter (1968) placed a small suprathreshold flashing test in the receptive field of a goldfish ganglion cell. Another small spot was placed in various positions, and at each position its intensity was adjusted to reduce the response to the test to threshold. The intensity required at each position defined the adaptation receptive field. in the frog. Burkhardt and Berntson (1972) used similar methods to plot adaptation receptive fields for test spots centered in the receptive fields of retinal ganglion cells. The findings of Cleland and Enroth-Cugell (1968) and Enroth-Cugell and Shapley (1973) in the cat were based on the size of Ricco’s law summation areas for response and adaptation. Since different methods have been used on different animals, it is hard to know whether to ascribe various resuhs to differences between methods or differences between the retinas of mammals and cold blooded vertebrates. A recent study of ours (Green, Tong and Cicerone. 1977) seemed to imply that rats have smaller adaptation pools than excitation receptive fields. In Green ef al., a small steady adaptation spot was placed within the rat ganglion cell receptive field, and its effect on the receptive field sensitivity profile was measured. The adaptation spot was found to produce a local decrease in sensitivity at and around the adapting spot. The experiments reported here were designed to determine the spatial extent of adaptive pooling by directly measuring the adaptation receptive field of rat retinal ganglion cells using the same methods that Easter (1968) and Burkhardt and Bemtson (1972) had used on cold-blooded retinas.