Abstract
Visual neurons adjust their sensitivity to the immediate environment through the process of adaptation. Adaptation increases sensitivity when signals are weak, to improve the signal-to-noise ratio, and decreases sensitivity when signals are strong, to avoid response saturation. In the retina, cells adapt to the mean intensity but also to the contrast, or the range of light intensities relative to the mean. Contrast adaptation is not present at the output of photoreceptors and thus must arise within retinal circuitry. I will describe experiments in mammalian retinal ganglion cells, measured in vitro, that aim to elucidate the cellular mechanisms for contrast adaptation. A linear systems approach is used to quantify a gain change at high contrast; the analysis is robust in the presence of known nonlinearities in the subthreshold or spiking responses. When contrast triples, the gain in the spiking response reduces by about a factor of two. Intracellular measurements show that the gain change is partially an intrinsic property of the cell, related to spike generation, and partially a synaptic mechanism that arises within the presynaptic bipolar cells. Different ganglion cell types express differing degrees of adaptation, and this variation must relate to each cell type's unique pattern of intrinsic properties and synaptic connections.