Adaptive Processes of the Limulus Lateral Eye

Our ability to see spatiotemporal fluctuations in contrast over widespread levels of illumination is due to retinal gain controls that constantly modulate the properties of cellular and synaptic mechanisms in our eyes. At present two general forms of gain control have been described in the vertebrate retina, one of which adapts to mean luminance and the other to luminance variance (contrast). We sought to test for both processes of adaptation in an invertebrate retina by recording the spike trains of individual optic nerve cells in the horseshoe crab to white noise stimuli of different luminance and contrast. The stimuli were also input to conceptual models of the retina with linear-nonlinear elements in assorted configurations and to a cell-based computer model that accurately simulates the retinal response to underwater movies. We find that the crab eye shows both luminance and contrast adaptation. Only the former was reproduced by the cell-based model but the latter could be mimicked by linear-nonlinear cascade model, which suggests the two adaptive processes are related or a heretofore unknown mechanism underlies contrast adaptation. That such a simple eye can produce seemingly complex behaviors like luminance and contrast adaptation makes the finding of broad importance to vision research.