Linearity and Non-linearity in the Retinal Light Response

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This continuing grant for three years is on mechanisms of visual response in the eye. The retina of the eye is the sensory tissue containing the photoreceptor cells connected to several kinds of nerve cells. The simplest subunit of human visual processing is the response from a uniform patch of retina illuminated when it is stimulated by light. This response to light is not simple, but subject to a sequence of events which can be modelled mathematically as linear and non-linear processes. Over the past century, a set of psychophysical "laws" have been established to relate dynamic visual responses to stimulus changes over time. But some of these laws imply simple linear addition of stimulus effects, and some imply that the effects cannot be added linearly. This project will try to clarify what kinds of processing in the retina could satisfy all of these laws under non-extreme conditions. The question of linearity of the response at each level of the visual system, and conditions under which the linearity fails have complementary significance un unravelling the sequence of visual processing. The general approach is to use the nonlinear stages as markers to determine the response dynamics before and after each such marker. Human observers will be tested by measuring their perception of the intensity of flickering lights, while varying the intensity and duration of the stimulus. The first set of experiments will examine the linearity of the mechanism of phototransduction, in which the light signal is first converted to a neural response signal. intensity levels high enough to saturate the system will be used to explore the time course of photochemical reactions preceding the saturation. The second experiments will test how non- linearity applies to the change in threshold for perception of a test pulse when there is a large abrupt "step" change in light intensity. Analysis of the data will include mathematical modelling of the functions describing the stimulus-response relationships. This integrated approach is a novel and very sophisticated one, with a strong theoretical component. The results should have impact on visual research in many areas, and on models of information processing in other systems.