We propose to develop and exploit new psychophysical methods for analyzing the spatial and temporal properties of early visual processes, with the goal of establishing causal connections between the psychophysics and the underlying neural mechanisms. Treating the visual process as a causal chain, we use measurements of visual phenomena and visual performance to make inferences about how signals are transformed as they flow through the visual system. In particular we propose to exploit a nonlinearity recently demonstrated in the local visual response that precedes the convergence of signals from different photo-receptors, to analyze the characteristics and visual consequences of optical and neural processes that precede this nonlinearity and of the neural processes that come after it. It is possible to monitor the effects of this early local nonlinearity selectively (without intrusion by later nonlinearities, which are pervasive in the visual system), by stimulating the eye with grating patterns too fine to be resolved except at or near the receptoral level, where processing is still strictly local. When such patterns are briefly presented, keeping space-average luminance constant, the spatially modulated stimulus penetrates to and acts upon only those stages that can resolve the stripes. But an early nonlinear process, transforming the signal at a stage where resolution is still preserved, can change the space-average excitation of later poorly resolving elements to an extent that depends upon the modulation of the unresolved grating. In many of our experiments we proceed by manipulating, more or less independently, the spatial and temporal characteristics of the stimulus gratings and of the distortion product derived from them. In others, we examine how the behavior of the nonlinear mechanism (as monitored through the visibility of its distortion products) is affected by the context of other stimulation. The stimuli used are typically interference fringes that are formed on the retina at high contrast without substantial attenuation by the optics of the eye. Difference-frequency gratings and contrast-modulation flicker are the main nonlinear distortion products that we use for this purpose. Using both threshold and nulling methods to measure these phenomena, we are defining the spatial organization and the dynamics of visual adaptation to light, and partitioning spatial and temporal resolution losses into those preceding the adaptive nonlinearity and those following it. We will also study the compressive nonlinearity in visual contrast responses. This will provide a new psychophysical tool to derive human contrast-response functions, and has important implications for lightness perception.
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