The main focus of the proposed work is on the question: what limits visual resolution? Treating the vision as a causal chain, measurements of visual phenomena and visual performance are being used to make inferences about how signals are transformed as they flow through the visual system. In particular, we are exploiting a nonlinearity demonstrated in the 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 act 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 products derived from them. In others, we examine how the behavior of the nonlinear mechanism (as monitored by the visibility of its distortion products) is affected by the context of other stimulation. We use laser interferometry to by- pass contrast attenuation by the eye's optics to produce fine stimuli of high retinal contrast, which can be used to characterize the spatial organization of dynamic visual sensitivity regulation to light. Further experiments will reveal the consequences of early visual nonlinearity for the definition of luminance, for color disc rimination and for form perception and motion processing; the first two of which have important implications for users and designers of computer displays. The central nonlinear process of pattern adaptation is also proving useful as an indicator of the quality of the visual representation as it is transmitted from eye to brain, with our recent discovery that patterns too fine for the subject to resolve subjectively can nevertheless penetrate to the cortex and activate pattern-specific neurons there. Proposed experiments exploit this to further advance the analysis of visual resolution losses.
Showing the most recent 10 out of 69 publications
