OPTICAL AND RETINAL LIMITS TO VISUAL PERFORMANCE

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DESCRIPTION (Investigator's abstract): The long term goal of this research program is to determine how optical and retinal factors affect vision. Specific aims for the next project period fall into three categories: acuity, aliasing and optics. Acuity is the most important measure of visual function in clinical practice and it plays a key role in current debates about the limits to performance imposed by the neural architecture f the visual system. Previous experiments have shown that in peripheral vision, resolution acuity is limited by the ambiguity of aliasing due to neural undersampling of the retinal image, probably by the relatively sparse array of retinal ganglion cells. The investigators propose to test this ganglion cell hypothesis by mapping the resolution acuity of the human observer over the entire visual field using the aliasing criterion, a psychophysical task which is known to be sampling-limited, and then comparing the results with recent anatomical estimates of ganglion cell density. In this way the investigators aim to establish the functional significance of ganglion cell density for normal peripheral acuity, and thus the consequences of nerve fiber loss in the diseased eye. Aliasing is the false perception of a pattern too fine to be resolved. Such illusory percepts occur naturally in peripheral vision and are due to neural undersamplng. Evidence of the functional utility of aliased images will be south by comparing visual performance before and after anti-alias filtering of visual stimuli. Resolution and detection of harmonically-rich patterns will be measured for peripheral vision and the results compared with that obtained for sinusoidal grating to determine how the higher frequency, non-resolvable components affect performance. This approach will be adapted for clinical testing of peripheral vision by employing a striped-letter optotype which is visible due to aliasing independently of the size of the letter. With such targets the investigators expect to demonstrate aliasing in a clinical context and thus exploit an essential difference between foveal and peripheral vision which may have diagnostic value for glaucoma and other ocular diseases characterized by the death of retinal ganglion cells. Optical filtering of the retinal image by the eye represents a potential limiting factor for any visual task. Recently the investigators have developed a simple schematic eye which accounts well for the effects of chromatic aberration and diffraction on vision. The only free parameters of the model involve the position of the pupil relative to the eye's nodal point and visual axis. The investigators now propose to survey the distribution of pupil locations in a large number of individual eyes using a previously developed, two-color vernier-alignment technique. The investigators also propose to examine the importance of axial pupil location on the modulation transfer function of the model eye and to extend these theoretical calculations to include color contrast.