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Krasnow Institute > Monday Seminars > Abstracts COMPARATIVE ANALYSIS OF SPATIAL VISION AND THE RETINA William Hodos The sharpness of vision is often measured by determining visual acuity, such as is done with the familiar physicians' eye chart. This measure, however, only assesses one aspect of spatial vision; i.e., the ability to discriminate the fine details of small, high contrast objects, such as small black letters against a white background. When objects with many fine details are analyzed for their spatial frequency content, they are found to contain many high spatial frequencies. On the other hand, the overall size and gross shape of an object (ignoring the fine details) contain many low spatial frequencies. Humans and animals need high contrast to detect both their high and the low spatial frequencies. Far less contrast (several log units less), however, is required to detect intermediate spatial frequencies. A plot of an animal's or person's ability to detect contrast over a range of spatial frequencies is known as a "contrast sensitivity function" and characteristically has the shape of an inverted U. In the experiments to be described, contrast sensitivity functions of humans are compared to those of a variety of other vertebrates (fish, mammals, and birds). The data show considerable variability across species in terms of the range of spatial frequencies, the amount of contrast required to detecting various frequencies, and the frequency at which the least amount of contrast is required for detection; i.e, the peak frequency. A remarkable finding was that although birds have excellent visual acuity (high-frequency detection at high contrast), they were relatively insensitive to contrast in comparison with mammals whatever their acuity. In an attempt to account for the variability, peak spatial frequency and maximum sensitivity to contrast were plotted as a function of stimulus variables (target area, target brightness), optical variables (pupil diameter, posterior nodal distance, retinal-image brightness), and retinal variables (photoreceptor density, ganglion-cell density) to determine which variables were the most influential in accounting for the data. Target area and the density of photoreceptors accounted for the much of the variance in the data, but could not account for the low sensitivity to contrast among the birds, which may reflect differences in the internal circuitry of their retinas.
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