Scotopic Vision: Behavioral Sensitivity and Underlying Optical and Neural Factors

163 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006.Contrast sensitivity at a para-central visual location was measured for cats and humans utilizing the same visual stimuli and similar behavioral paradigms over an 8 log-unit range of adaptation levels spanning scotopic and mesopic vision. Over the same luminance range, contrast thresholds were measured for neurons in the cat dorsal lateral geniculate (LGNd) and medial interlaminar (MIN) nuclei. Behavioral sensitivity was measured for 7 spatial frequencies (0, 1/8, 1/4, 1/2, 1, 2 and 4 cyc/deg) using drifting Gabor functions (4 Hz) presented at 8-degree eccentricity along the horizontal meridian. Neural sensitivity was estimated for 3 spatial frequencies (1/8, 2 and 4 cyc/deg) with sine gratings drifting at 4 Hz. Cats had better sensitivity for spatial frequencies below 1/2 cyc/deg, whereas humans were more sensitive above this. In the scotopic range, cats detected stimuli, on average, at an adaptation level 0.72 log-unit lower than humans for spatial frequencies at or below 1/4 cyc/deg. However, contrast sensitivities of the two species in these stimulus conditions were quite similar when compared in terms of retinal illuminance adjusted for three major optical factors (pupil size, focal length and the cat's tapetum). For both species, scotopic increment thresholds were proportional to the square root of retinal illuminance, in accordance with the de Vies-Rose law, which holds when noise in stimuli is the major limiting factor to sensitivity. A comparison of the cat's sensitivity with that of thalamic cells derived from ROC analyses applied to mean firing rate (F0 response) and temporally-modulated activity (F1 response) indicated that the cell class and neural code critical for behavioral sensitivity vary with spatial frequency and adaptation level. For stimuli within the cat's acuity, F1 responses dominated, with average Y-cell sensitivity matching behavioral sensitivity at 1/8 cyc/deg in the scotopic range, and the most sensitive X cells accounting for behavior at 2 cyc/deg from high scotopic through mesopic vision. For 4 cyc/deg (near the acuity limit of mesopic vision), behavioral sensitivity was predicted by F0 responses of Y cells. MIN Y cells were, on average, more sensitive than LGNd Y cells by 0.25 log units in scotopic conditions.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD