The goal of this experiment was to obtain difference limens at several combinations of background brightness and retinal eccentricity. Background brightnesses were 0.0001, 0.001, 0.01, 0.1, 1, and 10 ft-L; retinal locations were the fovea and 2°, 6°, and 10° in the nasal portion. Monocular difference limens were determined using a 1° test stimulus superimposed at the center of a 27° background. The technique of threshold determination was the ascending series of the Method of Limits. The findings suggest that the curve describing log ΔB vs log background brightness becomes steeper as retinal eccentricity increases. At the three higher background brightnesses log ΔB increased as a function of eccentricity; at the three lower brightnesses log ΔB decreased with increasing eccentricity. In both cases the curves were negatively accelerated. It is suggested that the rising functions represent cone and the falling ones, rod thresholds. It appeared that the slopes of the curves relating ΔB to eccentricity were not as great at intermediate background brightnesses as they were at extreme values. This finding suggests a gradual rather than a sudden transition from rod to cone activity as brightness increases.
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