The Influence of Intensity, Color and Retinal Location on the Fusion Frequency of Intermittent Illumination

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Abstract
Measurements of the critical fusion frequency for white light, when made with the rod-free area of the fovea yield a single, sigmoid relation between critical frequency and log I. Similar measurements made with a retinal area 5[degree] off-center containing rods and cones show a relation possessing two clearly separated sections. The high intensity section resembles the one obtained for the fovea, while the low intensity section is new. The two sections are separate functions of the rods at low intensities and of the cones at high intensities. This is borne out by measurements made with retinal areas 15[degree] and 20[degree] from the fovea where the ratio of rods to cones is anatomically greater than at 5[degree] off-center. As a result the two sections of the data become still further separated than at 5[degree] off-center. Confirmation of the rod-cone character of the peripheral data is furnished by measurements of flicker for 5o off-center, with different portions of the visible spectrum. Following the established variations in relative sensibility of the rods and cones at different wave-lengths, the two sections of the data show a corresponding separation along the intensity axis for different wave-lengths.

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