The Variation of Critical Fusion Frequency with Brightness at Various Retinal Locations*

The purpose of this experiment is to investigate the variation in critical fusion frequency with brightness for various positions in the test field.Critical frequency curves for brightnesses between −3 and +3 log millilamberts were determined for a 2° test field of green light at the fovea and at 10°, 20°, 30°, and 40° in the periphery. The procedure used allowed an image of a 2-mm circular aperture to be focused in the plane of the pupil. Tests showed that astigmatism did not influence the data obtained for the peripheral stimuli.The curve of log critical frequency against log brightness for foveal observation does not show an inflection in its rise to a maximum. The curves for peripheral observations rise to a plateau at low brightnesses, dip at medium to low brightnesses, and rise again to a maximum, thus giving the conventional "cone" curve at high brightnesses. The maximum is lower and lower for locations increasingly distant from the fovea.Data on color thresholds indicate that rods probably make their contributions to the initial part of the "cone" branch.Comparison of data for green and white central fields shows that these colors give data that superimpose for equivalent foveal stimulation. For peripheral stimulation a slight shift of the curve for green (with respect to the curve for white) seems to occur in the direction of lower brightnesses. The latter effect is interpreted to be a Purkinje shift.The Purkinje shift is greatly emphasized when orange and green peripheral fields are used. Green light, matched foveally with orange light, is more effective than orange in providing peripheral flicker.The data are interpreted to mean that rods function and interact with cones at levels of brightness that are often considered to be photopic.