Foveal simultaneous brightness contrast as a function of inducing, and test-field luminances.

Abstract

Simultaneous brightness contrast was investigated in the fovea as a function of the inducing- and test-field luminances. The inducing and the test fields were adjacent squares, each subtending 33[image] on a side, and presented to the subject''s right eye. A match field of equal size was presented to the subject''s left eye. The subject was directed to set the luminance of the match field so that its brightness appeared equal to that of the test field. As the inducing field was varied from below to above that of the test field, the apparent brightness of the test field, and thus the contrast effect, was measured by the actual luminance of the match field. Results showed the contrast function to be of the following nature. The test-field apparent brightness was depressed only slightly for inducing field luminances less than that of the test field. For inducing-field luminances greater than that of the test field the depression of the test-field apparent brightness seemed directly proportional to the luminance of the inducing field. Thus, a "bend" in the function occurred when the inducing-field luminance was approx. equal to that of the test field. This relation was consistent over a large range of test-field luminances. A theoretical formulation based upon the relation of the test-field apparent brightness to the activity in the visual field completely surrounding the test field led to an equation satisfactorily fitting the data. The physiological nature of this surround activity was undetd. The possibility of binocular contrast effects between the inducing and the match fields was considered and shown to be insignificant in a control expt.