Areal effects in foveal brightness discrimination.

Abstract

The effect on foveal brightness discrimination of changes in the diam. of the test- ([DELTA]I-) and adapting-(I-) fields was investigated. For constant sizes of test- and adapting-field, the curve relating log [DELTA]lt/I and log I falls rapidly at low values of log I; it then levels off at medium values of log I. At high values of log I, when the adapting-field is not larger than the rod-free area of the fovea, the curve usually shows a small final rise. For a given brightness and constant test-field size, [DELTA]lt/I decreases as the size of the adapting field increases. For a given brightness and constant adapting-field size, increasing the size of a very small test-field lowers [DELTA]lt/I. For a moderately large test-field an increase in size has little effect on [DELTA]It/I. For a test-field almost as large as its adapting-field, an increase in size up to the limits of the adapting-field may result in increases in [DELTA]lt/I, especially at high adapting-field brightnesses.