Length and width summation in human vision at different background levels.

The effect of background intensity on the spatial summation of rectangular stimuli of varying length and width was studied in human psychophysical experiments and compared with the known effects of light adaptation on the spatial summation of circular stimuli. Both the detection threshold and the threshold of orientation identification were measured. In agreement with previous data, summation was more efficient along the line stimulus than across it. This asymmetry was found to exist at all adaptation levels studied (1-1000 trolands). The adaptation level affected both length and width summation; the change in the length of summation was twice as great as the change in the diameter of summation with circular stimuli. Orientation selectivity was reduced for short lines presented on a dim background. The results suggest the existence of mechanisms of light adaptation at the level of cortical orientation-selective units.