Binocular brightness: a suppression-summation trade off.

In two experiments, 13 and 9 subjects estimated binocular brightness of targets of large visual extent. On each trial one eye was presented with a fairly intense luminance of 800 cd/m2, and the other eye with one of 12 luminances ranging from zero to 800 cd/m2. The first experiment, using ganzfeld stimuli (stimuli of uniform luminance that cover the entire visual field), produced a large amount of binocular brightness summation and very little Fechner's paradox, a decrease in binocular brightness that occurs when the luminances to the two eyes differ greatly. The second experiment, using a smaller target with very low spatial frequencies, produced greater Fechner's paradox than the ganzfelder, but more binocular summation and less Fechner's paradox than what is usually reported for small targets with abrupt contours. The results suggest a trade off between suppressive and summative mechanisms involving binocular cells that are spatially tuned. The trade off is controlled in the vector-sum model by the angle between vectors, which reflects the total inhibition in spatially tuned, binocular channels.