Monoptic and dichoptic metacontrast across the vertical meridian.

Abstract That type of visual masking known as metacontrast was investigated with a viewing arrangement that imaged the test stimulus and the masking stimulus on different sides of the vertical meridian for both monoptic and dichoptic conditions. If the human visual pathways are in fact organized such that there is a “sharp” nasotemporal division along the vertical meridian, then in this experiment the test stimulus and the masking stimulus ought to have been available to different primary visual cortices, and any masking observed must have occurred beyond area 17. Such across-the-meridian masking was present in both the monoptic and dichoptic conditions; its magnitude was slightly greater monoptically than dichoptically when the test stimulus and the masking stimulus were juxtaposed, but monoptic and dichoptic masking were approximately equal when the test and masking stimuli were separated by 9.5′ or 19′ visual angle. Some difficulties with the classical accounts of the vertical meridian are discussed in relation to these results.