Inhibition of the brightness of illumination when an adjacent visual field is illuminated.
This is an apparently paradoxical phenomenon because it consists of a reduction in subjective brightness of a flash of light which is caused by a second flash following shortly afterward in an adjacent region of the visual field. The effect depends upon the duration, intensity, surface areas of the two flashes, the retinal area stimulated, and particularly the interval of time between the two flashes. The phenomenon appears most clearly with an interval of about 0.1s and disappears when that interval reaches 0.3-0.4s. Syn. backward masking (this term is used to indicate when the test stimulus and the masking stimulus overlap spatially). A flash of light can also be made to appear slightly less bright when it is preceded by another flash in an adjacent region of the visual field and the interval of time is of the order of 0.05s. This second phenomenon is called paracontrast. Syn. forward masking (this term is used when the test stimulus and the masking stimulus overlap spatially). See masking.