COLOR PROPERTIES OF CONTRAST FLASH EFFECT - MONOPTIC VS DICHOPTIC COMPARISONS

The visibility of a brief flash of light can be greatly reduced if it is followed within a few milliseconds by a 2nd flash falling on an adjacent portion of the retina. This type of backward masking is widely known as metacontrast. The essential features of the color mechanism work of Alpern and Rushton using dichoptic as well as monoptic presentation were replicated on a small scale. When the test flash is red, a red contrast flash raises its threshold much more than a green contrast flash of the same luminance. A given contrast flash is equally effective in raising test flash threshold whether presented monoptically or dichoptically. In this respect, contrast flashes act quite differently than steady backgrounds. Contrast flashes also differ from steady backgrounds in having what appears to be a much narrower action spectrum: the difference in effectiveness between a red light and a green one, as measured by the threshold elevation of a red test flash, is much greater when these lights are used as contrast flashes than when they are used as steady backgrounds. Thus both monoptically and dichoptically the results are incompatible with the assumption that masking takes place within a single .pi. mechanism. The contrast flash effect cannot always be reliably assumed to reflect a receptor-specific inhibitory signal acting within the human retina, even when the color properties of the effect seem superficially to be consistent with such a model.