Abstract

We have shown previously that the losses of foveal flicker sensitivity following extinction of a bleach can be either precipitous or gradual, depending critically on the spectral characteristics of both the bleach and test.¹ These losses, measured with 18-Hz flicker, also depend on the size and retinal location of the stimuli. At an eccentricity of 4.5°, precipitous losses of sensitivity to 20′ diam tests occur earlier than they do at the fovea. As the parafoveal tests are enlarged, the precipitous losses of sensitivity occur later [e.g., at 1 vs 3 min for 20′ vs 1° diam long wavelength tests that follow extinction of a 4.90-log troland (Td) 540-nm bleach]. As the tests are enlarged more (e.g., to 2° following extinction of the 540-nm bleach), the losses become less precipitous, often resembling the gradual losses found at the fovea for certain spectral combinations of bleaches and 20′ diam tests (the same diameter tests we used previously). On the other hand, when stimuli which result in gradual losses foveally (e.g., a 20′ diam 578-nm test following a 580-nm 4.85-log Td bleach) are moved to the parafovea, the losses can become early and precipitous. Making these foveal tests smaller can also result in precipitous losses. The data suggest that (a) precipitous losses are associated with relatively small tests, while delayed or gradual losses are associated with relatively large tests and (b) the relevant spatial dimensions depend on the spectral parameters of the bleach and test as well as on the retinal locus.

© 1988 Optical Society of America