Abstract

Using a seven-point scale, observers rated their perceptions of small (1-min of arc), very dim flashes of light and blanks presented foveally against a luminous background chosen to isolate either the long-wavelength sensitive or the middle-wavelength sensitive cones. The rating scale comprised three levels of uncertainty below a neutral rating, and three levels of perceived brightness above, for stimuli seen with some certainty. For stimulus intensities below a usual threshold value (~60% seen), the distributions of the rating scores are described well by Poisson statistics, and the expectations of the distributions increase linearly with average stimulus intensity. An increase of one point in the expected rating score required an increase in average stimulus intensity of about 300 photons incident on the cornea. These results are consistent with the hypothesis that observers are able to distinguish perceptions differing by some unitary neural event for stimuli dimmer than those usually described as seen. For stimuli seen with greater certainty, more complex behavior is evident.

© 1989 Optical Society of America