Abstract

Evidence indicates that the spatial tuning properties of luminance increments and decrements may not be equivalent and that stimulus onset-offset characteristics can differentially affect contrast thresholds. The present study investigated whether this is the case for letter identification. Individual Sloan letters were presented as luminance increments or decrements on a gray-scale display of a Macintosh IIfx computer, with contrasts controlled by an ISR video attenuator. The temporal mode of presentation was either a rapid onset with a Gaussian offset, or the reverse. Contrast sensitivity for letter identification was measured with a forced-choice staircase procedure. When plotted in Rayleigh (Michelson) units, contrast sensitivity was higher for letters of positive than of equivalent negative contrast, and the contrast sensitivity function for letters of positive contrast extended to smaller letter sizes. In Weber units, however, contrast sensitivity was equivalent for increments and decrements of equal magnitudes. Performance at Weber contrasts greater than unity was predictable from a negative exponential fit to the letter contrast sensitivity functions. Onset-offset characteristics had no differential effect on letter contrast sensitivity. These results indicate that the Weber definition provides the better metric for specifying letter contrast, and the findings provide a framework for predicting the effect of contrast polarity on tasks that involve letter identification.

© 1992 Optical Society of America