The effect of contrast polarity on letter identification

To determine the effect of contrast polarity on the spatial characteristics of letter identification, we measured contrast sensitivity for individual Sloan letters that were presented either as luminance increments or luminance decrements relative to a continuously presented adapting field. The temporal mode of presentation consisted of either a rapid onset with a Gaussian offset, or the reverse. When contrast was specified in terms of Rayleigh (Michelson) units, the contrast sensitivity function for letters of positive contrast extended to smaller letter sizes than the function for letters of negative contrast. However, when contrast was defined in Weber terms, letter contrast sensitivity functions were identical for letters of positive and negative contrast, indicating that letter identification was equivalent for luminance increments and decrements that had equal absolute magnitude. Onset-offset characteristics had no differential effect on letter contrast sensitivity by either contrast definition. These findings provide a basis for predicting the effect of contrast polarity on tasks that involve letter identification.