Categorical perception requires spatially distributed attention

A yes/no discrimination task and a 4AFC task were performed with letter-like stimuli in order to demonstrate that categorical perception only occurs for letters of the alphabet under conditions of spatially distributed attention. The requirement of spatially distributed attention was studied further under conditions requiring focused (within-category stimulus pair) or distributed (between-categories) attention. Reaction times were measured for detection of an 'odd' stimulus in a display of either 2, 4, or 6 letter-like stimuli. Two different modes of perception were demonstrated. Sloped plots of reaction times, indicating no categorical perception, were produced for within-category stimulus pairs. Flat plots of reaction time, indicating categorical perception, were produced for between-categories stimulus pairs. A 4AFC experiment was carried out with simple vertical lines in order to assess whether categorical perception would occur with a 4AFC design regardless of the stimuli used. Vertical lines would not be expected to demonstrate categorical perception. No categorical perception was found for these stimuli. Spatially distributed attention is necessary for categorical perception of letter-like stimuli to occur.