This research will investigate whether aphasic patients demonstrate the same sensitivity to internal phonetic category structure as do normal subjects, and whether this sensitivity is subject to the same contextual effects. Specifically, the proposed studies will examine the extent to which aphasic patients structure phonetic categories in terms of a central prototype with surrounding exemplars both near the category boundary and at the non-distinctive end of the category, and the influence of speaking rate as cued by vowel duration on this distribution of exemplars. These questions will be addressed by analyzing reaction times on tests of phoneme discrimination based on a number of temporal parameters which have been extensively examined in normal subjects (i.e. voice-onset time in voicing contrasts for initial stop consonants, duration of formant transitions in stop-glide contrasts; and vowel duration in voicing contrasts for final consonants). In addition, the effects of rate-dependent phonetic category structure on lexical access for both normal subjects and aphasic patients will be examined in a semantic priming paradigm, where speaking rate and sub-phonetic properties of prime words will be manipulated independently. The influence of the sub-phonetic manipulations on semantic priming will then be analyzed as a function of rate context, to determine the nature of the rate effect in normals and the extent to which any deficit in performance observed in aphasic patients reflects an impairment in phonetic category structure.