Distinguishing the neural bases of lexical access

Much has been discovered about broad brain and language relationships from examining patterns of brain damage and their corresponding patterns of aphasia. Considerably less is known about the specifics of neural systems supporting normal access of the sound and meaning of words. The four experiments reported here concern lexical (word-level) processes, with the overall goal of distinguishing neural systems supporting lexical phonological (word form) access from those supporting lexical semantic (word meaning) access.; In the first experiment, neural correlates of lexical phonological access were addressed by manipulating word frequency in an overt picture naming task. Production of lower frequency words is thought to require additional resources primarily at the level of lexical phonological access. A group of left-sided areas including posterior superior temporal gyrus (pSTG), inferior frontal gyrus (IFG), and occipito-temporal cortex (OT) showed activation for retrieving lower frequency words. When compared to areas which were more active for naming pictures with less familiar concepts (a semantic-level variable), effects of lower frequency words and less familiar concepts overlapped in IFG and OT, but not pSTG, identifying pSTG most specifically with lexical phonological access.; Two converging experiments examined word frequency effects during overt word pronunciation, and during auditory pseudoword repetition, the latter after incorporating a prospective "frequency" effect generated by exposing subjects to some pseudowords more than others. Effects of word frequency in the word pronunciation experiment overlapped those in picture naming in IFG, and the effect of pseudoword frequency overlapped that from picture naming in pSTG.; The fourth and final experiment examined lexical semantic access using a lexical decision task, where the variable of interest was word meaning relatedness. Left-hemisphere activation occurred in OT, overlapping with effects of concept familiarity in the picture naming task, identifying this area most specifically with accessing lexical semantic representations.; These results are consistent with a model in which retrieval of lexical phonology is processed by pSTG, and lexical semantics by OT, with the two processes sharing and possibly being bound together during production by IFG.