Understanding the role of the prefrontal cortex in phonological processing.

Lesion studies have demonstrated impairments of specific types of phonological processes. However, results from neuropsychological studies of speech sound processing have been inconclusive as to the role of specific brain regions because of a lack of a one-to-one correspondence between behavioural patterns and lesion location. Functional neuroimaging studies have contributed more detailed information about the involvement of specific brain regions in a wide range of phonological tasks. A framework developed by Hickok and Poeppel to account for these neuropsychological and neuroimaging results is evaluated in light of a series of phonological studies in which cognitive load is manipulated by changing the acoustic properties and lexical status of stimuli, as well as the type of phonological judgement. Overall, the findings for speech stimuli are consistent with the view that tasks that require increased articulatory recoding result in increased activation of the posterior aspect of the inferior frontal gyrus (BA 44). However, similar activation patterns for tone sequences as compared to speech may challenge whether the recoding is speech-specific. Implications of these investigations for future neuroimaging studies of individuals with aphasia are discussed.