A systematic investigation of the functional neuroanatomy of auditory and visual phonological processing.

Neuroimaging studies of auditory and visual phonological processing have revealed activation of the left inferior and middle frontal gyri. However, because of task differences in these studies (e.g., consonant discrimination versus rhyming), the extent to which this frontal activity is due to modality-specific linguistic processes or to more general task demands involved in the comparison and storage of stimuli remains unclear. An fMRI experiment investigated the functional neuroanatomical basis of phonological processing in discrimination and rhyming tasks across auditory and visual modalities. Participants made either "same/different" judgments on the final consonant or rhyme judgments on auditorily or visually presented pairs of words and pseudowords. Control tasks included "same/different" judgments on pairs of single tones or false fonts and on the final member in pairs of sequences of tones or false fonts. Although some regions produced expected modality-specific activation (i.e., left superior temporal gyrus in auditory tasks, and right lingual gyrus in visual tasks), several regions were active across modalities and tasks, including posterior inferior frontal gyrus (BA 44). Greater articulatory recoding demands for processing of pseudowords resulted in increased activation for pseudowords relative to other conditions in this frontal region. Task-specific frontal activation was observed for auditory pseudoword final consonant discrimination, likely due to increased working memory demands of selection (ventrolateral prefrontal cortex) and monitoring (mid-dorsolateral prefrontal cortex). Thus, the current study provides a systematic comparison of phonological tasks across modalities, with patterns of activation corresponding to the cognitive demands of performing phonological judgments on spoken and written stimuli.