ERP mapping in phonological and lexical semantic monitoring tasks: A study complementing previous PET results

Previous PET results identified distinct neural systems involved with phonology (vicinity of the left sylvian fissure) and lexical semantics (left inferior temporal, left superior frontal, bilateral inferior parietal regions). In the time domain, the phonological task was thought to involve serial parsing of pseudo-words, whereas the lexical semantic task would correspond to probabilistic automated access to meaning. Auditory event-related potentials (ERPs) elicited by the same tasks were explored on 32 channels in 12 male volunteers. Subjects had to categorize as target a second element of a pair of pseudo-words or words if a preceding target was detected in the first element. Depending on the absence/presence of target in the first element, a RELEASE condition and a HOLD condition were distinguished. RELEASE and HOLD ERPs split earlier in the semantic task than in the phonological task (300 versus 412 ms after SOT, respectively), although words lasted longer than pseudo-words. Corresponding paired t test maps showed a predominance of differences over left perisylvian regions for the phonological task and posterior bilateral regions for the semantic task. Underlying generators were investigated using BESA (Scherg, 1990) with global task ERPs. Six dipoles-constrained according to PET clusters-brought residual variance down to 0.36%, from 364 to 565 ms after SOT, in both tasks. Relative dipole amplitudes suggested a left-sided functional asymmetry for phonology. These results support the hypothesis of left perisylvian serial processing for phonology contrasting with bihemispheric parallel access for semantics and substantiates BESA for temporally tackling cognitive processes.