Segregating Semantic from Phonological Processes during Reading

TLDR

Previous neuroimaging links left inferior frontal activation to semantic processing, yet frontal lobe damage does not critically impair semantic knowledge. The study aims to distinguish semantic knowledge from the strategic processes needed for verbal decision making. PET was used to compare neural activity during semantic versus phonological decisions on the same visually presented words, with tasks differing only in attention to meaning or phonological segmentation. Semantic decisions activated left extrasylvian temporal regions (temporal pole, BA 39 near the angular gyrus), whereas phonological decisions engaged supramarginal gyri, left precentral sulcus, and cuneus, supporting the dissociation of semantic knowledge and phonological segmentation.

Abstract

A number of previous functional neuroimaging studies have linked activation of the left inferior frontal gyms with semantic processing, yet damage to the frontal lobes does not critically impair semantic knowledge. This study distinguishes between semantic knowledge and the strategic processes required to make verbal decisions. Using positron emission tomography (PET), we identify the neural correlates of semantic knowledge by contrasting semantic decision on visually presented words to phonological decision on the same words. Both tasks involve identical stimuli and a verbal decision on central lingual codes (semantics and phonology), but the explicit task demands directed attention either to meaning or to the segmentation of phonology. Relative to the phonological task, the semantic task was associated with activations in left extrasylvian temporal cortex with the highest activity in the left temporal pole and a posterior region of the left middle temporal cortex (BA 39) close to the angular gyrus. The reverse contrast showed increased activity in both supramarginal gyri, the left precentral sulcus, and the cuneus with a trend toward enhanced activation in the inferior frontal cortex. These results fit well with neuropsychological evidence, associating semantic knowledge with the extrasylvian left temporal cortex and the segmentation of phonology with the perisylvian cortex.