Time course of semantic processes during sentence comprehension: an fMRI study

Abstract

The ability to create new meanings from combinations of words is one important function of the language system. We investigated the neural correlates of combinatorial semantic processing using fMRI. During scanning, participants performed a rating task on auditory word or pseudoword strings that differed in the presence of combinatorial and word-level semantic information. Stimuli included normal sentences comprised of thematically related words that could be readily combined to produce a more complex meaning, semantically incongruent sentences in which content words were randomly replaced with other content words, pseudoword sentences, and versions of these three sentence types in which syntactic structure was removed by randomly re-ordering the words. Several regions showed greater BOLD signal for stimuli with words than for those with pseudowords, including the left angular gyrus, left superior temporal sulcus, and left inferior frontal gyrus, suggesting that these areas are involved in semantic access at the single word level. In the angular and inferior frontal gyri these differences emerged early in the course of the hemodynamic response. An effect of combinatorial semantic structure was observed in the left angular gyrus and left lateral temporal lobe, which showed greater activation for normal compared to semantically incongruent sentences. These effects appeared later in the time course of the hemodynamic response, beginning after the entire stimulus had been presented. The data indicate a complex spatiotemporal pattern of activity associated with computation of word and sentence-level semantic information, and suggest a particular role for the left angular gyrus in processing overall sentence meaning.

Figure 1

Maps of group activation at time points 4, 6, 8, 10, 12, 14, and 16 seconds after stimulus onset. The activations are shown projected onto a left-hemisphere, surface-modeled, inflated brain based on the ICBM Single Subject MRI Template. Activations are significant at a corrected p-value of .05. Contrasts from top to bottom include: congruent sentences versus rest, congruent sentences versus random sentences, random sentences versus pseudoword sentences, congruent sentences versus pseudoword sentences, congruent word lists versus random word lists, random word lists versus pseudoword word lists, congruent word lists versus pseudoword word lists.

Figure 2

Graphs of mean BOLD activation within clusters across time for the six conditions. Congruent sentences are shown in blue, congruent word lists in red, random sentences in green, random word lists in yellow, pseudo word sentences in cyan, and pseudoword word lists in magenta. Error bars represent standard error across subjects. Clusters include: (A) inferior frontal gyrus, (B) angular gyrus, (C) inferior frontal gyrus, and (D) superior temporal sulcus and middle temporal gyrus.