Examining the neural architecture of semantic processing: An fMRI investigation

There is considerable interest within the language literature in understanding how conceptual knowledge is stored and processed in the brain. Evidence from clinical case reports and neuroimaging studies suggests that a distributed left hemisphere network is involved in semantic processing, including frontal, temporal, and posterior parietal cortex. Although there is a large body of research aimed at localizing the semantic network, the majority of this research has been limited to the study of concrete nouns, or concepts defined primarily by sensory features. Very few studies have examined the neural substrates of abstract noun processing. Abstract nouns fundamentally differ from concrete nouns in that they lack sensory referents and tend to be defined largely through their association with other words. The purpose of the present study was three-fold: (1) to expand on the current body of literature aimed at localizing the semantic network, (2) to examine the neural systems involved in processing concrete and abstract nouns for meaning, and (3) to examine the effects on brain activation of manipulating semantic and phonological processing demands. Whole-brain, event-related fMRI was acquired while 28 participants performed a series of tasks that were similar in basic structure but that placed differential weight on semantic, phonological and visuoperceptual processing. Task specific processing demands were parametrically manipulated in each condition. The results show that concrete and abstract noun semantic representations are processed and perhaps stored in very different cortical regions. Semantic processing of concrete nouns relies on heteromodal cortex in the ventral temporal, posterior parietal, and posterior cingulate region, whereas processing of abstract nouns engaged perisylvian temporal and frontal areas. These findings are consistent with the dual encoding theory (Paivio, 1971, 1978, 1991) and suggest that concrete nouns are closely linked to multimodal sensory representations in semantic memory while abstract nouns are primarily linked to verbally encoded representations. Finally, the brain activation changes observed in response to manipulating task demands provide further support for the specialized role of the left dorsal prefrontal cortex, posterior parietal cortex, and inferior temporal region in semantic processing. In contrast, the left precentral gyrus and anterior insula appear to be involved in phonology-specific operations. In summary, the results from this study argue against a uniform or homogenous organization to the semantic network. Rather, our results suggest that the semantic network is organized into regions specialized in processing qualitatively different kinds of conceptual knowledge.