Monday, November 12, 2012

Complementary taxonomic and thematic semantic systems

I am happy to report that my paper with Kristen Graziano (a Research Assistant in my lab) showing cross-task individual differences in strength of taxonomic vs. thematic semantic relations is in this month's issue of the Journal of Experimental Psychology: General (Mirman & Graziano, 2012a). This paper is part of a cluster of four articles developing the idea that there is a functional and neural dissociation between taxonomic and thematic semantic systems in the human brain.  

First, some definitions: by "taxonomic" relations I mean concepts whose similarity is based on shared features, which is strongly related to shared category membership (for example, dogs and bears share many features, in particular, the cluster of features that categorize them as mammals). By "thematic" relations I mean concepts whose similarity is based on frequent co-occurrence in situations or events (for example, dogs and leashes do not share features and are not members of the same category, but both are frequently involved in the taking-the-dog-for-a-walk event or situation).

Regarding the functional dissociation, I described in an earlier post our finding (Kalenine et al, 2012) that thematic relations are activated faster than taxonomic relations (at least for manipulable artifacts). In this most recent paper we show that the relative degree of activation of taxonomic vs. thematic relations during spoken word comprehension predicts  - at the individual participant level - whether that participant will tend to pick taxonomic or thematic relations in an explicit similarity judgement task. In other words, for some people, taxonomic relations are more salient and for other people thematic relations are more salient, and this difference is consistent across two very different task contexts.

Regarding the neural dissociation, in a voxel-based lesion-symptom mapping study of semantic picture naming errors (i.e., picture naming errors that were semantically related to the target), we found that lesions in the anterior temporal lobe were associated with increased taxonomically-related errors relative to thematically-related errors and lesions in the posterior superior temporal lobe and inferior parietal lobe (a region we refer to as "temporo-parietal cortex" or TPC) were associated with the reverse pattern: increased thematically-related errors relative to taxonomically-related errors (Schwartz et al., 2011). In a follow-up study, we found that individuals with TPC damage showed reduced implicit activation of thematic relations, but not taxonomic relations, during spoken word comprehension (Mirman & Graziano, 2012b).

I think these findings add some important pieces to the puzzle of semantic cognition and we're now working on a theoretical and computational framework for explaining these complementary semantic systems. Kalénine S., Mirman D., Middleton E.L., & Buxbaum L.J. (2012). Temporal dynamics of activation of thematic and functional knowledge during conceptual processing of manipulable artifacts. Journal of Experimental Psychology: Learning, Memory, and Cognition, 38 (5), 1274-1295 PMID: 22449134
Mirman D., & Graziano K.M. (2012a). Individual differences in the strength of taxonomic versus thematic relations. Journal of Experimental Psychology: General, 141 (4), 601-609 PMID: 22201413
Mirman D., & Graziano K.M. (2012b). Damage to temporo-parietal cortex decreases incidental activation of thematic relations during spoken word comprehension. Neuropsychologia, 50 (8), 1990-1997 PMID: 22571932
Schwartz M.F., Kimberg D.Y., Walker G.M., Brecher A., Faseyitan O.K., Dell G.S., Mirman D., & Coslett H.B. (2011). Neuroanatomical dissociation for taxonomic and thematic knowledge in the human brain. Proceedings of the National Academy of Sciences of the United States of America, 108 (20), 8520-8524 PMID: 21540329