When we attempt to find the word for something, related words are also accessed (as in word association, priming, freudian slips, and simple errors). But these related words are of two types, taxonomic and thematic:
Across all types of speakers and all manner of testing, semantic naming errors overwhelmingly reflect taxonomic relations; that is, the predominant error is a category coordinate (apple named as pear or grape), superordinate (apple → fruit), or subordinate (apple → Granny Smith). A small subset are thematic errors, such as apple → worm or bone → dog, in which the target and error are from different taxonomic categories but frequently play complementary roles in the same actions or events.
Does this reflect a difference in semantic memory for the two types or not? The researchers of a recent paper, Schwartz etal. (see citation below), used the errors made by stroke victims compared to the location of their brain damage to show a difference between taxonomic and thematic storage in the brain. Their results:
We found that taxonomic errors localized to the left anterior temporal lobe and thematic errors localized to the left temporoparietal junction. This is an indication that the contribution of these regions to semantic memory cleaves along taxonomic- thematic lines. Our findings show that a distinction long recognized in the psychological sciences is grounded in the structure and function of the human brain.
What is the relationship between these two ways of retrieving the right word?
Although many thematic errors in our corpus do involve objects with complementary functions in action events (dog → bone; zipper → jacket), many others are linked by other types of relation, such as spatial relations (e.g., anchor → sea) or causal relations (e.g., ambulance → fire). This goes along with a broader role for this TPJ area in the representation of relational information, which may be what undergirds its essential contribution to sentence comprehension. We suggest that in the process of identifying an object for naming, relevant event representations are retrieved or simulated that create a momentary linkage between the target concept and others in the event context. This process probably takes place bilaterally in the TPJ, but it is the component on the left that conveys information about these linked concepts to left-lateralized lexical-phonological systems. Lesions here render this communication noisier or less precise, thereby reducing the natural advantage of the target concept over its contextual associates and encouraging an error in which one of these associates is named in place of the target.
we propose that the ATL and TPJ are each multimodal hubs that extract somewhat different relationships. The ATL extracts perceptual feature similarity for the purpose of object processing, whereas the TPJ extracts role relations for the purpose of event processing. The ATL system is the dominant one in naming, which explains why taxonomic errors predominate over thematic errors.
I find this very interesting in the context of how we make/understand sentences and how we reason in metaphors. The separation of conceptual structures from the elements that comprise them is indicated. It seems like something deep about how we think is nearing the surface.
Schwartz, M., Kimberg, D., Walker, G., Brecher, A., Faseyitan, O., Dell, G., Mirman, D., & Coslett, H. (2011). From the Cover: Neuroanatomical dissociation for taxonomic and thematic knowledge in the human brain Proceedings of the National Academy of Sciences, 108 (20), 8520-8524 DOI: 10.1073/pnas.1014935108