The primacy of taxonomic semantic organization over thematic semantic organization during picture naming.

Different organizational structures have been argued to underlie semantic knowledge about concepts; taxonomic organization, based on shared features, and thematic organization based on co-occurrence in common scenes and scenarios. The goal of the current study is to examine which of the two organizational systems are more engaged in the semantic context of a picture naming task. To address this question, we examined the representational structure underlying the semantic space in different picture naming tasks by applying representational similarity analysis (RSA) to electroencephalography (EEG) datasets. In a series of experiments, EEG signals were collected while participants named pictures under different semantic contexts. Study 1 reanalyzes existing data from semantic contexts directing attention to taxonomic organization and semantic contexts that are not biased towards either taxonomic or thematic organization. In Study 2 we keep the stimuli the same and vary semantic contexts to draw attention to either taxonomic or thematic organization. The RSA approach allows us to examine the pairwise similarity in scalp-recorded amplitude patterns at each time point following the onset of the picture and relate it to theoretical taxonomic and thematic measures derived from computational models of semantics. Across all tasks, the similarity structure of scalp-recorded neural activity correlated better with taxonomic than thematic measures, in time windows associated with semantic processing. Most strikingly, we found that the scalp-recorded patterns of neural activity between taxonomically related items were more similar to each other than the scalp-recorded patterns of neural activity for thematically related or unrelated items, even in tasks that makes thematic information more salient. These results suggest that the principle semantic organization of these concepts during picture naming is taxonomic, at least in the context of picture naming.