A representational asymmetry for composition in the human left-middle temporal gyrus

Natural language is notable amongst representational systems for the rich internal structure of phrase and sentence-level expressions. Here, we provide evidence from two fMRI studies that a region of the left Middle Temporal Gyrus (MTG) exhibits a surprising representational asymmetry: verbs and patients (to whom was it done?) are bound to form a representation, but verbs and agents (who did it?) are not. Within MTG, BOLD signal to novel combinations of familiar components can be modeled by combining learned verb-patient conjunctive representations with more general agent representations, but not by the converse (verb-agent + patient). This asymmetry is not predicted by an abstract propositional representation of the event (e.g., chased(dog,cat), nor by a theory which derives conjunctions from the experienced statistical co-occurences between verbs and nouns. However, this asymmetry is predicted by various linguistic accounts of the internal structure of event descriptions (e.g., Williams, 1981; Marantz,1984; Grimshaw, 1990; Kratzer, 1996). These results provide evidence for the time-varying instantiation of re-usable representations of structure in MTG, consistent with the principle of compositionality, as well as accounts of verb-argument structure.