Activations of "motor" and other non-language structures during sentence comprehension.

In this paper we report the results of an experiment in which subjects read syntactically unambiguous and ambiguous sentences which were disambiguated after several words to the less likely possibility. Understanding such sentences involves building an initial structure, inhibiting the non-preferred structure, detecting that later input is incompatible with the initial structure, and reactivating the alternative structure. The ambiguous sentences activated four areas more than the unambiguous sentences. These areas are the left inferior frontal gyrus (IFG), the right basal ganglia (BG), the right posterior dorsal cerebellum (CB) and the left median superior frontal gyrus (SFG). The left IFG is normally activated when syntactic processing complexity is increased and probably supports that function in the current study as well. We discuss four hypotheses concerning how these areas may support comprehension of syntactically ambiguous sentences. (1) The left IFG, right CB and BG could support articulatory rehearsal used to support the processing of ambiguous sentences. This seems unlikely since the activation pattern associated with articulatory rehearsal in other studies is not similar to that seen here. (2) The CB acts as an error detector in motor processing. Error detection is important for recognizing that the wrong sentence structure has been chosen initially. (3) The BG acts to select and sequence movements in the motor domain and in cognitive domains may serve to inhibit competing and completed plans which is not unlike inhibiting the initially non-preferred structure or "unchoosing" the initial choice when incompatible syntactic input is received. (4) The left median SFG is relevant for the evaluation of plausibility. Evaluating the plausibility of the two possibilities provides an important basis for choosing between them. The notion of the use of domain general cognitive processes to support a linguistic process is in line with recent suggestions that the a given area may subserve a specific cognitive task because it carries out an appropriate sort of computation rather than because it supports a specific cognitive domain.