Distinct frontal regions for processing sentence syntax and story grammar

Time is a fundamental dimension of cognition. It is expressed in the sequen tial ordering of individual elements in a wide variety of activities such a s language, motor control or in the broader domain of long range goal-direc ted actions. Several studies have shown the importance of the frontal lobes in sequencing information. The question addressed in this study is whether this brain region hosts a single supramodal sequence processor, or whether separate mechanisms are required for different kinds of temporally organis ed knowledge structures such as syntax and action knowledge. Here we show t hat so-called agrammatic patients, with lesions in Broca's area, ordered wo rd groups correctly to form a logical sequence of actions but they were sev erely impaired when similar word groups had to be ordered as a syntacticall y well-formed sentence. The opposite performance was observed in patients w ith dorsolateral prefrontal lesions, that is, while their syntactic process ing was intact at the sentence level, they demonstrated a pronounced defici t in producing temporally coherent sequences of actions. Anatomical reconstruction of lesions from brain scans revealed that the sen tence and action grammar deficits involved distinct, non-overlapping sites within the frontal lobes. Finally, in a third group of patients whose lesio ns encompassed both Broca's area and the prefrontal cortex, the two types o f deficits were found. We conclude that sequence processing is specific to knowledge domains and involves different networks within the frontal lobes.