Is left fronto-temporal connectivity essential for syntax? Effective connectivity, tractography and performance in left-hemisphere damaged patients

Syntactic processing typically engages left inferior frontal gyrus and posterior middle temporal gyrus, and damage to these regions is associated with syntactic deficits. What has not been directly determined, however, is whether it is the effective connectivity between these regions - and therefore also the integrity of the white matter tracts that connect them - that underpins successful syntactic analysis. We addressed these related issues by obtaining measures of the psycho-physiological interaction between frontal and temporal regions and of the integrity of the major white matter tracts that directly connect them - the arcuate fasciculus and extreme capsule fibre system. We correlated these estimates with performance measures of syntax in a group of patients with left hemisphere damage and varying degrees of syntactic impairment. Good syntactic function was associated with enhanced effective connectivity and increased tract integrity, suggesting that intact connectivity between left frontal and temporal regions is essential for syntactic analysis rather than the activation of these regions per se.