Hyperdirect connectivity of opercular speech network to the subthalamic nucleus

The importance of the basal ganglia in modulating cognitive and motor behaviors is well known, yet how the basal ganglia participate in the uniquely human behavior of speech is poorly understood. The subthalamic nucleus (STN) is well positioned to facilitate two basal ganglia functions critical for speech: motor learning and gain modulation. Using a novel paradigm to study cortical-subcortical interactions during speech in patients undergoing awake DBS surgery, we found evidence for a left opercular hyperdirect pathway in humans by stimulating in the STN and examining antidromic evoked activity in the left temporal, parietal and frontal opercular cortex. These high resolution cortical and subcortical mapping data provided evidence for hyperdirect connectivity between Brocas area (typically corresponding to pars triangularis and pars opercularis of the inferior frontal gyrus) and the STN. In addition, we observed evoked potentials consistent with the presence of monosynaptic projections from areas of opercular speech cortex that are primarily sensory, including auditory cortex, to the STN. These connections may be unique to humans, evolving alongside the ability for speech. SIGNIFICANCEUsing intracranial recordings from the basal ganglia and cortex in subjects undergoing deep brain stimulation, this study provides evidence for monosynaptic cortical inputs from motor planning, motor sensory, and auditory sensory cortices to the subthalamic nucleus. These observations suggest that in humans, the cortical-basal ganglia hyperdirect pathway is uniquely positioned to participate in speech production. Moreover, the existence of a monosynaptic connection between human sensory cortical areas and the subthalamic nucleus indicates a need to update traditional models of information transfer within cortical-basal ganglia-thalamocortical circuitry, which has significant implications for understanding other human cognitive behavior or dysfunction.