BASAL GANGLIA PATHWAYS: BEYOND THE CLOSED-LOOP CIRCUITS WITH THE CEREBRAL CORTEX

Concepts of basal ganglia (BG) functions have been strongly influenced by their anatomical interconnections with the cerebral cortex. Views regarding these interconnections have changed dramatically over the past century. Specifically, advances in transneuronal tracing with neurotropic viruses have demonstrated that the BG participate in parallel closed-loop circuits with cerebral cortical areas that underlie motor and cognitive functions (Middleton and Strick, 2000b). Using transneuronal tracing techniques, we have identified two new pathways that allow the BG to influence motor and cognitive processes. First, we used the retrograde transneuronal transport of rabies virus (RV) to show that the BG participates in open-loop circuits with the dorsal prefrontal cortex (PFC). Specifically, the ventral striatum (VStr) projects to the dorsal PFC, but does not receive input back from the dorsal PFC. Our results expand on the finding that there exist open-loop circuits between the BG and motor cortical areas (Kelly and Strick, 2004; Miyachi et al., 2006; Saga et al., 2011). These open-loop circuits provide a pathway for BG limbic processing to influence both motor and cognitive functions. Second, we used retrograde transneuronal transport of RV to reveal a pathway that enables BG output to influence cerebellar (CB) function. Specifically, the subthalamic nucleus (STN) sends a disynaptic projection to the CB cortex. These results are important because until recently, it was generally accepted that the BG and the CB were not directly connected. The pathway from the BG to the CB complements the recent discovery that the CB sends a disynaptic projection to the striatum (Hoshi et al., 2005). Together, these pathways provide the anatomical substrate for substantial interactions between the BG and the CB, in both the motor and nonmotor domains. Overall, we identified two novel output pathways from the BG: from the VStr to the dorsal PFC and from the STN to the CB cortex. These pathways provide the BG with the potential to influence motor and nonmotor processes, outside of the traditional closed-loop circuits with the cerebral cortex. Considerable evidence suggests that these pathways are likely to have important effects on both normal and abnormal aspects of behavior

Self-paced movements induce high-frequency gamma oscillations in primary motor cortex

There has been increasing interest in the functional role of high-frequency (> 30 Hz) cortical oscillations accompanying various sensorimotor and cognitive tasks in humans. Similar “high gamma” activity has been observed in the motor cortex, although the role of this activity in motor control is unknown. Using whole-head MEG recordings combined with advanced source localization methods, we identified high-frequency (65 to 80 Hz) gamma oscillations in the primary motor cortex during self-paced movements of the upper and lower limbs. Brief bursts of gamma activity were localized to the contralateral precentral gyrus (MI) during self-paced index finger abductions, elbow flexions and foot dorsiflexions. In comparison to lower frequency (10–30 Hz) sensorimotor rhythms that are bilaterally suppressed prior to and during movement (Jurkiewicz et al., 2006), high gamma activity increased only during movement, reaching maximal increase 100 to 250 ms following EMG onset, and was lateralized to contralateral MI, similar to findings from intracranial EEG studies. Peak frequency of gamma activity was significantly lower during foot dorsiflexion (67.4 ± 5.2 Hz) than during finger abduction (75.3 ± 4.4 Hz) and elbow flexion (73.9 ± 3.7 Hz) although markedly similar for left and right movements of the same body part within subjects, suggesting activation of a common underlying network for gamma oscillations in the left and right motor cortex. These findings demonstrate that voluntary movements elicit high-frequency gamma oscillations in the primary motor cortex that are effector specific, and possibly reflect the activation of cortico-subcortical networks involved in the feedback control of discrete movements