IS 18. Interneuron networks

Abstract

Transcranial magnetic stimulation (TMS) is known to activate several populations of interneuron networks in cerebral cortex as demonstrated by the existence of the well-known early and later indirect wave (I-wave) pathways which can be differentially stimulated by changing the direction of induced current in the brain. The question we ask here is whether these “circuits” are epiphenomena that only function when activated by a non-physiological TMS pulse, or whether they represent activity in intrinsic collections of interneurons that contribute differentially to specific types of motor behaviour. That is, can the electroanatomy of TMS tell us something about the functional anatomy of the cerebral cortex? A series of experiments suggests that this may be the case. First, we have devised a new intracortical paired associative stimulation (icPAS), to show that the difference in synaptic inputs recruited by anterior-to-posterior (AP) and posterior-to-anterior (PA) stimulation determines the direction of change in presumed synaptic plasticity. Second, we have recently found that variation in response to plasticity probing protocols, such as theta burst stimulation (TBS), is strongly influenced by which neuronal networks are likely to be recruited by each TMS pulse. The responses to TBS protocols were highly variable between individuals: about 50% of this variation was predicted by our postulated marker for the efficiency of late I-wave recruitment. Finally, the differential modulation of these circuits can interact with motor practice, measured by enhancement of acceleration of ballistic finger movements. These findings highlights the importance of differential role of interneuron networks within primary motor cortex in stimulation induced plasticity as well as voluntary motor learning.