Motor imagery (MI) is similar to overt movement, engaging common neural substrates and facilitating the corticomotor pathway, however it does not result in excitatory descending motor output. Transcranial magnetic stimulation (TMS) can be used to assess inhibitory networks in the primary motor cortex via measures of 1 ms short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI) and late cortical disinhibition (LCD). These measures are thought to reflect extrasynaptic GABAA tonic inhibition, postsynaptic GABAB inhibition and presynaptic GABAB disinhibition respectively. The behavior of 1 ms SICI, LICI and LCD during MI has not yet been explored. This study aimed to investigate how 1 ms SICI, LICI and LCD are modulated during MI and voluntary relaxation (VR) of a target muscle. Twenty-five healthy young adults participated. TMS was used to assess non-conditioned motor evoked potential (MEP) amplitude, 1 ms SICI, 100 and 150 ms LICI and LCD in the right abductor pollicis brevis and right abductor digiti minimi during rest, MI and VR of the hand. Compared to rest, MEP amplitudes were facilitated in APB during MI. SICI was not affected by task or muscle. LICI100 decreased in both muscles during VR but not MI, while LCD was recruited in both muscles during both tasks. This indicates that VR modulates postsynaptic GABAB inhibition, while both tasks modulate presynaptic GABAB inhibition in a non-muscle-specific way. This study highlights further neurophysiological parallels between actual and imagined movement, which may extend to voluntary relaxation.
- Transcranial magnetic stimulation
- Motor cortex
- Motor imagery
- Copyright © 2016, Journal of Neurophysiology