The role of synchrony and oscillations in the motor output

Abstract

There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20-30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20-30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date.

Time-varying covariance of neural activities recorded in striatum and frontal cortex as monkeys perform sequential-saccade tasks

“…This result suggests that the LFP responses were affected by multiple cognitively integrated aspects of the tasks performed with a sensitivity equivalent to the spike activity of single units. Notably, the patterns of task-related temporal crosscovariance that we observed across striatal and cortical sites were visible without reference to oscillatory activity within particular frequency bands (19)(20)(21)(22).…”

The reorganization of corticomuscular coherence during a transition between sensorimotor states

“…Although oscillations in both frequency bands are e↵ectively carried down the corticospinal tract (Baker et al, 2003), most studies using sustained contractions find that only beta-band oscillations are coherent between motor cortex and muscle activity Halliday et al, 1998;Baker et al, 1997;Gross et al, 2000). Corticomuscular beta-band coherence is most prominent during tonic muscle contractions and disappears during movement (Baker et al, 1997;Riddle and Baker, 2006;Kilner et al, 2000;Baker et al, 1999) and beta-band activity is enhanced when higher precision is required Kristeva-Feige et al, 2002;Witte et al, 2007;Gilbertson et al, 2005). These findings suggest that the beta-band activity is related to a mechanism that maintains the current sensorimotor state (Baker, 2007;Engel and Fries, 2010;Van Wijk et al, 2012).…”

A beta2-frequency (20–30 Hz) oscillation in nonsynaptic networks of somatosensory cortex

“…The origin of these in vivo beta rhythms is unclear; however, pyramidal tract neurons (lying in layer V; ref. 7) and motor cortex local field potentials exhibit coherence at beta2 frequencies with hand and forearm electromyographic activity, in monkeys performing a precision grip task (8,9), suggesting that beta2 oscillations originate in layer V in vivo. In addition, layer V neurons form a major input pathway to basal ganglia, which also demonstrate beta rhythms (10).…”