Task‐dependent modulation of 15‐30 Hz coherence between rectified EMGs from human hand and forearm muscles

Abstract

The planning and execution of movements involve areas of the cerebral cortex that are separated in their activity both temporally and spatially. However, the mechanism by which these areas combine to produce a co-ordinated movement is unknown. Studies of the activity of neurones in the sensorimotor cortex in monkeys and humans have shown synchronous oscillatory activity in the 15-30 Hz range (Murthy & Fetz, 1992, 1996aSanes & Donoghue, 1993;Baker et al. 1997;Donoghue et al. 1998). These synchronous oscillations have been suggested to link the disparate motor signals together in a manner analogous to the perceptual 'binding' of stimulus attributes in the visual cortex (Singer & Gray, 1995), where the observed neuronal synchronization is attributed to neurones that participate in the encoding of related information. Sanes & Donoghue (1993) and Murthy & Fetz (1996a) have shown that oscillations in monkey sensorimotor cortex are synchronous over large distances (up to 14 mm), suggesting the involvement of large neuronal populations. However, Murthy & Fetz (1996b) concluded that the oscillatory episodes had no consistent relationship with a variety of motor tasks, and suggested that rather than being involved directly in 'binding' during movement execution, the oscillations could be a neuronal correlate of attention during sensorimotor tasks. 1. Recent reports have shown task-related changes in oscillatory activity in the 15-30 Hz range in the sensorimotor cortex of human subjects and monkeys during skilled hand movements. In the monkey these oscillations have been shown to be coherent with oscillatory activity in the electromyographic activity of hand and forearm muscles. 2. In this study we investigated the modulation of oscillations in the electromyogram (EMG) of human volunteers during tasks requiring precision grip of two spring-loaded levers. 3. Two tasks were investigated: in the 'hold' task, subjects were required to maintain a steady grip force (ca 2·1 N or 2·6 N) for 8 s. In the 'ramp' task, there was an initial hold period for 3 s (force ca 2·1 N) followed by a linear increase in grip force over a 2 s period. The task ended with a further steady hold for 3 s at the higher force level (ca 2·6 N). 4. Surface EMGs were recorded from five hand and forearm muscles in 12 subjects. The coherence of oscillatory activity was calculated between each muscle pair. Frequencies between 1 and 100 Hz were analysed. 5. Each subject showed a peak in the coherence spectra in the 15-30 Hz bandwidth during the hold task. This coherence was absent during the initial movement of the levers. During the ramp task the coherence in the 15-30 Hz range was also significantly reduced during the movement phase, and significantly increased during the second hold period, relative to the initial hold. 6. There was coherence between the simultaneously recorded magnetoencephalogram (MEG) and EMG during steady grip in the hold task; this coherence disappeared during the initial lever movement. Using a single equivalent current dipole source...

Task-and Phase-related Changes in Cortico-muscular Coherence

“…This has similarly been reported for finger muscles in monkey 4 and in man. 8 The decrease in 15-35 Hz coherence during the ramp phase may be related to event-related desynchronization and the increase in the coherence during hold phase may be related to sychronization of EEG activity.…”

“…It was most prominent during the steady hold period of the precision grip task, but was abolished during digit movement. 4,8,9 Coherence was shown to be largest when a steady hold period immediately followed movement, and covaried positively with the compliance of the object being gripped; it was smallest during isometric grip of a solid object. 9 This raised the possibility that 15 -30 Hz cortico-muscular coherence may encode the 'motor set' needed to maintain steady grip of compliant objects…”

“…Kilner et al found et al found et al that coherence between the cortex and finger muscles was larger when subjects had to squeeze a compliant object rather than a more rigid object. 8,9 This suggests that corticomuscular coherence may be related to functional parameters of the performed task. It may also play a functional role in the activation of the muscles.…”

“…1,[4][5][6][7][8][9] However, the functional significance of this corticomuscular coherence is unclear. Kilner et al found et al found et al that coherence between the cortex and finger muscles was larger when subjects had to squeeze a compliant object rather than a more rigid object.…”

Source Separation and Clustering of Phase-Locked Subspaces

“…We assume that a set of sources undergoes linear mixing as in (3), and also that the data x has been whitened as in (4). Let the estimated sources be denoted by y(t) ≡ W T x(t) = W T As(t).…”