A novel method for reducing the effect of tonic muscle activity on the gamma band of the scalp EEG.

Judith F Nottage,Dominic H Ffytche,Paul D Morrison,Steve C R Williams

doi:10.1007/s10548-012-0255-9

Abstract

Neural oscillations in the gamma band are of increasing interest, but separating them from myogenic electrical activity has proved difficult. A novel algorithm has been developed to reduce the effect of tonic scalp and neck muscle activity on the gamma band of the EEG. This uses mathematical modelling to fit individual muscle spikes and then subtracts them from the data. The method was applied to the detection of motor associated gamma in two separate groups of eight subjects using different sampling rates. A reproducible increase in high gamma (65–85 Hz) magnitude occurred immediately after the motor action in the left central area (p = 0.02 and p = 0.0002 for the two cohorts with individually optimized algorithm parameters, compared to p = 0.03 and p = 0.16 before correction). Whilst the magnitude of this event-related gamma synchronisation was not reduced by the application of the EMG reduction algorithm, the baseline left central gamma magnitude was significantly reduced by an average of 23 % with a faster sampling rate (p < 0.05). In comparison, at left and right temporo-parietal locations the gamma amplitude was reduced by 60 and 54 % respectively (p < 0.05). The reduction of EMG contamination by fitting and subtraction of individual spikes shows promise as a method of improving the signal to noise ratio of high frequency neural oscillations in scalp EEG.

Highlights

It has become increasingly evident from animal studies that frequencies above 20 Hz play a key role in the active functioning of the brain
The second blow was the demonstration by Yuval-Greenberg et al (2008), in the following year, that most of the widely reported induced broad-band gamma peak at 200–300 ms after a visual stimulus does not originate in the brain (Yuval-Greenberg et al 2008; Keren et al 2010), which followed concerns raised by Trujillo et al (2005), a few years earlier, about possible saccade related gamma band artefacts
Neither group showed a significant difference between the magnitudes of the baseline corrected, left central, gamma event related synchronisation (ERS) before and after Saccade Muscle Potential (SMP) reduction

Summary

Introduction

It has become increasingly evident from animal studies that frequencies above 20 Hz play a key role in the active functioning of the brain. Instead the primary source of the gamma peak is the extra-ocular muscles of the eye during micro-saccades, and its timing corresponds to the rebound in micro-saccades after the initial poststimulus inhibition of saccades. This saccade associated artefact has been found to even contaminate intra-cranial recordings, (Jerbi et al 2009; Kovach et al 2011) and large surface EMG bursts can sometimes be detected in

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Discussion

Conclusion