MEG studies of motor cortex gamma oscillations: evidence for a gamma "fingerprint" in the brain?

Abstract

The human motor cortex exhibits transient bursts of high frequency gamma oscillations in the 60-90 Hz range during movement. It has been proposed that gamma oscillations generally reflect local intracortical activity. However, movement-evoked gamma is observed simultaneously in both cortical and subcortical (basal ganglia) structures and thus appears to reflect long-range cortical-subcortical interactions. Recent evidence suggests that gamma oscillations do not simply reflect sensory reafference, but have a facilitative role in movement initiation. Here we summarize contributions of MEG to our understanding of movement-evoked gamma oscillations, including evidence that transient gamma bursts during the performance of specific movements constitutes a stereotyped spectral and temporal pattern within individuals-a gamma "fingerprint"-that is highly stable over time. Although their functional significance remains to be fully understood, movement-evoked gamma oscillations may represent frequency specific tuning within cortical-subcortical networks that can be monitored non-invasively using MEG during a variety of motor tasks, and may provide important information regarding cortical dynamics of ongoing motor control.

Keywords: MEG; basal ganglia; frequency tuning; gamma oscillations; motor cortex.

Figure 1

(A) Gamma band responses during right finger abductions in a right-handed subject. Synthetic Aperture Magnetometry (SAM) source localization was used to localize the generator of gamma power in right motor cortex, transformed to MNI coordinates with 4 mm resolution (left). A time-frequency plot of the source activity (right) over the range of 1–150 Hz shows a brief burst of gamma band activity around 80 Hz immediately following EMG onset (t = 0 s) as a percent change above baseline (−1.5 to −1 s). (B) Time-frequency analysis of source activity from contralateral motor cortex for left and right index finger movements in three selected right-handed subjects, showing the peak frequency (F_peak) and standard error in each subject, estimated from 5000 bootstrap resampling (with replacement) of the single trial time-frequency estimates. Beamformer source analysis was carried out using the BrainWave analysis toolbox (http://cheynelab.utoronto.ca).

Figure 2

(A) Left: Averaged motor fields for all 5 recording sessions for left and right index finger movements (time-locked to button press). Right: Corresponding time-frequency analysis of the single trial power from 60 to 90 Hz for the peak locations of gamma activity. (B) Bootstrapped mean time-frequency plots for the 5 repeated sessions for right index finger movements. F_peak indicates the mean peak frequency ± standard error. (C) Peak motor cortex gamma frequencies for right index finger (RIGHT, blue markers) and left index finger (LEFT, red markers) movements repeated for 5 sessions over a one-week period for two subjects. Sessions taken at different time intervals separated by approximately 4–5 years are also shown. Each data point indicates the bootstrap resampled mean peak frequency, using 5000 re-samplings of two-thirds of the single trial time-frequency estimates. Error bars indicate the 95% confidence intervals based on the bootstrap standard error.