Corticomuscular coherence between motor cortex, somatosensory areas and forearm muscles in the monkey

Abstract

Corticomuscular coherence has previously been reported between primary motor cortex (M1) and contralateral muscles. We examined whether such coherence could also be seen from somatosensory areas. Local field potentials (LFPs) were recorded from primary somatosensory cortex (S1; areas 3a and 2) and posterior parietal cortex (PPC; area 5) simultaneously with M1 LFP and forearm EMG activity in two monkeys during an index finger flexion task. Significant beta-band ( approximately 20 Hz) corticomuscular coherence was found in all areas investigated. Directed coherence (Granger causality) analysis was used to investigate the direction of effects. Surprisingly, the strongest beta-band directed coherence was in the direction from S1/PPC to muscle; it was much weaker in the ascending direction. Examination of the phase of directed coherence provided estimates of the time delay from cortex to muscle. Delays were longer from M1 ( approximately 62 ms for the first dorsal interosseous muscle) than from S1/PPC ( approximately 36 ms). We then looked at coherence and directed coherence between M1 and S1 for clues to this discrepancy. Directed coherence showed large beta-band effects from S1/PPC to M1, with smaller directed coherence in the reverse direction. The directed coherence phase suggested a delay of approximately 40 ms from M1 to S1. Corticomuscular coherence from S1/PPC could involve multiple pathways; the most important is probably common input from M1 to S1/PPC and muscles. If correct, this implies that somatosensory cortex receives oscillatory efference copy information from M1 about the motor command. This could allow sensory inflow to be interpreted in the light of its motor context.

Keywords: oscillations; sensorimotor.

Figure 1

Using directed coherence to determine bi-directional coupling and delays. (A) coherence and directed coherence analysis of two simulated white band signals with uni-directional coupling (A → B with a 20-ms delay). (B) coherence and directed coherence analysis of two simulated white band signals with bi-directional coupling (A → B with a 20-ms delay and B → A with a 30-ms delay). (a) coherence spectra. Dotted lines show 95% significance level. (b) coherence phase spectra. Phase plotted three times to avoid wrap-around effects. Black lines show best fit with text indicating the phase delay. (c) directed coherence spectra. Dotted lines show 95% significance level. (d) directed coherence phase spectra. Phase plotted three times to avoid wrap-around effects. Black lines show best fit with text indicating the phase delay.

Figure 2

Data from a single session recording in M1 in monkey L. (A) histological reconstruction of approximate recording sites for monkey L. X marks approximate position of electrode used to record LFP in (B). (B) raw traces showing LFP, EMG and lever position for two consecutive trials. Dotted lines indicate the hold periods used for analysis. (C) LFP–FDP coherence spectrum. Dotted line indicates P < 0.05 significance level. (D) phase spectrum for LFP–FDP coherence. Error bars show 95% confidence limits. Dotted lines show region used to fit regression analysis, solid lines show best fit. (E) LFP → EMG and EMG → LFP directed coherence spectra. Dotted line represents P < 0.05 significance limit. (F), phase for frequencies with significant directed coherence. In (D) and (F), each phase point has been plotted three times separated by 2π to avoid wrap-around effects when fitting regression lines.

Figure 3

LFP–EMG coherence. (A) overlaid phase – frequency plots from individual recording sites for LFP–1DI coherence from each area in monkey M. (B,C) LFP–1DI and LFP–FDP coherence combined across all available sessions in monkeys M and L respectively. Dotted lines represent P < 0.05 significance levels. (D,E) phase spectra for corticomuscular coherence with the two monkeys shown by different symbols for muscle 1DI (D) and muscle FDP (E). Error bars represent 95% confidence limits on phase (some error bars cannot be seen since they are smaller than the plot markers). Dotted lines in (D,E) show regions used for regression analysis and black lines represent best fit.

Figure 4

LFP–EMG directed coherence. (A,B) LFP → EMG directed coherence for the four cortical areas in monkey M and L respectively. (C,D) EMG → LFP directed coherence for the four cortical areas in monkey M and L respectively. Dotted lines in (A–D) represent P < 0.05 significance levels. (E), phase spectra for LFP → EMG directed coherence with data from monkey M and L overlaid, for the 1DI muscle. (F) as (E) for the FDP muscle. Dotted lines in (E,F) show regions used for regression analysis and black lines represent best fit.

Figure 5

LFP–M1 LFP coherence. (A) coherence spectra between LFP in areas 3a, 2 and 5 and M1 for monkey M. (B) as (A) but for monkey L. Dotted lines represent P < 0.05 significance levels. (C) Phase spectra with results from monkeys M and L overlain. Maximum 95% confidence limits 0.02 for area 3a, 0.10 for area 2 and 0.08 for area 5 (not shown since smaller than size of symbols).

Figure 6

LFP–M1 LFP directed coherence. (A) directed coherence spectra for monkey M for 3a/2/5 → M1 direction. (B) as (A), but for M1 → 3a/2/5 direction. The highest significance level out of area 3a, area 2 and area 5 is shown but the difference in 95% significance levels between the different areas is small compared to the size of the directed coherence. (C,D) as (A,B) for monkey L. (E,G) phase spectra for areas 3a/2/5 → M1 directed coherence for monkeys M and L respectively. (F,H) phase spectra for M1 → areas 3a/2/5 directed coherence for monkeys M and L respectively. For (E–H) regression analysis was used to fit phases at frequencies between dotted lines; black line represents best fit.

Figure 7

Schematic showing possible pathways underlying coherence between S1/area 5 and EMG. Lettered labels are referred to in text.