Cortico force coherence of the finger and toe with slight rhythmic pressure on force sensors using electroencephalography

Abstract

We investigated the usefulness of cortico-force coherence (CFC) between electroencephalography (EEG) and force signals for assessing sensorimotor cortex function. Fourteen healthy participants performed slight rhythmical pressing with the right finger and right toe at a self-paced rate of 1-3 Hz under the active condition of CFC using a force sensor and electromyography (EMG). For passive CFC, the experimenter pressed the participant's right finger at a rate similar to that in the active condition. As control, the conventional corticokinematic coherence (CKC) was recorded for the right finger using an accelerometer (ACC). We also recorded CFC in the active condition by pressing the right toe. In all participants, coherence spectra between the 32-channel EEG signals and force, ACC, and EMG signals showed significant peaks (P < 0.01) at the movement frequency peaks or their harmonics. Finger CFC peak value did not differ among the three conditions (active CFC, passive CFC, and CKC). Finger CFC, force sensor, and EMG values showed no differences. Additionally, finger CFC did not significantly differ from toe CFC. The CFC approach with EEG appears promising and useful for the functional assessment of the sensorimotor cortex, with a clinical advantage of conducting measurements using less force and without obvious kinematics.

Keywords: Corticokinematic coherence; Electroencephalography; Motor-evoked fields; Primary sensorimotor cortex; Sophisticated movements; Voluntary movement.

Fig. 1

(a) Raw signals of EEG (top traces), EMG (second traces), force (third traces), and ACC (last traces) from a single participant (participant 5) for finger CFC in the active (upper panel) and passive (lower panel) conditions. EEG, force, and ACC signals were bandpass filtered with 1–10 Hz, and EMG signal was bandpass filtered with 10 Hz–100 Hz. EEG traces are from C3 channel (left SM1) contralateral to the right finger. During the active condition of CFC, EMG bursts are time-locked to pressing force peaks, whereas no EMG activity is present during the passive condition. ACC signals are also detected at the rhythms of the presses in both active and passive conditions. (b) Power spectra of movement signals obtained through force and EMG signals for finger CFC in the active conditions of a single participant (participant 5). The scale of the x-axis is 10 Hz. Force and EMG spectra are plotted on a logarithmic scale. Note that the peak frequency occurs in the same frequency band of finger movement and its harmonic frequency band, i.e., at 2.8 Hz and 5.5 Hz, in both the force and EMG results (indicated by arrows). ACC accelerometer, CFC cortico-force coherence, EEG electroencephalographic signal, EMG electromyographic signal, Force force signal, SM1 primary sensorimotor cortex.

Fig. 2

CFC waveform for the active (left trace) and passive (middle trace) conditions and CKC (right trace) from C3 channel (left SM1), contralateral to the right finger of a single participant (participant 5). The scale of the x-axis is 10 Hz. The horizontal dashed line indicates a significance level of 99%. The CKC peak is observed at 2.8 Hz in the CFC in the active condition, at 2.5 Hz and 4.8 Hz in the passive condition, and at 4.2 Hz in the CFC; these are around the moving and/or harmonic frequency band of the finger movements. CFC cortico-force coherence, CKC cortico-kinematic coherence, SM1 primary sensorimotor cortex.

Fig. 3

Power spectra of force signals (upper panels) and coherence waveform (second panels) for finger CFC in the active conditions from all participants. The scale of the x-axis is 10 Hz. Force spectra are plotted on a logarithmic scale. The scale of the x-axis is 10 Hz.

Fig. 4

(a) Mean peak values of coherence for the finger CFC in both active and passive conditions and finger CKC across subjects. The peak value of finger CFC in the active condition was not significantly different from that in the passive condition and CKC. (b) Mean peak values of coherence over the left hemisphere for the CFC of the finger and toe. The peak value of finger CFC was not significantly different from that of the toe CFC. CFC cortico-force coherence, CKC cortico-kinematic coherence.

Fig. 5

Topoplot maps for the CFC of the finger and toe at the peak frequency bands of a single participant (participant 14). Coherence between all 32 EEG signals and force signals is detected over the finger and toe areas of the senosorimotor cortex. The topoplot distribution of the CFC is organized in an orderly manner from the top to the sides of the head, along with the toe and hands, consistent with the classical homunculus. CFC cortico-force coherence, EEG electroencephalographic signal.

Fig. 6

Topoplot maps for the CFC of the finger and toe at the peak frequency bands of all participants. CFC cortico-force coherence, EEG electroencephalographic signal.