Predominantly left-lateralized EEG-EMG associations following gamma-theta stimulation in older adults with mild cognitive impairment.

INTRODUCTION: Mild cognitive impairment (MCI) is frequently accompanied by impairments in postural control and trunk muscle function. Although gamma-theta rhythmic stimulation has been reported to modulate neural oscillatory activity, its influence on brain-muscle interactions in individuals with MCI remains poorly understood. This exploratory pilot study investigated whether changes in EEG connectivity following gamma-theta stimulation were associated with alterations in trunk muscle electromyographic (EMG) activity in older adults with MCI.

METHODS: Nine older adults with MCI underwent two separate 10-min sessions of combined 40 Hz visual flicker and theta-frequency mechanical stimulation. Resting-state EEG and trunk muscle EMG were assessed before and after stimulation. EEG connectivity changes were calculated using Spearman correlations between predefined electrode pairs. Supplementary analyses using phase-lag index (PLI) and imaginary coherence (iCoh) were performed to address potential volume conduction effects. EMG activity was recorded bilaterally from the multifidus and lumbar paraspinal muscles during trunk extension tasks. Correlation analyses examined associations between EEG connectivity changes and EMG changes in RMS amplitude, mean frequency, and spectral power measures.

RESULTS: Although no predefined EEG connectivity pair demonstrated significant group-level change following stimulation, several EEG connectivity change scores were significantly associated with concurrent EMG changes. The most robust associations involved left multifidus activity: FC5-F3 connectivity changes were associated with left multifidus RMS amplitude (ρ = +0.800, p = 0.010), and T7-T8 connectivity changes were associated with left multifidus mid-frequency spectral power (ρ = -0.800, p = 0.010). Leave-one-out sensitivity analyses indicated that both associations remained statistically significant in all nine subsets. Supplementary phase-based connectivity analyses provided limited convergent support for the primary findings. Although several contralateral EEG-EMG associations were identified, the overall pattern was characterized by a predominance of ipsilateral associations.

DISCUSSION: These findings suggest that gamma-theta stimulation may influence coordinated brain-muscle responses in older adults with MCI, with a predominance of left-hemisphere EEG-EMG associations indicating relative hemispheric predominance rather than strict hemispheric exclusivity. Associations involving broader measures of muscle activation and spectral distribution were more robust than those involving specific spectral indices. Given the small sample size, absence of sham control, non-simultaneous EEG-EMG recording, and exploratory statistical approach, these findings should be interpreted as hypothesis-generating and require confirmation in larger controlled studies.