Current implications of EEG and fNIRS as functional neuroimaging techniques for motor recovery after stroke

Abstract

Persistent motor deficits are highly prevalent among post-stroke survivors, contributing significantly to disability. Despite the prevalence of these deficits, the precise mechanisms underlying motor recovery after stroke remain largely elusive. The exploration of motor system reorganization using functional neuroimaging techniques represents a compelling yet challenging avenue of research. Quantitative electroencephalography (qEEG) parameters, including the power ratio index, brain symmetry index, and phase synchrony index, have emerged as potential prognostic markers for overall motor recovery post-stroke. Current evidence suggests a correlation between qEEG parameters and functional motor outcomes in stroke recovery. However, accurately identifying the source activity poses a challenge, prompting the integration of EEG with other neuroimaging modalities, such as functional near-infrared spectroscopy (fNIRS). fNIRS is nowadays widely employed to investigate brain function, revealing disruptions in the functional motor network induced by stroke. Combining these two methods, referred to as integrated fNIRS-EEG, neural activity and hemodynamics signals can be pooled out and offer new types of neurovascular coupling-related features, which may be more accurate than the individual modality alone. By harnessing integrated fNIRS-EEG source localization, brain connectivity analysis could be applied to characterize cortical reorganization associated with stroke, providing valuable insights into the assessment and treatment of post-stroke motor recovery.

Keywords: electroencephalography; functional near-infrared spectroscopy; functional neuroimaging; motor function; stroke.

Figure 1:

An overview of the EEG and fNIRS parameters commonly employed in the assessment of motor recovery after stroke. Through the electrodes attached to the scalp, EEG mainly detects the voltage fluctuations in the cortex. The parameters of qEEG and HD-EEG used for motor recovery after stroke encompass PSD and PRI, BSI and PSI, as well as FC. TMS-EEG involves the application of a single TMS pulse to the M1, whereby the combined EEG records the TEP (P30-N45-P60-N100-P200). The data recorded from the Cz electrode is represented by the red line, while the blue lines show data from all channels. FNIRS measures HbO and HbR levels, as well as FC, by the optodes positioned across various regions of the scalp during resting-state or task-state. EEG, electroencephalogram; fNIRS, functional near infrared spectroscopy; qEEG, quantitative EEG; HD-EEG, high-density EEG; PSD, power spectrum density; PRI, power ratio index; BSI, brain symmetry index; PSI, phase synchrony index; FC, functional connectivity; TMS-EEG, transcranial magnetic stimulation combined with EEG; TEP, TMS-evoked potential; HbO, oxygenated hemoglobin; HbR, deoxygenated hemoglobin.

Figure 2:

Integration of fNIRS-EEG for motor function assessment and rehabilitation after stroke. FNIRS data is analyzed (for example by GLM method). EEG signals analyses use a sliding window scheme. FNIRS informed-EEG source localization is used to investigate the brain connectivity, brain network dynamic process and the brain controllability analysis. GLM, general linear model.