Abstract
The paper presents a point of care testing device for neurovascular coupling (NVC) from simultaneous recording of electroencephalogram (EEG) and near infra red spectroscopy (NIRS) during anodal transcranial direct current stimulation (tDCS). Here, anodal tDCS modulated cortical neural activity leading to hemodynamic response can be used to identify impaired cerebral microvessels functionality. The impairments in the cerebral microvessels functionality may lead to impairments in the cerebrovascular reactivity (CVR) where severely reduced CVR predicts the chances of transient ischemic attack (TIA) and ipsilateral stroke. The neural and hemodynamic responses to anodal tDCS were studied through joint imaging with EEG and NIRS where NIRS provided optical measurement of changes in tissue oxy-( ) and deoxy-( ) haemoglobin concentration and EEG captured alterations in the underlying neuronal current generators. Then, a cross-correlation method for the assessment of neurovascular coupling (NVC) underlying the site of anodal tDCS is presented. The feasibility studies on healthy subjects and stroke survivors showed detectable changes in the EEG and NIRS responses to a 0.526A/m2 of anodal tDCS. The NIRS system was bench tested on 15 healthy subjects that showed a statistically significant (p<0.01) difference in the signal to noise ratio (SNR) between the on and off states of anodal tDCS where the mean SNR of the NIRS device was found to be 42.33±1.33dB in the on state and 40.67±1.23dB in the off state. Moreover, the clinical study conducted on 14 stroke survivors revealed that the lesioned hemisphere with impaired circulation showed significantly (p<0.01) less change in than the non-lesioned side in response to anodal tDCS. The EEG study on healthy subjects showed a statistically significant (p<0.05) decrease around "individual alpha frequency" in the Alpha band (8-13Hz) following anodal tDCS. Moreover, the joint EEG-NIRS imaging on 4 stroke survivors showed an immediate increase in the Theta band (4Hz-8Hz) EEG activity after the start of anodal tDCS at the non-lesioned hemisphere. Furthermore, cross-correlation function revealed a significant (95 percent confidence interval) negative cross-correlation only at the non-lesioned hemisphere during anodal tDCS where the log-transformed mean-power of EEG within 0.5Hz-11.25Hz lagged response in one of the stroke survivors with white matter lesions. Therefore, it was concluded that anodal tDCS can perturb local neural and vascular activity (via NVC) which can be used for assessing regional NVC functionality where confirmatory clinical studies are required.