Graduate student Bioengineering Mansfield, Texas, United States
Introduction:: Neuronal and vascular activity in the human brain communicate with each other in both directions. This interaction is known as the neurovascular coupling (NVC). Spontaneous fluctuations in vasomotion consist of hemodynamic activities linked to three anatomical layers with three specific infraslow oscillation (ISO) frequencies: (i) endothelial layer in the endogenic band (0.005-0.02 Hz), (ii) neurons in the neurogenic band (0.02-0.04 Hz) and (iii) smooth muscles in the myogenic band (0.04-0.2 Hz). Furthermore, the five commonly-analyzed rhythm bands of electroencephalogram (EEG) are delta (1-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), beta (13-30 Hz), and gamma ( >30 Hz) frequencies. This study aimed to evaluate the interactive coupling among various neurophysiological activities of the prefrontal cortex in healthy humans in the resting state. The study utilized concurrent measurements of two brain-sensing techniques, two-channel broadband near-infrared spectroscopy (2-bbNIRS) and 19-channel EEG, to acquire both hemodynamic and electrophysiological signals. The significance of this quantification in the prefrontal neurophysiological network is to identify potential markers for detecting the early stages of Alzheimer’s disease (AD).
Materials and Methods:: A group of healthy younger (n=22) and older (n=20) adults were recruited for the study with written informed consent. After the participants sat comfortably on a chair, two sensors of the 2-bbNIRS were placed bilaterally on their foreheads. Each sensor had two fiber bundles, one for sending light to the forehead and the other for gathering the light reflected from the brain tissue. Meanwhile, a 19-channel EEG device was attached to the participant’s head, and EEG data were recorded throughout the measurement period. The measurement lasted 14 minutes with 7-min eyes open and 7-min eyes closed for older adults, while it lasted 7 min with eyes closed for younger adults. The experimental setup is shown in Fig. 1.
Data analysis included the following stages: (1) EEG data preprocessing, followed by EEG data downsampling and beta-band (13-30 Hz) filtering. A time series of EEG beta power with a 1.5-s temporal resolution (0.67 Hz) was created using the area under the power spectral density (PSD) curve in the beta frequency region (13-30 Hz); (2) 2-bbNIRS spectral processing to determine temporal concentrations of oxygenated hemoglobin [HbO] and cytochrome c oxidase [CCO]; (3) construction of a neurophysiological network and unilateral coupling quantification using generalized partial directed coherence (GPDC). A 3-node neurophysiological network was then built by integrating the causal interactions between several physiological representations of local brain activity (such as the left or right prefrontal cortex) and hemodynamic, metabolic, and
electrophysiological inputs. A neurophysiological network was constructed for each ISO frequency band.
Results, Conclusions, and Discussions:: Results:
Neurophysiological networks spanning the same prefrontal cortical (PFC) region (i.e., left or right PFC) were built using three separate physiological signals, namely [HbO], [CCO], and down-sampled EEG beta power, in three different ISO frequency bands (endo, neuro and myo band). In the resting state, our preliminary results derived from younger adults showed a strong coupling between metabolic and electrophysiological activities in the endogenic band (see Fig. 2(a)), indicating a closely coupled neurophysiological activity among the three nodes of the network on both lateral PFC. In contrast, all three connections in the myogenic band of both lateral PFC were significantly weaker ( Fig. 2(c)). The directionality was also calculated among the three nodes for each band. In general (Fig. 2), the results illustrated a strong coupling of HbO to EEG and CCO to HbO, whereas the reverse, a weak coupling, was evident.
Discussion
In the resting state, the neurophysiological networks among hemodynamic, metabolic, and electrophysiological activity in the endogenic rhythm are linked or coupled well. This observation is in line with the neurophysiological source of oscillation in the endogenic band, which controls hemodynamics, metabolism, and neural activity in the human brain through contraction and relaxation of the endothelial layer of the vessel wall. This frequency band is primarily responsible for controlling the oxygen supply-demand balance between brain hemodynamics vs neuronal and metabolic activities. Therefore, the oxygen supply provided by HbO in this frequency band is driven by the oxygen demand from EEG and CCO activity. In this investigation, we were able to discover and quantify directed connections across three cerebral neurophysiological entities of the PFC as potential properties of the resting human brain, using a novel, dual-mode brain-sensing technology and multivariate coherence analysis. These characteristics and network neurophysiology in general can be researched further and applied to other clinical populations, such as age-related changes, neurological conditions, early stages of AD, and other brain diseases.
Conclusion
Dual-mode bbNIRS with electroencephalography (EEG) is a promising noninvasive methodology. It facilitates the quantification of directed neurophysiological networks in the prefrontal cortex of humans after being combined with generalized partial directed coherence analysis.
