(C-92) Age-dependent prefrontal cortical connectivity in the younger and older human brain:a broadband NIRS study

Introduction:: The degree to which the lateral sides of the prefrontal cortex oscillate synchronously or coherently is indicated by the bilateral prefrontal connections in the human brain. Higher connectivity levels indicate bilateral or global coherent oscillations, while lower connectivity levels indicate locally motivated activity. The supply demand balance between local oxygenated hemodynamics and metabolism is controlled by unilateral hemodynamic–metabolic coupling. Aging causes a reduction in or loss of cognitive function, and variations in cognitive function result in different degrees of mental abilities. Accordingly, we speculated that the decline in cognition might be associated with a decrease in bilateral connectivity and unilateral coupling of the prefrontal cortex, which controls a major part of cognition. Furthermore, it has been reported that infraslow oscillations (ISO) of cerebral hemodynamics is driven by relaxation-contraction cycles of blood vessel walls. ISOs are divided into three frequency bands: (i) endogenic (0.005–0.02 Hz), (ii) neurogenic (0.02–0.04 Hz), and (iii) myogenic (0.04–0.2 Hz) rhythms. The endogenic band can be used to observe the dilation-contraction cycles of the endothelium layer. Neurons emit neurotransmitters and vasoactive ions, which regulate blood vessel dilation-contraction cycles and cause oscillations in the neurogenic band. Finally, the smooth muscle surrounding the blood vessels engages in relaxation-contraction cycles inside the myogenic band (Shahdadian, S., 2022). This study aimed to investigate age-dependent prefrontal cortical connectivity and identify potential markers in the younger and older human brains.


Materials and Methods:: For younger subjects, 22 participants with ages ranging from 19-22 and 20 older participants with ages ranging from 88-75 were recruited. The resting state and bilateral measurements were performed from the consented participants using a two-channel broadband near-infrared spectroscopy (2-bbNIRS) system. The probes from 2-bbNIRS were positioned bilaterally on the participants' foreheads to record prefrontal ISO signals of oxygenated concentration, [HbO], and oxidized cytochrome c oxidase, [CCO]. Each channel consisted of two fiber bundles, one for delivering light to the forehead and the other for collecting backscattered light from the cortical tissue. The two bundles were separated by 3 cm. Data were collected over a course of 14 minutes with 7-min eyes open and 7-min eyes closed. The data acquisition rate was 0.667 Hz (1.5 s between two time points).

Data analysis was based on the modified Beer-Lambert law. The measured spectra within 780 nm-900 nm were used to determine temporal [HbO] and [CCO] for the 7-min eyes-open and eyes-closed periods, respectively. The coherence between each pair of two time series was calculated using a frequency-domain analysis approach, that is, utilizing the multi-taper method to determine coherence values. Specifically, we quantified four pairs of spectral coherence for the human forehead in the resting state: (i) bilateral coherence of [HbO] (hemodynamic); (ii) bilateral coherence of [CCO] (metabolic); (iii) unilateral coherence between [HbO] and [CCO] on the left forehead, and (iv) unilateral coherence between [HbO] and [CCO] on the right forehead.


Results, Conclusions, and Discussions:: Results

The hypothesis of this study was that a decline in prefrontal connectivity in older adults exists and can be measured non-invasively compared to younger adults. The results from the younger participant group are shown in Fig. 1, which shows bilateral connectivity (left panel) and unilateral coupling (right panel) in three different frequency bands (endogenic, neurogenic, and myogenic). We expect that several or all of the four prefrontal coherence values (reflecting bilateral connectivity and unilateral coupling) would have significantly different values between the older and younger adult groups, in some or all three ISO (i.e. endogenic, neurogenic, and myogenic) frequency bands.

Discussion and Conclusion

The main finding is expected to show significant differences in bilateral connectivity and/or unilateral coupling in each ISO frequency band between older and younger healthy participants. The oscillations of the blood vessels decrease with age, indicating less blood flow through the cortex. The brain requires oxygenated blood to perform active cognitive functions properly. The oscillation reductions in older participants may be related to a decrease in the supply of oxygenated blood, leading to lower connectivity and lower cognitive performance due to the aging process. A better understanding of age-dependent prefrontal cortical connectivity derived from 2-bbNIRS may facilitate the development of a noninvasive and portable device that can be used for clinical applications, such as neurophysiological monitoring of age-related changes, neurological conditions, early stages of AD, and other brain diseases.


Acknowledgements (Optional): :

References (Optional): : Shahdadian, S., Wang, X., Kang, S., Carter, C., Chaudhari, A., & Liu, H. (2022). Prefrontal cortical connectivity and coupling of infraslow oscillation in the resting human brain: a 2- channel broadband NIRS study. Cerebral Cortex Communications, 3(3), tgac033.