Photobiomodulation (PBM) uses the delivery of light in the visible (400–700 nm) or near-infrared (700–1100 nm) range to stimulate or inhibit biological cell and tissue function [5]. During PBM, the cytochrome c oxidase activities are increased due to near-infrared light absorption, increasing electron transport, mitochondrial membrane potential, and adenosine triphosphate production, thus boosting oxygen metabolism and utilization rate, and energy production in the cells [6]. In human studies, increased cerebral concentrations of oxidized cytochrome c oxidase, oxygenated hemoglobin, and total hemoglobin [7], and increased strength of electrophysiological oscillations [8] were observed during and after PBM. Together, these results suggested that PBM may improve cell metabolism. 

 

Recent studies have tested PBM in patients with cognitive or psychological disorders and have yielded some encouraging results. Early studies on PBM have shown positive effects on patients with traumatic brain injury [9], as indicated by improved executive function, verbal learning, and memory after 18 treatments over the course of 6 weeks. After PBM, improvements in cognitive or neurological functioning were also reported in stroke patients [10–12]. A significant effect has been found in patients with dementia as well. Following a 12-week course of PBM therapy, five patients with mild to moderately severe dementia showed improvements in cognitive performance according to the Mini-Mental State Examination and the Alzheimer’s Disease Assessment Scale [13]. In another study, 11 patients with Alzheimer’s disease showed improved cognitive function (i.e., executive function and memory) after 28 sessions of PBM [14]. Furthermore, a patient with Alzheimer’s disease showed significant improvement in cognitive functions, as measured by the Montreal Cognitive Assessment and Working Memory Questionnaire, after applying PBM therapy twice daily for 4 weeks [15].  

 

Our research team has been investigating the effect of Photobiomodulation (PBM) on improving cognitive functions for several years. We have explored the beneficial effects of PBM among older adults. In this study, 30 older adults were recruited and divided into either the real or the sham PBM group. After a single PBM trial, we found that only the participants in the real PBM group improved action selection ability, inhibitory control, and mental flexibility, as measured by the Eriksen flanker task and the category fluency test.  On the contrary, participants in the sham PBM group, who did not receive a real PBM, got no significant improvement [16]. 

 

Furthermore, another study was conducted to examine the possible underlying mechanism associated with PBM, in this study, a group of young adults was recruited and their hemodynamic change associated with the work memory task was measured by the fNIRS [17]. It was found that participants who received a real PBM exhibited enhanced neural efficiency, indicated by a significant reduction of oxygenated hemoglobin (HbO) in the bilateral prefrontal cortex during a difficult working memory task (i.e. the 3-back task), and with a comparable task performance as before receiving the PBM.  This result was not observed in subjects who have received sham stimulation. These results suggest that PBM may reduce the cognitive efforts needed to complete the tasks with a high working memory load.  

 

In addition, our research team has also studied PBM on older adults with mild cognitive impairment (MCI) [18]. Eighteen individuals who meet the diagnostic criteria of MCI proposed by the National Institute of Aging-Alzheimer’s Association workgroups were recruited and were randomly assigned to control and experimental groups. The hemodynamic changes, as a measure of functional brain activity, were evaluated using functional near-infrared spectroscopy (fNIRS). A single real or sham PBM session was administered to the forehead of each patient in the experimental and control groups, respectively. All subjects performed a visual memory span test before and after the stimulation, and their hemodynamic responses during the tasks were measured using fNIRS.  The results showed that among the MCI subjects, only those who received PBM, but not those who received the sham stimulation, demonstrated significant improvement in the visual memory performance and a reduction in the hemodynamic response during the tasks. These findings suggest that PBM may reduce the cognitive efforts needed to complete tasks that require high memory loads and thus improve the cognitive performance of individuals with MCI. 

 

Apart from studies with single PBM trials, we have also investigated the effect of PBM in multiple trials. In a case series of three amnestic MCI patients who received 18 sessions of PBM [19], it is found that one participant got improved verbal memory from the 1st percentile to the 67th percentile. For another two participants, they got improved visual memory from the 4th percentile to 26th percentile, and from 11th percentile to 54th percentile, respectively. Based on the results of our published studies, it is believed that the beneficial effects of PBM on healthy and MCI individuals may also be seen in patients with TBI. 

 

Observing the results of our previous studies, not all older adults are responsive to the PBM intervention.  That is, some showed significant improvement in a relatively short intervention period, while the others have not shown any improvement even after 18 sections.  The present study is to examine the possible factors that may predict the response of the PBM stimulation.

 

Lee, T. L., Ding, Z., & Chan, A. S. (2023) Can transcranial photobiomodulation improve cognitive function? A systematic review of human studies. Ageing Research Reviews, 83, 101786

 

Chan, A. S., Lee, T., Hamblin, M., & Cheung, M. (2021). Photobiomodulation enhances memory processing in older adults with mild cognitive impairment: A functional Near-Infrared spectroscopy study. Journal of Alzheimer's Disease, 83(4), 1471-1480. 

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Chan, A. S., Lee, T. L., Hamblin, M. R., & Cheung, M. C. (2021) Photoneuromodulation makes a difficult cognitive task less arduous. Scientific Reports, 11(1), 13688.

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Chan, A. S., Lee, T. L., Yeung, M. K., & Hamblin, M. R. (2019). Photobiomodulation improves the frontal cognitive function of older adults. International Journal of Geriatric Psychiatry, 34(2), 369-377.

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Chan, A. S., Yeung, M. K., & Lee, T. L. (2019). Can photobiomodulation enhance brain function in older adults? In M. Hamblin & Y. Y. Huang (Eds.), Photobiomodulation in the Brain (pp. 427-446). Academic Press.