Dose response of transcranial near infrared light stimulation on brain functional connectivity and cognition in older adults- a randomised comparison.

'Photobiomodulation, also called low-level light therapy, has been reported in animal studies to have an effect on brain activity and cognition. However, studies in humans regarding its effect on cognition and brain functional connectivity, and the required dose threshold for achieving the same have been very limited. We compared the effects of different doses of photobiomodulation (PBM) on cognition and resting state brain functional connectivity in 25 cognitively normal adults aged 55 to 70 years. They were randomized to a single session of the sham group, 'low-dose' and 'high-dose' groups receiving NIR light with transcranial fluence of 26 and 52 J/cm 2 respectively, and intranasal fluence of 9 and 18 J/cm 2 respectively. There was a significant increase in resting state functional connectivity of the left superior frontal gyrus (SFG) with the left planum temporale (PT), p = 0.0016, and with the left inferior frontal gyrus, pars triangularis, p = 0.0235 in the 'high-dose' group only compared to the 'sham' group. There was also a significant improvement in visual search and processing speed (p = 0.012) in the 'high-dose' group. Replication of these findings in an adequately powered randomised sham-controlled study in healthy older adults can pave the way for clinical application of NIRL as a therapeutic modality in patients with Alzheimer's disease. This article is protected by copyright. All rights reserved.