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The aim of the present study was to investigate the electrophysiological effects of the photobiomodulation (PBM) by the quantitative electroencephalography (qEEG) as a diagnostic method. The neurotherapeutic potential of transcranial PBM has been recently investigated in preclinical and clinical studies. According to the PBM mechanisms of action on increasing the cerebral blood flow and the neuronal firing, a change may occur in cortical electrical activity after transcranial PBM that could be revealed in qEEG. A total of 30 participants (15 males and 15 females) were included in this experimental study in a convenience sampling method. A 19-channel EEG was obtained from subjects, before and after receiving sham or real 850-nm PBM by light emitting diode (LED) array on the right prefrontal cortex (PFC). An attentional task also was completed by the participant before and after the irradiation. Results presented that the effect of PBM on the reaction time was significant (p = 0.001) in favor of the real-treatment group (p < 0.05). For the absolute power, repeated-measures ANOVA showed a significant interaction of group × time × frequency (p = 0.04). In the real-treatment group, absolute power of delta band was significantly reduced in all electrodes (p < 0.05). Also, a similar significant interaction of group × time × frequency was seen for relative power (p = 0.04). Post-hoc analysis showed a significant decrease in delta band after PBM in the real treatment group (p < 0.05). The study presented that light irradiation with 850-nm LED source on right PFC could change brain electrical activity and has beneficial effects on attentional performance. [ABSTRACT FROM AUTHOR]