biostudies-literatureWojtkiewicz SNalecz Institute of Biocybernetics and Biomedical EngineeringNarodowe Centrum Nauki1869-1887https://www.ebi.ac.uk/biostudies/studies/S-EPMC9045899biostudies-literatureUnknown13(4)Here we show a method of the lock-in amplifying near-infrared signals originating within a human brain. It implies using two 90-degree rotated source-detector pairs fixed on a head surface. Both pairs have a joint sensitivity region located towards the brain. A direct application of the lock-in technique on both signals results in amplifying common frequency components, e.g. related to brain cortex stimulation and attenuating the rest, including all components not related to the stimulation: e.g. pulse, instrumental and biological noise or movement artefacts. This is a self-driven method as no prior assumptions are needed and the noise model is provided by the interfering signals themselves. We show the theory (classical modified Beer-Lambert law and diffuse optical tomography approaches),Biomedical optics expressLock-in functional near-infrared spectroscopy for measurement of the haemodynamic brain response.PMC90458992012/05/B/ST7/011622020/39/D/ST7/03425ST213/20212016/21/N/ST7/03117Bejm KWojtkiewicz SLiebert AfalseLock-in functional near-infrared spectroscopy for measurement of the haemodynamic brain response.Here we show a method of the lock-in amplifying near-infrared signals originating within a human brain. It implies using two 90-degree rotated source-detector pairs fixed on a head surface. Both pairs have a joint sensitivity region located towards the brain. A direct application of the lock-in technique on both signals results in amplifying common frequency components, e.g. related to brain cortex stimulation and attenuating the rest, including all components not related to the stimulation: e.g. pulse, instrumental and biological noise or movement artefacts. This is a self-driven method as no prior assumptions are needed and the noise model is provided by the interfering signals themselves. We show the theory (classical modified Beer-Lambert law and diffuse optical tomography approaches),2022-01-01T00:00:00Z2022 Apr2026-06-12T05:47:54.454Z2026-06-12T03:08:19.543ZS-EPMC90458993551926010.1364/boe.44803810.1364/BOE.448038