Functional brain signals: a photon counting system for brain activity monitoring

Abstract

A simplified in vivo near infrared spectroscopy (NIRS) system for functional brain analysis and a protocol for the study of visual evoked potentials in the human brain is presented. A novel NIRS system bases on a simple photon counting technique using a CW light source (laser diode at 780 nm), fibre optodes delivering light to the subject and from the subject to detector, a photomultiplier tube (PMT) for high infra-red (IR) response and the 800 MHz Gated Photon Counter/multichannel scaler (MCS) for data acquisition. A chequerboard stimulus was used to elicit a response signal from the visual cortex. This photon signal arising from the cortical systems of the brain was processed to detect features indicative of the neural processing systems involved.

Multi-timescale measurements of brain responses in visual cortex during functional stimulation using time-resolved spectroscopy

“…Therefore, in this work, two non-invasive measurement techniques for visual stimulation studies in human adults are described. This study continues research done previously [13,14] on the development of an alternative optical system for acquisition of multi timescale evoked potentials in the human brain and the main outline of the current work is done in a comparative study to obtain those signals on ms-timescales. Before descriptions of the experimental methods and results are presented, a brief introduction to the physiology of the human visual processing system is given along with a background on previously known work for its analysis.…”

“…A single photon counting system described in detail elsewhere [13,14] makes it possible to record multi-timescale changes in the human brain with high resolution. Electroencephalography was applied to pick up brain neural activity in parallel and to prove that the chosen protocol did actually evoke some visual evoked potentials (VEP).…”

“…The interrogation of biological tissue with near-infrared light (700-1300 nm) allows non-invasive evaluation of important brain metabolic processes such as tissue oxygenation, haemoglobin content, and cytochrome oxydase (CytO 2 ) [2,5,9,13,16]. A number of studies undertaken by leading laboratories have shown that it is possible to measure brain responses during motor, cognitive, and visual stimulation [9,14,17,18,19].…”

“…However, with this approach, currently achieved best value for the sampling rate is ~ 200 Hz and the technique is very expensive. On the "slow" time scale NIRS has proved its ability to record hemodynamic changes in the human brain [3,9,10,13,16,18], however on the "fast" timescale which is important for neural event detection it is only just beginning to make progress [11,12,14,20]. The previously described slow biochemical changes in the human brain (~ 10 s) are indirectly associated with neuronal firing.…”