An Optical Brain Computer Interface

An optical brain computer interface (OBCI) is a device, which translates physiological measures of thought processes, derived through optical interrogation techniques, into control signals capable of driving external computers independent of the peripheral nervous system. The optical method used in our current system is near-infrared spectroscopy. An optical window at 650-900nm allows light, within this range, to usefully penetrate the intact cranium and reach sufficient depths to probe the surface of the cerebral cortex [1]. The photons are mostly scattered and absorbed, but some are back-reflected and exit the head at positions up to a few centimeters away from their original source location. Both experimental and theoretical studies have shown that the photons travel in an arc-shaped path from source to detector [2]. Nearinfrared spectroscopy can provide information regarding both haemodynamic and neural activity. The optical response due to haemodynamic activity is a much easier signal to acquire as it leads to 1-2% changes in signal amplitude, whereas the optical response due to neural firing only yields changes of 0.01-0.1% [3]. The cerebral haemodynamic response results from an autoregulatory process responding to the metabolic demands of neuronal firings and takes 5-8sec. The faster optical response relating to neuronal firing is of the order of milliseconds but because it is of much smaller magnitude it requires averaging techniques to achieve acceptable signal to noise ratios. Franceschini and Boas [3] reported averaging blocks of 1000 trials and observing a 0.05% change in light intensity. It is for this reason that we have chosen to use the more accessible slow optical response related to cerebral haemodynamics for the first prototype OBCI.