Functional frequency-domain near-infrared spectroscopy detects fast neuronal signal in the motor cortex

Millisecond changes in the optical properties of the human brain during stimulation were detected in five volunteers using noninvasive frequency-domain near-infrared spectroscopy. During a motor stimulation task we found highly significant signals, which were directly related to neuronal activity and exhibited much more localized patterns than the slow hemodynamic signals that are also detected by the near-infrared method. We considerably reduced the noise in the instrumental system and improved data analysis algorithms. With the achieved signal-to-noise ratio, single subject measurements were feasible without the requirement of particularly strong stimuli and within a reasonable period of measurement of 5 min at a mean signal-to-noise ratio of 3.6. The advantage of this non-invasive technique with respect to electrical recording is that it is able to detect neuronal activity with the relatively high spatial resolution of 8 mm.