A novel near infra-red spectrophotometry system using microprobes: its evaluation and application for monitoring neuronal activity in the visual cortex

A novel near-infrared (NIR) spectrophotometry system with microprobes of optical transmitter and receivers (550 and 410 μm in diameter, respectively) has been developed. A three-dimensional profile of the signal source estimated in in vitro experiments showed two spindle-shaped regions around the respective probes, suggesting that the signal detected by the present system comes from a relatively restricted region around each probe. Next, we examined how the concentration of oxygen in inspired gas affected the NIR signals in the rat cerebral cortex in vivo. Calculated concentrations of oxyhemoglobin (HbO 2) and deoxyhemoglobin (Hb) in the rat cortex changed in mirror-image fashion in response to the change in the O 2 concentration in the inspired gas. Finally, NIR responses to visual stimulation were recorded from the striate cortex of conscious adult cats that had been monocularly deprived since the peak of the sensitive period. Corresponding to the results of previous electrophysiological studies, stimulation of the normal eye induced significant NIR signals, whereas that of the deprived eye evoked no response. These results indicate that this new NIR system can be applied to study changes in oxygen metabolism in relatively restricted regions following neuronal activation in the brain.