Reflectance of rat brain structures mapped by an optical fiber technique

A method to detect differences in reflectance as a pair of glass fibers is advanced through various brain structures at a constant speed is described. The system has been used to develop a technique for accurate and easy placement of electrodes or cannulae into a limited brain region. The probe consisted of two thin (50 micrometer in diameter) glass optic fibers. A 5 W lamp was used to transmit light down one of these fibers. The relative intensity of light reflected from brain tissue into the other optic fiber was monitored with a photomultiplier and ink-writing recorder. Different brain structures were shown to vary in the amount of reflectance, with white matter having much higher reflectance than gray matter. It was demonstrated in a total of 239 penetrations that the patterns of successive changes in the reflected light response, as the probe was lowered into the brain, was characteristic for each frontal plane. The probe may additionally be itself used as an electrode if plated with silver.