A Low-Level High-Speed Switching System for Brain Mapping

In recent experiments J. C. Lilly has obtained contour maps, in rapid sequence, depicting electrical activity over selected areas of the cortex of an animal's brain. The present work has been an attempt to improve the resolution in both time and space of this mapping technique. Adequate resolution requires more than 200 electrodes covering a square centimeter of the cortex. A complete map must be observed 500 to 1,000 times a second. In order to handle the required number of information channels, a time-division switching system has been devised. The electrodes are divided into groups of sixteen. Simultaneously operating switching units commutate the information within the respective groups. An amplifier for each group passes a train of samples representing the potentials at the electrodes of that group. These simultaneous sample trains can be recorded on multitrack magnetic tape. Later the sampled information can be displayed in map form, e.g., on a cathode-ray screen. The electronic switching of the 16 electrode signals in a group takes place after a single stage of preamplification for each electrode. The switching elements are matched pairs of silicon junction diodes connected, with resistances, in bridge circuits. A master pulse generator operates all the switching units. The system is sensitive to electrode potentials of about 30μv.