A vertical stereotaxic approach to auditory cortex in the unanesthetized monkey.

A procedure is described for chronic single-unit recording from monkey auditory cortex. The cortex is approached in a vertical stereotaxic plane and subsequent histology is performed on tissue sections made in the same plane. The procedure maximizes the probability of locating and identifying auditory cortical areas during the chronic unit recording sessions and of identifying individual electrode tracks in subsequent histologic examination. Average stereotaxic coordinates for the center of area A1 are A-P +5, M-L 17.5 and D-V +20, but variation of +/-5 mm can occur across subjects. The superior temporal plane is sloped at an angle of about 30 degrees in the A-P dimension but is relatively flat in the M-L dimension. Microelectrode penetrations made with this procedure were found, on the average, to terminate within 0.7 mm of the intended site. Procedures for improving this accuracy and for identifying closely-spaced penetrations are discussed.