(Communicated by Yasuji KATSUxi, M. J. A., Oct. 12, 1988) Previous studies have shown that rats hear sounds between 0.5 and 65 kHz,2>,12) and emit ultrasonic sounds at about 22 kHz and 50 kHz to communicate.8 However, there were no systematic studies on the tonotopic representation of the rat auditory cortex, except for a few partial studies.?>,11) In this study, we describe in detail the tonotopic representation of the rat auditory cortex.Methods. Ten male Wistar-strain rats (220-300 g) with tympanic membranes of no sign of infection were used for the experiments. They were anesthetized with Nembutal (40 mg kg) intraperitoneally, the trachea was cannulated for aspirating mucosal products, and the femoral vein was catheterized for injections of additional doses of the anesthesia (5 mg/kg/hr) . The outer auditory meatus was opened to enable an ultrasonic earphone) to be inserted, which was connected to an ear bar. The rat's head was clamped onto a stereotaxic frame (Narishige, SN-2) and a hole was made on a lateral part of the skull to expose the left auditory cortex. To minimize brain movements, dura matter was left intact, and a small metal chamber covering the hole was glued onto the bone and filled with saline and paraffin. Rectal temperature was kept at 37.5-38.5°C using a servo-controlled heater. All experiments were conducted in a sound-proofed room.Auditory stimuli were delivered to the ear contralateral to the recording side. Tone burst signals (5 ms of rise-fall time, 500 ms of duration and a 1/s repetition rate) were generated by an oscillator (Wavetek, model 116), an electronics switch, attenuator and amplifier. Sound pressure levels near the tympanic membrane in each animal were measured with a 1/4 inch microphone (Bruel & Kjaer 4135) and expressed in dB SPL (sound pressure level in dB re 20 µPa) .At the beginning of the experiment, veins and arteries along the cortical surface were sketched using a 20 X dissection microscope (Olympus MTX) with a cross-sectioned scale in the eyepiece. Tungsten microelectrodes (1-3 µm of tip diameters) were inserted vertical to the auditory cortical surface by a motordriven micromanipulator (Narishige, , and the positions where the electrodes were inserted, were plotted on the diagram. Extracellular potentials of a single neuron or a tiny cluster of 2 or 3 neurons in response to the tone bursts were amplified and monitored with an oscilloscope and an audio-monitor. The best frequency (i.e., the frequency at the minimal threshold) of each neuron was measured audiovisually.Results. The auditory cortex of the rat was located posterior to an ascending branch of the inferior cerebral vein in the caudal part of the temporal cortex (Fig. 1B), corresponding to area 41 of Krieg's map of the rat brain.3>Neurons in response to the tone bursts were encountered in the auditory
