The properties of the orientation sound (pulse) of the Jamaican mustached bat, Pteronotus parnellii parnellii is the same as the Panamanian mustached bat, P.p. rubiginosus. It consists of four harmonics, each containing a long constant-frequency (CF) component followed by a short frequency-modulated (FM) component. Thus, there are eight components in total: CF1-4 and FM1-4. The combination-sensitive area of the auditory cortex in P.p. parnellii consists of two major divisions (FM-FM and CF/CF areas) as in P.p. rubiginosus. The FM-FM area projects to the dorsal fringe (DF) and other areas. Response latencies of neurons in the DF area are longer than those in the FM-FM area. The distribution of latencies is unimodal for the FM-FM area, but bimodal for the DF area. In this electrophysiological study of the response properties of neurons in the DF and FM-FM areas, our aim was to find out how signal processing might be different between the two areas. Both the FM-FM and DF areas consist of three types of FM-FM combination-sensitive neurons: FM1-FM2, FM1-FM3, and FM1-FM4. They do not respond or respond poorly to pulse alone, echo alone, single CF tones or single FM sounds. But they show strong facilitation of response to the echo when it is delivered with particular delays from the pulse. The essential elements in the pulse-echo pair for facilitation are the FM1 of the pulse and FM2 or FM3 or FM4 of the echo. In both the FM-FM and DF areas, the great majority of neurons show short-lasting facilitation, and other neurons show long-lasting facilitation. FM-FM neurons are tuned to particular echo delays, i.e., target ranges. In both the FM-FM and DF areas, the width of a delay-tuning curve is linearly related to the value of a best delay. There is no sign that processing of range information is more specialized in the DF area than the FM-FM area. In both the FM-FM and DF areas, three types of FM-FM neurons form independent clusters. Along the major axis of each cluster, best delays for facilitative responses of neurons systematically change according to the loci of the neurons. The more posterior the location, the longer the best delay is. Therefore, there are six time (i.e., range) axes in total. The time axis in the DF area is shorter than that in the FM-FM area.(ABSTRACT TRUNCATED AT 400 WORDS)

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(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

Neural activity of multiple fields in the auditory cortex of anesthetized guinea pigs in response to pure tones was visualized by optical recording using a voltage-sensitive dye (RH795). Ten auditory fields were identified based on the tonotopic organization and response latency: the core fields consisting of the primary (AI) and secondary (AII) fields and surrounding belt areas consisting of dorso-anterior (DA), dorsal (D), dorso-posterior (DP), posterior (P), ventro-posterior (VP), ventro-medial (VM), ventro-anterior (VA) and ventral (V) fields. Tonotopic organization was observed in all the fields apart from DA, D, DP and V. Spatio-temporal displays suggest that the auditory information spreads from the core fields of AI and AII to belt fields via three distinct (dorsocaudal, caudal and ventrorostral) pathways.

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