Tonotopic representation in the rat auditory cortex.

Abstract

(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

Multiparametric Auditory Receptive Field Organization Across Five Cortical Fields in the Albino Rat

“…In keeping with previous reports in the rat, we observed a mirror-reversal in tonotopy at the low-frequency border of AI, marking the caudal boundary of PAF (Doron et al 2002;Horikawa et al 1988). The response properties of units recorded within PAF and AI were markedly different.…”

“…We found that AI is composed of neurons with well-tuned tonal receptive fields and firing rates that most often increased monotonically with the intensity of tonal or broadband stimuli as previously described in the rat (Doron et al 2002;Horikawa et al 1988;Kilgard and Merzenich 1999;Phillips and Kelly 1989;Polley et al 2004Sally and Kelly 1988;Wu et al 2006). The AI map featured small, spatially independent modules of sharp spectral tuning or nonmonotonic rate-level functions.…”

“…To date, tonal receptive fields, intensity-response profiles, and basic excitatory response properties were characterized for three fields of the normative rat auditory cortex: the primary auditory cortex (AI), the posterior auditory field (PAF), and the anterior auditory field (AAF) (Doron et al 2002;Horikawa et al 1988;Kilgard and Merzenich 1999;Phillips and Kelly 1989;Rutkowski et al 2003;Sally and Kelly 1988). Anatomical tracer studies conducted in the rat confirmed that AI lies in cortical area TEI and receives heavy projections from the ventral division of the medial geniculate body (MGv) into the middle cortical layers (Kimura et al 2003;Roger and Arnault 1989;Romanski and LeDoux 1993b;Winer et al 1999).…”

Spectral and Temporal Processing in Rat Posterior Auditory Cortex

“…Although electrophysiological studies have generally focused on response characteristics of primary auditory cortex neurons (Horikawa et al 1988;Sally and Kelly 1988;Thomas and Tillein 1997;Kilgard and Merzenich 1999), considerable evidence suggests the existence of a functionally distinct PAF (Zhang et al 2001;Doron et al 2002;Rutkowski et al 2003). The purpose of this study is to compare spectral and temporal processing in the primary and PAFs.…”

Fine functional organization of auditory cortex revealed by Fourier optical imaging

“…The areas are thought to correspond to different auditory pathways and contribute toward distinct aspects of auditory information processing (13,14). The number of fields in each species is itself controversial, ranging in the rat from estimates as low as two or three (15) to as many as five (2,4).…”