Multi-unit electrophysiological mapping was used to establish the area of the left- and right-hemisphere auditory cortex (AC) of the mouse and to characterize various fields within the AC. The AC of the left hemisphere covered a significantly larger (factor of 1.30) area compared to that of the right side. Based on best-frequency (BF) maps and other neuronal response characteristics to tone and noise bursts, five fields (primary auditory field, anterior auditory field, second auditory field, ultrasonic field, dorsoposterior field) and two small non-specified areas could be delimited on both hemispheres. The relative sizes of these fields and areas were similar on both sides. The primary and anterior auditory fields were tonotopically organized with counter running frequency gradients merging in the center of the AC. These fields covered BF ranges up to about 45 kHz. Higher BFs up to about 70 kHz were represented non-tonotopically in the separate ultrasonic field, part of which may be considered as belonging to the primary field. The dorsoposterior and second auditory fields were non-tonotopically organized and neurons had special response properties. These characteristics of the mouse AC were compared with auditory cortical maps of other mammals.

Electrophysiological mapping was used to study frequency representation in the inferior colliculus (IC) of the mouse. In the lateral nucleus (LN) only part of the frequency range of hearing was represented and tonotopicity was separate from that in the rest of the IC. Highest frequencies occupied the medial part (M) of the central nucleus (CN). A single complete representation of the hearing range was present only if representations in the dorsal cortex (plus dorsomedial nucleus) and CN (including M) were combined. Continuous isofrequency planes making up these nuclei (without the lateral part of the CN) were reconstructed. They tilted from medial to lateral and from caudal to rostral. The steepness of the slopes increased from caudal to rostral and from dorsal to ventral (i.e., with increasing frequency). Isofrequency planes had similar angles of deviation from the horizontal plane as described for dendritic laminae in the CN. Differences of mapping in the lateral part of the CN from that in the rest of the CN could be explained by the different organization of laminae in this part. The relative amounts of IC depth and volume occupied by parts of the mouse audible frequency range were quantified. Frequency representation along IC depth was not proportional to that along cochlear length. Compared with the relative density of afferent nerve fiber supply within given frequency ranges represented along the basilar membrane, there is a relative under-representation in the IC up to 15-20 kHz and an over-representation of higher frequencies. Highest absolute tone sensitivity (lowest threshold) was found in neurons forming a column (running perpendicular to isofrequency planes) in the center of the IC. Results are discussed with regard to frequency representation, intrinsic neuronal organization, and functional segregation in the IC of mammals.

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