Inferior colliculus of the house mouse. I. A quantitative study of tonotopic organization, frequency representation, and tone‐threshold distribution

Abstract

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.

Topography and physiology of ascending streams in the auditory tectothalamic pathway

“…However, some caution may be warranted with respect to the boundaries established in this manner because their correlation with functional borders established in vivo is tentative (2,22,23). In addition, recorded neurons were localized based on a combination of the gross anatomical features, cell morphology, and intrinsic physiology.…”

Antidromic Activation Reveals Tonotopically Organized Projections From Primary Auditory Cortex to the Central Nucleus of the Inferior Colliculus in Guinea Pig

“…Thus the ICC should provide access to neural circuits essential for hearing restoration by an auditory prosthesis. The ICC has a highly organized tonotopic structure (Geniec and Morest 1971;Malmierca et al 1995;Merzenich and Reid 1974;Rose et al 1963;Schreiner and Langner 1997;Serviere et al 1984;Stiebler and Ehret 1985), which should allow for systematic access to different frequency regions using a penetrating electrode array auditory midbrain implant (AMI). In fact, the AMI should be able to access low-frequency regions (Ͻ1 kHz) in the ICC, which is usually not achievable in the cochlea with current CIs.…”

Excitatory, inhibitory and facilitatory frequency response areas in the inferior colliculus of hearing impaired mice

“…In normal hearing mice, there exists a topographic frequency map that originates at the inner ear and is preserved in all major auditory nuclei (Ehret and Romand, 1997;Muller et al, 2005). In the mouse IC, this tonotopic map normally represents lower frequencies dorsolaterally and progressively higher frequencies ventromedially (Stiebler and Ehret, 1985). Willott (1984) observed that the tonotopic map in IC is severely disrupted in mice suffering from sensorineural hearing loss.…”

“…Neurons in the ventral IC normally respond to high frequencies, but as sensorineural hearing loss progresses, their sensitivity shifts to lower frequencies (Stiebler and Ehret, 1985;Willott, 1984). Several mechanisms have been proposed that may underlie changes in tonotopic organization in mice with sensorineural hearing loss (Willott, 1984;Willott, 1986).. One explanation for altered tonotopy involves a change in sensitivity in the cochlea (Willott, 1984).…”

“…The most dramatic change in the IC of C57 mice over the age of six months occurs physiologically and is demonstrated by a shift in frequency tuning of neurons in the ventral region (Willott et al, 1988), where high frequencies are represented in normal hearing mice (Stiebler and Ehret, 1985). As the ventral region in the IC loses its high frequency inputs due to the basal cochlear pathology, there is a shift in neural sensitivity from high to low frequencies.…”