Audible frequencies of sound are encoded in a continuous manner along the length of the cochlea, and frequency is transmitted to the brain as a representation of place on the basilar membrane. The resulting tonotopic map has been assumed to be a continuous smooth progression from low to high frequency throughout the central auditory system. Here, physiological and anatomical data show that best frequency is represented in a discontinuous manner in the inferior colliculus, the major auditory structure of the midbrain. Multiunit maps demonstrate a distinct stepwise organization in the order of best frequency progression. Furthermore, independent data from single neurons show that best frequencies at octave intervals of approximately one-third are more prevalent than others. These data suggest that, in the inferior colliculus, there is a defined space of tissue devoted to a given frequency, and input within this frequency band may be pooled for higher-level processing.

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Acoustic experiences significantly shape the functional organization of the auditory cortex during postnatal ‘critical periods’. Here, we investigate the effects of a non-traumatic augmented acoustic environment (AAE) on the central nucleus of the inferior colliculus (ICC) and lower brainstem nuclei in rat during the critical period. Our results show that an AAE during P9–P28 had a persistent effect on the evoked auditory brainstem responses leading to a decreased latency and an increased amplitude of the response at and above the frequency of the stimulus used for the AAE. These findings are correlated with increased numbers of sites in the ICC that responded to the AAE frequency and show higher thresholds. There also were persistent effects in neurons with a best frequency higher than the AAE stimulus. These neurons showed decreased activity at low sound levels in the low frequency tail of the frequency response area. This was at, below and above the AAE stimulus frequency. Less often, increased activity at higher sound levels also was seen. Together, these findings suggest multifaceted interactions between activity-dependent plasticity, homeostasis, and development in the brainstem during the initial stages of hearing. A neonate exposed to an altered auditory environment may experience long-lasting change over the entire network of the auditory system.

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