Neuronal Coding And Plastic Changes Of Sound Duration In Auditory Midbrain Of Guinea Pigs

Temporal and frequency features in acoustic signals are important for language recognition. Most acoustic information contained in signals for communication, such as speech, is embodied in the temporal fluctuations in both of the amplitude and spectra. Therefore, temporal features are important substrate in converting information. However, temporal processing mechanisms in the central auditory system are far less understood than those for frequency and intensity processing. This study aimed to explore the duration coding behaviors of neurons in the inferior colliculus of guinea pigs and involvement of local inhibition in this coding; demonstrate the plastic changes in duration tuning for neurons in low frequency region after high frequency cochlear lesion.Duration tuning has been well documented in central auditory system of several species of bats. Recently, duration tuning has also been found in non-echolocation mammals in different stations along the auditory passage. However, the numbers of duration tuning neurons in the inferior colliculus (IC) of non-echolocation mammals were found to be greatly smaller than in bats in these studies. This superficial weakness in duration coding in the non-echolocation mammals may be resulted from the inappropriate use of a long analyzing window. When a short window is used to calculate the spike rate in the time peak of responses, the number of duration tuning neurons in non-echolocation mammals is comparable to that in bats. The result indicates that the duration tuning is a transient behavior in the IC of guinea pigs.Previous studies have shown that the interplay between the excitation and inhibition in the IC contributes to auditory processing and shapes the selectivity or tuning of many parameters of the acoustic signal. In the studies of bats, the role of inhibitory inputs in duration tuning was clearly demonstrated. To the best of our knowledge, however, the role of inhibition in duration tuning has not yet been verified in non-echolocating mammals. In this study, we find that similar to echolocating bats, the interaction between inhibition and excitation is the key for shaping the duration tuning in the IC of guinea pigs, a non-echolocating mammal. This interaction is much stronger in the early excitation of the tonal responses that are mediated by AMPA receptors. Correspondingly, while the total responses of these non-offset responders are not duration tuned, the early component of the responses from these neurons is more likely to be duration tuned. Accumulating evidences demonstrated that cochlear lesion will cause functional and plastic changes in center auditory system. However, most studies till now focused on the changes in frequency and intensity coding. Little is known about the changes in temporal coding features in the central auditory system after cochlear lesion. In this study, we found that duration tuning of neurons in low frequency region diminished after cochlear lesion induced by a high frequency tone exposure, presumably through an off-channel effect.This study indicates that duration tuning is also important to non-echolocating mammal; the mechanism under duration tuning in non-echolocating mammal is different to that in echolocating animals and the duration coding in guinea pigs is also subject to plastic changes after cochlear lesion. This study provides more insight on duration tuning in non-echolocating mammals and the plasticity in temporal processing. The knowledge obtained in this study is also helpful in understanding the problem of communication after high frequency hearing loss..