Hyperactivity following unilateral hearing loss in characterized cells in the inferior colliculus.

Hyperactivity (increased spontaneous firing rates) following cochlear trauma and hearing loss has been well documented in the inferior colliculus (IC). This hyperactivity is associated with frequency regions in the IC that are closely related to regions of peripheral hearing loss. In other auditory nuclei, notably cochlear nucleus, hyperactivity has been shown to be more prevalent in particular cell types but this has not been investigated in the IC. Single-neuron spontaneous firing rates were recorded in the IC of animals after acoustic trauma (10-kHz tone at 124dB for 2h) and in sham surgery controls. Single-neuron recordings were made 2weeks later. Evoked responses to ipsi- and contralateral sound were used for classification. Classifications were based on peri-stimulus time histograms, input-output functions, frequency response areas and monaural/binaural responses. Results showed increased spontaneous firing rates in the IC following trauma, in regions corresponding to the frequencies at which there was peripheral hearing loss (12-20kHz). Most response categories, with the exception of cells showing an onset response classification, showed a significantly increased average spontaneous firing rate. These data suggest that hyperactivity in the IC is not confined to a particular response type in contrast to findings in the cochlear nucleus. This may be the result of factors intrinsic to the IC, or because of convergent input to the IC from a range of other auditory structures.