Functional reorganization in chinchilla inferior colliculus associated with chronic and acute cochlear damage.

This paper describes some of the unexpected functional changes that occur in the inferior colliculus (IC) following noise- and drug-induced cochlear pathology. A striking example of this is the compensation that is seen in IC responsiveness after drug-induced selective inner hair cell (IHC) loss. Despite a massive reduction in the compound action potential (CAP) caused by partial IHC loss, the evoked potential amplitude from the IC shows little or no reduction. Acoustic trauma, which impairs cochlear sensitivity and tuning, also reduces the CAP amplitude. Despite this reduced neural input, IC amplitude sometimes increases at a faster than normal rate and the response amplitude is enhanced at frequencies below the hearing loss. Single unit recordings suggest the IC enhancement phenomenon may be due to the loss of lateral inhibition. After an acute traumatizing exposure to a tone located above the characteristic frequency (CF), approximately 50% of IC neurons show a significant increase in their spike rate, a significant expansion of the low frequency tail of the tuning curve and a significant improvement in sensitivity in the tail of the tuning curve. These changes suggest that IC neurons receive inhibition from a high frequency side band and that this inhibition is diminished by acoustic trauma above CF. To determine if side band inhibition was locally mediated, specific antagonist(s) to inhibitory neurotransmitters were applied and found to produce effects similar to acoustic trauma. The results suggest that lesioned-induced central auditory plasticity could contribute to several symptoms associated with sensorineural hearing loss such as loudness recruitment, tinnitus and poor speech discrimination in noise.