Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: effects of changing the azimuthal location of an unmasked pure-tone stimulus.

Azimuth functions (discharge rates evoked by tone bursts as a function of stimulus azimuth) were obtained from neurons in the inferior colliculus (IC) of C57 mice aged 2, 7 and 12 months. Because of a gene that affects the cochlea, C57 mice exhibit high-frequency sensorineural hearing loss at 7 and 12 months. Azimuth functions were examined for differences that might be related to the decline in localization acuity that accompanies hearing loss in this strain. Irrespective of age group, nearly all neurons in the central area of the IC were sensitive to the azimuth of a best frequency (BF) stimulus, as revealed by azimuth functions in which firing rates varied by more than 50% from maximum to minimum at one or more intensities. The age groups were similar in many respects (e.g., there were no significant differences in the proportion of functions meeting the criterion for direction sensitivity, the proportion of neurons with direction sensitive functions over a range of intensities, azimuth function shapes, the locations or stability of 'borders' separating angles evoking high versus low discharge rates). However, in 7- and 12-month-olds: the proportion of IC neurons in which the strongest excitatory driving was evoked by ipsilateral stimulation was significantly larger; azimuth function borders were more likely to be 'reversed' (i.e., the high rates being evoked by the more ipsilateral angle); and a greater proportion of azimuth functions met the criterion for direction sensitivity only minimally. The findings suggest that binaural excitatory-inhibitory interactions are altered in IC neurons of hearing-impaired mice.