Interaural timed differences (ITD) are the dominant cue for localizing low frequency sounds in the azimuthal plane. In mammals, ITDs are initially encoded by neurons of the medial superior olive (MSO). Our recent in vivo recordings from gerbil MSO neurons with pharmacological blockade of glycinergic inhibition (Brand et al. 2002) show that binaural excitation leads to ITD sensitivity with maximal discharges around zero ITD. Consequently, the maximal slopes of the resulting ITD functions are mostly outside the physiological relevant range of ITDs. Exquisitely well-timed glycinergic inhibition, however, adjusts these ITD functions to peak at 0.12 cycles of each neuron’s best frequency, thereby bringing the steep slope into the relevant range. It has been shown earlier that this glycinergic input to the MSO undergoes an experience-dependent redistribution in the first days after hearing onset (Kapfer et al. 2002). We hypothesized that this structural change should be related to the function of inhibition, hence, the precise and proper timing. In animals with abnormal distributions of inhibitory inputs ITD sensitivity should not be adjusted as in control animals. To test this, we recorded from ITD-sensitive auditory brainstem neurons in gerbils with normal development and compared it with that in animals that were reared in omnidirectional noise and animals that were exposed to the same noise as adults. The latter group of animals showed ITD tuning similar to that found in the control group. However, in noise-reared animals, the ITD sensitivity was not adjusted to bring the maximal slope into the physiologically relevant range and resembled that found in the MSO during blockade of glycinergic inhibition.

Our findings show that well-timed inhibition is essential for the fine-tuning of ITD functions. Moreover, auditory experience is essential for developing the proper timing of inhibitory inputs, apparently by selective removal of inhibitory inputs on MSO cells.

This work was supported by the Deutsche Forschungsgemeinschaft (GR 1205).