- Binaural evoked responses were recorded with glass micropipettes from the central nucleus of the rat's inferior colliculus (ICC) before and after transection of the commissure of Probst (CP) with a microsurgical knife. The peak-to-peak amplitude of the averaged evoked response was measured for binaural clicks with interaural time differences (ITDs) between -1.0 and +30.0 ms (positive values reflecting ipsilateral-leading-contralateral click pairs). Before transection, the amplitude of the evoked response decreased as the ITD was shifted in favor of larger ipsilateral lead times. After transection of the CP, acoustic stimulation of the ipsilateral ear was much less effective in reducing evoked response amplitude. Responses to both short (+/-1.0 ms) and long (1.0-30.0 ms) ITD intervals were affected. After recordings were made, both anterograde and retrograde tract tracing methods were used to verify that the CP was completely transected and that all crossed projections from the dorsal nucleus of the lateral lemniscus (DNLL) to ICC were destroyed. The surgery completely eliminated the retrograde transport of fluorogold from the ICC to the opposite DNLL and blocked the anterograde transport of biotinylated dextran to contralateral DNLL and ICC. The physiological consequences of CP transection are attributed to the complete destruction of decussating, inhibitory (GABAergic) efferent projections from the DNLL.