Input-output relationships of the dorsal nucleus of the lateral lemniscus: possible substrate for the processing of dynamic spatial cues.

One organizing principle of the auditory system is the progressive representation of best tuning frequency. Superimposed on this tonotopy are nucleotopic organizations, some of which are related to the processing of different spatial cues. In the present study, we correlated asymmetries in the outputs of the dorsal nucleus of the lateral lemniscus (DNLL) to the two inferior colliculi (ICs), with asymmetries in the inputs to DNLL from the two lateral superior olives (LSOs). The positions of DNLL neurons with crossed and uncrossed projections were plotted from cases with unilateral injections of retrograde tracers in the IC. We found an orderly dorsal-to-ventral progression to the output that recapitulated the tonotopy of DNLL. In addition, we found a nucleotopic organization in the ventral (high-frequency) part of DNLL. Neurons with projections to the ventromedial (high-frequency) part of the contralateral IC were preferentially located ventrolaterally in DNLL; those with projections to the ventromedial part of the ipsilateral IC were preferentially located ventromedially in DNLL. This partial segregation of outputs corresponded with a partial segregation of inputs from the two LSOs in cases which received closely matched bilateral injections of anterograde tracers in LSO. The ventral part of DNLL received a heavy projection medially from the opposite LSO and a heavy projection laterally from the ipsilateral LSO. The findings suggest a direct relationship in the ventral part of the DNLL between inputs from the two LSOs and outputs to the two ICs. Possible roles for this segregation of pathways in DNLL are discussed in relation to the processing of static and dynamic spatial cues.