Sensitivity to Interaural Time Differences in the Dorsal Nucleus of the Lateral Lemniscus of the Unanesthetized Rabbit: Comparison With Other Structures

Abstract

Interaural time differences, a cue for azimuthal sound location, are first encoded in the superior olivary complex (SOC), and this information is then conveyed to the dorsal nucleus of the lateral lemniscus (DNLL) and inferior colliculus (IC). The DNLL provides a strong inhibitory input to the IC and may serve to transform the coding of interaural time differences (ITDs) in the IC. Consistent with the projections from the SOC, the DNLL and IC had similar distributions of peak- and trough-type neurons, characteristic delays, and best ITDs. The ITD tuning widths of DNLL neurons were intermediate between those of the SOC and IC. Further sharpening is seen in the auditory thalamus, indicating that sharpening mechanisms are not restricted to the midbrain. The proportion of neurons that phase-locked to the tones delivered to each ear progressively decreased from the SOC to the auditory thalamus. The degree of phase-locking for a large majority of DNLL neurons was too weak to support their involvement in processing monaural inputs to generate a sensitivity to ITDs. The response rates of DNLL neurons were on average approximately 60% greater than in the IC or SOC, indicating that the inhibitory input provided to the IC by the DNLL is robust.