Processing of Interaural Temporal Disparities in the Medial Division of the Ventral Nucleus of the Lateral Lemniscus

Abstract

The medial division of the ventral nucleus of the lateral lemniscus (VNLLm) contains a specialized population of neurons that is sensitive to interaural temporal disparities (ITDs), a potent cue for sound localization along the azimuth. Unlike many ITD-sensitive neurons elsewhere in the auditory system, neurons in the VNLLm respond only at the onset of tones. An onset response may be significant for behavior because, under echoic conditions, tones require sharp onsets for accurate localization. In contrast, noise can generally be localized even with gradual onsets, presumably because transients occur at random intervals in noise. We recorded responses of neurons in the VNLLm to tones and noise in unanesthetized rabbits. We found that although tones elicited a transient response, noise elicited a sustained response as if it was a sequence of transients. The responses to tones indicate that these neurons represent a secondary stage in the processing of ITDs. The onset response to tones was only weakly synchronized to the phase of the tone, indicating that neurons in the VNLLm inherit their sensitivity to ITDs from their inputs. The latencies were short (∼8 ms), implying that the ITD sensitivity is derived from ascending inputs. Most neurons in the VNLLm discharged maximally at the same ITD at all frequencies, a characteristic shared with neurons of the medial superior olive. However, the latency of neurons in the VNLLm to interaurally delayed stimuli is linked strongly to the timing of the contralateral stimulus. This suggests that these neurons receive a suprathreshold, contralateral input that is modulated by a subthreshold input conveying information about ITDs. Other stations in the auditory pathway contain a subset of neurons that respond transiently to tones and are sensitive to ITDs. These neurons may represent a novel pathway that assists in localizing sounds in the presence of reflections.