TEMPORAL PRECISION OF THE LSO NEURONS AND THEIR INPUT

The mammalian auditory system exploits interaural time differences (ITDs) and interaural level differences (ILDs) to localize sound sources in the azimuthal plane. Neurons in the lateral superior olive (LSO) receive excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear (EI neurons) and have traditionally been regarded as ILD encoders. However, accumulating evidence indicates that LSO neurons are also highly sensitive to temporal disparities between converging binaural inputs.
Current-clamp recordings in brain slices revealed that LSO neurons exhibit transient firing responses to sustained current injections. An input spike-timing model revealed that these responses depend on both (1) the rate of membrane depolarization and (2) temporal jitter–induced noise in the synaptic inputs. To assess the functional relevance of these parameters in vivo and to further elucidate the mechanisms underlying this dual coding strategy, we performed extracellular in vivo recordings from LSO neurons in mice.
We employed transposed tone pulse stimuli to independently manipulate envelope shape and pulse rate, thereby systematically varying stimulus transience (e.g., transforming a 100-Hz sinusoidal amplitude-modulated stimulus into a 100-Hz click-like pulse train). In vivo recordings demonstrate that LSO neurons are highly sensitive to envelope transience and can reliably encode such features at high modulation rates. Importantly, ILD encoding remains robust and is not compromised at these elevated modulation rates.
Together, the in vitro and in vivo findings provide a mechanistic framework for understanding how synaptic integration in LSO neurons supports the high perceptual acuity of sound source localization, particularly for transient acoustic signals.