Bilateral asymmetry of excitatory synaptic properties shapes ITD processing in gerbil MSO
P. E. Jercog, G. Svirskis, V. C. Kotak, D. H. Sanes, J. Rinzel

Interaural time difference (ITD) is thought to be the primary cue encoded by Medial Superior Olivary (MSO) neurons. MSO neurons receive synaptic inputs that are driven by the ipsi- and contralateral ear, and discharge rate depends on the arrival time differences of these two inputs. In vivo recordings from individual gerbil MSO neurons demonstrate that ITD tuning is asymmetric and peak discharge rate is produced just beyond the physiological range (approx.150 us in gerbil), on the contralateral-leading side. The observations support a slope code rather than a place code. Based on experimental data and simulations, we present a new model to explain the asymmetric location of the ITD range. We propose a mechanism that relies on bilateral differences in arrival-time statistics of the excitatory inputs, e.g., greater jitter on the contralateral side. A broader distribution of arrival times for synaptic inputs from the contralateral side creates summated EPSPs in the soma more spread in time from that side. This asymmetry in the arrival times leads to an asymmetry in the ITD function. Using a thick (approx. 450 um) horizontal brain slice preparation, we activated the afferent bundles from either ipsi- or contralateral ventral cochlear nucleus (VCN) while recording intracellularly from MSO neurons. We found larger variability in EPSP amplitudes, halfwidths and rising times for contralateral, as compared to ipsilateral, stimulation in most neurons (N=14 from N=21). Our experimental results combined with simulations suggest that bilateral differences in the arrival time statistics of the EPSPs contribute to shaping and positioning the ITD response function.