Dendritic delay lines shape the computation of sound location in neurons of the gerbil medial superior olive

In mammals, neurons of the medial superior olive encode information about sounds arising from discrete spatial locations along the horizon. This tuning requires that an internal delay in the brain must offset acoustic disparities to ensure coincident arrival of excitatory inputs driven from the two ears. The source of this optimal internal delay, originally assumed to arise from axonal delay lines, is currently controversial in mammals. Here we use 2-photon guided paired dendritic and somatic recordings together with compartmental modeling of 40 complete MSO neuron morphologies to demonstrate that the dendrites themselves serve as a significant source of internal delay. We show that most MSO neurons exhibit morphological asymmetries that impose different EPSP delays across dendrites and shifts in optimal interaural time differences. Dendrite-based delays in the mammalian MSO are heterogeneous within each isofrequency laminae and provide a stable, structural mechanism to help tune individual neurons to sounds from different azimuthal locations.