A DEVELOPMENTAL DISSECTION IN THE AUDITORY BRAINSTEM OF MONGOLIAN GERBILS OF NEURAL DELAYS UNDERLYING SOUND LOCALIZATION

Medial superior olivary neurons (MSO) are sensitive to sound directionality by their sub-millisecond sensitivity to the interaural time difference (ITD) in sound arrival. These neurons function as coincidence detectors of synaptic inputs from spherical bushy cells (SBCs) from both cochlear nuclei (CN). The precise timing of SBC’s firing relative to sound arrival plus the CN-MSO propagation delay determines the timing of synaptic activity at the MSO. The development of MSO’s sensitivity to sound source direction closely follows the development in ear-MSO latencies (Sierksma & Borst 2024). We previously found that the contralateral ear-MSO latency decreased more than its ipsilateral counterpart, but the origin of this difference is unclear.
To dissect the latencies, we made in vivo recordings from SBCs in developing and adult Mongolian gerbils. Latencies up to the SBC presynaptic action potential, SBC synaptic delays, SBC-ipsilateral MSO propagation delays and inter-MSO propagation delays all decreased developmentally, but this decrease was more prominent for the inter-MSO propagation delay than for the CN-ipsilateral MSO delay. By reconstructing CNs and MSOs in 3D we estimated inter-nuclei distances, allowing us to calculate conduction velocities. Our results suggest that all velocities increase during development, but more so between MSOs.
This differential speedup thus provides a possible explanation for the differential decrease in ipsi-and contralateral latencies. The ability to accurately detect sound direction thus at least partially depends on differential speedup within the same axon.