Mechanisms of absolute and relative timing in circuit processing of auditory space
What are the mechanisms regulating the timing of inputs in microcircuits? One central, yet often neglected parameter is myelination, which shapes the conduction velocity of action potentials along the axon. In the last funding period, we gained crucial novel insight into the functional specificity of myelination and its effects on spike conduction and transmission in vivo: We revealed pronounced differences in axonal and myelination morphology within sub-populations of auditroy brainstem axons based on their respective neuronal frequency tuning and determined that this morphological diversity results in significantly different conduction times of the axon subgroups (Ford et al, 2015, Nature Communications). Moreover, by comparing two species with differential hearing ranges, we revealed that the specializations in myelination and conduction velocity are highly specific to the functional requirements of low frequency sound localization (Stange-Marten et al., 2017, PNAS). We furthermore established in vivo that specifically the low-frequency tuned, fast conducting axons exhibit level-constant synaptic delays, which is a prerequisite for coincidence detection in the microsecond range. These results significantly advance our understanding of the role of myelination for the temporal tuning of inputs in circuit processing, but also raise important questions that we aim to address in the next funding period:
1. What is the role of intrinsic and extrinsic factors and activity in shaping myelination patterning during development?
2. Is there a structural basis for constant synaptic delays?
3. How does relative and absolute timing (e.g. level-constant and -dependent synaptic delays) shape circuit function and population coding?