Multi-electrode recordings of delay lines in nucleus laminaris of the barn owl

Abstract

Event Abstract Back to Event Multi-electrode recordings of delay lines in nucleus laminaris of the barn owl Nico Lautemann1*, Paula Kuokkanen2, Richard Kempter3 and Hermann Wagner1, 4 1 Rheinisch-Westfaelische Technische Hochschule, Department for Zoology and Animal Physiology, Georgia 2 Humboldt University, Institute for Theoretical Biology, Germany 3 Bernstein Center for Computational Neuroscience, Germany 4 University of Maryland , Faculty of Electrical Engineering and Computing, United States Barn owls (Tyto alba) are nocturnal hunters that are able to catch their prey in complete darkness by using auditory cues. The cue used to localize the azimuthal position of a sound source is the interaural time difference (ITD). ITD is the difference of the arrival time of a sound at the two ears. The time pathway computing the ITD starts in the cochlear nucleus magnocellularis (NM). The axons of NM neurons project bilaterally to nucleus laminaris (NL), making NL the first binaural stage in the time pathway. The NL neurons are narrowly tuned to sound frequency and act as coincidence detectors. Simultaneous inputs from the right and left side cause the neurons to be maximally active. Their firing frequency changes periodically in dependence on an imposed phase shift between the left and right inputs. Nucleus laminaris contains both, a tonotopic map and a map of ITD. The projections from the ipsi- and contralateral NM are supposed to form delay lines. The ipsilateral axon collaterals contact and penetrate NL from dorsal, while the contralateral axon collaterals run on the ventral side and transverse NL from ventral to dorsal. In the barn owl the map of ITD results from the synapses of the axon collaterals with NL neurons at different dorso-ventral depths. In this way a time-code, present in the NM collaterals, is converted into a place-code in NL neurons. The key elements and features of such a sound-localization circuit have been proposed by Jeffress in 1948 [1]. Since then a large amount of evidence has been accumulated, supporting the hypothesis that this model is realized in birds. However, the existence of delay lines in the barn owl has not yet been directly shown. To do so, we used acute coronal slice preparations of the NM-NL circuit of the barn owl brainstem and recorded the extracellular multi-unit activity in the network at many different positions with planar multi-electrode arrays (MEA) while electrically stimulating the NM fibers (Fig. 1A). We demonstrate the propagation of the response along and within NL, directly showing the existence of delay lines (Fig. 1B). The delays inside and outside of NL were quantified by determining propagation velocities, showing different propagation velocities of the fibers in- and outside NL. Since the network is still developing in the first weeks, we used animals of different ages (P2-P11) to study the maturation of the delay lines, taking place in the first days post hatch.