Maps of ITD in the nucleus laminaris of the barn owl.

Axons from the nucleus magnocellularis (NM) and their targets in nucleus laminaris (NL) form the circuit responsible for encoding interaural time differences (ITDs). In barn owls, NL receives bilateral inputs from NM such that axons from the ipsilateral NM enter NL dorsally, while contralateral axons enter from the ventral side. These afferents and their synapses on NL neurons generate a tone-induced local field potential, or neurophonic, that varies systematically with position in NL. From dorsal to ventral within the nucleus, the best interaural time difference (ITD) of the neurophonic shifts from contralateral space to best ITDs around 0 µs. Earlier recordings suggested that in NL, iso-delay contours ran parallel to the dorsal and ventral borders of NL (Sullivan WE, Konishi M. Proc Natl Acad Sci U S A 83:8400-8404, 1986). This axis is orthogonal to that seen in chicken NL, where a single map of ITD runs from around 0 µs ITD medially to contralateral space laterally (Köppl C, Carr CE. Biol Cyber 98:541-559, 2008). Yet the trajectories of the NM axons are similar in owl and chicken (Seidl AH, Rubel EW, Harris DM, J Neurosci 30:70-80, 2010). We therefore used clicks to measure conduction time in NL and made lesions to mark the 0 µs iso-delay contour in multiple penetrations along an isofrequency slab. Iso-delay contours were not parallel to the dorsal and ventral borders of NL; instead the 0 µs iso-delay contour shifted systematically from a dorsal position in medial NL to a ventral position in lateral NL. Could different conduction delays account for the mediolateral shift in the representation of 0 µs ITD? We measured conduction delays using the neurophonic potential and developed a simple linear model of the delay-line conduction velocity. We then raised young owls with time-delaying earplugs in one ear (Gold JI, Knudsen EI, J Neurophysiol 82:2197-2209, 1999) to examine map plasticity.