Forum of European Neuroscience 184.20, 2002.

Neurophonics in the nucleus laminaris of the barn owl as measured by stimulation with acoustic clicks.

H. Wagner¹, S. Brill¹,R. Kempter², and C. E. Carr³

¹ Institut für Biologie II, RWTH Aachen, Aachen, Germany
² Institute for Theoretical Biology, Humboldt University Berlin, Germany
³ Dept of Biology, University of Maryland, College Park, USA

Some 50 years ago Jeffress proposed that interaural time differences may be derived in the brain by coincidence-detecting neurones receiving inputs from axons forming delay lines. In the 1980's this model was shown to be surprisingly similarly realized in the avian brainstem. In the nucleus laminaris, incoming axons from the nucleus magnocellularis form dorso-ventrally extended delay lines, while laminaris neurons act as coincidence detectors. So far delay lines have been measured by calculating the changes of phase measured in response to a tonal stimulus as an electrode tip moved parallel to the path of the axons. Latencies of the signal to the borders of nucleus laminaris have been derived from phase-frequency relations. All these measurements of latencies were indirect. We have now measured delays directly by using click stimuli in three anaesthetized owls. Recording sites were verified by reconstruction of lesions. Condensation clicks of 40 microseconds duration and zero rise/fall time were presented. The neurophonic, a field potential, was recorded in and close to nucleus laminaris. The analog waveform of the response was stored. The neural response to clicks could be divided into two main components based on spectral content: A low-frequency component and a high- frequency component. The high-frequency component matched the frequency-tuning measured with pure tones. The amplitudes of these two components changed in a counter-rotating fashion: The low-frequency component was strong at the top and the bottom of the nucleus, while the high-frequency component had the highest amplitude in the middle of the nucleus. The low-frequency component reversed its phase while the electrode moved from top to bottom of the nucleus. In both components the time of arrival as determined by fitting of the signals with a Gabor function changed with depth. We are currently testing, how each of these components contributes to creating delay lines in nucleus laminaris.