J. Neurophysiol. 108:2837-2845, 2012. doi:10.1152/jn.00366.2012
G. Ashida, K. Funabiki, P. T. Kuokkanen, R. Kempter, C. E. Carr.
Owls use interaural time differences (ITDs) to locate a sound source. They compute ITD in a specialized neural circuit that consists of axonal delay lines from the cochlear nucleus magnocellularis (NM) and coincidence detectors in the nucleus laminaris (NL). Recent physiological recordings showed that tonal stimuli induce oscillatory membrane potentials in NL neurons (Funabiki et al., J. Neurosci., 2011). The amplitude of these oscillations varies with ITD and is strongly correlated to the firing rate. The oscillation, termed the sound analogue potential, has the same frequency as the stimulus tone, and is presumed to originate from phase-locked synaptic inputs from NM fibers. To investigate how these oscillatory membrane potentials are generated, we applied recently developed signal-to-noise ratio (SNR) analysis techniques (Kuokkanen et al., J. Neurophysiol., 2010) to the intracellular waveforms obtained in vivo. Our theoretical prediction of the band-limited SNRs agreed with experimental data for mid-to-high-frequency (>2 kHz) NL neurons. For low-frequency (<= 2 kHz) NL neurons, however, measured SNRs were lower than theoretical predictions. These results suggest that the number of independent NM fibers converging onto each NL neuron, and/or the population-averaged degree of phase-locking of the NM fibers, could be significantly smaller in the low-frequency NL region than estimated for higher best frequency NL.