Phase Shifts in Monaural Field Potentials of the Medial Superior Olive

Abstract

Jeffress (J Comp Physiol Psychol 41:35-39, 1948) proposed that external interaural time differences (ITDs) are compensated by internal, axonal delays allowing ITD to be represented by a population of coincidence detectors in the medial superior olive (MSO). The MSO shows a strong extracellular field potential: the neurophonic. Studies in the barn owl reported a phase shift in the neurophonic along the nucleus laminaris and concluded that this phase shift is consistent with axonal delay lines as proposed by Jeffress. We recorded the neurophonic in the MSO of the cat at various locations along its short, dendritic axis. A phase shift of about 0.5 cycles was observed at depths close to the amplitude maxima, sometimes accompanied by localized amplitude minima. Current source density analysis for contralateral (ipsilateral) stimulation shows a current source close to a neurophonic amplitude maximum and a sink 100 μm ventromedially (dorsolaterally). These results indicate that some of the features of the neurophonic may be caused by a dipole field. Contralateral (ipsilateral) excitation causes a current sink at the ventromedial (dorsolateral) dendrites and a source at the soma and dorsolateral (ventromedial) dendrites. The difference in phase at the sink and source is 0.5 cycles, which closely resembles the phase shift that has been reported in the barn owl. Our interpretation in terms of a dipole field raises the question whether the neurophonic phase shift reported in the barn owl reflects axonal delays or simply a nucleus laminaris dipole configuration.