Oscillatory Dipoles As a Source of Phase Shifts in Field Potentials in the Mammalian Auditory Brainstem
Year: 2010
Type: article
Abstract: A popular model of binaural processing, proposed by Jeffress (1948), states that external interaural time delays (ITDs) are compensated by internal axonal delays allowing ITD to be spatially represented by a population of coincidence detectors in the medial superior olive (MSO). Isolating single-neuron responses in MSO is difficult because of the presence of a strong extracellular field potential known as the neurophonic, so that few studies have tested Jeffress's key prediction. Phase delays in the nucleus laminaris neurophonic in owls have been observed and are consistent with a Jeffress-like model. Here, we recorded neurophonic responses in cat MSO to monaural tones at locations along its dendritic axis. Fourier analysis of the neurophonic was used to extract amplitude and phase at the stimulus frequency. Amplitude, as a function of depth, showed two peaks separated by a dip. A half-cycle phase shift was observed at depths close to the dip, over a wide frequency range. Current source density analysis for contralateral (ipsilateral) stimulation shows a current source close to the neurophonic amplitude peak and a sink a few hundred micrometers ventromedially (dorsolaterally). These results are consistent with a dipole configuration: contralateral (ipsilateral) excitation causes a current sink at the ventromedial (dorsolateral) dendrites and a source at the soma and dorsolateral (ventromedial) dendrites. Incorporating these results in a dipole model explains the phase and amplitude patterns observed. We conclude that the half-cycle phase shift is consistent with a current dipole, making it difficult to derive measurements of axonal delays from the neurophonic.
Source: Journal of Neuroscience
Institution: KU Leuven
Language: English
Subfield: Sensory Systems
Field: Neuroscience
Domain: Life Sciences
Open Access status: hybrid
Funder: Vlaamse regering
OpenAlex ID: W2078699371