title: Human cortical responses to slow and fast binaural beats reveal multiple mechanisms of binaural hearing.
abstract: When two tones with slightly different frequencies are presented to both ears, they interact in the central auditory system and induce the sensation of a beating sound. At low difference frequencies, we perceive a single sound, which is moving across the head between the left and right ears. The percept changes to loudness fluctuation, roughness, and pitch with increasing beat rate. To examine the neural representations underlying these different perceptions, we recorded neuromagnetic cortical responses while participants listened to binaural beats at a continuously varying rate between 3 Hz and 60 Hz. Binaural beat responses were analyzed as neuromagnetic oscillations following the trajectory of the stimulus rate. Responses were largest in the 40-Hz gamma range and at low frequencies. Binaural beat responses at 3 Hz showed opposite polarity in the left and right auditory cortices. We suggest that this difference in polarity reflects the opponent neural population code for representing sound location. Binaural beats at any rate induced gamma oscillations. However, the responses were largest at 40-Hz stimulation. We propose that the neuromagnetic gamma oscillations reflect postsynaptic modulation that allows for precise timing of cortical neural firing. Systematic phase differences between bilateral responses suggest that separate sound representations of a sound object exist in the left and right auditory cortices. We conclude that binaural processing at the cortical level occurs with the same temporal acuity as monaural processing whereas the identification of sound location requires further interpretation and is limited by the rate of object representations.
authors.affiliation: Rotman Research Institute, Baycrest Centre, Toronto, Ontario, Canada; Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada; bross@research.baycrest.org.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Ross
authors.affiliation: Rotman Research Institute, Baycrest Centre, Toronto, Ontario, Canada;
authors.forename: Takahiro
authors.initials: T
authors.lastname: Miyazaki
authors.affiliation: International Laboratory for Brain, Music and Sound Research, Department of Psychology, University of Montreal, Montreal, Quebec, Canada; and.
authors.forename: Jessica
authors.initials: J
authors.lastname: Thompson
authors.affiliation: Rotman Research Institute, Baycrest Centre, Toronto, Ontario, Canada;
authors.forename: Shahab
authors.initials: S
authors.lastname: Jamali
authors.affiliation: Center for Computer Research in Music and Acoustics, Stanford University, Stanford, California.
authors.forename: Takako
authors.initials: T
authors.lastname: Fujioka
mesh_terms: D000161:Acoustic Stimulation; D000328:Adult; D001303:Auditory Cortex / Q000502:physiology*; D001307:Auditory Perception / Q000502:physiology*; D005072:Evoked Potentials, Auditory; D005260:Female; D065818:Gamma Rhythm; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D013997:Time Factors; D055815:Young Adult
keywords: auditory cortex; binaural beat; binaural hearing; gamma oscillation; hemispheric asymmetry; magnetoencephalography; opponent-channel code; phase synchrony; sound localization; steady-state responses