Human cortical responses to slow and fast binaural beats reveal multiple 1 mechanisms of binaural hearing

22 When two tones with slightly different frequencies are presented to both ears, they interact in the 23 central auditory system and induce the sensation of a beating sound. At low difference 24 frequencies, we perceive a single sound, which is moving across the head between the left and 25 right ears. The percept changes to loudness fluctuation, roughness, and pitch with increasing beat 26 rate. To examine the neural representations underlying these different perceptions, we recorded 27 neuromagnetic cortical responses while participants listened to binaural beats at continuously 28 varying rate between 3 Hz and 60 Hz. Binaural beat responses were analyzed as neuromagnetic 29 oscillations following the trajectory of the stimulus rate. Responses were largest in the 40-Hz 30 gamma range and at low frequencies. Binaural beat responses at 3 Hz showed opposite polarity 31 in the left and right auditory cortices. We suggest that this difference in polarity reflects the 32 opponent neural population code for representing sound-location. Binaural beats at any rate 33 induced gamma oscillations. However, the responses were largest at 40-Hz stimulation. We 34 propose that the neuromagnetic gamma oscillations reflect post synaptic modulation that allows 35 for precise timing of cortical neural firing. Systematic phase differences between bilateral 36 responses suggest that separate sound representations of a sound object exist in the left and right 37 auditory cortices. We conclude that binaural processing at the cortical level occurs with same 38 temporal acuity as monaural processing whereas the identification of sound location requires 39 further interpretation and is limited by the rate of object representations. 40