Ross Bernhard, Miyazaki Takahiro, Thompson Jessica, Jamali Shahab, Fujioka Takako
Rotman Research Institute, Baycrest Centre, Toronto, Ontario, Canada; Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada;
Rotman Research Institute, Baycrest Centre, Toronto, Ontario, Canada;
J Neurophysiol. 2014 Oct 15;112(8):1871-84. doi: 10.1152/jn.00224.2014. Epub 2014 Jul 9.
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.
当向双耳呈现频率略有不同的两个音调时，它们会在中枢听觉系统中相互作用，并引发跳动声音的感觉。在低频差时，我们会感知到一个单一的声音，它在头部左右耳之间移动。随着拍频增加，感知会变为响度波动、粗糙度和音高。为了研究这些不同感知背后的神经表征，我们在参与者聆听3赫兹至60赫兹连续变化频率的双耳节拍时记录了神经磁皮层反应。双耳节拍反应被分析为跟随刺激率轨迹的神经磁振荡。反应在40赫兹的伽马频段和低频时最大。3赫兹时的双耳节拍反应在左右听觉皮层中显示出相反的极性。我们认为这种极性差异反映了用于表示声音位置的对立神经群体编码。任何频率的双耳节拍都会诱发伽马振荡。然而，在40赫兹刺激时反应最大。我们提出神经磁伽马振荡反映了允许皮层神经放电精确计时的突触后调制。双侧反应之间的系统相位差异表明，声音对象的独立声音表征存在于左右听觉皮层中。我们得出结论，皮层水平的双耳处理与单耳处理具有相同的时间敏锐度，而声音位置的识别需要进一步解释，并且受对象表征速率的限制。
