Magnetoencephalographic study of human auditory steady-state responses to binaural beat
Introduction
Presentation of one sinusoid to each ear with a small difference of frequency (Δf) provides an orderly and continuously changing interaural relative phase through one cycle of Δf. When Δf is smaller, a single auditory image moves toward the leading ear to which a tone of higher frequency is presented [1], [2]. As Δf is increased, subjective periodic fluctuations called binaural beats (BBs) are elicited at a rate equal to Δf. Perrott and Nelson [3] have reported that, when the frequency of sinusoidal tone presented unilaterally to one ear is 250 Hz, BBs are detected when Δf at the other ear is between about 2 and 30 Hz. BBs demonstrate that the discharges of the auditory nerve fibers preserve information on the phase of the acoustic stimuli. Neural spikes tend to occur at a particular phase of the sinusoidal waveform (phase locking) and the central auditory system has capacity for preserving temporal information (frequency coding). In most mammals, phase locking becomes progressively less precise at frequencies above 1 kHz, and it disappears completely at approximately 4–5 kHz [4]. BBs are essentially a low-frequency phenomenon and are heard most distinctly for frequencies between 300 and 600 Hz [5]. Perception of BBs depends on detecting the continuously changing IPD. The goal of this study is to demonstrate the cortical representation of fluctuation of BBs by magnetoencephalography (MEG) and to confirm that IPD is coded in the human auditory cortex.
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Subjects
Six normal-hearing, right-handed subjects (four males, two females; ages 24–57 years; mean±S.D. age of 40.3±11.5 years) participated in this study. Subjects had no history of otological and neurotological disorders and had normal audiological status.
Stimulation
Continuous pure tones were played on an Apple personal computer via MOTU 828 (Mark of the Unicorn, Massachusetts, USA) audio interface and led to foam insert earphones through plastic tubes. As frequency of BB, 4 and 6.66 Hz were employed, and the
Results
Steady-state responses with dominant amplitudes in bilateral temporal areas were observed in all the subjects. In some channels of these regions, four peaks were clearly recognized in the time window which corresponds to four cycles of BBs (Fig. 1a). Spectral analysis detected evident peaks of the frequency of BB (4 or 6.66 Hz) in several channels of the bilateral temporal areas (Fig. 1b).
Discussion
BBs provide a classic example of binaural interaction, considered to result from neural interaction in the central auditory pathway that receives input from both ears. Some papers reported that the central auditory system utilizes the information of continuously changing IPD when BBs are presented to mammals. Kuwada et al. [2] found that the responses of cat inferior colliculus neurons are phase-locked to the frequency of BBs. Reale and Brugge [6] studied the interaural phase difference
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