In contrast with binaural beats, neuronal correlates of acoustic beats, physically present in the acoustic signal, have been found in the cochlear nuclei, the earliest relay of the auditory pathway (Draganova et al., 2008). Moreover, similar studies have suggested the involvement of the medial superior olive and the inferior colliculus in their generation mechanism (Draganova et al., 2008). In humans, magnetoencephalographic (MEG) studies have recorded auditory steady-state responses to binaural beats from various sources in the parietal, frontal and temporal areas of the cerebral cortex, including the auditory cortices (Karino et al., 2006). Moreover, these studies have disclosed responses in the inferior colliculus of the midbrain that are phase-locked to the binaural-beat frequency (Kuwada et al., 1979 McAlpine et al., 1996). In animal studies, single-unit recordings have disclosed the earliest responses evoked by binaural-beat stimulation in neurons of the superior olivary complex of the brainstem (Wernick and Starr, 1968 Spitzer and Semple, 1998), the first nucleus in the ascending auditory pathway receiving bilateral input. There is agreement on the involvement of the auditory cortex and the brainstem in the neural mechanisms behind binaural beats perception. This latter effect is known as the “binaural beat”, and can be perceived if the carrier frequency is lower than 1000 Hz, and the two frequencies differ from each other by approximately less than 35 Hz (Licklider et al., 1950).įirst described by Dove ( 1841) and further characterized by Thompson ( 1877), binaural-beat phenomena reflect the convergence of neural activity from the auditory nerves in binaurally sensitive networks (Moore, 1997). In contrast, when the same two tones with slightly different frequencies are played binaurally, one to each ear, the same beat is perceived, although no physical combination of these tones occurs outside the auditory system, hence pointing to a central nervous origin.
In such a phenomenon, known as “acoustic beat”, the two input frequencies are physically mixed in the signal before they reach the auditory system. When two pure-tone sinewaves with slightly different frequencies (e.g., 300 and 305 Hz) are presented simultaneously to the same ear, a periodic two-tone complex with a frequency corresponding to the frequency difference between the two tones (e.g., 5 Hz) can be perceived as a “beat”. Our results do not support binaural-beat stimulation as a potential tool for the enhancement of EEG oscillatory activity, nor to induce changes in emotional arousal.
BINAURAL BEATS FEMALE AROUSAL SKIN
In the psychophysiological measures, no changes in heart rate and skin conductance were observed for any of the beat frequencies presented. Additional analysis of spectral EEG topographies yielded negative results for the effect of binaural beats in the scalp distribution of EEG spectral power. For all the beat frequencies used for stimulation, no significant changes between Baseline and Beat epochs were observed within the corresponding EEG bands, neither with binaural or with acoustic beats. In each of these epochs, we analyzed the EEG spectral power, as well as calculated the heart rate and skin conductance response (SCR). Beats of five different frequencies (4.53 Hz -theta-, 8.97 Hz -alpha-, 17.93 Hz -beta-, 34.49 Hz -gamma- or 57.3 Hz -upper-gamma) were presented binaurally and acoustically for epochs of 3 min (Beat epochs), preceded and followed by pink noise epochs of 90 s (Baseline and Post epochs, respectively). Additionally, we analyzed the effects of binaural-beat stimulation on two psychophysiological measures related to emotional arousal: heart rate and skin conductance. In the present study, we explored the potential contribution of binaural beats to the enhancement of specific electroencephalographic (EEG) bands, as previous studies suggest the potential usefulness of binaural beats as a brainwave entrainment tool.
If these two tones are presented one to each ear, they still produce the sensation of the same beat, although no physical combination of the tones occurs outside the auditory system.
When two pure tones of slightly different frequencies are delivered simultaneously to the two ears, is generated a beat whose frequency corresponds to the frequency difference between them.
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