What Are Binaural Beats?
Binaural beats are auditory brainstem responses
which originate in the superior olivary nucleus of each hemisphere. They
result from the interaction of two different auditory impulses,
originating in opposite ears, below 1000 Hz and which differ in frequency
between one and 30 Hz (Oster, 1973).For example, if a pure tone of 400 Hz
is presented to the right ear and a pure tone of 410 Hz is presented
simultaneously to the left ear, an amplitude modulated standing wave of 10
Hz, the difference between the two tones, is experienced as the two wave
forms mesh in and out of phase within the superior olivary nuclei. This
binaural beat is not heard in the ordinary sense of the word (the human
range of hearing is from 20-20,000 Hz). It is perceived as an auditory
beat and theoretically can be used to entrain specific neural rhythms
through the frequency-following response (FFR)--the tendency for cortical
potentials to entrain to or resonate at the frequency of an external
stimulus. Thus, it is theoretically possible to utilize a specific
binaural-beat frequency as a consciousness management technique to entrain
a specific cortical rhythm.
How It Works On The Brain
When signals of two different frequencies are
presented, one to each ear, the brain detects phase differences between
these signals. "Under natural circumstances a detected phase
difference would provide directional information. The brain processes this
anomalous information differently when these phase differences are heard
with stereo headphones or speakers. A perceptual integration of the two
signals takes place, producing the sensation of a third "beat"
frequency. The difference between the signals waxes and wanes as the two
different input frequencies mesh in and out of phase. As a result of these
constantly increasing and decreasing differences, an amplitude-modulated
standing wave -the binaural beat- is heard. The binaural beat is perceived
as a fluctuating rhythm at the frequency of the difference between the two
auditory inputs. Evidence suggests that the binaural beats are generated
in the brainstem’s superior olivary nucleus, the first site of
contralateral integration in the auditory system (Oster, 1973). Studies
also suggest that the frequency-following response originates from the
inferior colliculus (Smith, Marsh, & Brown, 1975)" (Owens &
Atwater, 1995). This activity is conducted to the cortex where it can be
recorded by scalp electrodes.
The Discovery Of Binaural Beats
Binaural beats were discovered in 1839 by a
German experimenter, H. W. Dove. The human ability to "hear"
binaural beats appears to be the result of evolutionary adaptation. Many
evolved species can detect binaural beats because of their brain
structure. The frequencies at which binaural beats can be detected change
depending upon the size of the species’ cranium. In the human, binaural
beats can be detected when carrier waves are below approximately 1000 Hz (Oster,
1973). Below 1000 Hz the wave length of the signal is longer than the
diameter of the human skull. Thus, signals below 1000 Hz curve around the
skull by diffraction. The same effect can be observed with radio wave
propagation. Lower-frequency (longer wave length) radio waves (such as AM
radio) travel around the earth over and in between mountains and
structures. Higher-frequency (shorter wave length) radio waves (such as FM
radio, TV, and microwaves) travel in a straight line and can’t curve
around the earth. Mountains and structures block these high-frequency
signals. Because frequencies below 1000 Hz curve around the skull,
incoming signals below 1000 Hz are heard by both ears. But due to the
distance between the ears, the brain "hears" the inputs from the
ears as out of phase with each other. As the sound wave passes around the
skull, each ear gets a different portion of the wave. It is this waveform
phase difference that allows for accurate location of sounds below 1000
Hz(9). Audio direction finding at higher frequencies is less accurate than
it is for frequencies below 1000 Hz. At 8000 Hz the pinna (external ear)
becomes effective as an aid to localization. In summary it’s the ability
of the brain to detect a waveform phase difference is what enables it to
perceive binaural beats.
Altered States
Binaural beats can easily be heard at the
low frequencies (< 30 Hz) that are characteristic of the EEG spectrum (Oster,
1973). This perceptual phenomenon of binaural beating and the objective
measurement of the frequency-following response (Hink, Kodera, Yamada,
Kaga, & Suzuki, 1980) suggest conditions which facilitate entrainment
of brain waves and altered states of consciousness. There have been
numerous anecdotal reports and a growing number of research efforts
reporting changes in consciousness associated with binaural-beats.
"The subjective effect of listening to binaural beats may be relaxing
or stimulating, depending on the frequency of the binaural-beat
stimulation" (Owens & Atwater, 1995). Binaural beats in the delta
(1 to 4 Hz) and theta (4 to 8 Hz) ranges have been associated with reports
of relaxed, meditative, and creative states (Hiew, 1995), and used as an
aid to falling asleep. Binaural beats in the alpha frequencies (8 to 12
Hz) have increased alpha brain waves (Foster, 1990) and binaural beats in
the beta frequencies (typically 16 to 24 Hz) have been associated with
reports of increased concentration or alertness (Monroe, 1985) and
improved memory (Kennerly, 1994).
Passively listening to binaural beats may
not spontaneously propel you into an altered state of consciousness.
One’s subjective experience in response to binaural-beat stimulation may
also be influenced by a number of mediating factors. For example, the
willingness and ability of the listener to relax and focus attention may
contribute to binaural-beat effectiveness in inducing state changes.
"Ultradian rhythms in the nervous system are characterized by
periodic changes in arousal and states of consciousness (Rossi, 1986;
Shannahoff-Khalsa, 1991; Webb & Dube,
1981). These naturally occurring shifts may underlie the anecdotal reports
of fluctuations in the effectiveness of binaural beats. External factors
are also thought to play roles in mediating the effects of binaural
beats" (Owens & Atwater, 1995). The perception of a binaural beat
is, for example, said to be heightened by the addition of white noise to
the carrier signal (Oster, 1973), so white noise is often used as
background. "Music, relaxation exercises, guided imagery, and verbal
suggestion have all been used to enhance the state-changing effects of the
binaural beat" (Owens & Atwater, 1995). Other practices such as
humming, toning, breathing exercises, autogenic training, and/or
biofeedback can also be used to interrupt the homeostasis of resistant
subjects (Tart, 1975).