| LANE, J. D., S. J. KASIAN, J. E. OWENS AND G. R.
MARSH. Binaural auditory beats affect vigilance
performance and mood. PHYSIOL BEHAV 63(2) 249
252, 1998. - When two tones of slightly different
frequency are presented separately to the left and
right ears the listener perceives a single tone that
varies in amplitude at a frequency equal to the frequency
difference between the two tones, a perceptual phenomenon
known as the binaural auditory beat. Anecdotal reports suggest that binaural auditory
beats within the electro-encephalograph frequency
range can entrain EEG activity and may affect states
of consciousness, although few scientific studies
have been published. This study compared the effects
of binaural auditory beats in the EEG beta and EEG
theta/delta frequency ranges on mood and on performance
of a vigilance task to investigate their effects
on subjective and objective measures of arousal. Participants (n = 29) performed a 30-min visual
vigilance task on three different days while listening
to pink noise containing simple tones or binaural
beats either in the beta range (16 and 24 Hz) or
the theta/delta range (1.5 and 4 Hz). However, participants
were kept blind to the presence of binaural beats
to control expectation effects. Presentation of beta-frequency
binaural beats yielded more correct target detections
and fewer false alarms than presentation of theta/delta
frequency binaural beats. In addition, the beta-frequency
beats were associated with less negative mood. Results suggest that the presentation of binaural
auditory beats can affect psychomotor performance
and mood. This technology may have applications for
the control of attention and arousal and the enhancement
of human performance. © 1998 Elsevier Science Inc. Keywords: binaural auditory beats, vigilance performance,
mood, frequency-following response WHEN two pure auditory signals of similar frequency
are mixed together, the phase interference between
their waveforms produces a composite signal with
a frequency midway between the upper and lower frequencies
and an amplitude modulation that occurs with a frequency
equal to the difference between the two original
frequencies. For example, mixing tones of 100 Hz
and 110 Hz yields a signal with a perceived frequency
of 105 Hz that rises and falls in amplitude with
a frequency of 10 Hz. The amplitude-modulated composite
signal is called an auditory beat. A similar phenomenon occurs when auditory signals
of similar frequency are presented separately to
the left and right ear through stereo headphones.
Although each ear hears only one of the frequencies,
the listener perceives the middle frequency and the
amplitude modulation, even though the auditory beat
does not exist in physical space. This phenomenon,
called a "binaural auditory beat," and described
more than 25 years ago (6), is created by the brain's
processing of the two separate auditory signals at
the level of the olivary nuclei of the brainstem. Binaural auditory beats provide a mechanism for
stimulating the auditory system at very low frequencies,
below the frequency threshold of hearing. Such very
low frequency auditory stimuli might be capable of
eliciting an entrainment of EEG frequencies, similar
to that known to occur during low frequency photic
stimulation (photic-driving). Anecdotal evidence
does suggest that presentation of low-frequency binaural
auditory beats can elicit a variety of changes in
the listener's state of consciousness that might
have a broad range of practical applications (5,7). For example, the presentation of binaural auditory
beats in the delta and theta frequency ranges is
said to be associated with enhanced creativity and
improved sleep. Preliminary experimental studies
suggest that binaural auditory beats in the EEG beta
frequency range can enhance attention and memory
task performance (3), and that those in the alpha
frequency range may increase alpha EEG production
and subjective relaxation (2). A recent study examined the effects of delta and
theta frequency binaural auditory beats on EEG spectral
patterns in healthy volunteers. EEG spectra were
compared between a period of wakeful rest and a period
in which participants listened through stereo headphones
to pure tones designed to produce binaural beats
in the theta and delta range. During the stimulus
period participants produced significantly less spectral
power in the alpha and beta EEG bands and significantly
more power in the theta and delta bands, evidence
of possible EEG entrainment by the binaural beat
stimuli. During stimulation participants reported
subjective experiences similar to meditative, trance,
or hypnogogic states. Taken together, the anecdotal, clinical, and preliminary
experimental evidence suggests that the presentation
of binaural auditory beats may produce controllable
changes in EEG and/or subjective states of consciousness.
Only the most recent studies include sufficient experimental
controls and can be considered as scientific investigations.
Even so, the value of potential applications of a
technology for self-control of EEG patterns and states
of consciousness argues for continued investigation
of the binaural beat phenomenon and its psychophysiological
effects. The present study was designed to investigate whether
different patterns of binaural-beat stimulation could
produce changes in level of arousal and alertness
manifested in behavior and mood. A double-blind cross-over
design was used to compare two distinct Patterns
of binaural-beat signals, one containing binaural
beats in the EEG-beta frequency range and the other
binaural beats in the EEG-delta/theta range. These
patterns were selected because these EEG frequency
bands are typically associated with states of alertness
versus drowsiness, and entrainment of these frequencies
might thus enhance or impair alertness. The binaural-beat signals were presented continuously
during the performance of a 30-min vigilance task
that required continuous video monitoring and responses
to infrequent targets. We predicted that presentation
of binaural-beat signals in the EEG beta frequency
range would elicit better task performance in this
monotonous task (more correct detection of targets
and fewer false alarms) than presentation of binaural
beat signals that entrained EEG frequencies in the
theta/ delta range. We also expected that differential
stimulation would affect the mood changes associated
with the monotonous task, especially those related
to subjective alertness and fatigue. Materials and Methods Subjects Volunteers were recruited by advertisement from
the Duke University community. They were required
to be in good health, have normal hearing and vision
(corrected or uncorrected), and be free from acute
illness or use of medications. Thirty-two people
were recruited and 29 completed the protocol. This
group had a mean age (±SD) of 32 (± IO) years with
a range from 19 to 51 years. The group contained
19 females and 10 males; 20 whites, 8 blacks, and
1 Asian; 18 employed workers and 11 students. All
volunteers were nonsmokers. Each received $30 for
completion of the study. Materials Binaural Beat Stimulation Binaural beat signals were presented stereophonically
by cassette tape. Three different tapes were prepared
as follows. All three tapes contained a background
of "pink noise" with uniform amplitude in the frequency
spectrum from 40-320 Hz and decreasing amplitude
(12 db/octave) at frequencies above and below these
limits. Tapes also contained carrier tones at 100,
200, 250, and 300 Hz, which had amplitudes 15 db
above the amplitude of the pink noise. The tape constructed for the training session contained
no binaural beat stimuli, but the tapes for the two
experimental treatments did. For the delta/theta
condition the 100-Hz tone was presented with a 1.5-Hz
binaural beat, the 200 and 250 Hz tones were presented
with 4-Hz binaural beats, and the 300-Hz tone was
presented with no binaural beat. Thus, this tape included binaural beats at 1.5 and
4 Hz. For the beta condition the 200-Hz tone was
presented with a 16-Hz binaural beat and the 300-Hz
tone was presented with a 24-Hz binaural beat. The
100 and 250-Hz tones were presented with no binaural
beat. The tape for the beta condition contained binaural
beats at 16 and 24 Hz. Subjectively the three tape
recordings sounded exactly alike, described by subjects
as similar to the constant monotonous roar of a waterfall
or the sound inside a large propeller-driven airplane. The presence of binaural beats was very difficult
to detect when the tapes were listened to by the
experimenters, and none of the participants reported
noticing them. The tapes were played to subjects
through stereo headphones, and volume was set to
a comfortable listening level. Vigilance Task A continuous performance vigilance task was administered
using it personal computer (Compaq 386 SX), which
contained a multifunction' data acquisition and timing
card (ADAI 100; Real Time Devices, State College,
PA) configured to measure response times with a precision
of I ms. The vigilance task was administered using
a special-purpose computer program written by J.
D. L. It can be summarized as follows. The participant watched the VGA video monitor as
individual stimuli of 5-cm height were displayed
at a rate of 1/s and a duration of 100 ms. The stimuli
were capital letters that were selected at random
from a list of 20 capitals that excluded those with
similar shapes (e.g. 0 and Q). On 10% of stimulus
presentations, the previous letter was repeated.
This repetition of a stimulus was the target for
the participant to detect. The computer program presented
1 target in each block of 10 stimuli (every 10-s
interval) to insure that 6 targets were presented
each minute, although the position of the target
within the block was random. The intervals between
targets ranged from 0 to 18 stimuli. The participant pressed the spacebar of the keyboard
as quickly as possible each time a target was detected.
The total duration of the vigilance task was 30 min.
Instructions emphasized the importance of continuous
monitoring for targets, rapid responding, and the
importance of maintaining good performance throughout
the entire task. The computer program administered
all stimuli and recorded the parameters of each stimulus
trial. Response latency was measured for all keypresses
and recorded with stimulus data for later analysis. Mood assessment The Profile of Mood States (POMS; EDITS, San Diego,
CA) was used to assess changes in mood. The POMS
contains 65 adjective rating items (O to 4 scale)
that describe feelings people experience (e.g., friendly,
tense, grouchy, etc.). Item ratings can be summarized
on standard scales that represent six general moods:
tension-anxiety; depression-dejection; anger-hostility;
vigor-activity; fatigue-inertia; and confusion-bewilderment
(4). This inventory was administered before and after
the vigilance task to assess task-related changes
in mood. Procedure Participants were kept blind to the true purpose
of the study. When volunteers were recruited, they
were told that the study was intended to evaluate
a new computerized vigilance task and to assess how
stable performance was over several days. Throughout
the study, they were told that task conditions were
identical across days and that the tape-recorded
sounds were intended to provide a uniform monotonous
auditory background that would blackout any external
sounds. Participants were not told about the differences
in the treatment conditions or the presence of auditory
binaural beats on the tape recordings. This deception was judged to be necessary to prevent
expectation bias regarding treatment effects. Furthermore,
keeping participants unaware of the presence of binaural-beat
stimulation prevented the distraction of actively
listening to the tape recordings in order to determine
their content, which could help to maintain arousal
during the task and interfere with the development
of a vigilance decrement. Use of this deception was
approved by the Medical Center Institutional Review
Board, and participants were debriefed at the conclusion
of the study. |
Each volunteer
took part in three experimental sessions that were
identical except for the treatment condition. Sessions
were scheduled beginning between 1300 and 1600 hours,
and all sessions for a participant were scheduled at
the same time of day. Participants were asked to abstain
from recreational drugs and alcohol for at least 24
h prior to testing and to get a normal night's sleep.
Compliance was confirmed by self-report.
The first experimental session was intended
for training and to provide a stable level of performance
for the two subsequent test sessions. The control tape
recording, which contained the same sounds but no binaural
beats, was presented during the training session. The
beta and theta/delta treatment conditions were presented
in the second and third sessions. The tape cassettes
were blind-coded so that treatments were presented
double-blind, and the order of treatments was counterbalanced
across subjects. Each session began with
the completion of a short battery of questionnaires.
The first session included completion of informed consent
procedures followed by completion of demographic and
health history forms. During the second and third sessions
different psychological questionnaires were completed
during this time. The POMS was completed at the end
of this battery each day, immediately before the vigilance
task, with instructions to describe feelings at that
moment.
The computer program displayed instructions for the
vigilance task on the monitor and presented samples
of the stimuli. The experimenter reviewed the instructions
with the participant, and the participant's questions
were answered. Participants then completed a ]-min
practice/warm-up trial of the vigilance task, and performance
feedback was provided upon completion. When the experimenter
was convinced that the participant understood how to
perform the task, the actual task was begun.
The participant performed the task while seated at
a desk in a swivel chair. The room was dimly lit. The
experimenter adjusted the stereo headphones and started
the tape playback. Auditory volume was adjusted to
a comfortable listening level for the participant that
would block perception of external sounds. Then the
experimenter left the room, and the participant began
the 30-min vigilance task after a brief delay. The
tape-recorded binaural-beat stimulation was presented
continuously during the task. Immediately after completion
of the task, the participant completed a second POMS
to indicate how she or he felt at that moment. The
experimenter reviewed a summary of performance to insure
that instructions had been followed and reasonable
levels of success obtained. However, participants received
only general positive feedback each day.
Results
Vigilance Performance
Task performance was scored as the number of correct
target detections (out of a possible 180 targets) and
the number of false alarms (when a keypress response
was made to a nontarget stimulus). The number of hits
and false alarms in the beta and theta/delta binaural
beat conditions were compared by paired t-test. Because
we proposed a directional hypothesis, that beta frequency
beats would improve performance compared to theta/delta
frequency beats, a one-tailed test was used to maximize
statistical power from our sample.
A total of 180 targets were presented during the 30-min
task Participants detected a significantly larger number
of targets when exposed to the beta-frequency binaural
beats (mean = 153.5, SD = 23.6) than when exposed to
theta/delta-frequency binaural beats (mean = 147.6,
SD = 34.7). The difference in the number of correct
detections was 5.9 ± 3.4 (mean -- SEM), which yielded
t(28) = 1.7 (p < 0.05). In contrast, participants produced
more false alarms in the theta/delta condition (mean
= 8.7, SD = 12.2) than in the beta condition (mean
= 6.6, SD = 9.4). The difference in false alarms was
2.0 -- 0.9 (mean ± SEM), which yielded t(28) = 2.26
(p < 0.02). Thus, the binaural beat treatments had
the predicted effects on vigilance task performance.
To determine whether the treatments had differential
effects on performance decrements during the vigilance
task, performance scores for six 5-min periods were
analyzed with a two-condition (beta versus theta/delta)
by 6-period repeated-measures analysis of variance,
using Greenhouse-Geisser corrections. The effect of
period was significant for correct detections (F(5,
135) = 7.63, p < 0.0008), but the condition by period
interaction was not (F(5, 135) = 1.40, p < 0.24); Although
there was a significant decrement in correct detections
over time during the task, the rate of decrement did
not differ significantly between the beta and theta/delta
conditions. For false alarms, neither the period effect
or the interaction were significant (both p > 0.20).
Subjective Mood
POMS scale scores were evaluated by two condition
X two period repeated-measures analysis of variance,
in which the interaction tested the hypothesis that
the binaural-beat stimuli would alter how the vigilance
task affected mood. The main effect of period represented
the effects of the vigilance task itself, regardless
of treatment. We did not propose directional hypotheses
for each of the six mood scales of the POMS, and thus
used this omnibus approach to detect treatment effects.
As demonstrated by significant interactions, the binaural-beat
condition affected scores for confusion/bewilderment
(F(l, 28) = 7.30, p < 0.01) and fatigue/inertia (F(l,
28) = 4.07, p < 0.05), with a trend observed in scores
for depression/dejection (F(l, 28) = 3.81, p < 0.06).
Scores for confusion/bewilderment rose more from the
beginning to the end of the vigilance task when the
participant listened to theta/delta binaural beats
(mean = 1.9, SE = 0.4, p < 0.0001), than when beta
binaural beats were presented (mean = 0.9, SE = 0.4,
p < 0.03). Moreover, scores for fatigue/inertia also
rose more when the participant listened to theta/delta
binaural beats (mean = 3.6, SE = 0.7, p < 0.0001),
than when beta binaural beats were presented (mean
= 2.3, SE = 0.8, p < 0.005). In contrast, depression/
dejection scores rose slightly (mean = 0.3, SE = 0.2)
when participants listened to the theta/delta binaural
beats during the vigilance task and dropped slightly
(mean = -0.4, SE = 0.4) when they listened to beta
binaural beats.
Scores for vigor/activity did not contain a significant
condition by period interaction, although there was
a significant period effect (F(l, 28) = 25.02, p < 0.0001).
Scores dropped from the beginning to the end of the
task (mean = -2.9).
Discussion
The results of this study provide evidence that presentation
of simple binaural auditory beat stimuli during a 30-min
vigilance task can affect both the task performance
and the changes in mood associated with the task. The
observed effects were consistent with our predictions
regarding differential effects on alertness and mood.
Binaural beats in the beta EEG frequency range were
associated with relative improvements in target detection
and reduction in the number of false alarms compared
to binaural beats in the theta/delta EEG frequency
range. Moreover, beta binaural beats were associated
with smaller increases in task-related confusion and
fatigue compared to theta/delta beats, and the two
conditions had different effects on scores for depression/dejection.
Scores on the confusion/bewilderment scale increased
under both conditions, but rose significantly more
during theta/delta frequency stimulation. This scale
includes the items "confused," I unable to concentrate," "muddled," "bewildered," "efficient" (scored
in reverse). "forgetful," and "uncertain about things." It
appears to represent "a self-report of cognitive efficiency" (4).
Changes observed in this study suggest that the theta/delta
binaural beats produced a subjective impairment in
the ability to think clearly.
Performance of the vigilance task also increased scores
for fatigue/inertia in both conditions, but more so
for the theta/delta condition. This scale describes "a
mood of weariness, inertia, and low energy level" (4)
and includes "worn-out," "listless," "fatigued," "exhausted," "sluggish," "weary," and "bushed" as
its items. The depression/dejection scale represents
depressed mood accompanied by a sense of inadequacy,
and includes "unhappy," "sorry," "sad," "miserable," "hopeless," "unworthy," "discouraged," "desperate," and "worthless" among
its items. Together these scales suggest that the negative
changes in mood produced by a monotonous task may have
been partially ameliorated by the presentation of beta-frequency
binaural beats.
These effects on behavior and mood were observed in
the absence of participant expectations, and experimental
controls ruled out other "placebo" effects. Not only
were participants unaware of their treatment condition,
they were unaware that different binaural-beat treatments
were being presented during the three days of testing.
Although experimenters knew the true nature of the
study, they were careful to maintain the cover story
throughout the study. Moreover, they were also blind
to the order in which the experimental treatments were
administered and thus could not systematically bias
the results.
We presume that the behavioral and mood effects were
mediated by changes in level of central nervous system
arousal induced by binaural-beat stimulation. It is
plausible that these signals entrained corresponding
EEG frequencies and increased relative EEG spectral
power in the beta or theta/delta bands. Such an interpretation
is consistent with earlier studies that suggest apparent
EEG changes in response to binaural beat stimulation
(2), although the evidence of such effects remains
preliminary. The present study lacked EEG measurements
that could confirm this interpretation, but future
studies can test this hypothesis directly.
It is interesting to note that similar changes in
performance of a vigilance task were observed when
normal volunteers were trained using biofeedback to
increase or suppress EEG theta activity (1). Those
trained experimental groups did differ both in theta
activity and in vigilance performance during testing,
and suppression of theta activity during the task was
associated with relatively better vigilance performance.
Perhaps binaural-beat stimulation provides alternative
means of suppressing theta activity, or enhancing beta
Activity, to enhance performance. If so, it has the
distinct advantage that it requires neither extensive
training nor intent to self-control EEG for its successful
application.
The observations in the present study have interesting
implications. If binaural beat auditory stimulation
can influence behavior and mood, then such stimulation
may have useful applications for the self-control of
arousal, attention, and performance. There may be potential
applications of these performance enhancing signals
in situations that demand high levels of continuous
sustained attention and performance, such as commercial
highway driving or air traffic control. Performance
enhancing stimulation may prove useful in other occupational
tasks as well. Conversely, binaural-beat stimulation
that decreases arousal may have applications in the
treatment of insomnia or stress.
The phenomenon of binaural auditory beat stimulation
and its psychophysiological consequences deserves further
study. Additional controlled studies will be required
to determine what behavioral, affective, and cognitive
effects different patterns of binaural beats might
have and how any associated changes in physiology,
behavior, or subjective experience might be used. Little
is known about the mechanisms that may be involved
in the transduction of simple auditory signals into
changes in mood and performance demonstrated here.
However, the results of this study demonstrate clearly
that simple binaural-beat auditory stimulation can
influence psychomotor and affective processes, even
when people are unaware that such signals are being
presented.
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