Auditory Beat Stimulation and its Effects on Cognition and Mood States

Abstract

Auditory beat stimulation may be a promising new tool for the manipulation of cognitive processes and the modulation of mood states. Here, we aim to review the literature examining the most current applications of auditory beat stimulation and its targets. We give a brief overview of research on auditory steady-state responses and its relationship to auditory beat stimulation (ABS). We have summarized relevant studies investigating the neurophysiological changes related to ABS and how they impact upon the design of appropriate stimulation protocols. Focusing on binaural-beat stimulation, we then discuss the role of monaural- and binaural-beat frequencies in cognition and mood states, in addition to their efficacy in targeting disease symptoms. We aim to highlight important points concerning stimulation parameters and try to address why there are often contradictory findings with regard to the outcomes of ABS.

Full text

REVIEW
published: 12 May 2015
doi: 10.3389/fpsyt.2015.00070
Edited by:
Alkomiet Hasan,
Ludwig-Maximilians-University
Munich, Germany
Reviewed by:
Felipe Ortuño,
Clínica Universidad de Navarra, Spain
Joseph Kambeitz,
Ludwig-Maximilians-University
Munich, Germany
*Correspondence:
Leila Chaieb,
Department of Epileptology, University
of Bonn, Sigmund-Freud Str. 25,
Bonn D-53105, Germany
leila.chaieb@uni.ukb-bonn.de
Specialty section:
This article was submitted to
Neuropsychiatric Imaging and
Stimulation, a section of the journal
Frontiers in Psychiatry
Received: 02 February 2015
Accepted: 27 April 2015
Published: 12 May 2015
Citation:
Chaieb L, Wilpert EC, Reber TP and
Fell J (2015) Auditory beat stimulation
and its effects on cognition and mood
states.
Front. Psychiatry 6:70.
doi: 10.3389/fpsyt.2015.00070
Auditory beat stimulation and its
effects on cognition and mood states
Leila Chaieb*, Elke Caroline Wilpert , Thomas P. Reber and Juergen Fell
Department of Epileptology, University of Bonn, Bonn, Germany
Auditory beat stimulation may be a promising new tool for the manipulation of cognitive
processes and the modulation of mood states. Here, we aim to review the literature
examining the most current applications of auditory beat stimulation and its targets. We
give a brief overview of research on auditory steady-state responses and its relationship
to auditory beat stimulation (ABS). We have summarized relevant studies investigating
the neurophysiological changes related to ABS and how they impact upon the design
of appropriate stimulation protocols. Focusing on binaural-beat stimulation, we then
discuss the role of monaural- and binaural-beat frequencies in cognition and mood
states, in addition to their efficacy in targeting disease symptoms. We aim to highlight
important points concerning stimulation parameters and try to address why there are
often contradictory findings with regard to the outcomes of ABS.
Keywords: auditory beat stimulation, monaural beats, binaural beats, cognition, mood states, vigilance, memory,
auditory steady-state response
Introduction
Auditory beat stimulation (ABS) has long been of interest for a wide array of applications, ranging
from investigating the auditory steady-state response (ASSR) and measuring audiometric parame-
ters in the brain, to understanding mechanisms of sound localization (1). In addition to this, a few
studies also suggest that ABS can be used to modulate cognition (2), to reduce anxiety levels, as
well as to enhance mood states (3). Other clinical targets also include traumatic brain injury (4) and
attention-deficit hyperactivity disorder (5). Resulting studies have reported contradictory findings
as to the effects of applied monaural-beat frequencies and binaural-beat frequencies, which have
somewhat hampered the progress of further investigations addressing potential effects on cognition
and mood effects amongst other possible targets. Here, we review relevant studies and look to
highlight the most promising directions for future approaches.
Literature Search Strategy
This review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-
Analyses (PRISMA) criteria (6). The electronic databases PubMed and MEDLINE were searched
initially using the single search terms “auditory beat stimulation, monaural beat, binaural beat,
auditory steady-state response,” and then the combination of these search terms with the terms
“cognition, memory, attention, mood, vigilance, anxiety, and creativity.” The strategy was not limited
to human studies. In addition to the electronic search strategy, the reference lists of the manuscripts
that were reviewed were examined to identify any additional articles not captured by the main search
strategy. A total of 920 articles were identified in the initial search. Analysis of the papers followed the
inclusion and exclusion criteria recommended by the PRISMA Guidelines. Articles that presented
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Chaieb et al. Beat stimulation and mood states
a combination of at least two terms from the list of search terms,
“auditory beat stimulation, monaural beat, binaural beat, cogni-
tion, memory, attention, mood, vigilance, anxiety and creativ-
ity” were included. Manuscripts in English, original articles, and
experimental studies were considered. Exclusion criteria were
other study designs (case reports and case series), non-original
studies including editorials, book reviews, and letters to the editor,
and studies not specifically designed and focused on monaural
and/or binaural-beat stimulation. Abstracts were screened for rel-
evance and then full texts were assessed against inclusion criteria.
After screening, a total of 30 articles were selected. During the
construction of the manuscript, several other references were
added, mainly referring to basic neurophysiological findings.
Auditory Beat Stimulation
The effects of ABS have been predominantly investigated using
monaural and binaural beats (Figure 1). The main differences
between binaural and monaural beats are listed in Table1. Monau-
ral and binaural beats are generated when sine waves of neigh-
boring frequencies and with stable amplitudes are presented to
either both ears simultaneously (monaural beats) or to each ear
separately (binaural beats). Monaural beats are physical beats,
which are objectively heard when the combination of two sine
waves at neighboring frequencies (e.g., 400 and 440Hz) are sum-
mated and presented to each ear at the same time resulting in
an amplitude modulated (AM) signal. The beat corresponds to
the difference between the two frequencies (in this case 40 Hz).
Binaural beats are generated when the sine waves within a close
range are presented to each ear separately. For example, when the
400 Hz tone is presented to the left ear and the 440 Hz tone to the
right, a beat of 40 Hz is perceived, which appears subjectively to
be located “inside” the head. This is the binaural beat percept.
The binaural beat percept was first reported by H. W. Dove in
1839 and outlined in detail by Oster over five decades ago (7).
Oster reported that the binaural beats were detected only when the
FIGURE 1 |Application of monaural and binaural beats. The
superposition of amplitude modulated signals of nearby frequencies
delivered either to one ear or both ears simultaneously (monaural beats),
or neighbouring frequencies to each ear separately (binaural beats) are
shown here. Carrier tones of 440 and 480 Hz generating a 40 Hz beat is
shown here as an example [Courtesy of Becher et al. (41)].
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Chaieb et al. Beat stimulation and mood states
TABLE 1 |Monaural and binaural-beat stimulation: main characteristics.
Monaural beats Binaural beats
Physical/objective beat Subjective percept
Presentation of composite
frequencies to one ear or both ears
simultaneously
Presentation of neighboring frequencies
to each ear separately
Peripheral Central
Demodulated in the cochlea Processed in the medial superior olivary
nuclei
Able to be perceived in either one or
both ears
Require combined action of both ears
Heard across a wider beat frequency
range and at higher carrier tones
Present when beat frequencies are low
and with carrier tones below 1000 Hz
The main differences between monaural and binaural beats are summarized and outlined.
carrier frequency was below 1000 Hz, a finding that confirmed an
earlier study by Licklider and colleagues (8). This indicates that
beat carrier frequencies have to be sufficiently low enough to be
temporally encoded by the cortex (9).
Neurophysiology of Auditory Beat
Processing
Acoustic stimuli are heard when the peripheral components of the
auditory pathway (ears, cochlea, and inner hair cells) convert pres-
sure waves into neural action potentials via mechano-electrical
transduction. This is the first order of auditory processing prior to
sound waves being encoded (or rather re-encoded) by the primary
auditory cortex. Auditory information is further processed at a
number of subcortical structures. Auditory nerve fibers leaving
the cochlea converge with the vestibulocochlear nerve and enter
the cochlear nucleus (CN) in the brainstem and bifurcate. As the
nerve fibers branch they form synapses with different subtypes of
neurons – spherical bushy cells, globular bushy cells, and stellate
cells, each of which differ in their temporal and spectral response
properties (10). Information is then relayed to either the inferior
colliculus (IC) via outputs from the stellate and dorsal cochlear
nucleus (DCN) cells, or by an indirect route to the superior olivary
complex (SOC). Bushy cells of the anteroventral cochlear nucleus
(AVCN) project outputs via this route (11). The SOC processes
convergent information from the left and right ears and cues
related to sound localization (12). The left and right IC has a
commissural connection, which allows for binaural interactions
within the ascending pathway, and is comprised of numerous
subnuclei, the largest of which is the central nucleus (ICC) (13,
14). Here, the temporal integration window between the IC and
the auditory cortex enables processing of monaural characteristics
such as amplitude modulation (15). From here, outputs travel
to the medial geniculate nucleus (MGN) of the thalamus, where
thalamic output fibers connect to the auditory cortex located in
the temporal lobes (13,14).
The neurophysiological processing of binaural and monaural
beats differs slightly. Draganova and colleagues underlined these
differences by referring to monaural beats as “peripheral” – as they
interacted at the cochlear level, and binaural beats as “central,”
i.e., the binaural beat percept being the result of the effect of
a central interaction which mostly likely occurs in the superior
olivary nuclei (16). Monaural beats are heard when a composite
auditory stimulus is presented to both ears simultaneously, which
is detected by the cochlear and relayed to the brain stem and
auditory cortex. Binaural beats, however, are only subjectively
perceived when two sine waves of nearby frequencies are delivered
to each ear separately. Brainstem neurons in the SOC, which are
sensitive to phase shifts between both ears, fire action potentials at
a rate corresponding to the phase difference between both ears and
generate the binaural-beat percept. Thus the binaural-beat percept
is caused by the major neural mechanism which enables sound
localization (1).
The Auditory Steady-State Response
The ASSR is a composite auditory evoked potential which can be
elicited using repetitive acoustic stimuli which continually persist
over a time period. The ASSR follows the envelope of a complex
stimulus, and it has been suggested that the steady-state response
drives the background activity of the EEG (17). Regan defines the
steady-state response as “an evoked potential whose constituent
discrete frequency components remain constant in amplitude and
phase over an extended time period” (18). In a seminal study,
Galambos and colleagues recorded click-related potentials (ERPs)
with latencies between 8 and 80 ms post stimulus onset extracted
from the EEG recorded from electrodes placed at the forehead and
ear. These ERPs are termed the middle latency (MLR) responses,
and can be acquired after the earlier brainstem responses. It was
observed that the ERP was most evident when the clicks were
delivered at a rate of40/s, a response that they subsequently named
the 40 Hz ERP [which is a composite of the several transient
waves comprising the MLR (19)]. As the steady-state response
consists of the superposition of the subcomponents of the MLR,
studies have sought to predict the steady-state response from
its transients (20). However, the prediction of the steady-state
response from the superposition of its transient waveforms is not
always accurate, and therefore provides evidence of oscillatory
entrainment (17). Since this early study, many more have inves-
tigated ASSRs with the aim of examining their role in attentional
processes (21–23) and possible clinical applications (24,25). In
a study examining the effect of selective attention on the 40 Hz
transient response, Tiitinen et al. (21) demonstrated that the 40 Hz
transient response was larger over frontal and central areas when
participants were told to pay attention to tone stimuli, than when
they were instructed to ignore them. This demonstrated that the
transient 40 Hz response is enhanced under active attention. In
a later study, Ross and colleagues (22) showed that the ASSR
was also enhanced under conditions of active attention. Partici-
pants performed a modulatory discrimination task in which they
had to indicate when the standard stimuli presented (40 Hz AM
tones) changed in modulation frequency (to 30 Hz, the target
stimuli). Auditory stimuli were delivered monaurally to the right
ear. Magnetoencephalography (MEG) recordings showed that the
ASSR was enhanced during auditory attention, and was more pro-
nounced in the left hemisphere contralateral to stimulation (22).
Another study reported different ASSR patterns to attended versus
unattended AM tones (20 and 45Hz) delivered to the left and
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Chaieb et al. Beat stimulation and mood states
right ear simultaneously (26). The authors reported an attention-
dependent modulation of the ASSR patterns only for the 20 Hz
stimulation, but not for the 45 Hz stimulation. Together, these
findings indicate that directed attention has an impact on ASSR
amplitudes, but that these effects likely dependent on modulation
frequency.
Auditory Steady-State Responses to
Monaural and Binaural Beats
Beat stimulation has been used to examine the source and origin
of the ASSR, an often controversial debate due to contradictory
findings (27,28). More recent studies have sought to clarify this
issue. For example, the 40 Hz ERP has been used to investigate
the cortical sources of the ASSR, as well as the characteristics and
effects of monaural and binaural-beat stimulation frequencies.
Ross and colleagues recorded ASSRs to AM tones with modula-
tion frequencies of ~40 Hz using MEG. They reported that ASSR
amplitudes decreased with increasing carrier frequencies, with
the ASSR amplitude at 250 Hz to be a magnitude of three times
larger than at 4000 Hz. Importantly, they observed that when beats
at 39 and 41 Hz are simultaneously presented both beats can be
perceived at the same time (29).
To probe the cortical representation of binaural-beat frequen-
cies, Karino et al. applied modulation frequencies of 4.00–6.66 Hz
while recording magnetic fields using MEG. The authors reported
that the binaural beat ASSR arose from the superior temporal, pos-
terior parietal, and frontal cortices, in addition to the auditory cor-
tex (30). Another study applied a similar technique to that of Pan-
tev et al. (31), by comparing a transient of the MLR–N1m to ASSR
responses to monaural and binaural-beat stimuli, recorded using
MEG (16). Their findings showed that ASSR to both monaural-
and binaural-beat stimuli are located anterior and medially to
Heschl’s gyri within the Sylvian fissure, and when compared with
the N1m source, place the ASSR generating network within the
primary auditory cortex, which is also in line with other stud-
ies (29,32). The authors also observed that the magnetic field
amplitudes of the ASSR elicited by monaural beats were ~5 greater
than those of the ASSR to binaural beats (16). A recent study
has also reported similar findings concerning the magnitude of
responses to monaural and binaural beats, and that stimulation
conditions were reflected in interhemispheric phase differences
(33). Schwartz and Taylor also reported a lesser ASSR amplitude
response to binaural-beat stimuli compared to monaural beats. A
40 Hz binaural beat ASSR was evoked with a carrier frequency
at 400 Hz but was undetectable above 3000 Hz. This was not the
case for the monaural beat stimulation frequencies, which could
be detected above 3000 Hz (9).
Two studies by Pratt and colleagues have examined cortical
evoked potentials first to binaural-beat frequencies, and then
a following study reported responses to monaural beats. The
first study aimed to explore ERP responses to binaural beats of
3 and 6 Hz using two different carrier frequencies of 250 and
1000 Hz. Similar to other studies (29), the authors reported that
beat-evoked responses were higher in amplitude to beats with a
low carrier frequency (250 Hz) and also to the lower beat fre-
quency itself (3 Hz). In the second study, Pratt et al. (34) recorded
beat-evoked ERPs again, this time to both monaural and binaural
beats at the same frequencies (3 and 6 Hz), as well as at the same
base carrier frequencies (250 and 1000 Hz), but using different
onset phases, i.e., by applying monaural beats that differed in
phase by a quarter of the beat period. The amplitudes of the
beat-evoked oscillations were higher in response to monaural
beats and also to the lower carrier and beat frequencies (250
and 3 Hz). The sources of the beat-evoked responses both to the
monaural and binaural-beat stimuli originated in the temporal
lobe regions and were lateralized to the left hemisphere, regard-
less of the difference in phase-onset between the stimuli. These
findings suggest that the processing of binaural and monaural
beats occurs in the same cortical regions regardless of the onset
phase (34).
In a study aiming to map the origin of the ASSR, Pastor and
colleagues applied a train of monaural stimuli at 12 different
stimulation rates. Using positron emission tomography [(PET)-
H215O] and EEG data, the authors reported an increase in regional
cerebral blood flow (rCBF) together with oscillatory responses
peaking at 40 Hz. These findings indicate that the ASSR is related
to an increase in overall synaptic activity in the auditory cortex
at this frequency, and is not just due the superposition of MLRs
(35). Interestingly, an earlier study by Pantev et al. compared the
transients of the MLR and the 40Hz SSR, and found that the two
evoked responses originate from different sources in the auditory
cortex, which were tonotopically segregated with regard to carrier
frequencies (31).
Altogether, these findings suggest that binaural and monaural
beats are processed in the same cortical areas, and that the beat-
generated ASSR is related to changes in synaptic activity in the
auditory cortex as well as to the superposition of MLR transients,
also originating in the primary auditory cortex.
Frequency-Specific EEG Changes Due to
Beat Stimulation
Studies reporting significant effects after application of binaural
beats using continuous EEG have shown changes in only cer-
tain frequency bands, these include gamma (9,35) and alpha
(36). A recent study seeking to investigate these effects in the
alpha and beta ranges has reported no significant changes. Ver-
non and colleagues applied binaural beats in the alpha (10 Hz)
and beta (20 Hz) frequency ranges for 1 min duration over ten
trials to evaluate whether a frequency following response (FFR)
could be elicited to these frequencies in two separate participant
groups. Each trial was interleaved with exposure to a pure tone
played at 400 Hz while EEG was recorded from the left and
right temporal regions. The authors observed a slight decrease
in resting baseline amplitudes for both beat frequencies, from
the pre- to post-entrainment and during the experimental ses-
sion, and participants exhibited reduced alpha activity during
the binaural beat on-phase compared to the off-phase (36). In a
similar study, Gao et al. (37) investigated the effects of binaural
beats applied for a 5min duration at 1, 5, 10, and 20Hz. To
detect EEG changes due to binaural beats, they analyzed rela-
tive power (RP), phase locking values (PLVs), and cross-mutual
information (CMI). Relative power in the theta and alpha bands
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Chaieb et al. Beat stimulation and mood states
increased during delta and alpha binaural-beat frequency stim-
ulation while it decreased in the beta band. During alpha and
delta binaural-beat stimulation, reduction in CMI was detected
among right temporal, frontal, and occipital areas 3.5 min after
stimulation onset. However, during beta-beat stimulation, an
increase and subsequent decrease in CMI was observed – this
occurred between the left temporal and frontal areas (increase),
and between right temporal and centro-parietal areas (decrease),
and in the case of theta beat stimulation, an increase over left
temporal and central cortical areas was observed. These data
suggest that application of binaural beats in theta, alpha, delta, and
beta frequencies is able to alter functional connectivity between
brain regions (37).
Another study looking at the effect of putatively inhibitory and
facilitatory binaural-beat frequencies (15 and 7Hz, respectively)
on meditation practice, reported significant entrainment effects
(38). Application of binaural beats at a theta frequency (7 Hz)
increased left temporal lobe delta power in experienced medita-
tors, whereas this effect was not recorded in the novice participant
group. When the beta beat frequency was applied novice partic-
ipants showed more gamma power increase during meditation
than the experienced mediator group (38). These results indicate
that the effects of binaural beats depend upon prior experience
and individual skills.
ABS and Phase Synchronization
A study investigating binaural and monaural beat ASSRs com-
pared the phases of the 40 Hz sinusoids derived from EEG record-
ings at various electrode locations (9). The authors observed a
fronto-occipital phase shift in both the binaural and monaural
beat ASSRs, of ~3–7 msec. These phase shifts indicate that the
monaural and binaural beat ASSRs are generated by more than
one neuronal network at different locations. The authors suggest
that either the rostro-caudal phase shift of gamma oscillations
may demonstrate a sequential recruitment of relevant cortical
regions, similar to findings from a study looking at thalamocor-
tical oscillations (39), or that the data could have been generated
by more than one source with different orientations (9). Bilat-
eral phase differences could be detected for the binaural beat,
while monaural beats evoked responses of equal phase in both
left and right hemispheres (33). The phase delays reported by
Ross et al. and Schwarz and Taylor (9) may also reflect alter-
ations in phase synchronization. Phase synchronization occurs
when oscillations in two brain regions have a constant phase
relation over some time period. Phase synchronization is integral
to cognition as it supports the processes of neural communi-
cation, neuroplasticity and memory formation (40). If auditory
beats do indeed induce phase synchronization, this may indicate
a role for monaural and binaural beats in modulating memory
processes. Recent findings based on intracranial EEG record-
ings in humans suggest that auditory beats are able to specifi-
cally alter not only EEG power, but also phase synchronization.
For instance, an increased temporo-lateral phase synchroniza-
tion was observed due to 5 Hz binaural-beat stimulation, while
a decreased mediotemporal synchronization was detected during
5 Hz monaural beats (41).
Studies Examining the Effects of ABS
The application of ABS to manipulate cognitive processes or for
the modulation of mood or pain responses has been investigated,
but has yielded contradictory results, especially with regard to
binaural beats. Studies that reported statistically significant effects
state that they are often weak and short-lived, and in addition,
there is very little discussion as to which mechanisms are involved
in the generation of these effects. This may be in part due to
the nature of the stimuli itself, i.e., the binaural beat being a
weak percept, and that most studies did not employ the use of
measurement techniques like EEG or MEG to quantify result-
ing electrophysiological effects. Another possible reason for the
reported inconsistencies may be due to the incomparability of the
methodological approaches.
Cognitive Effects of ABS
Memory
Two studies applying binaural-beat stimulation at the theta fre-
quency reported opposite effects on memory (2,42). Wahbeh
and colleagues tested verbal memory performance using the Rey
Auditory Verbal Learning Test. Participants are asked to repeat
a list of 15 unrelated words over several different trials. The test
reflects both working and long-term memory processes (43). They
reported that binaural-beat stimulation at 7 Hz, for a single 30-
min session, resulted in decrease in immediate verbal memory
recall in the experimental condition compared to control condi-
tion (42). In contrast, application of 5 Hz binaural-beat stimula-
tion for 15 min, twice per day for 15 days, resulted in a significant
increase in the number of words recalled post-stimulation, as
measured using the Wechsler III Memory Scale, when compared
to the other stimulation conditions (13 Hz binaural beats and a
white noise control condition) (2). The Wechsler II Memory Scale
assesses different working and long-term memory functions using
a battery of tests. In this study, immediate recall of word lists was
assessed (44). These results may suggest that prolonged exposure
to ABS may affect verbal memory recall.
Creativity
Creativity has been suggested to be related to divergent thinking,
as opposed to convergent thinking. Divergent thinking refers to
the generation of multiple answers to a given problem, while
convergent thinking means aiming for a single, correct solution to
a problem (45). A recent study reported positive effects of binaural
beats applied at the alpha (10Hz) and gamma (40Hz) ranges on
creativity. Creativity was assessed using the divergent thinking
[Alternate Uses Task (AUT)] and convergent thinking [Remote
Associations Task (RAT)] tasks, which were correlated with the
spontaneous Eye Blink Rate (EBR), a marker of dopamine levels
in the brain. The divergent thinking task (AUT) involved partic-
ipants being asked to name as many uses for certain household
objects as they possibly could. The task assesses four components:
originality, fluency, flexibility, and elaboration. In the convergent
thinking task (RAT), participants were required to name a single
compound word which matched three seemingly unrelated words.
In addition to these tasks, participants were also required to fill out
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Chaieb et al. Beat stimulation and mood states
a Positive and Negative Affect Scale questionnaire. Beat stimuli
were applied for 3min prior to the tasks. The results of this
study indicated that binaural beats at both frequencies affected
performance in the divergent, but not convergent thinking tasks.
Authors noted that participants with a low EBR benefited from
alpha binaural-beat stimulation, whereas those with high EBRs
were either unaffected or impaired by alpha and gamma binaural-
beat stimulation. In this study, background white noise was also
added to each stimulus in order to amplify the binaural-beat
percept (46).
Attention
In a pilot study, Kennel et al. investigated the potential use of
binaural-beat stimulation to reduce the symptom of inattention
in children and adolescents with attention-deficit/hyperactivity
disorder (ADHD) (5). ADHD is a developmental neuropsychi-
atric disorder diagnosed in children and adolescents. Individuals
affected by ADHD exhibit the core symptoms of inattention,
hyperactivity, and impulsivity in varying degrees of severity (47).
Participants were either required to listen to commercial record-
ings of binaural beats embedded in natural sounds or a sham
recording containing pink noise for 20 min, three times a week
for 3 week duration. The Test of Variables of Attention (TOVA)
and the Children’s Color Trails Test 1 and 2 (CCTT1 and 2)
were performed to measure changes in attention over time and
course of treatment. For this study the binaural-beat stimulation
did not have a significant impact on attention, but participants
reported subjectively experiencing less problems associated with
inattention during the study period (5). Unfortunately, one of
the main limitations of this study was that the beat stimulation
parameters of the audio program administered were not reported.
Anxiety, Mood States and Vigilance as
Targets of ABS
Anxiety
Two types of anxiety can be differentiated. State anxiety is a tem-
porary increase in anxiety levels related to an event or situation.
Trait anxiety, however, is a continually heightened level of anxiety
which is a personal characteristic (48). In an interesting study,
Padmanabhan and colleagues applied binaural-beat stimulation
to patients suffering from pre-operative anxiety (3). Patients were
assessed using the State-Trait Anxiety Inventory (STA-I) ques-
tionnaire, and beat stimuli were administered via a compact disk
player with either binaural beats or a sham-like audio recording.
The recordings contained binaural beat recordings within a delta
frequency range. The authors reported a 26.3% decline in anxiety
scores in the post-stimulation STA-I assessment for the binaural
beat audio group when compared to a 11.1% decline in the placebo
audio group (3). A similar approach was taken by Weiland et al.
who used sound compositions of either natural settings, or with
an embedded binaural-beat frequency of 10 Hz (49). The interven-
tion was applied for 20min and patients were requested to com-
plete the STA-I in order to assess anxiety scores. They reported
significant decreases in anxiety scores post-intervention in those
patients who received the binaural-beat stimulation compared to
the patient group who did not (49). Le Scouarnec et al. used a
commercial binaural-beat recording for a pilot study examining
levels of anxiety (50). Participants with mild anxiety disorders
were asked to listen to a recording of binaural beat stimuli in
the delta/theta range, daily for 30min for a total of 1 month
while detailing their anxiety ratings before and post-treatment
using STAI-I scores. The authors reported that patients recorded
a reduction in anxiety ratings and an increase in the number of
times the patients chose to listen to the recordings (50). In a later
study, it was also reported that patients who received binaural-beat
stimulation in the delta frequency for 30min daily over 60 days
showed a significant decrease in trait anxiety scores assessed with
the STA-I (51).
Mood States
Mood is a temporary, conscious state of mind or predominant
feeling (52). Mood states are often dependent upon external fac-
tors. Several studies have sought to modulate mood states by
binaural-beat stimulation. Two studies by Wahbeh and colleagues
looked at the effect of binaural beats at theta (7Hz) and delta
(0–4 Hz) frequencies on mood states (42,51). Binaural beats were
presented either daily over 60days (at delta frequency) or once
for 30 min (at theta frequency). Changes in mood states were
assessed using the Profile of Mood States (POMS) questionnaire,
given before and after stimulation. The POMS is a 65-item self-
report questionnaire that contains total mood score and six sub-
scales: Tension–Anxiety, Depression–Dejection, Fatigue–Inertia,
Anger–Hostility, Vigor–Activity, and Confusion–Bewilderment
(53). They reported a decrease in total mood disturbance, as well
as a decrease in tension, anxiety, confusion, and fatigue subscales
after delta beat stimulation compared the control condition. How-
ever, there was an increase in the depression and vigor subscales
(51). In the second study, after 30 min of theta binaural-beat stim-
ulation there was an increase of the POMS depression subscale
in the experimental condition relative to the control condition
(42). In a similar study, Lane et al. (54) reported decreases in
POMS depression subscales after binaural-beat stimulation in the
beta range (16 and 24 Hz), compared to presentation of beats in
the theta/delta range (1.5 and 4 Hz). The authors suggested that
perception of beta frequency beats is associated with less negative
mood (54).
Vigilance
Vigilance is the ability to maintain focus of attention and to
remain alert to stimuli over prolonged periods of time (55). Vig-
ilance tasks typically comprise of monotonic sensory processing
requiring continuous attention. A recent study aimed to explore
the impact of binaural beats on vigilance and personality traits
assessed according to the Five Factor Model (FFM) (56). The FFM
of personality identifies five traits: Neuroticism (N), Extraversion
(E), Openness to experience (O), Agreeableness (A), and Con-
tentiousness (C). A previous study had identified a correlation
between O and C personality traits and cortical entrainment in
the theta and beta bands using photic driving (57). Binaural-beat
stimuli were applied at theta (7 Hz) and beta (16Hz) frequencies
while participants performed a vigilance task, in which they were
required to respond to the presentation of a target stimulus (a
number or letter), presented on a computer screen out of a list of
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Chaieb et al. Beat stimulation and mood states
serially presented stimuli, with a button press. EEG was recorded
throughout the experiment and binaural beats were applied for a
total of 4 min during the execution of the task. A baseline of white
noise was played in between each stimulation epoch. The authors
hypothesized that beats within the beta range would sustain or
increase levels of vigilance (indicated by a reduction in response
time during the vigilance task), based on earlier studies (54). In
line with previous findings (57), they also suggested that O and
C personality traits would be more susceptible to entrainment in
the theta and beta beat frequency ranges, and that individuals who
scored higher in trait category A would demonstrate higher beta
power in the left temporal and central cortical regions (58). The
authors reported no significant effects of the stimulation frequen-
cies, either on vigilance or any interaction with personality. They
suggested the short duration exposure to the binaural-beat stimuli
was insufficient to alter vigilance or entrain cortical frequencies.
However, another investigation also examining the effects of
binaural beats on vigilance and mood, reported significant effects.
Lane et al. applied binaural beats at beta (16 and 24 Hz) and
theta/delta (1.5 and 4 Hz) ranges for 30 min throughout a psy-
chomotor vigilance task. The authors reported that beats in the
beta range were associated with a less negative mood (see above)
and improved performance in a vigilance task. However, the
beat stimuli presented in this study contained a background of
pink noise and had lower carrier tones to that of the previous
study reporting negative effects of binaural-beat frequencies on
vigilance (54). It may be that the choice of carrier tone impacts
upon the efficacy of beat stimulation, as it was reported that lower
carrier tones as well as beat frequencies produce more robust
effects (34). The addition of a pink or white noise background may
also have had an effect upon the processing of the beat percept
(46). These differences, along with the length of the stimulation
duration may account for the discrepancies seen in the outcomes
of these similar studies.
Conclusion
This review has aimed to give a brief overview of ABS and its role
in cognition and potential use as a therapeutic tool for modulating
mood states. While findings for most putative applications up to
now are either solitary or contradictory, several studies consis-
tently report a diminishing impact of binaural-beat stimulation
on anxiety levels. The underlying neural mechanisms are still yet
to be unraveled. Understanding how and where the binaural-beat
percept is generated and which cortical networks are most affected
will aid in the optimization of both monaural and binaural-beat
stimulation as a tool to modulate cognitive and mode states. Many
studies employing ABS as either a mechanistic tool or potential
therapeutic aid, report contrasting findings. Further research,
including more accurate reporting of experimental protocols,
especially those studies undertaken in a clinical setting, will help
to clarify the most promising effects. In a recent study, Ross and
colleagues reported that inconsistencies relating to monaural and
binaural beats at low frequencies, as well as at the 40 Hz frequency,
could possibly be attributed to earlier investigations suggesting
that they share common neural mechanisms (33). Many factors
may impact upon the efficacy of beat stimulation, including the
duration of the applied stimulus. Carrier frequencies may also play
a role, as well as the addition of background white or pink noise,
which may amplify the beat percept (46).
A study examining the effects of aging showed that regardless of
age, a binaural-beat percept in the gamma range could be detected,
but with less accuracy by older individuals (59). Some investiga-
tions also reported gender differences concerning binaural-beat
perception (7) and alterations in auditory perception during the
menstrual cycle (60). Other studies suggest that attending to the
stimulus may play role (9). As many factors impact upon the
efficacy of monaural and binaural-beat stimulation, a more in-
depth reporting of beat stimulation parameters and protocols
would offer the possibility to limit the methodological incon-
sistencies that may explain many of the contradictory outcomes
reported in the literature. Most importantly, electrophysiological
investigations comparing the effects of auditory beats under dif-
ferent stimulation conditions and parameters are still rare. Such
studies are necessary as a fundament to allow the development
of mechanistic hypotheses explaining the behavioral outcomes of
beat stimulation.
Acknowledgments
The authors would like to thank Dr. Hui Zhang for her helpful
comments and constructive criticism of the manuscript. The work
of LC and JF is supported by the Deutsche Forschungsgemein-
schaft (FE 366/6-1 and SFB1089).
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Conflict of Interest Statement: The authors declare that the research was con-
ducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Copyright © 2015 Chaieb, Wilpert, Reber and Fell. This is an open-access article
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