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Review ArticleNeurophysiological Effects of Meditation Based onEvoked and Event Related Potential Recordings
Nilkamal Singh and Shirley Telles
Patanjali Research Foundation, Patanjali Yogpeeth, Haridwar, Uttarakhand 249405, India
Correspondence should be addressed to Shirley Telles; shirleytelles@gmail.com
Received 28 November 2014; Revised 28 January 2015; Accepted 8 February 2015
Academic Editor: Carlo Miniussi
Copyright 2015 N. Singh and S. Telles. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.
Evoked potentials (EPs) are a relatively noninvasive method to assess the integrity of sensory pathways. As the neural generators formost of the components are relatively well worked out, EPs have been used to understand the changes occurring duringmeditation.Event-related potentials (ERPs) yield useful information about the response to tasks, usually assessing attention. A brief review ofthe literature yielded eleven studies on EPs and seventeen on ERPs from 1978 to 2014. The EP studies covered short, mid, andlong latency EPs, using both auditory and visual modalities. ERP studies reported the effects of meditation on tasks such as theauditory oddball paradigm, the attentional blink task, mismatched negativity, and affective picture viewing among others. Both EPand ERPs were recorded in several meditations detailed in the review. Maximum changes occurred in mid latency (auditory) EPssuggesting that maximum changes occur in the corresponding neural generators in the thalamus, thalamic radiations, and primaryauditory cortical areas. ERP studies showed meditation can increase attention and enhance efficiency of brain resource allocationwith greater emotional control.
1. Introduction
Meditation has been described as a training in awarenesswhich over long periods of time produces definite changesin perception, attention, and cognition. The neurophysio-logical correlates of meditation have been determined byelectrophysiological recordings (from the 1960s to the presenttime) and more recently by neuroimaging studies (fromthe 1980s till the present time). Among electrophysiologicalvariables sensory evoked potentials (EPs) provide a relativelynoninvasive way of studying changes in specific sensorypathways during meditation [1]. It is believed that meditationalters cortical functioning and corticofugal controls whichmay significantly modify the processing of information atbrainstem and thalamic levels [24]. Hence short, mid, andlong latency EPswould be expected to helpmap changes fromthe brainstem up to the association or secondary corticalareas [5]. The present review was undertaken to determinewhich modalities and latencies of EPs were recorded inmeditation and the conclusions derived.
EPs are evoked spontaneously with repetitive sen-sory stimulation and can provide information about brain
resource allocation and the speed of stimulus processing,whereas event-related potentials (ERPs) are not spontaneousbut elicited with cognitive task processing [68]. Hence ERPscan provide additional information regarding the discrim-inative ability of the brain and neurocognitive processingrelated to shifting attention.The present review also discussesstudies which evaluated the effect of meditation on differentmodalities of ERPs.
2. Methods
2.1. Search Strategy for Meditation and Evoked PotentialStudies. Thedatabase searched was PubMed using the searchwords Meditation, Evoked Potentials. Fifty-eight citationswere obtained from PubMed. To be included in this reviewarticles had to be written in English (8 articles were excludedas they were written in other languages). Articles wereexcluded from the review if (i) they reported event-relatedpotentials (ERPs) rather than EPs (19 articles were excludedfor this reason) and (ii) they did not deal directly withthe subject of meditation (5 articles were excluded for thisreason), (iii) the articles were not experimental studies but
Hindawi Publishing CorporationBioMed Research InternationalVolume 2015, Article ID 406261, 11 pageshttp://dx.doi.org/10.1155/2015/406261
2 BioMed Research International
were review articles or descriptive (5 articles were excludedfor this reason), and (iv) the study recorded variables otherthan EPs such as EEG, MRI, and spectroscopy studies(10 articles were excluded for this reason). Eleven articlesreported evoked potential changes in different meditationsand are reviewed here for their study design, method ofmeditation, and conclusions derived.
2.2. Search Strategy forMeditation and Event-Related PotentialStudies. The databases searched were PubMed using thesearch words Meditation, Event-Related Potentials. Sixtycitations were obtained from PubMed. To be included in thisreview articles had to be written in English (8 articles wereexcluded as they were written in other languages). Articleswere excluded from the review if (i) they reported evokedpotentials (EPs) rather than event-related potentials (ERPs)(15 articles were excluded for this reason) and (ii) they didnot deal directly with the subject of meditation (3 articleswere excluded for this reason), (iii) the articles were notexperimental studies but were review articles or descriptive(5 articles were excluded for this reason), and (iv) the studyrecorded variables other than ERPs such as EEG, MRI,and spectroscopy studies (11 articles were excluded for thisreason). Details of one study were not available. Seventeenarticles reported event-related potential changes in differentmeditations and are reviewed here for their study design,method of meditation, and conclusions derived.
2.3. Method of Review. The whole papers were obtained andthe details related to (i) stimulus modality for EPs or natureof the ERP, (ii) sweep width or latency of the EPs, (iii) type ofmeditation, (iv) study design, and (v) changes (if any) in EPor ERP components and the corresponding changes in theneural generators were noted.
3. Results
3.1. Details of the Evoked Potential Studies. Out of the elevenstudies 10 used auditory stimuli while one used visual stimuli.With regard to sweep width out of the nine studies tworeported short latency EPs, four mid latency EPs, two longlatency EPs, and one long latency visual evoked potential(LLVEP) and there were also two combinations of (i) shortlatency and mid latency auditory EPs and (ii) short, mid, andlong latency auditory EPs. Details about the components ofEPs are given in Table 1.
The meditation techniques studied were all eyes closedpractices; two were transcendental meditation (TM), twoQigong, five meditations on the Sanskrit syllable OM, one amoving meditation called cyclic meditation, and one Sahajayogameditation.Apart from themovingmeditation the othereight techniques directed the thoughts in a fixed pattern forall practitioners towards either a single syllable or phraseor a set of thoughts. In the moving meditation as well, thesequence of thoughts was fixed and played on a CD [9].
Hence in all the studies the meditation techniquesinvolved directing the attention in a specific way in allpractitioners.The next point considered was the study designwhich included the controls used.
The range of experience in meditation was from nave totwenty years.The sample size variedwidely in the nine studieswith a range between five and sixty practitioners and groupaverage S.D. was 27.8 18.4.
3.2. Details of the Event-Related Potential Studies. Out ofthe seventeen studies four used auditory oddball paradigm,three used the attentional blink task, two used mismatchednegativity, two used affective picture viewing, one usedglobal-to-local target task, one used the Stroop task, oneused visual A-X continuous performance task, one usedrow/column speller, one used anticipatory and pain evokedERPs, and one used discrimination of the imagery of the handmovement.
The meditation techniques used were Vipassana medita-tion in six studies, Sahaja yoga meditation, open monitoringmeditation, Sudarshan Kriya yoga meditation, musical med-itation, meditative mindfulness, cyclic meditation, mindful-ness based cognitive therapy one each in seven studies, andtwo mixed traditions and 2 did not specify the meditationtechniques used. All meditation techniques were performedwith eyed closed.
3.3. Observations Common to Both EP and ERP Researchin Meditation. In meditation research it is often difficult tofind an appropriate control group which does not practicemeditation but is otherwise comparable to the meditationgroup in other ways. This is because meditators often haveother changes in their lifestyle such as abstaining fromcaffeine, nicotine, and other stimulating substances, differenthours of rising and sleeping, and other differences. For thisreason in some studies themeditators themselves are assessedunder identical conditions on another day as a control, whichmay be described as the self-as-control [10].
This design was followed in four of the EP studies whichare presented in Table 2 [5, 1113]. There were two singlegroup studies [1, 14].These were the earliest studies on evokedpotentials and meditation. The absence of a comparison orcontrol state or group is an obvious disadvantage, as theeffect of time during a session can be expected to influencethe evoked potential components and the meditation effectwould not be separated from this. In five other papers [1519]the approach taken was to have separate groups of partici-pants. In only one trial [17] the participants were randomizedas three groups. The advantage of the separate groups designover the self-as-control is that nonmeditators would not getinadvertently into meditation which is a disadvantage of theself-as-control design, where experiencedmeditatorsmay getinto the meditative state automatically, even during a controlsession.
Out of the seventeen ERP studies, 12 studies had a two-group design, four studies had a self-as-control design, andone had mixed design in which the study was conductedin two phases, phase 1 was a cross sectional study andphase 2 was a longitudinal study. This is described in Table 3.The disadvantages and advantages are comparable to thosedescribed above for EP studies. With ERP studies there isan additional point; since the participants are given a task
BioMed Research International 3
Table 1: Neural generators of evoked potential components.
S. number Name of thecomponentsLatencies(msec) Neural generators
1 Short latency auditoryevoked potential
Wave I 1.9 Auditory portion of the eighth cranial nerve
Wave II 3.6 Near or at the cochlear nucleus. A portion from the eighth nerve fibersaround the cochlear nucleusWave III 4.2 The lower pons through the superior olive and trapezoid bodyWave IV 5.2 The upper pons or lower midbrain, in the lateral lemniscus and the inferior
colliculus; a contralateral brainstem generator for wave V is suggestedWave V 5.8Na 1419 Medial geniculate bodyPa 2532 Superior temporal gyrus
2 Mid latency auditoryevoked potential
Nb 3565 Dorso-posterior-medial part of Heschls gyrus that is the primary auditorycortexN1 4060ms Secondary auditory cortex in the lateral Heschls gyrusP1 80115ms Bilateral parts of the auditory superior cortex
Long latency auditoryevoked potential
N2 140180ms Mesencephalic-reticular activating system (RAS)P2 220280ms Anterior cingulate cortex
it is usual to record before and after meditation, not duringmeditation, as doing the taskwould interferewithmeditation.
3.4. Results of EPs
3.4.1. Auditory Evoked Potentials. Discussing the shortlatency or brainstem auditory evoked potentials first, therewere three studies. In the earliest study [14] the peak latencyof wave V which corresponds to the inferior colliculus variedwith the intensity of the stimuli and hence could not beconsidered as an effect of the transcendentalmeditation (TM)practiced. In the second study which reported short latencyauditory evoked potentials in Qigong meditators [15] therewas a significant increase in the peak amplitudes of waves I toV of short latency auditory evoked potentials. An increase inamplitude is suggestive of recruitment of increased neuronsin the underlying neural generators [20]. This suggested thatQigong meditation activates areas in the brainstem. Therewas no change in short latency auditory evoked potentialsin patients with epilepsy who practiced Sahaja yoga possiblybecause they were novices to meditation [17]. In the otherstudy which reported changes in short latency auditoryevoked potentials the peak latency of wave V increasedduring random thinking, focusing and meditative focusing,but not during meditation [11]. An increase in peak latencysignifies delayed transmission through the respective neuralgenerators [5], in this case located in the brain stem. Hence itwould appear as if random thinking, focusing, andmeditativefocusing all delay auditory information transmission at thebrainstem level but this does not happen during meditationon OM.
Themid latency auditory evoked potentials correspond toneural generators in the thalamus, primary auditory cortex,andHaschls gyrus [24].WithQigongmeditation the ampli-tudes of Na and Pa components decreased during meditation[15]. The Pa wave peak amplitude also decreased duringmeditation on OM in sixty practitioners [12] but there was
an increase in peak amplitude of Na wave during meditationon OM in a separate study [18]. The Na wave is believed tobe due to activity at the mesencephalic and diencephalic level[21] and the Pa wave corresponds to activity at the superiortemporal gyrus [22]. A third component of the mid latencyauditory evoked potentials is the Nb wave.This wave appearsrelatively localized in the dorso-posterior-medial area ofHeschls gyrus, that is, the primary auditory cortex [23]. TheNb wave peak amplitude increased and peak latencies of Paand Nb waves decreased after the moving meditation calledcyclic meditation (CM), which incorporates movement [5].The peak latency of Nb wave reduced during meditationon OM as well [19]. Hence the moving meditation alteredthe auditory pathway at the level of superior temporal gyrusand Heschls gyrus with a delay in auditory transmissiondespite an increase in number of neurons recruited. Duringmeditation on OM the delay in auditory transmission wasfound only in the dorso-posterior-medial area of Heschlsgyrus.
Long latency auditory evoked potentials showed nosignificant change in an early study on transcendentalmeditation practitioners [1]. The P2 component showed adecrease in peak amplitude during Qigong meditation anda decrease in peak latency during meditation on OM. TheP2 wave partly reflects auditory output of the mesencephalicactivation system [24, 25]. From MEG and EEG data basedon depth electrodes in patients, the neural generators for theP2 componentwere localized in the planum temporale as wellas the auditory association complex (Brodmann area 22) [2629]. Also it is speculated that the P2 component may receivecontributions from cortical areas in the depth of the Sylvianfissure. Hence this area, which is associated with complexauditory functions, changes during both Qigong and OMmeditations.
3.4.2. Summary of Changes in Auditory Evoked Potentials.Out of the four studies which evaluated short latency auditory
4 BioMed Research International
Table2:Detailsof
thee
vokedpo
tentialstudies.
S.nu
mber
Reference
Mod
ality
audi-
tory/visu
al/som
atosensory
andlatency
Type
ofmeditatio
nMeditatio
nexperie
nce,du
ratio
nCom
ponentsa
lteredandbrain
area
Samplea
nddesig
n
1
Electro
enceph
alogr
Clin
Neuroph
ysiol.
1978,45(5):
671673[1]
Auditory
Long
latency
Transcendentalmeditatio
n18
mon
thsto6years
Nosig
nificantchange
Sing
legrou
p
2
Intern
JNeuroscience.
1980,10(2-3):
165170[14
]
Auditory
Shortlatency
Transcendentalmeditatio
n6to
9years
WaveV
latencyincreasedin
mod
erateintensitystimuliand
waveV
latencydecreasedin
high
intensity
stimuli
Sing
legrou
p
3Am
JChinMed.
1990,18(3-4):
95103
[15]
Auditory
Shortlatency,m
iddle
latency,and
long
latency
Qigon
gmeditatio
nwhich
involves
theinitia
lconcentrating,
asub
sequ
ent
circ
ulating,
andfin
ally
thedispersio
nof
Qi
1to20
years
Therew
asas
ignificantincrease
inam
plitu
dein
waveI-V
ofBA
ER,N
aand
Pawaveo
fMLR
decreased,andP2
waveo
fLL
AER
also
decreaseddu
ring
meditatio
n
3grou
ps:B
AER
,MLA
ER,and
LLAER
4Am
JChinMed.
1993,21(3-4):
243249[16]
Visual
Corticalevoked
potentials
Qigon
gmeditatio
nin
which
the
practitionerc
oncentrateso
nthe
Dantian
2.3yearsfor
the
experie
nced
grou
p,1.9
mon
thsfor
the
learning
grou
p
Peak-to
-peakam
plitu
deof
N80-P115-N150and
N150-P2
00-N
280increasedin
thee
xperienced
grou
p
3grou
ps:practition
ergrou
plearning
grou
p,andcontrolgroup
5IntJ
Neurosci.
1994;76(1-2):
8793[18]
Auditory
Middlelatency
evoked
potentials
OMm
editatio
nin
which
the
participantsmeditatedwith
effortless
absorptio
nin
thes
ingle-thou
ghtstateof
theo
bjecto
fmeditatio
n,thatis,
OM.
10years
Experie
nced
meditatorshad
significantincreaseinpeak
amplitu
deof
Naw
aved
uring
meditatio
nandsig
nificant
redu
ctionin
Naw
avep
eak
amplitu
dedu
ringcontrolsessio
n
Two-grou
pstu
dy(experienced
meditators
andno
nexperienced)
6Indian
JMed
Res.
1993;98:2379[19
]
Auditory
Middlelatency
evoked
potentials
OMm
editatio
nin
which
the
participantsmeditatedwith
effortless
absorptio
nin
thes
ingle-thou
ghtstateof
theo
bjecto
fmeditatio
n,thatis,
OM.
520
years
Experie
nced
meditatorshada
significantreductio
nin
thep
eak
latencyof
theN
bwave
Two-grou
pstu
dy(experienced
meditators
andno
nexperienced)
7
Appl
Psycho
physiol
Biofeedb
ack.2000,
25(1):112
[17]
Auditory
BAEP
MLA
EP
Sahajayoga
inwhich
thep
articipants
makec
ertain
mentalassertio
nsby
placing
theh
andon
different
partso
fthe
body.
Not
experie
nced
=10
(Sahajay
ogag
roup
),
=10
(mim
icking
exercise
grou
p),and
=12
(con
trolgroup
)
Sign
ificant
increase
inNa-Pa
amplitu
deof
MLR
following
meditatio
npractice
Rand
omized
controlled
study
8Clin
EEG
Neurosci.2009,40
(3):190
195[5]
Auditory
Mid
latency
Cyclicm
editatio
nin
which
aserieso
fasanas
(posturesinterspersedwith
relaxatio
ntechniqu
es)arepracticed
with
awareness.
648
mon
ths
Afterc
yclic
meditatio
ntherew
asas
ignificantincreaseinthep
eak
latencyof
theP
awavea
ndof
the
Nbwave;peak
amplitu
deof
the
Nbwavea
lsoincreased
Selfas
controldesign
BioMed Research International 5
Table2:Con
tinued.
S.nu
mber
Reference
Mod
ality
audi-
tory/visu
al/som
atosensory
andlatency
Type
ofmeditatio
nMeditatio
nexperie
nce,du
ratio
nCom
ponentsa
lteredandbrain
area
Samplea
nddesig
n
9IntJ
Yoga.20103
(2):374
1[11]
Auditory
Brainstem
auditory-evoked
potentials(BAEP
s)
OMm
editatio
nin
which
the
participantsmeditatedwith
effortless
absorptio
nin
thes
ingle-thou
ghtstateof
theo
bjecto
fmeditatio
n,thatis,
OM.
6mon
ths
WaveV
peak
latency
significantly
increasedin
cancalata,ekagrata,and
dharana,
butn
ochange
occurred
durin
gthed
hyanas
essio
n
Selfas
control
10Clin
EEG
Neurosci.2012,43
(2):154
60[12]
Auditory
Mid
latency
OMm
editatio
nparticipantswere
instructed
tokeep
theire
yesc
losedand
dwellonthou
ghtsof
OM,w
ithou
tany
effort,particularlyon
thes
ubtle
(rather
than
physical)attributes
and
conn
otations
ofthes
yllable.
660
mon
ths
Sign
ificant
increase
inthep
eak
latencieso
fNaa
ndPa
waves
durin
gmeditatio
nandthep
eak
amplitu
deof
Pawavew
assig
nificantly
decreaseddu
ring
meditatio
n
Selfas
controldesign
11Clin
EEG
Neurosci.2014,pii:
1550059414544737
[13]
Auditory
Long
Latency
OMm
editatio
nin
which
participants
wereinstructedto
keep
theire
yesc
losed
anddw
ellonthou
ghtsof
OM,w
ithou
tanyeffort,particularlyon
thesub
tle(ratherthanph
ysical)attributes
and
conn
otations
ofthes
yllable.
660
mon
ths
Decreaseinthep
eaklatencyof
theP
2waved
uringandafter
meditatio
nSelfas
controldesign
6 BioMed Research International
Table3:Detailsof
eventrelated
studies.
S.nu
mber
Citatio
nParticipants
Natureo
fthe
ERP
task
Design
Interventio
nFind
ings
1Neuroscience.2014,
281C:195201
[31]
Health
yexperie
nced
meditatorsand
nonm
editators
Affectivep
icture
view
ing
Two-grou
pstu
dy
Nointerventio
nwas
given
butthisstudy
compared
betweenlong-te
rmexperie
nced
Sahajayoga
meditatorsand
nonm
editators
Mid
latency(14
0400m
s)ER
Pswerea
ttenu
ated
forb
othpo
sitivea
ndnegativ
epicturesa
ndas
tronger
ERPnegativ
ityin
the
timew
indo
w200
300m
swas
foun
din
meditatorsregardlessof
picturev
alence.W
eassum
ethat
long-te
rmmeditatio
npracticee
nhancesfrontaltop-do
wncontroloverfastautom
aticsalience
detection,basedon
amygdalafunctio
ns.
2IntJ
Psycho
physiol.
2013,90(2):207214[32]
Health
yexperie
nced
meditatorsaged
206
1years
Auditory
oddb
all
task
with
twotones
(stand
ardandtarget)
One
grou
pwas
assessed
intwoseparate
cond
ition
s(selfas
control)
Vipassanam
editatio
nand
rand
omthinking
TheV
ipassana
expertssho
wed
greaterP
3bam
plitu
destothetargetton
eafterm
editatio
nthan
they
didbo
thbeforemeditatio
nandafter
then
o-meditatio
nsession.Th
eseresultssuggestthat
expertVipassanam
editatorsshow
edincreasedattentionalengagem
entafterm
editatio
n.
3SocC
ognAffect
Neurosci.2013,8
(1):
100
111[34]
Health
yVipassana
meditatorsEx
p.=
2.540
years
Three-stimulus
auditory
oddb
alltask
One
grou
pwas
assessed
intwoseparate
cond
ition
s(selfasc
ontro
l)
Vipassanam
editatio
nand
instr
uctedmindwanderin
g
Meditatio
ncomparedto
controlcon
ditio
nhaddecreasedevoked
delta
(24
Hz)po
wer
todistr
acterstim
ulicon
comitantlywith
agreater
event-r
elatedredu
ctionof
late(500
900m
s)alph
a-1(810Hz)activ
ity,w
hich
indexedaltereddynamicso
fatte
ntionalengagem
entto
distr
acters.A
ddition
ally,
stand
ardstimuliw
erea
ssociatedwith
increasedearly
event-r
elated
alph
aphase
synchron
y(in
tertria
lcoh
erence)and
evoked
theta(
48H
z)ph
ases
ynchrony,
suggestin
genhanced
processin
gof
theh
abitu
ated
stand
ardbackgrou
ndstimuli.Finally,during
meditatio
n,therew
asag
reater
differentialearly-evokedgammap
ower
tothed
ifferentstim
ulus
classes.C
orrelatio
nanalysisindicatedthatthiseffectstemmed
from
ameditatio
nsta
te-related
increase
inearly
distr
acter-evoked
gammap
ower
andph
ases
ynchrony
specifictolonger-te
rmexpertpractitioners.Th
efind
ings
suggestthatV
ipassana
meditatio
nevokes
abrain
stateof
enhanced
perceptualcla
rityanddecreasedautomated
reactiv
ity.
4Fron
tHum
Neurosci.
2012,6:133
[38]
Health
ymeditators
andno
nmeditators
Global-to-local
targettask
Stud
ycond
uctedin
two
phases
Phase1:crosssectional
study
Phase2
:lon
gitudinal
study
Openmon
itorin
gmeditatio
n
Meditatorsshow
edan
enhanced
processin
gof
targetlevelinformation.In
contrastwith
control
grou
p,which
show
edalocaltargetselectioneffecto
nlyin
theP
1and
aglobaltargetselectio
neffectintheP
3compo
nent,m
editatorsshow
edeffectsof
localinformationprocessin
gin
theP
1,N2,andP3
andof
glob
alprocessin
gforthe
N1,N2,andP3
.Thus,m
editatorsseem
todisplay
enhanced
depthof
processin
g.In
thelon
gitudinalexperim
ent,meditatio
nmod
ulates
attentionalreadyafter
a4-day
meditatio
nretre
at.Together,theseresultssuggestthatp
racticingmeditatio
nenhances
thes
peed
with
which
attentioncanbe
allocatedandrelocated,thus
increasin
gthed
epth
ofinform
ationprocessin
gand
redu
cing
respon
selatency
5SocC
ognAffect
Neurosci.2013,8
(1):
8592[45]
Health
ymeditators
andno
nmeditators
Stroop
task
Two-grou
pstu
dy
Com
paris
onbetween
meditatorsand
nonm
editators,m
editators
arefrom
vario
ustradition
s
Meditatorsshow
edgreatere
xecutiv
econ
trol(i.e.,fewer
errors),ah
ighere
rror
relatednegativ
ity(ERN
),andmoree
motionalacceptancethancontrols.
6BM
CPsychiatry.2012,
12:15[44]
Patientsw
ithbipo
lar
disorder
andno
rmal
healthyparticipants
AvisualA-X
continuo
usperfo
rmance
task
Two-grou
pstu
dyMindfulnessbasedcogn
itive
therapy(M
BCT)
MBC
Tin
bipo
lard
isorder
improved
attentionalreadinessandattenu
ated
activ
ationof
nonrele
vant
inform
ationprocessin
gdu
ringattentionalprocesses
7JN
euralE
ng.2011,8(2):
025019
[42]
Health
yindividu
als
Row/colum
nspeller
task
Two-grou
pstu
dyMeditativ
eMindfulness
Indu
ction(M
MI)and
non-MMIcon
trolgroup
MMIsub
jectsw
eres
ignificantly
morea
ccuratethancontrolsub
jectsa
ndthey
prod
uced
significantly
larger
P300
amplitu
desthancontrolsub
jectsa
tCza
ndPO
7
8NeurosciR
es.2011,71
(1):444
8[46]
Health
ymeditators
andno
nmeditators
Emotionalloadof
stimuli(IAPS
pictures)
Two-grou
pstu
dy
Nointerventio
nwas
given
butthisstudy
compared
with
experie
nced
meditators
andno
nmeditators
Theresultsho
wed
different
emotionalprocessingin
meditatio
npractitioners:ath
ighlevelsof
processin
gmeditatorsarelessa
ffected
bystimuliw
ithadversee
motionalload,whileprocessin
gof
positives
timulirem
ains
unaltered
9Pain.2010,150(3):
428438[47]
Health
ymeditators
andno
nmeditators
Anticipatoryand
pain-evokedER
PsTw
o-grou
pstu
dy
Nointerventio
nwas
given
butthisstudy
compared
with
experie
nced
meditators
andno
nmeditators;
meditatorswerefrom
different
tradition
s
Meditatio
nredu
cesthe
anticipationandnegativ
eapp
raisa
lofp
ain
BioMed Research International 7
Table3:Con
tinued.
S.nu
mber
Citatio
nParticipants
Natureo
fthe
ERP
task
Design
Interventio
nFind
ings
10JN
eurosci.2009,29
(42):1341813427
[37]
Health
ymeditators
andno
nmeditators
Attentionblinktask
andattention
auditory
task
Two-grou
pstu
dyVipassanaa
ndloving
kind
nessmeditatio
n
Threem
onthso
fintensiv
emeditatio
ntraining
redu
cedvaria
bilityin
attentionalprocessingof
targettonesa
ndredu
cedreactio
ntim
evariability.Th
oseind
ividualswith
greatestincrease
inneuralrespon
seconsistency
hadlargestd
ecreaseinbehavioralrespon
sevaria
bility.Re
duced
varia
bilityin
neuralprocessin
gwas
observed
regardlessofwhether
thed
eviant
tone
was
attend
edor
unattend
ed.
significantly
affectatte
ntionandbrainfunctio
n.
11Con
fProcIEE
EEn
gMed
BiolSoc.2008,
2008:6626
65.[48]
Meditatorsand
nonm
editators
Disc
riminationof
theimaginativeh
and
movem
entand
the
idlesta
te
Two-grou
pstu
dyType
ofmeditatio
nno
tspecified
Them
editatio
npracticec
anim
provethe
classificatio
naccuracy
ofEE
Gpatte
rns.Th
eaverage
classificatio
naccuracy
was
88.73%
inthem
editatio
ngrou
p,whileitwas
70.28%
inthec
ontro
lgrou
p.Anaccuracy
ashigh
as98.0%was
achieved
inthem
editatio
ngrou
p.
12IntJ
Psycho
physiol.
2009,72(1):516
0.[33]
Health
yexperie
nced
meditators
Auditory
oddb
all
task
with
twotones
(stand
ardandtarget)
One
grou
pwas
assessed
intwoseparate
cond
ition
s(selfas
control)
Vipassanam
editatio
nand
rand
omthinking
Duringmeditatio
nN1amplitu
defro
mthed
istracter
was
redu
cedfro
ntally;P
2am
plitu
desfrom
both
thed
istracter
andod
dballstim
uliw
eres
omew
hatreduced;P
3aam
plitu
defro
mthe
distr
acterw
asredu
ced.Th
emeditatio
n-indu
cedredu
ctionin
P3aa
mplitu
dehadap
ositive
correlationwith
theq
ualityandexperie
nceo
fmeditatio
n
13JC
ognNeurosci.2009,
21(8):15361549.[36]
Health
ymeditators
andno
nmeditators
Thea
ttentionalblin
ktask
Two-grou
pstu
dyVipassana
Meditatio
n
Thetap
hase
lockingin
consciou
stargetp
erceptionandsuggestthatafterm
entaltrainingthe
cogn
itive
syste
mismorer
apidlyavailabletoprocessn
ewtargetinform
ation.Mentaltrainingwas
notassociatedwith
changesinthea
mplitu
deof
T2-in
ducedrespon
seso
roscillatoryactiv
itybeforetask
onset
14Neurorepo
rt.2007,18
(16):17091712.[39]
Health
ymeditators
andno
nmeditators
Them
ismatch
negativ
ity(M
MN)
paradigm
Two-grou
pstu
dySudarshankriyay
oga
meditatio
n
Meditatorswerefou
ndto
have
larger
MMNam
plitu
desthanno
nmeditators.Th
emeditatorsalso
exhibitedsig
nificantly
increasedMMNam
plitu
desimmediatelyafter
meditatio
nsuggestin
gtransie
ntsta
techangeso
wingto
meditatio
n.
15PL
oSBiol.2007,5(6):
e138.[35]
Health
ymeditators
andno
nmeditators
Thea
ttentionalblin
ktask
Two-grou
pstu
dyVipassana
Threem
onthso
fintensiv
ementaltrainingresultedin
asmallera
ttentionalblin
kandredu
ced
brain-resource
allocatio
nto
thefi
rsttarget,as
reflected
byas
mallerT
1-elicitedP3
b,a
brain-po
tentialind
exof
resource
allocatio
n.Th
oseind
ividualsthatshow
edthelargestdecrease
inbrain-resource
allocatio
nto
T1generally
show
edtheg
reatestreductio
nin
attentionalblin
ksiz
e.Th
eseo
bservatio
nsproviden
ovelsupp
ortfor
thev
iewthatthea
bilityto
accuratelyidentify
T2depend
supo
nthee
fficientd
eploym
ento
fresou
rces
toT1.Th
eresultsalso
demon
stratethat
mentaltrainingcanresultin
increasedcontroloverthe
distr
ibutionof
limitedbrainresources.
16IntJ
Neurosci.2006,116
(12):14191430.[43]
Health
yindividu
als
Auditory
oddb
all
task
with
twotones
(stand
ardandtarget)
One
grou
pwas
assessed
intwoseparate
cond
ition
s(selfas
control)
Cyclicm
editatio
nTh
erew
asredu
ctionin
thep
eaklatencieso
fP300after
cyclicm
editatio
natFz,C
z,andPz
comparedto
the
prevalues.Th
eP300peak
amplitu
desa
fterC
Mwereh
ighera
tFz,Cz
,and
Pzsites
comparedto
the
prevalues.
17Ch
inMed
SciJ.1999,4
(2):7579.[40]
Health
ymeditators
andno
nmeditators
Thea
udito
rymism
atch
negativ
ity(M
MN)and
P300
Two-grou
pstu
dyMusicalmeditatio
nMMNam
plitu
desinthetrained
child
renwerelargerthanthoseinthec
ontro
lgroup
.In
additio
n,theM
MNam
plitu
desw
ereidenticalin
attend
andigno
recond
ition
sfor
both
grou
ps.
8 BioMed Research International
evoked potentials changes occurred in brainstem evokedpotentials only in Qigong [15], not in transcendental medita-tion [14], in Sahaja yoga [17], or in OMmeditation [11].Withrespect to mid latency auditory evoked potentials assessed infour meditations changes were seen in all four meditations[5, 12, 15, 1719], that is, Qigong, Sahaja yoga, meditation onOM, and cyclic meditation. This suggests that meditationmodifies neural generators at the level of specific thalamicnuclei, thalamic radiation, and primary sensory corticesirrespective of the meditation techniques. Long latency AEPschanged in two out of the three meditations in which theywere recorded. Hence there were changes during Qigong andOM meditation but not during TM.
3.4.3. Visual Evoked Potentials. Visual evoked potentials wererecorded in a single study on Qigong meditators [16]. Therewas a significant increase in peak-to-peak amplitude ofN80-P115-N150 and N150-P200-N280 waves during Qigongmeditation. The authors suggest that N80-P115-N150-P200recorded may have corresponded to N70-P100-N130-P170reported by Vaughan in 1996 [30]. The first positive compo-nent is believed to be generated within thalamocortical radi-ations; the subsequent negative component is generated inlamina IV cb.The next positive component reflects inhibitoryactivity within this lamina and the later positive componentreflects extra striate cortex activity.This suggests that Qigongmeditation increases the activity in the visual pathway fromthalamocortical radiations up to the extra striate cortex withall the relay centers in between being included.
3.5. Results of ERPs. Healthy experienced Sahaja yoga med-itators and nonmeditators were assessed in a two-groupstudy using an affective picture viewing task [31]. In thiscomparison between long-term Sahaja yoga meditators andnonmeditators, mid latency ERPs were attenuated for bothpositive and negative pictures and a stronger ERP negativitybetween 200 and 300ms was found in meditators regardlessof picture valence.
There were two separate studies on healthy experiencedVipassana practitioners using an auditory oddball task [32,33]. In both the studies the groups were assessed in twoseparate conditions (the self as control design), that is,Vipassana meditation and random thinking. In one of thestudies Vipassana practitioners showed greater P3b ampli-tudes to the target tone after meditation than they did bothbefore meditation and after the nonmeditation session [32].The other study reported changes in multiple components inresponse to the standard and target stimuli; the meditation-induced reduction in P3a amplitude had a positive correlationwith the quality and experience of meditation [33].
In another study healthyVipassanameditators were givena three-stimulus auditory oddball task [34]. The group wasassessed in two separate conditions, that is, Vipassana medi-tation and instructedmind wandering.Meditation comparedto the control condition had decreased evoked delta power todistracter stimuli concomitantly with a greater event-relatedreduction of late (500900ms) alpha-1 (810Hz) activity,suggestive of a modification of attentional engagement todistracters. Additionally, standard stimuli were associated
with increased early event-related alpha phase synchrony(intertrial coherence) and evoked theta phase synchrony.Finally, during meditation, there was a greater differentialearly-evoked gamma power to the different stimuli. Correla-tion analysis indicated that this effect from ameditation state-related increase in early distracter-evoked gamma powerand phase synchrony was specific to longer-term expertpractitioners.
Two separate studies compared healthy Vipassana med-itators and nonmeditators using the attentional blink task[35, 36]. In one study three months of intensive mentaltraining resulted in a smaller attentional blink and reducedbrain-resource allocation to the first target, as reflected by asmaller T1-elicited P3b, a brain-potential index of resourceallocation [35]. This efficiency in brain resource allocationmight explain the findings of the other study which reportedchanges, especially in theta phase locking in conscious targetperception [36].
Healthy practitioners of Vipassana and loving kindnessmeditation and nonmeditators were assessed in an attentionblink task and an attention auditory task [37]. Hence this two-group study compared practitioners of Vipassana and lovingkindness meditation with nonmeditators. Three months ofintensive meditation training reduced variability in atten-tional processing of target tones and reduced reaction timevariability. Those individuals with greatest increase in neuralresponse consistency had largest decrease in behavioralresponse variability. Reduced variability in neural processingwas observed regardless of whether the deviant tone wasattended to or unattended. Meditation can significantly affectattention and brain function.
Performance in a Global-to-Local target task was com-pared between healthy open monitoring meditators andnonmeditators [38]. The study was conducted in two phases.Phase 1 was a cross sectional study. Phase 2 was a longitudinalstudy. Open monitoring meditation practitioners showed anenhanced processing of target level information, in contrastto the control group, which showed a local target selectioneffect only in the P1 and a global target selection effect inthe P3 component; meditators showed effects of local infor-mation processing in the two other components (other thanP3) and of global processing for two other components.Thus,meditators seem to display enhanced depth of processing. Inthe longitudinal experiment, meditationmodulated attentionafter a 4-day meditation retreat.
Two studies reported the effect of meditation on mis-match negativity (MMN) [39, 40] Healthy practitioners ofSudarshan Kriya yoga and nonmeditators were comparedusing the mismatch negativity (MMN) paradigm [39]. Inthis two-group, comparative study Sudarshan Kriya yogameditators were found to have larger MMN amplitudesthan nonmeditators. Meditators also exhibited significantlyincreased MMN amplitudes immediately after meditationsuggesting transient state changes owing to meditation. TheMMN is related to neuropsychological functioning, partic-ularly to executive functions [41]. Another study on healthycomparing children with training in musical meditation andnonmeditators using auditory mismatch negativity (MMN)and P300 [40] reported similar results.
BioMed Research International 9
Apart from the mismatch negativity (MMN) paradigmP300 can also be a useful indicator for neuropsychologicalfunctioning such as brain resource allocation for attentionalprocessing. Two separate studies on two different types ofmeditation supported the findings mention in the aboveparagraph [42, 43]. Healthy practitioners ofmeditativemind-fulness and a control groupwere assessed using a row/columnspeller task [42]. In this two-group study, the meditativemindfulness induction (MMI) and non-MMI control groupwere compared. MMI subjects were significantly more accu-rate than control subjects and they produced significantlylarger P300 amplitudes than control subjects at the vertex andparietooccipital region. In a separate study, practitioners of amoving meditation called cyclic meditation were assessed ontwo separate days, practicing cyclic meditation or an equalduration of supine rest for comparison [43]. On both daysthey were given an auditory oddball task with two tones(standard and target). There was a reduction in the peaklatency of P300 after cyclic meditation at the frontal region,vertex, and parietal region. Also the P300 peak amplitudesafter CM were higher at the same sites.
In addition to the studies on healthy participants theeffect of meditation was evaluated on patients with bipolardisorder which is characterized by a number of attentionalabnormalities. Patients with bipolar disorder and normalhealthy participants were assessed with a visual A-X con-tinuous performance task [44]. In this two-group study,mindfulness based cognitive therapy in bipolar disorderimproved attentional readiness and attenuated activationof nonrelevant information processing during attentionalprocesses.
Four separate studies compared meditators and non-meditators using different tasks for ERPs [4548]. In oneof the studies healthy meditators of various traditions andnonmeditators performed the Stroop task [45]. Meditatorsshowed greater executive control (i.e., fewer errors), a highererror related negativity, and emotional acceptance thancontrols. Another study used emotionally loaded stimuli(i.e., the International Affective Picture System) [46]. Theresult showed different emotional processing in meditationpractitioners. At high levels of processing meditators areless affected by stimuli with an adverse emotional load,while processing of positive stimuli remains unaltered. Inanother study healthy meditators from different traditionsand nonmeditators were assessed using anticipatory andpain-evoked ERPs [47]. Meditation was found to reducethe anticipation and negative appraisal of pain. Practitionersof imaginative meditators and nonmeditators were assessedduring hand movement(s) and the idle thinking state [48].Imagining moving the hand is associated with event-relateddesynchronization of the beta EEG.This occurredmore oftenand predictably in meditators compared to controls.
3.5.1. Summary of Event-Related Potential Studies. The resultsof the study on Sahaja yoga practitioners showed meditationcan enhance frontal top-down control over fast automaticsalience detection, based on amygdala functions followinglong-term meditation [31]. Studies on Vipassana meditators
showed increased attentional engagement after meditation,enhanced perceptual clarity, decreased automated reactivity,increased efficiency in distribution of limited brain resources,and switching attention [3237]. Similar findings werereported in studies done on open monitoring meditation,Sudarshan Kriya yoga, musical meditation, and meditativemindfulness and cyclic meditation [3843]. Meditators werealso reported to have greater emotional acceptance, wereless affected by stimuli with an adverse emotional load, andreduced the anticipation and negative appraisal of pain [4547]. Hence meditation can induce a mental state whichis characterized by efficient brain resource allocation withgreater emotional control.
4. Limitations of EP and ERP Studies Reviewed
One of the main limitations of the studies reviewed is in thestudy design. Out of the 11 EP studies there was just one [17]which randomized the participants to the three experimentalconditions. The other multiple-group studies [16, 1821] didnot use randomization. Three other early studies [1, 14, 15]had a single group which is an obvious disadvantage. Theremaining 4 studies assessed the meditators as their owncontrol on a separate day or days. In one out of four studiesthe assignment to meditation or the control interventionwas random [5], whereas in the other 3 studies [1113] thesequence was fixed and hence the effect of one interventionon another could not be ruled out. Also a disadvantage of theself-as-control design is that meditators with long experiencein meditation may get into a meditative state even duringthe nonmeditation control sessions. Apart from the disadvan-tages in study design another disadvantage especially in theearly studies [1, 1416] was small sample sizes (range between5 and 15). This is not seen in more recent studies [12, 13]( = 60 and = 48, resp.). On the whole evoked potentialsoffer poor spatial and temporal resolution compared to fMRIandMEG. Also the localization of changes is restricted to thesensory pathway activated by the specific stimulus. Despitethese limitations evoked potentials continue to be usefulto studies on practices like meditation as they are far lessdistracting and do not involve a drastic change in posture asis required for fMRI.
In the ERP studies reviewed the number of multiple-group studies (12 studies) was higher than the EP studiesreviewed in the present study. However the evidence of theERP studies is also with similar limitations as none of theERP studies used randomization as a method of allocatingthe participants to different groups.
5. Limitations of the Present Study
Thepresent study has several limitations: (i) the present studydid not review studies onmeditation and EEG, (ii) the studiesonmeditation and EPswere not homogenous, (iii) the studiesreviewed in the present study were not categorized accordingto the Jadad scale, and (iv) the studies included in the presentstudy were searched from only one database, that is, PubMed.
10 BioMed Research International
6. Directions for Future ResearchSuggested by This Study
The present study has shown that EPs are useful in localizingchanges in meditation to areas such as the brainstem, tha-lamus, thalamocortical radiations, primary sensory cortices,and association cortical areas and ERPs can provide usefulinformation about neurocognitive processing of attentionand brain resource allocation. Most of the studies are limitedby small sample sizes, lack of proper controls, no objectiveway of assessing the quality ofmeditation, and awide range ofvariation in the practitioners sampled. This can be correctedin future studies. Also studies can be specifically designedto verify whether the findings of EP studies which suggestthat changes occur in the thalamus, thalamocortical connec-tions, and primary relays and enhancement in attention andincreased efficiency of brain resource allocation as suggestedbyERP studies are indeed correct. To verify these resultsmorerigorous studies with a better design, larger sample size andstudies of EPs and ERPs in combination with neuroimagingduring meditation are recommended.
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper.
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