Summary: A new study shows that different processes are responsible for maintaining relevant and suppressing irrelevant information. By promoting the suppression of irrelevant information, memories may be enhanced

Robertson, Edwin M.

English

SummaryA new study shows that different processes are responsible for maintaining relevant and suppressing irrelevant information. By promoting the suppression of irrelevant information, memories may be enhanced

Elsevier - Publisher Connector 

Brain Rhythms: Enhancing Memories 

Current Biology Vol 19 No 21
R992Brain Rhythms: Enhancing Memories
A new study shows that different processes are responsible for maintaining
relevant and suppressing irrelevant information. By promoting the suppression
of irrelevant information, memories may be enhanced.
Edwin M. Robertson
Everyday we are bombarded with
information from the media,
colleagues, spouses and friends. Some
of this information is relevant and
important, but other information is
irrelevant and should have no influence
on our behaviour. When listening to
the weather forecast, only the weather
predicted for our region is relevant to
our behaviour. Adapting our behaviour
to the predicted conditions — for
example, driving rather than walking
into work when severe rain is
forecast — may depend upon us
encoding the forecast in our region
and ignoring the weather conditions
in other regions. Distinct neural
processes, as reported by Sauseng
et al. [1] in this issue ofCurrent Biology,
are responsible for encoding relevant,
whilst suppressing irrelevant,
information.
The successful retention of relevant
information depends upon the
synchronization of specific brain
rhythms — a coupling between theta
and gamma rhythms — whereas the
suppression of irrelevant information
depends upon a single, distinct rhythm,
the alpha rhythm. The evidence for this
double dissociation comes not just
from correlations: a functional link has
been established between these brain
rhythms and memory processing. In
their work, Sauseng et al. [1] found that
promoting the alpha rhythm increased
suppression of irrelevant information
and so enhanced memory
performance. Thus, a simple model of
memory formation in which relevant
information is maintained and
irrelevant information is suppressed
has been shown through correlative
and causative work to have
a biological basis.
In the new study [1], participants
were asked to learn an array of
coloured squares appearing on one
half of the screen, while suppressing
the irrelevant coloured squares,
appearing on the other half of the
screen. Later, following a retention
interval, participants viewed a set of
‘probe’ stimuli and were asked to
decide whether those stimuli matched
the relevant stimuli. The use of this
probe tested whether learning of the
relevant coloured stimuli had occurred.
During the retention interval, the brain
rhythms associated with learning the
relevant and suppressing the irrelevant
stimuli were identified by recording
electrical brain activity (with an
electroencephalogram).
The retention of relevant information
was related to the synchronization
between theta and gamma oscillations.
Prior work has implicated theta
oscillations and their interaction with
gamma oscillations in the neuroplastic
processes that underlie memory
formation [2,3]. Promoting theta
oscillations can also enhance memory
formation [4]. Yet, to perform well on
this task, it was not sufficient to recall
relevant stimuli, but also necessary
to suppress irrelevant stimuli.
Suppression of information processing
has been associated with the alpha
rhythm, and consistent with those
earlier observations, the alpha
rhythm was found to be related to
the suppression of irrelevant stimuli
[1,5]. Thus, relevant information is
maintained by neural processes which
are distinct from those suppressing
irrelevant stimuli.
Observations from this study extend
beyond correlation to show a causative
link between alpha rhythms and the
suppression of irrelevant stimuli.
Transcranial magnetic stimulation
(TMS) is a technique which allows
the non-invasive stimulation of the
brain, and when applied at 10 Hz,
it has been shown to entrain alpha
rhythm [6,7]. Sauseng et al. [1] found
that promoting alpha rhythm, by
applying TMS at 10 Hz, increased
participants’ capacity to suppress
irrelevant stimuli. Thus, the alpha
rhythm is not a mere epiphenomenon
of memory processing but instead
seems likely to play a critical functional
role in the suppression of irrelevant
information.
An increasing number of studies
have shown that TMS and related
techniques can enhance human
performance (for example, [4,8–11]). Inthe work of Sauseng et al. [1], TMS was
used to enhance the alpha rhythm to
increase the suppression of irrelevant
information and so ultimately improve
performance (Figure 1) [1]. Other
studies have used non-invasive
stimulation techniques to directly
improve the plasticity of neural
circuits and so enhance memory
formation [11,12]. TMS has also been
used to disrupt a brain area that was
inhibiting another area, and through
this disinhibition mechanism produce
enhanced performance (for example,
[8,9]). A challenge for future studies
is to understand the relationship
amongst these different descriptions
of how TMS enhances performance
(Figure 1).
Each description could be a distinct
mechanism; alternatively, each
description may offer a different
perspective of the same underlying
mechanism. The latter possibility of
having only a single mechanism
explaining all of the effects of TMS,
whilst attractive from a parsimonious
perspective, seems unlikely. Enhanced
performance through suppressing
irrelevant items was dependent
upon the frequency at which TMS
was applied [1]; yet TMS can
disrupt a brain area at any frequency,
and so rivalry between brain areas
ought to be diminished, and so
performance enhanced, regardless
of stimulation frequency. Showing
that the enhancement of memory
is frequency-dependent, whilst
certainly not definitive, is an
important first step towards showing
that rivalry between brain areas and
the induction of alpha rhythms are
distinct mechanisms.
Future work will deepen our
understanding of the relationship
between artificial and naturally
occurring forms of memory
enhancements [13,14]. Memories can
be enhanced over sleep, and the
mechanisms engaged can, at least
in part, be successfully reproduced
to provide memory enhancement
over wakefulness [4]. Similarly,
the termination of other critical
processes — a disengagement
between memory systems — can
be successfully replicated over
wakefulness to produce memory
enhancements [9,15,16]. Thus, natural
and artificial mechanisms of memory
enhancement may have key
components in common.
Dispatch
R993The apparent ease with which human
performance can be enhanced implies
that the brain is consistently operating
below its maximum capacity. Such
inefficiency may at first appear odd;
after all, it would be beneficial to have
the greatest possible performance at
all times. However, this would be to
ignore the hidden cost of improving
human performance. For example,
a motor skill memory can be enhanced
by disrupting a declarative memory
[9,15,16]. Thus, improved motor
performance can occur at the cost of
losing another type of knowledge. Yet
retaining declarative knowledge for
a motor skill would allow the motor skill
to be described to other members of
a group ensuring that the skill can
benefit a wider society.
Redundantly encoding knowledge
can also have other benefits. Finding
your way around a new city involves
simultaneously learning spatial
landmarks and specific routes between
locations. Consequently, when spatial
landmarks are obscured, for example
by fog, it is still possible to navigate
based upon route learning. Redundant
encoding therefore provides flexibility
in responding to challenges from the
environment; however, this comes
with the cost of slowed learning due
to simultaneously acquiring knowledge
of both landmarks plus routes [17].
Suppressing irrelevant information
and so enhancing performance may
also come with a cost if initially
irrelevant information becomes
relevant. For instance, unbeknownst
to you, your boss plans to send you
out of the region that day; having
maintained only the weather forecast
for your region and suppressing those
for other regions prevents you from
responding flexibly to the new
situation. So, enhanced performance
may be limited to a particular context,
and come with the cost of impaired
performance outside of that context.
Perhaps any enhanced performance
comes at a cost which, in turn, implies
that impaired performance may have
hidden benefits. For instance, the
interference between memories when
they are acquired in quick succession
impairs recall; but this impairment
may come with benefits to memory
processing [18,19].
In summary, maintaining relevant
and suppressing irrelevant information
are correlated with and functionally
dependent upon distinct neural
processes. Understanding the brain
rhythms necessary for learning
provides insight into memory
organization, allowing the optimal
conditions for encoding a specific
memory to be recreated and so
performance enhanced. Artificially
enhancing performance may tap into
mechanisms such as those engaged
over sleep, which naturally enhance
memories. Any enhanced performance
may come with a cost — for example,
a reduced capacity to respond flexibly
to changing circumstances — while
impaired performance may have
hidden benefits for memory
processing.
References
1. Sauseng, P., Klimesch, W., Heise, K.,
Gruber, W.R., Holz, E., Karim, A.A.,
Glennon, M., Gerloff, C., Birbaumer, N., and
Hummel, F.C. (2009). Human brain
oscillations dissociate memory storage
2
4
1
TMS
Recall
A B
D
P
D
P
TMS
α rhythm
−
−
−
Δ 
du
e 
to
 T
M
S
Current Biology
Figure 1. Possible routes to memory enhancement.
(A) A sequence can be learnt as a procedural series of movements (P; button selections, grey
highlighted boxes), and as declarative knowledge (D) for the sequence (e.g., 2-4-1). Disrupting
the declarative knowledge for the sequence— for example, by applying TMS to the dorsolateral
prefrontal cortex — leads to an enhancement of procedural motor skill (see box graph) [9],
which implies that there is a competitive interaction between different types of memory
[15,16,20]. Similarly, there may be a competitive interaction between hemispheres (i.e., cerebral
rivalry). By applying TMS the balance of this rivalry is altered, producing, for example, improved
attention on one side [8]. (B) In contrast, a potentially distinct mechanism is to use TMS to
promote the alpha rhythm and so suppress information processing within a hemisphere. Sup-
pressing the processing of relevant information (cued to participants by an arrow; see above)
will impair performance, whereas, suppressing the processing of irrelevant information will
enhance performance [1].When applied to the right hemisphere TMSwill suppress the irrelevant
visual cues presented on the left visual field, and so diminish their recall (recall; see above).
Current Biology Vol 19 No 21
R994from information selection processes and
predict short-term memory capacity. Curr.
Biol. 19, 1846–1852.
2. Fell, J., Klaver, P., Elfadil, H., Schaller, C.,
Elger, C.E., and Fernandez, G. (2003).
Rhinal-hippocampal theta coherence during
declarative memory formation: interaction with
gamma synchronization? Eur. J. Neurosci. 17,
1082–1088.
3. Kahana, M.J. (2006). The cognitive correlates
of human brain oscillations. J. Neurosci. 26,
1669–1672.
4. Kirov, R., Weiss, C., Siebner, H.R., Born, J., and
Marshall, L. (2009). Slow oscillation electrical
brain stimulation during waking promotes
EEG theta activity and memory encoding.
Proc. Natl. Acad. Sci. USA 106,
15460–15465.
5. Buzsaki, G. (2006). Rhythms of the Brain
(Oxford: Oxford University Press).
6. Klimesch, W., Sauseng, P., and Gerloff, C.
(2003). Enhancing cognitive performance with
repetitive transcranial magnetic stimulation at
human individual alpha frequency. Eur. J.
Neurosci. 17, 1129–1133.
7. Thut, G., and Miniussi, C. (2009). New insights
into rhythmic brain activity from TMS-EEG
studies. Trends Cogn. Sci. 13, 182–189.
8. Hilgetag, C.C., Theoret, H., and
Pascual-Leone, A. (2001). Enhanced visual
spatial attention ipsilateral to rTMS-induced
‘virtual lesions’ of human parietal cortex.
Nat. Neurosci. 4, 953–957.
9. Galea, J.M., Albert, N.B., Ditye, T., and
Miall, R.C. (2009). Disruption of the dorsolateral
prefrontal cortex facilitates the consolidation of
procedural skills. J. Cogn. Neurosci. May 4
[Epub ahead of print].
10. Reis, J., Schambra, H.M., Cohen, L.G.,
Buch, E.R., Fritsch, B., Zarahn, E., Celnik, P.A.,
and Krakauer, J.W. (2009). Noninvasive cortical
stimulation enhances motor skill acquisition
over multiple days through an effect on
consolidation. Proc. Natl. Acad. Sci. USA
106, 1590–1595.
11. Hummel, F., Celnik, P., Giraux, P., Floel, A.,
Wu, W.H., Gerloff, C., and Cohen, L.G. (2005).
Effects of non-invasive cortical stimulation on
skilled motor function in chronic stroke.
Brain 128, 490–499.
12. Bolognini, N., Pascual-Leone, A., and Fregni, F.
(2009). Using non-invasive brain stimulation to
augment motor training-induced plasticity.
J. Neuroeng. Rehabil. 6, 8.
13. Maquet, P. (2001). The role of sleep in
learning and memory. Science 294,
1048–1052.
14. Walker, M.P., and Stickgold, R. (2004).
Sleep-dependent learning and memory
consolidation. Neuron 44, 121–133.
15. Brown, R.M., and Robertson, E.M. (2007).
Inducing motor skill improvements with
a declarative task. Nat. Neurosci. 10,
148–149.
16. Robertson, E.M. (2009). From creation to
consolidation: a novel framework for memory
processing. PLoS Biol. 7, e19.
17. Schroeder, J.P., Wingard, J.C., and
Packard, M.G. (2002). Post-training reversible
inactivation of hippocampus reveals
interference between memory systems.
Hippocampus 12, 280–284.
18. Robertson, E.M., Pascual-Leone, A., and
Miall, R.C. (2004). Current concepts in
procedural consolidation. Nat. Rev. Neurosci.
5, 576–582.
19. Lee, J.L. (2008). Memory reconsolidation
mediates the strengthening of memories by
additional learning. Nat. Neurosci. 11,
1264–1266.
20. Brown, R.M., and Robertson, E.M. (2007).
Off-line processing: reciprocal interactions
between declarative and procedural memories.
J. Neurosci. 27, 10468–10475.
Berneson-Allen Center for Non-Invasive
Brain Stimulation, Harvard Medical School,
Boston, MA 02215, USA.
E-mail: emrobert@caregroup.harvard.edu
DOI: 10.1016/j.cub.2009.09.034


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Brain rhythms: enhancing memories

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Current Biology Vol 19 No 21R992Brain Rhythms: Enhancing MemoriesA new study shows that different processes are responsible for maintainingrelevant and suppressing irrelevant information. By promoting the suppressionof irrelevant information, memories may be enhanced.Edwin M. RobertsonEveryday we are bombarded withinformation from the media,colleagues, spouses and friends. Someof this information is relevant andimportant, but other information isirrelevant and should have no influenceon our behaviour. When listening tothe weather forecast, only the weatherpredicted for our region is relevant toour behaviour. Adapting our behaviourto the predicted conditions — forexample, driving rather than walkinginto work when severe rain isforecast — may depend upon usencoding the forecast in our regionand ignoring the weather conditionsin other regions. Distinct neuralprocesses, as reported by Sausenget al. [1] in this issue ofCurrent Biology,are responsible for encoding relevant,whilst suppressing irrelevant,information.The successful retention of relevantinformation depends upon thesynchronization of specific brainrhythms — a coupling between thetaand gamma rhythms — whereas thesuppression of irrelevant informationdepends upon a single, distinct rhythm,the alpha rhythm. The evidence for thisdouble dissociation comes not justfrom correlations: a functional link hasbeen established between these brainrhythms and memory processing. Intheir work, Sauseng et al. [1] found thatpromoting the alpha rhythm increasedsuppression of irrelevant informationand so enhanced memoryperformance. Thus, a simple model ofmemory formation in which relevantinformation is maintained andirrelevant information is suppressedhas been shown through correlativeand causative work to havea biological basis.In the new study [1], participantswere asked to learn an array ofcoloured squares appearing on onehalf of the screen, while suppressingthe irrelevant coloured squares,appearing on the other half of thescreen. Later, following a retentioninterval, participants viewed a set of‘probe’ stimuli and were asked todecide whether those stimuli matchedthe relevant stimuli. The use of thisprobe tested whether learning of therelevant coloured stimuli had occurred.During the retention interval, the brainrhythms associated with learning therelevant and suppressing the irrelevantstimuli were identified by recordingelectrical brain activity (with anelectroencephalogram).The retention of relevant informationwas related to the synchronizationbetween theta and gamma oscillations.Prior work has implicated thetaoscillations and their interaction withgamma oscillations in the neuroplasticprocesses that underlie memoryformation [2,3]. Promoting thetaoscillations can also enhance memoryformation [4]. Yet, to perform well onthis task, it was not sufficient to recallrelevant stimuli, but also necessaryto suppress irrelevant stimuli.Suppression of information processinghas been associated with the alpharhythm, and consistent with thoseearlier observations, the alpharhythm was found to be related tothe suppression of irrelevant stimuli[1,5]. Thus, relevant information ismaintained by neural processes whichare distinct from those suppressingirrelevant stimuli.Observations from this study extendbeyond correlation to show a causativelink between alpha rhythms and thesuppression of irrelevant stimuli.Transcranial magnetic stimulation(TMS) is a technique which allowsthe non-invasive stimulation of thebrain, and when applied at 10 Hz,it has been shown to entrain alpharhythm [6,7]. Sauseng et al. [1] foundthat promoting alpha rhythm, byapplying TMS at 10 Hz, increasedparticipants’ capacity to suppressirrelevant stimuli. Thus, the alpharhythm is not a mere epiphenomenonof memory processing but insteadseems likely to play a critical functionalrole in the suppression of irrelevantinformation.An increasing number of studieshave shown that TMS and relatedtechniques can enhance humanperformance (for example, [4,8–11]). Inthe work of Sauseng et al. [1], TMS wasused to enhance the alpha rhythm toincrease the suppression of irrelevantinformation and so ultimately improveperformance (Figure 1) [1]. Otherstudies have used non-invasivestimulation techniques to directlyimprove the plasticity of neuralcircuits and so enhance memoryformation [11,12]. TMS has also beenused to disrupt a brain area that wasinhibiting another area, and throughthis disinhibition mechanism produceenhanced performance (for example,[8,9]). A challenge for future studiesis to understand the relationshipamongst these different descriptionsof how TMS enhances performance(Figure 1).Each description could be a distinctmechanism; alternatively, eachdescription may offer a differentperspective of the same underlyingmechanism. The latter possibility ofhaving only a single mechanismexplaining all of the effects of TMS,whilst attractive from a parsimoniousperspective, seems unlikely. Enhancedperformance through suppressingirrelevant items was dependentupon the frequency at which TMSwas applied [1]; yet TMS candisrupt a brain area at any frequency,and so rivalry between brain areasought to be diminished, and soperformance enhanced, regardlessof stimulation frequency. Showingthat the enhancement of memoryis frequency-dependent, whilstcertainly not definitive, is animportant first step towards showingthat rivalry between brain areas andthe induction of alpha rhythms aredistinct mechanisms.Future work will deepen ourunderstanding of the relationshipbetween artificial and naturallyoccurring forms of memoryenhancements [13,14]. Memories canbe enhanced over sleep, and themechanisms engaged can, at leastin part, be successfully reproducedto provide memory enhancementover wakefulness [4]. Similarly,the termination of other criticalprocesses — a disengagementbetween memory systems — canbe successfully replicated overwakefulness to produce memoryenhancements [9,15,16]. Thus, naturaland artificial mechanisms of memoryenhancement may have keycomponents in common.DispatchR993The apparent ease with which humanperformance can be enhanced impliesthat the brain is consistently operatingbelow its maximum capacity. Suchinefficiency may at first appear odd;after all, it would be beneficial to havethe greatest possible performance atall times. However, this would be toignore the hidden cost of improvinghuman performance. For example,a motor skill memory can be enhancedby disrupting a declarative memory[9,15,16]. Thus, improved motorperformance can occur at the cost oflosing another type of knowledge. Yetretaining declarative knowledge fora motor skill would allow the motor skillto be described to other members ofa group ensuring that the skill canbenefit a wider society.Redundantly encoding knowledgecan also have other benefits. Findingyour way around a new city involvessimultaneously learning spatiallandmarks and specific routes betweenlocations. Consequently, when spatiallandmarks are obscured, for exampleby fog, it is still possible to navigatebased upon route learning. Redundantencoding therefore provides flexibilityin responding to challenges from theenvironment; however, this comeswith the cost of slowed learning dueto simultaneously acquiring knowledgeof both landmarks plus routes [17].Suppressing irrelevant informationand so enhancing performance mayalso come with a cost if initiallyirrelevant information becomesrelevant. For instance, unbeknownstto you, your boss plans to send youout of the region that day; havingmaintained only the weather forecastfor your region and suppressing thosefor other regions prevents you fromresponding flexibly to the newsituation. So, enhanced performancemay be limited to a particular context,and come with the cost of impairedperformance outside of that context.Perhaps any enhanced performancecomes at a cost which, in turn, impliesthat impaired performance may havehidden benefits. For instance, theinterference between memories whenthey are acquired in quick successionimpairs recall; but this impairmentmay come with benefits to memoryprocessing [18,19].In summary, maintaining relevantand suppressing irrelevant informationare correlated with and functionallydependent upon distinct neuralprocesses. Understanding the brainrhythms necessary for learningprovides insight into memoryorganization, allowing the optimalconditions for encoding a specificmemory to be recreated and soperformance enhanced. Artificiallyenhancing performance may tap intomechanisms such as those engagedover sleep, which naturally enhancememories. Any enhanced performancemay come with a cost — for example,a reduced capacity to respond flexiblyto changing circumstances — whileimpaired performance may havehidden benefits for memoryprocessing.References1. Sauseng, P., Klimesch, W., Heise, K.,Gruber, W.R., Holz, E., Karim, A.A.,Glennon, M., Gerloff, C., Birbaumer, N., andHummel, F.C. (2009). Human brainoscillations dissociate memory storage241TMSRecallA BDPDPTMSα rhythm−−−Δ due to TMSCurrent BiologyFigure 1. Possible routes to memory enhancement.(A) A sequence can be learnt as a procedural series of movements (P; button selections, greyhighlighted boxes), and as declarative knowledge (D) for the sequence (e.g., 2-4-1). Disruptingthe declarative knowledge for the sequence— for example, by applying TMS to the dorsolateralprefrontal cortex — leads to an enhancement of procedural motor skill (see box graph) [9],which implies that there is a competitive interaction between different types of memory[15,16,20]. Similarly, there may be a competitive interaction between hemispheres (i.e., cerebralrivalry). By applying TMS the balance of this rivalry is altered, producing, for example, improvedattention on one side [8]. (B) In contrast, a potentially distinct mechanism is to use TMS topromote the alpha rhythm and so suppress information processing within a hemisphere. Sup-pressing the processing of relevant information (cued to participants by an arrow; see above)will impair performance, whereas, suppressing the processing of irrelevant information willenhance performance [1].When applied to the right hemisphere TMSwill suppress the irrelevantvisual cues presented on the left visual field, and so diminish their recall (recall; see above).Current Biology Vol 19 No 21R994from information selection processes andpredict short-term memory capacity. Curr.Biol. 19, 1846–1852.2. Fell, J., Klaver, P., Elfadil, H., Schaller, C.,Elger, C.E., and Fernandez, G. (2003).Rhinal-hippocampal theta coherence duringdeclarative memory formation: interaction withgamma synchronization? Eur. J. Neurosci. 17,1082–1088.3. Kahana, M.J. (2006). The cognitive correlatesof human brain oscillations. J. Neurosci. 26,1669–1672.4. Kirov, R., Weiss, C., Siebner, H.R., Born, J., andMarshall, L. (2009). Slow oscillation electricalbrain stimulation during waking promotesEEG theta activity and memory encoding.Proc. Natl. Acad. Sci. USA 106,15460–15465.5. Buzsaki, G. (2006). Rhythms of the Brain(Oxford: Oxford University Press).6. Klimesch, W., Sauseng, P., and Gerloff, C.(2003). Enhancing cognitive performance withrepetitive transcranial magnetic stimulation athuman individual alpha frequency. Eur. J.Neurosci. 17, 1129–1133.7. Thut, G., and Miniussi, C. (2009). New insightsinto rhythmic brain activity from TMS-EEGstudies. Trends Cogn. Sci. 13, 182–189.8. Hilgetag, C.C., Theoret, H., andPascual-Leone, A. (2001). Enhanced visualspatial attention ipsilateral to rTMS-induced‘virtual lesions’ of human parietal cortex.Nat. Neurosci. 4, 953–957.9. Galea, J.M., Albert, N.B., Ditye, T., andMiall, R.C. (2009). Disruption of the dorsolateralprefrontal cortex facilitates the consolidation ofprocedural skills. J. Cogn. Neurosci. May 4[Epub ahead of print].10. Reis, J., Schambra, H.M., Cohen, L.G.,Buch, E.R., Fritsch, B., Zarahn, E., Celnik, P.A.,and Krakauer, J.W. (2009). Noninvasive corticalstimulation enhances motor skill acquisitionover multiple days through an effect onconsolidation. Proc. Natl. Acad. Sci. USA106, 1590–1595.11. Hummel, F., Celnik, P., Giraux, P., Floel, A.,Wu, W.H., Gerloff, C., and Cohen, L.G. (2005).Effects of non-invasive cortical stimulation onskilled motor function in chronic stroke.Brain 128, 490–499.12. Bolognini, N., Pascual-Leone, A., and Fregni, F.(2009). Using non-invasive brain stimulation toaugment motor training-induced plasticity.J. Neuroeng. Rehabil. 6, 8.13. Maquet, P. (2001). The role of sleep inlearning and memory. Science 294,1048–1052.14. Walker, M.P., and Stickgold, R. (2004).Sleep-dependent learning and memoryconsolidation. Neuron 44, 121–133.15. Brown, R.M., and Robertson, E.M. (2007).Inducing motor skill improvements witha declarative task. Nat. Neurosci. 10,148–149.16. Robertson, E.M. (2009). From creation toconsolidation: a novel framework for memoryprocessing. PLoS Biol. 7, e19.17. Schroeder, J.P., Wingard, J.C., andPackard, M.G. (2002). Post-training reversibleinactivation of hippocampus revealsinterference between memory systems.Hippocampus 12, 280–284.18. Robertson, E.M., Pascual-Leone, A., andMiall, R.C. (2004). Current concepts inprocedural consolidation. Nat. Rev. Neurosci.5, 576–582.19. Lee, J.L. (2008). Memory reconsolidationmediates the strengthening of memories byadditional learning. Nat. Neurosci. 11,1264–1266.20. Brown, R.M., and Robertson, E.M. (2007).Off-line processing: reciprocal interactionsbetween declarative and procedural memories.J. Neurosci. 27, 10468–10475.Berneson-Allen Center for Non-InvasiveBrain Stimulation, Harvard Medical School,Boston, MA 02215, USA.E-mail: emrobert@caregroup.harvard.eduDOI: 10.1016/j.cub.2009.09.034

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Brain rhythms: enhancing memories

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