4022 • The Journal of Neuroscience, March 12, 2014 • 34(11):4022– 4026
Brief Communications
Shaping Memory Accuracy by Left Prefrontal Transcranial
Direct Current Stimulation
Bastian Zwissler,1 Christoph Sperber,1 Sina Aigeldinger,1 Sebastian Schindler,2 Johanna Kissler,2
and Christian Plewnia1,3
Department of Psychiatry and Psychotherapy, Neurophysiology & Interventional Neuropsychiatry, University of Tübingen, 72076 Tübingen, Germany,
Department of Psychology, Affective Neuropsychology, University of Bielefeld, 33501 Bielefeld, Germany, and 3Werner Reichardt Centre for Integrative
Neuroscience, University of Tübingen, 72076 Tübingen, Germany
1
2
Human memory is dynamic and flexible but is also susceptible to distortions arising from adaptive as well as pathological processes. Both
accurate and false memory formation require executive control that is critically mediated by the left prefrontal cortex (PFC). Transcranial
direct current stimulation (tDCS) enables noninvasive modulation of cortical activity and associated behavior. The present study reports
that tDCS applied to the left dorsolateral PFC (dlPFC) shaped accuracy of episodic memory via polaritiy-specific modulation of false
recognition. When applied during encoding of pictures, anodal tDCS increased whereas cathodal stimulation reduced the number of false
alarms to lure pictures in subsequent recognition memory testing. These data suggest that the enhancement of excitability in the dlPFC
by anodal tDCS can be associated with blurred detail memory. In contrast, activity-reducing cathodal tDCS apparently acted as a noise
filter inhibiting the development of imprecise memory traces and reducing the false memory rate. Consistently, the largest effect was
found in the most active condition (i.e., for stimuli cued to be remembered). This first evidence for a polarity-specific, activity-dependent
effect of tDCS on false memory opens new vistas for the understanding and potential treatment of disturbed memory control.
Key words: brain stimulation; dorsolateral prefrontal cortex; executive functions; false memory; memory encoding; neuroenhancement
Introduction
Memory is a dynamic and sometimes creative process. The formation of episodic memories is especially prone to distortions
and errors, resulting in retrieved memories being markedly different from those that were initially encoded (Schacter and Slotnick, 2004). These inaccuracies predominantly concern the
details of memories instead of their “gist” or general thematic
content (Brainerd and Reyna, 2005; Payne et al., 2006), which
may be outcomes of an adaptive and economical rather than a
defective process (Schacter and Addis, 2007). The preference for
memory gist, however, can hamper performance in situations
requiring memory precision. Therefore, a balance between the
competing demands of memory efficiency, memory accuracy,
and its executive control ensures successful learning and behavior. The left dorsolateral prefrontal cortex (dlPFC), in particular,
Received Dec. 23, 2013; revised Jan. 21, 2014; accepted Feb. 9, 2014.
Author contributions: B.Z. and C.P. designed research; B.Z., C.S., S.A., and C.P. performed research; B.Z. and C.P.
analyzed data; B.Z., S.S., J.K., and C.P. wrote the paper.
This article was supported by the Werner Reichardt Centre for Integrative Neuroscience (CIN) at the Eberhard
Karls University of Tübingen. The CIN is an Excellence Cluster funded by the Deutsche Forschungsgemeinschaft (DFG)
within the framework of the Excellence Initiative (EXC 307). B.Z. was funded by the University Hospital Tübingen
(fortüne; 2086-1-0). C.P. was supported by the German Research Council (Deutsche Forschungsgemeinschaft; PL
525/1-1) and the CIN (PP 2011_11).
The authors declare no competing financial interests.
This article is freely available online through the J Neurosci Author Open Choice option.
Correspondence should be addressed to Dr. Christian Plewnia, Department of Psychiatry and Psychotherapy,
Neurophysiology & Interventional Neuropsychiatry, University of Tübingen, Calwerstrasse 14, D-72076 Tübingen,
Germany. E-mail:
[email protected].
DOI:10.1523/JNEUROSCI.5407-13.2014
Copyright © 2014 the authors 0270-6474/14/344022-05$15.00/0
has been shown to exert executive control over the acquisition of
episodic memories, presumably by maintaining information required for the formation of useful episodic traces (Rossi et al.,
2011; Hawco et al., 2013). However, imaging studies also suggest
that the left PFC is critically involved in the formation of both
accurate and false episodic memories (Kubota et al., 2006; Kim
and Cabeza, 2007).
To further advance knowledge on the dynamics of episodic
memory accuracy and the role of dlPFC activity in memory control, this study used transcranial direct current stimulation
(tDCS), which induces transient polarity-specific changes in cerebral excitability via weak electric currents applied to the scalp.
As initially documented in the motor system (Nitsche and
Paulus, 2000), cathodal tDCS decreases and anodal stimulation
increases, respectively, neuronal excitability and spontaneous firing rate by altering resting membrane potential. This technique
has since been widely used to explore brain network dynamics
and organization (Dayan et al., 2013), particularly concerning
cognition (Kuo and Nitsche, 2012) and memory (Brasil-Neto,
2012). Moreover, tDCS enables to enhance adaptive and to ameliorate maladaptive neuroplastic processes with potentially therapeutic effects on various neuropsychiatric disorders (Kuo et al.,
2014). However, the initial simple concept of “beneficial anodal”
and “inhibitory cathodal” effects of tDCS has not been confirmed
in the cognitive domain (Jacobson et al., 2012). Rather, it has
been suggested that tDCS exerts its effects predominantly by
modulating neuronal signal-to-noise ratio, with anodal tDCS increasing and cathodal tDCS decreasing noise leading to variable
Zwissler et al. • Shaping Memory Accuracy by Left Prefrontal tDCS
J. Neurosci., March 12, 2014 • 34(11):4022– 4026 • 4023
Figure 1. Experimental procedure, placement of tDCS electrodes, course of stimulus encoding/retrieval, cues, and sample pictures.
effects on cognitive performance (Antal et al., 2004; Dockery et
al., 2009; Miniussi et al., 2013). Importantly, this mechanism is
state-dependent, with the state of background brain activity predicting the functional relevance of noise addition or filtering by
tDCS (Miniussi et al., 2013).
In this study, we apply anodal, cathodal, and sham tDCS to the
left dlPFC during the presentation and encoding of images. Different postimage instructions for cognitive processing to control
participants’ focus within working memory (Nee and Jonides,
2009; Gazzaley and Nobre, 2012) are used to investigate the
brain-state dependency of tDCS effects. With an old/new recognition task, memory accuracy is quantified by correct and false
recognition rates. In sum, we sought to provide evidence for a
polarity-specific malleability of memory accuracy by tDCS and
its interaction with instruction-induced cognitive activity.
Materials and Methods
Participants. A total of 96 individuals (60 female; mean age 24.82 ⫾ 2.95
years) gave written informed consent to participate in the study. The
institutional ethical committee approved the protocol, and the study
was conducted in compliance with the Declaration of Helsinki. Participants, who were all right-handed according to the Edinburgh
Handedness Inventory (Oldfield, 1971; score ⬎ 40), were randomly
assigned to anodal (n ⫽ 24), cathodal (n ⫽ 24), or sham (n ⫽ 48)
stimulation. To account for double-blindedness and all experimental
variations (i.e., instruction assignment, image set assignment) under
all stimulation conditions, sham controls were required for both anodal and cathodal stimulation.
Semistructured interviews were conducted to identify participants
who receive frequent medical care or have psychological, psychiatric, or
neurological preconditions (e.g., psychotherapy, regular medication [except contraceptives], epileptic seizures, brain injuries, implants). Individuals for whom we could not clearly rule out all of these conditions
were not included in the experiment. As a consequence, one subject was
excluded from the study because of a concussion after a car accident. One
participant was identified as ambidextrous and therefore deselected. For
nine other participants, stimulation was terminated because of high impedance (⬎5 k⍀; as defined in the default mode of the stimulation device). Therefore, a total of 85 participants were included in the analyses
(anodal: n ⫽ 24, 14 females, mean age 25.33 years; cathodal: n ⫽ 22, 15
females, mean age 24.41 years; sham: n ⫽ 39, 22 females, mean age 24.87
years). The three groups did not differ in terms of gender (Pearson’s
2(2, N ⫽ 85) ⫽ 0.851, p ⫽ 0.654) and age (F(2,82) ⫽ 0.656; p ⫽ 0.521).
Participants received course credit or 3€ as a basic compensation and
earned an additional performance-dependent bonus.
tDCS. Direct current was generated by a battery-driven stimulator
(DC-STIMULATOR PLUS, NeuroConn) and delivered with a pair of
identical 5 ⫻ 7 cm 2 rubber electrodes covered with saline-soaked
sponges. Stimulation lasted 15 min (including a 5 min pretask idle time)
with a current of 1 mA, resulting in a current density of 0.028 mA/cm 2.
Stimulation was faded in and out with a 5 s ramp. For all participants, the
first electrode was placed on the left dlPFC (F3 according to the 10 –20
EEG system of electrode placement) and fastened with a standard EEG
cap, and the reference electrode was placed extracranially on the contralateral musculus deltoideus to avoid an opposite polarization in another brain area and thus ensure that tDCS effects could be traced back
exclusively to stimulation of the left dlPFC (Wolkenstein and Plewnia,
2013). Sham stimulation lasted for 30 s. Predefined codes assigned to
sham or verum stimulation were used to start the stimulation, allowing a
double-blind study design.
Procedure. After the start of the stimulation and a 5 min idle time,
participants began the learning phase of the experimental session (for
illustration of experimental design, see Fig. 1). The experimenter asked
participants to look closely at a randomized series of 90 neutral images
(2000 ms per image) showing everyday situations and objects (which
were pretested and rated in a previous study) (Zwissler et al., 2011).
Different postimage instructions for cognitive processing were used to
investigate the brain-state dependency of tDCS effects on memory accuracy: an active learning condition, as well as an active (Wylie et al., 2008)
and an inactive control condition. More concretely, each image was followed by one of three symbolic, single-colored cues (2000 ms per cue;
circle, triangle, or square). Two cues were instructed as relevant to successful task performance (active conditions), with one meaning “remember the preceding image” (R) and the other meaning “forget the
preceding image” (F). The third cue (passive condition) was not further
commented on (“irrelevant,” I). If participants did not understand the
instruction, the experimenter repeated that relevant cues should be focused on. Assignment of cue color, shape, and meaning was randomized
and balanced across stimulation conditions. After the learning phase,
participants were asked to perform a distractive attention test (“d2”;
Brickenkamp, 1994) to prevent further elaboration on stimuli from the
learning phase.
The “d2” (including its instructions) and the instructions for the old/
new recognition phase took 11 min. Previously seen images were intermingled with new, individually matched distracter images that differed
from the original images in detail but not in central aspects (i.e., gist).
Participants were told that they should try to accurately identify ALL
4024 • J. Neurosci., March 12, 2014 • 34(11):4022– 4026
Zwissler et al. • Shaping Memory Accuracy by Left Prefrontal tDCS
Figure 2. a, Correct recognition rates across groups and instructions. b, False alarm rates across groups and instructions. Error bars indicate SEM. R, pictures instructed to remember; F, pictures
instructed to forget; I, pictures designated as irrelevant. Error bars indicate SE. *p ⬍ 0.05. **p ⬍ 0.01. ***p ⬍ 0.001.
previously seen images regardless of their original instruction. Furthermore, they would earn 0.2€ for each correctly recognized image but
would lose the 0.2€ for each incorrectly recognized image. Therefore,
perfect performance could result in a maximum of 18€ (90 ⫻ 0.2€). This
procedure served to reinforce recognition accuracy and discourage
guessing. The 90 images presented during the learning phase and 90 new,
individually matched, and highly similar distracter images were displayed in a randomized order. Each image was shown for 300 ms, and
participants were instructed to decide as quickly as possible whether they
had previously seen the image. Although fast responses were encouraged,
there was no time limit. After a response was given, a fixation cross was
presented for 700 ms before the next image appeared. Experimental material was presented on a desktop computer (HP Compaq dc 7600) using
Presentation Software (Neurobehavioral Systems).
Statistics. Statistical analysis was performed using SPSS version 20.0
software (SPSS; www.spss.com). Data were tested for normal distribution (Shapiro-Wilk test) and sphericity (Mauchly’s test). Normal distribution was confirmed for all data, but there were a few expected
violations to sphericity that were corrected using Greenhouse-Geisser
adjustments. Analyses were performed using repeated-measures
ANOVAs with stimulationANODAL,SHAM,CATHODAL (three levels: anodal,
sham, cathodal) as a between-participants factor and instructionR,F,I
(three levels: remember, forget, irrelevant) as a within-participants factor, yielding a 3 ⫻ 3 matrix. Significant interaction effects were followed
up by one-way ANOVAs. Because of experimental groups of different
sizes, post hoc Student’s t tests were calculated using the Sidak correction
for multiple comparisons. An ␣ level of 0.05 was used for all statistical
tests.
Results
Correct and false recognition patterns (Fig. 2a,b) indicate that the
effect of tDCS on memory accuracy is the result of the modulation of false recognition. For correct recognition (Fig. 2a), only a
main effect of instructionR,F,I (F(2,164) ⫽ 61.92; p ⬍ 0.001; 2 ⫽
0.43) was found, replicating the classic directed forgetting
phenomenon (Basden et al., 1993). No instructionR,F,I ⫻ stimulationANODAL,SHAM,CATHODAL interaction (F(4,164) ⫽ 0.55, p ⫽ 0.68;
2 ⫽ 0.01) was observed.
By contrast, for false recognition (i.e., identifying an image
not previously seen as “old”; Fig. 2b), we found main effects of
both stimulationANODAL,SHAM,CATHODAL (F(2,82) ⫽ 7.01, p ⬍ 0.01;
2 ⫽ 0.15) and instructionR,F,I (F(2,164) ⫽ 29.86, p ⬍ 0.001; 2 ⫽
0.26), with F lures yielding significantly less false alarms than
both R ( p ⬍ 0.001; Cohen’s d ⫽ 0.74) and I ( p ⬍ 0.001; Cohen’s
d ⫽ 0.98) lures, which, in turn, did not differ ( p ⫽ 0.42; Cohen’s
d ⫽ 0.17). More importantly, there was also an interaction
of instructionR,F,I ⫻ stimulationANODAL,SHAM,CATHODAL (F(4,164) ⫽
2.81, p ⬍ 0.05; 2 ⫽ 0.06) (Fig. 2b). The effect of tDCS on false
recognition was most prominent in images instructed to-beremembered (R; stimulationANODAL,SHAM,CATHODAL: F(2,82) ⫽
10.14, p ⬍ 0.001; 2 ⫽ 0.20). Cathodal stimulation led to less
false recognition than sham ( p ⬍ 0.05; Cohen’s d ⫽ 0.72) or
anodal ( p ⬍ 0.001; Cohen’s d ⫽ 1.33) stimulation, and anodal
stimulation led to more false recognition than sham stimulation
( p ⬍ 0.05; Cohen’s d ⫽ 0.61). In images instructed to-beforgotten (F; stimulationANODAL,SHAM,CATHODAL: F(2,82) ⫽ 3.42,
p ⬍ 0.05; 2 ⫽ 0.08), anodal stimulation led to more false
recognition than sham stimulation ( p ⬍ 0.05; Cohen’s d ⫽
0.67), but there were no differences between cathodal and
sham ( p ⫽ 0.98) or cathodal and anodal ( p ⫽ 0.17) stimulation. No modulatory tDCS effects were found in irrelevant
images (I; stimulationANODAL,SHAM,CATHODAL: F(2,82) ⫽ 1.31,
p ⫽ 0.28; 2 ⫽ 0.03).
Importantly, there was no effect of stimulationANODAL,SHAM,CATHODAL on acquiescence bias (i.e., proportion of “yes” responses) (anodal, 0.45 ⫾ 0.07; sham, 0.43 ⫾ 0.08;
cathodal, 0.43 ⫾ 0.09; F(2,82) ⫽ 0.89, p ⫽ 0.42; 2 ⫽ 0.02). Moreover, tDCS did not affect reaction times for correct recognition
(anodal, 1183.01 ⫾ 37.77 ms; sham, 1156.13 ⫾ 29.63 ms; cathodal, 1201.26 ⫾ 39.44 ms; F(2,82) ⫽ 0.45, p ⫽ 0.64; 2 ⫽ 0.01) or
false recognition (anodal, 1260.50 ⫾ 47.21 ms; sham, 1266.47 ⫾
37.52 ms; cathodal, 1330.79 ⫾ 49.31 ms; F(2,82) ⫽ 0.68, p ⫽ 0.51;
2 ⫽ 0.02).
There was no effect of stimulationANODAL,SHAM,CATHODAL on
the performance in the attention test (F(2,82) ⫽ 0.56, p ⫽ 0.57;
2 ⫽ 0.01).
Discussion
The present findings are the first evidence of a polarity-specific,
activity-dependent malleability of memory accuracy by tDCS of
the left dlPFC. They add substantially to the understanding of the
functional neuroanatomy and neuronal processes underlying
memory accuracy, the feasibility and conditions of tDCS effects
on episodic memory, and the critical role of brain activation state
in the outcome of tDCS interventions.
However, some limitations of this study should be considered.
First, these findings are limited to pictorial episodic information.
Although these results may transfer to verbal and semantic information, this would need to be demonstrated by future studies.
Second, the topographic specificity of tDCS is relatively low, and
potential remote effects of stimulation have to be taken into ac-
Zwissler et al. • Shaping Memory Accuracy by Left Prefrontal tDCS
count. Although we aimed for a stimulation of the dlPFC, the size
of the stimulating electrode (35 cm 2) and individual variations in
anatomy do not rule out that adjacent areas of the left frontal
cortex were also affected by the stimulation. Moreover, transsynaptic effects of tDCS involving connected brain areas have also
been described (Lang et al., 2005; Chib et al., 2013). In particular,
it is conceivable that tDCS may modulate the influence of the left
dlPFC on remote brain regions (e.g., the medial temporal lobe)
that are actually responsible for encoding episodic memories
(Reber et al., 2002). However, previous tDCS studies on executive
control functions (Priori et al., 2008; Wolkenstein and Plewnia,
2013) have shown that the extracephalic localization of the reference electrode limiting the direct stimulation of other cortical
areas is effective. Third, tDCS effects outlasting the concurrent
stimulation and encoding phase may have also affected memory
retrieval. However, modulation of accuracy was most prominent
on items that received the “remember” instruction and thus
should have most likely taken place in the encoding phase.
Nevertheless, it could be conceivable that the likelihood of
identifying a new image as old was also influenced by interactions of prolonged tDCS effects and memories of different
strength. Therefore, the effects of tDCS on memory accuracy
cannot be attributed unambiguously to the encoding phase
alone. Nonetheless, a direct interaction between tDCS and
memory encoding is most likely because it does not require to
assume tDCS effects that outlast the attention test to interact
differentially with memory traces established according to the
specific encoding instruction.
In general, the present results suggest that the modulation of
neuronal excitability in the left prefrontal cortex interferes with
the regulation of memory encoding. It is important to note that
the effects were induced not only in the presence of stimuli but
particularly during their maintenance in working memory (Gazzaley and Nobre, 2012). In our study, memory encoding took
place in the absence of the images dependent to the cues presented after the stimulus. These retro-cues have been shown to
interact with memory encoding by reducing memory load (Duarte et al., 2013). It is most likely that the different instructions
initiate different encoding processes involving more or less elaborated rehearsal (Goodwin, 2007) of the presented pictures. Selective rehearsal of stimuli has been suggested to be a relevant
mechanism of memory control, particularly by improving discrimination accuracy of verbal and nonverbal material (Greene,
1987; Hourihan et al., 2009; Zwissler et al., 2011). Semantic elaboration (i.e., the integration of new information with semantic
knowledge) not only enhances episodic memory by involving the
left prefrontal cortex (Staresina et al., 2009) but also increases the
probability of falsely remembering previously unpresented
associates (Kim and Cabeza, 2007). The critical interaction of
elaborative cognitive processing and stimulation for memory
formation has been recently shown with repetitive transcranial
magnetic stimulation during memory encoding (Hawco et al.,
2013). High-strategy users showed reduced performance after
dlPFC stimulation, whereas low-strategy users tended to show
increased recall after dlPFC stimulation. However, the present
data do not allow for a differentiation between specific encoding
processes and their interaction with tDCS.
Notably, the polarity-specific alteration of false memory rate
demonstrates the feasibility of directed modulation of memory
accuracy by tDCS. Contrary to the simple mechanistic concept of
anodal stimulation as activity-enhancing and thus beneficial, we
found that anodal stimulation decreased and cathodal stimulation increased memory accuracy, respectively, by opposing mod-
J. Neurosci., March 12, 2014 • 34(11):4022– 4026 • 4025
ulations of false recognition rates. It has been proposed that the
modulation of neuronal excitability by tDCS is associated with
increases (anodal) or decreases (cathodal) in the amount of noise
in the stimulated structures (Antal et al., 2004; Dockery et al.,
2009; Miniussi et al., 2013). In the present study, the reduction of
noise by cathodal tDCS may have increased participants’ focus on
information encoded during stimulation (Weiss and Lavidor,
2012), leading to inhibited formation of false memory traces and
improved encoding of image details that are less likely to be activated by images with similar gist. By contrast, anodal tDCS may
have enhanced and further spread brain activity associated with
image processing by adding noise during memory encoding, resulting in less precise memories and thus a greater false recognition rate.
In this context, it is important to consider that the instruction
to forget the presented image primarily led to a decreased falsememory rate independent of stimulation (Fig. 2b). This points
toward an activation of inhibitory processes, particularly because
related lures of stimuli designated as irrelevant were more often
wrongly recognized as “old.” With anodal tDCS, this effect has
been counteracted, suggesting that additional activation to the
left prefrontal cortex interferes with a mechanism inhibiting
memory encoding susceptible for distortions. In turn, cathodal
stimulation associated with the instruction “forget” did not yield
a further inhibition of memory formation prone for errors, perhaps because of a floor effect. However, it has to be recognized
that, despite a clear polarity-specific modulation of false recognition, neither anodal nor cathodal stimulation exerted any effects
on correct recognition. Future studies might test whether, as it
seems, the threshold for stimulation effects differs for correct and
false recognition.
Finally, our finding that the modulatory effect of tDCS was
most prominent in the R condition, less prominent in the F condition, and absent in the I condition, underscores the critical
interaction of brain stimulation effects on ongoing brain activity
(Dockery et al., 2009; Andrews et al., 2011; Bikson et al., 2013). It
can be assumed that the instruction to remember and, to a lesser
extent the instruction to forget, activated specific encoding strategies associated with the activation of the left dlPFC. Apparently,
enhanced activity in this network made it preferentially sensitive
to modulation by tDCS as indicated by larger effects on memory
accuracy (Bikson et al., 2013). Therefore, the present data support the concept of metaplasticity based on dynamic interactions
between the level of activation in memory-encoding networks (as
modified by tDCS in the present study) and concurrent behavior
(e.g., memory encoding) (Floel and Cohen, 2007; Finnie and
Nader, 2012).
Together, our findings (1) provide new evidence for the critical role of the left PFC in the functional neuroanatomy of false
memory, (2) demonstrate the polarity-specific malleability of
memory accuracy by anodal and cathodal tDCS, (3) exemplify
the state dependency of brain stimulation effects in the cognitive
domain, and (4) open new perspectives for the investigation and
potential treatment of disorders associated with deficits in memory control.
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