The Routledge Handbook of Second Language Acquisition and Neurolinguistics

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DOI: 10.4324/9781003190912-8

USING NON- INVASIVE BRAIN

STIMULATION TO INVESTIGATE

SECOND LANGUAGE

Nick B. Pandža

Introduction

Learning a second language (L2) is difficult for adults, which is unsurprising due to the cognitive

demands it places on multiple memory systems, attentional mechanisms, and perceptual abilities.

These cognitive processes have commonly been targeted with behavioral training paradigms in

attempts to increase L2 learning rate and retention (e.g., Ingvalson et al., 2014). Stimulation of the

nervous system, or neurostimulation, is a category of methods that modulate neural activity and,

with the advent of non- invasive techniques, can also be used to investigate and enhance L2 learning

and processing. The two primary uses of neurostimulation to investigate L2 are through its ability

to localize behavior to a particular brain area or function and, in a more applied context, its ability

to actively modulate brain function to positively affect cognition and behavior, including language

learning outcomes.

A major distinction between neurostimulation and other methodological approaches addressed in

this volume (electroencephalography (EEG), see Dickson & Pelzl, this volume, and Mottarella & Prat

et al., this volume; magnetic resonance imaging (MRI), see Kousaie & Klein, this volume, and Rossi

et al., this volume) is that neurostimulation is inherently a neurocognitive intervention rather than a

neurocognitive measure. This paradigm shift is a fascinating new direction for neurolinguistics in

second language acquisition (SLA) as it allows us to leverage what we know about the neurocognition

of SLA, much of it foundationally built on observational neurocognitive methods, to causally affect

language learning. Compared to those other neurocognitive methods, neurostimulation research is

nascent and presently under- represented in research on L2 learning and processing.

Historically, neurostimulation has been a set of invasive and/ or sometimes painful techniques

developed for clinical applications, but painless, non- invasive ways to modulate neural activity have

been developed in recent decades using electric current and magnetic fields on the outside of the body.

This chapter reviews multiple methods of non- invasive brain stimulation (NIBS), including

transcranial magnetic stimulation (TMS), transcranial electrical stimulation (tES), and transcutaneous

peripheral nerve stimulation (PNS). In reality, these three are broad families of methods under which

differences in, for example, timing, intensity, stimulation pattern, or electrode placement can not

only affect the strength of any effects but even change the underlying mechanism of action (Polan�a

et al., 2018). Following a brief history of non- invasive neurostimulation methods to examine cogni-

tion, their use and potential in the investigation of L2 learning and processing is discussed, including

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Non-Invasive Brain Stimulation to Investigate Second Language

experimental paradigms, example studies, and advantages and disadvantages of specific techniques.

The chapter ends with a discussion of future directions for neurostimulation methods in the investi-

gation of L2.

Critical Definitions

Neurostimulation involves the application of stimulation (e.g., electrical, magnetic) to modulate the

activity of the nervous system. There are a variety of non- invasive techniques that target different

neurocognitive mechanisms, many of which support language learning. TMS and tES, being

transcranial, involve placing neurostimulators (e.g., electrodes) on or above the surface of the scalp

in order to affect cortical activity and have been evaluated for improving cognitive and language per-

formance (e.g., Miniussi et al., 2008). PNS, being stimulation of a peripheral cranial nerve, involves

the placement of electrodes on the surface of the skin in strategic locations such as the ear, neck, or

even forehead to electrically stimulate branches of cranial nerves to carry the stimulation back to

nuclei in the brainstem and facilitate the release of neurotransmitters with the intent to affect cogni-

tion and language (e.g., Colzato & Beste, 2020).

TMS uses a strong magnetic field produced by a coil, typically a combination of two circular coils

to optimize spatial resolution. These magnetic fields penetrate through the scalp and skull under the

coil’s position to the cortex and induce electrical fields that can stimulate neuronal activity (Sandrini

et al., 2011). A pulse of current can temporarily disrupt neural activity, and TMS has been used to

simulate lesions to localize the brain regions necessary for a given task, including several regions

necessary for language processing (Pascual- Leone et al., 2000). TMS provides a high degree of

accuracy in identifying where task- critical regions are in the brain (i.e., spatial localization), espe-

cially when combined with structural MRI (see Rossi et al., this volume). TMS pulses can also be

repeated over an extended period of time (repetitive TMS; rTMS) to facilitate or inhibit neural activity

(Miniussi et al., 2008). It has been used in people with aphasia to promote better language recovery

(e.g., Miniussi et al., 2008), and healthy individuals to facilitate picture naming and other language

tasks (e.g., Mottaghy et al., 1999). The potential mechanisms of action for TMS are still under active

investigation, and while there is some evidence it can elevate gamma- aminobutyric acid (GABA)

levels to suppress brain activity, the underlying cause of the virtual lesion effect is still unknown: it

could be the suppression of neural signals or an artifact of adding random noise to an ongoing process

(Sandrini et al., 2011).

tES uses electrical currents applied at low intensities to positively or negatively affect cortical

excitability. There are many techniques under the tES umbrella that are hypothesized to be mechan-

istically different based on differences in the stimulation pattern, including transcranial direct current

stimulation (tDCS), transcranial alternating current stimulation (tACS), and transcranial random

noise stimulation (tRNS). For example, the most commonly employed method, tDCS, uses con-

stant electrical currents applied at low intensities (~1– 2 milliamps (mA)) between electrodes on the

head such that the current passes through the cortex in between to facilitate or inhibit cortical excit-

ability by affecting resting membrane potentials (DaSilva et al., 2015; Miniussi et al., 2008). tDCS

has inferior spatial localization to TMS but is able to penetrate somewhat deeper brain structures

(DaSilva et al., 2015) and is a silent intervention, although it can produce a physical sensation on

the scalp. Stimulation has been found to facilitate long- term memory for word pairs (Marshall et al.,

2004) and vocabulary learning (e.g., Meinzer et al., 2014).

Rather than targeting specific cortical areas directly, peripheral nerve stimulation (PNS)

involves stimulating the peripheral branches of a cranial nerve to modulate cortical function more

broadly. Cranial nerves are sensory and motor neurons that project from the brainstem and supply

nerves to (i.e., innervate) the body, especially the head and neck. Stimulation of their peripheral

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Nick B. Pandža

branches leads to changes in the activity of neuromodulatory systems, which regulate nervous

system activity via neurotransmitters, such as changes in attention with the release of norepin-

ephrine (NE) throughout many areas of the cortex. While there are several types of cranial nerves

being targeted with neurostimulation, the most well- studied PNS to date— and thus, the type of

PNS in focus in this chapter— involves stimulation of the vagus nerve. Transcutaneous vagus

nerve stimulation (tVNS) is a type of PNS that involves electrical stimulation applied at low

levels to the skin over branches of the vagus nerve located in the ear (inner ear, tragus, or cymba

conchae) for transcutaneous auricular VNS (taVNS) or the neck for transcutaneous cervical VNS

(tcVNS) that carry nerve impulses back to the brain. The most well- studied mechanism underlying

VNS benefits for memory and cognition (Vonck et al., 2014) involves the nucleus of the solitary

tract’s innervation of the locus coeruleus (LC) brainstem nucleus, though other mechanisms are

also under investigation (e.g., George et al., 2008). The LC produces all of the neocortex’s supply

of the neurotransmitter norepinephrine (NE), and tVNS- related benefits may be due in part to the

LC- NE system’s role in optimizing behavior by controlling the trade- off between scanning and

focused states of attention (Colzato & Beste, 2020).

Relevant to the study of language learning, under certain parametrizations, TMS, tES, and PNS

have been mechanistically associated with long- term potentiation (LTP), or a facilitation of synaptic

transmission, which is arguably the major cellular mechanism underlying learning and memory for-

mation (Polan�a et al., 2018). Longer tDCS stimulation periods have also been associated with LTP

(Nitsche & Paulus, 2001), rTMS has been associated with LTP (Polan�a et al., 2018), and tVNS

has also been implicated in LTP given that tVNS is believed to indirectly release NE, which in turn

facilitates cortical LTP (Vonck et al., 2014).

Mechanistically, one of the ways by which LTP occurs is thought to be via modulation of brain-

derived neurotropic factor (BDNF), which is a protein encoded in the BDNF gene which has been

associated with rTMS- , tDCS- , and VNS- induced LTP (Cheeran, et al., 2008; Follesa et al., 2007;

Fritsch, et al., 2010) and a potential source of behavioral and physiological variability in NIBS-

facilitated effects (Polan�a et al., 2018). However, common to all NIBS methods, their growing popu-

larity continues to generate debates about their mechanisms of action, application techniques, ethics,

and applied use.

Table 6.1 features a reference list of the critical terms and acronyms related to non- invasive

neurostimulation research presented in this chapter. Note that this list is far from comprehensive, and

there is some variation in terminology in the literature.

Historical Perspectives

TMS, tES, and PNS were all first (and still are) investigated in clinical contexts. TMS currently

has FDA approval for conditions like obsessive- compulsive disorder and major depressive disorder,

tDCS is still being actively investigated for major depressive disorder, and VNS has FDA approval

for epilepsy and depression. In addition to these more or less primary uses, each of these methods has

been investigated for myriad other clinical, applied, and basic research topics.

Historically, TMS’s main use outside of clinical settings has been to disrupt neural activity to make

causal inferences about specific brain regions and cognitive functions (Sandrini et al., 2011). One of

the original and more well- known experimental paradigms for TMS is creating a virtual lesion, a safe,

temporary disruption of brain activity to imitate the effects of an actual brain lesion, such as from

stroke. For example, in the context of language research, a TMS- induced virtual lesion over Broca’s

Area would be expected to produce temporary deficits in language production that mimic Broca’s

Aphasia. TMS’s ability to disrupt brain activity and simulate lesions allow more methodologically

desirable research designs for localizing brain function than trying to find research participants with

rare brain lesions of comparable location and size.

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While TMS indirectly induces electrical fields in cortex, tES applies weak electrical currents

directly to the scalp. The concept of tES had its origin as early as the eighteenth century, being

investigated more seriously in clinical settings starting in the 1960s (Zoefel & Davis, 2017). The

most common tES technique, tDCS, uses a weak direct current between the electrodes applied to the

scalp, and the current affects the cortex through which it partially passes. TMS and tES both have

an extensive research history related to the enhancement of motor learning (Polan�a et al., 2018) and

substantial literature on their utility as a potential treatment for aphasia (Zoefel & Davis, 2017).

Unique among these three methods is PNS, which was first (and is still) an invasive manipula-

tion before non- invasive techniques were developed. VNS has been investigated invasively in clin-

ical populations since the mid- 1980s for its efficacy as an antiepileptic and antidepressant (Vonck

et al., 2014). More recently, its effects on auditory processing, memory, and cognition have also been

studied (see Colzato & Beste, 2020). The vagus nerve is the tenth cranial nerve and originates from

the medulla in the brainstem. Stimulation to the vagus nerve projects along nerve fibers to the nucleus

of the solitary tract in the brainstem. Recent innovations have led to user- friendly, non- invasive tVNS

technologies that stimulate the vagus by passing electrical current on the skin of the ears or neck

allowing its use and study with neurotypical populations.

Table 6.1 Reference List of Non- invasive Brain Stimulation Acronyms and Terms Presented in This Chapter

non- invasive

brain

stimulation

(NIBS)

methods

transcranial magnetic

stimulation (TMS)

repetitive TMS (rTMS)

high frequency rTMS (HF- rTMS)

low frequency rTMS (LF- rTMS)

theta burst stimulation

(TBS)

continuous TBS (cTBS)

intermittent TBS (iTBS)

transcranial electrical

stimulation (tES)

transcranial direct current

stimulation (tDCS)

anodal tDCS (atDCS)

cathodal tDCS (ctDCS)

transcranial alternating

current stimulation

(tACS)

transcranial random noise

stimulation (tRNS)

peripheral (cranial)

nerve stimulation

(PNS)

vagus nerve stimulation

(VNS)

transcutaneous VNS

(tVNS)

auricular tVNS

(taVNS)

cervical tVNS

(tcVNS)

trigeminal nerve

stimulation (TNS)

NIBS

mechanisms

of action

long- term potentiation

(LTP)

a proposed mechanism of action for cognitive effects of TMS, tES,

and PNS

brain- derived

neurotrophic factor

(BDNF)

a protein through which LTP is thought to occur

locus coeruleus-

norepinephrine

(LC- NE) system

the LC is a region in the brainstem responsible for producing NE, a

neurotransmitter implicated in PNS research

gamma- aminobutyric

acid (GABA)

a neurotransmitter implicated in TMS research

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Nick B. Pandža

Methods and Paradigms

There are multiple ways in which NIBS may be implemented, common ones including priming (i.e.,

conditioning or offline), concurrent (i.e., online), and peristimulus (i.e., stimulus time- locked) stimu-

lation. All three NIBS methods reviewed here can easily be implemented via priming, which involves

applying stimulation for a specified number of seconds or minutes prior to starting a critical task,

presumably inducing tonic shifts in arousal and thus cortical excitability that prepare the individual

to be in an optimal state for learning or performance throughout the task. For example, VNS studies

have observed an increase in activity in LC and related brain structures (Frangos et al., 2015), and

concentrations of norepinephrine in the cortex and hippocampus (Follesa et al., 2007). In TMS terms,

the rTMS technique can be used to prime participants before a task (Klooster et al., 2016). For tES,

studies have found cathodal tDCS (ctDCS) to have longer- lasting priming effects than anodal tDCS

(atDCS) (Monte- Silva et al., 2013), although more robust learning effects have been observed for

atDCS (Simonsmeier et al., 2018).

Peristimulus (peristim) stimulation involves delivering a pulse train of stimulation just prior to

the presentation of critical stimuli, presumably inducing phasic changes in task- related attention to

and consolidation of specific to- be- learned information. Work exploring peristim VNS has shown

effectiveness at improving low- level auditory processing (Engineer et al., 2011), and peristim (or

paired pulse) TMS is mainly used to assess cortical excitability (Klooster et al., 2016). Both delivery

methods may engage LTP but, while priming NIBS is possible with TMS, tES, and PNS, peristim

may be the most practical for language learning protocols with PNS given the noise artifacts of TMS

and the low temporal resolution of tDCS.

For TMS, there are numerous implementations of the method, although rTMS is perhaps most

relevant for investigating longer- lasting changes to cognition. Typically, low- frequency rTMS (LF-

rTMS) is administered ≤ 1 Hz while high- frequency rTMS (HF- rTMS) is administered ≥ 1 Hz, and

this distinction has been found in the motor system to decrease and increase cortical excitability,

respectively (Sandrini et al., 2011). rTMS can be administered concurrently to a task of interest or

beforehand, priming the participant before a task.

Another pattern of pulses, theta burst stimulation (TBS) utilizes trains of pulses applied with

short gaps that can also be used to induce longer- term effects of TMS on cognition. Continuous TBS

(cTBS) tends to be inhibitory whereas more intermittent TBS (iTBS) is excitatory (see Sandrini et al.,

2011 for a discussion of additional implementations).

Similar to TMS, changing the parameters of tES can create either facilitative or inhibitory effects.

Among tES techniques, tDCS is the most frequently used. Differences in atDCS and ctDCS refer

to differences in experimental setup. Both involve an anode (positively charged electrode) and a

cathode (negatively charged electrode), and so atDCS involves placing the anode near a brain region

of interest and the cathode at a control region, and ctDCS is the reverse. While tDCS broadly is a con-

stant current applied at about 1– 2 mA, tACS is applied at lower intensity (0.2– 1 mA) and is a bidir-

ectional/ biphasic current that can be applied at different frequencies (Simonsmeier et al., 2018). Even

lower in intensity is tRNS (- 500 to 500 microamps (μA)), which uses alternating current with random

amplitudes and frequencies (Moreno- Duarte et al., 2014). These are not an exhaustive list of tES

techniques, but are the most commonly employed, and the types included in a recent meta- analysis

of tES on language learning in healthy adults (Balboa- Bandeira et al., 2021). While there is a logic

to both pairing NIBS with learning (e.g., to potentially improve consolidation of new information)

and assessment (e.g., to potentially improve recall of learned information), a meta- analysis of tES

recently found a more robust effect when paired with learning than with assessment, and the effect

was only significant for anodal and not cathodal tDCS (Simonsmeier et al., 2018). Balboa- Bandeira

et al. (2021) conducted a meta- analysis for the effect of tES on adult language learning (nonwords,

artificial grammar, and foreign languages) and found a moderate effect of tES. While they did not

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find an effect on follow- up data or response times, there are a number of limitations in the review,

including the number of studies that could be included (11 in total) and disparate tES protocols,

as effects are assessed across studies encompassing tRNS, tACS, atDCS, ctDCS, and high density

tDCS. They rightfully call for further work to tease apart the potentially mechanistically different

effects of each tES technique, especially with multiple stimulation sessions. In particular they note

that a majority of the studies are tDCS and show effects with one day of training even though tRNS

may also have long- lasting effects on language learning processes. A number of studies included in

the meta- analysis involved continuous stimulation during the process of learning rather than before

as in a priming paradigm.

VNS can be implemented at the auricular or cervical branches, the former being more exten-

sively implemented to date, especially in language research. For the auricular branch of the vagus

nerve, there’s still an open question over the optimal location for stimulation. Yakunina et al. (2017)

compared three different stimulation points on the ear within individuals versus a sham control of

the earlobe: the outer ear canal, the inner tragus, and the cymba conchae. With confirmation from

functional MRI (fMRI; see Kousaie & Klein, this volume), they found stimulation of the cymba

conchae to produce the most reliable activation of the nucleus of the solitary tract and the locus

coeruleus in the brainstem compared to the other locations. However, in comparing this to other

research it’s worth noting that participants in this study were all delivered tVNS at 0.1 mA below

their pain threshold, and results here may reflect a higher tolerable threshold for the cymba conchae

than other stimulation sites on the ear, and results may have been different if stimulation was kept

below their sensory thresholds. For example, taVNS applied to the outer ear canal below sensory

threshold has been successfully implemented for both priming and peristim in the same training

study and has been found to both show positive effects but on different aspects of testing, reaction

time and accuracy, respectively (Pandža et al., 2020). These effects provide evidence for potentially

different mechanisms of action for taVNS on learning based solely on where in the learning process

NIBS is implemented.

For each of TMS, tES, and PNS, there are many ways for these methods and the techniques under

each of these umbrellas to be implemented. Care needs to be taken when deciding when, where, how,

and at what intensity NIBS are implemented so that effects can be reliably detected, the mechanism(s)

of action can be properly investigated, and the results can be replicated.

Example Studies

TMS

Of the three types of NIBS reviewed here, TMS is the most under- studied for the specific use case of

language learning. Of the few studies conducted to date, they are focused largely on the investigation

of neurocognitive mechanisms underlying language learning and/ or clinical application. Many report

interesting uses of rTMS in which it enhanced implicit learning mechanisms in adults. For example,

Ambrus et al. (2020) disrupted the left and right dorsolateral prefrontal cortex with inhibitory rTMS,

leading participants to better implicit statistical learning via consolidation of non- adjacent second-

order dependencies in an alternating serial reaction time task. In contrast, Sliwinska et al. (2017)

found positive effects of rTMS on explicit vocabulary learning and focused on implications for pro-

moting post- aphasia recovery. After first conducting a word learning study with fMRI to identify

functional networks of interest, the authors conducted a second experiment on a subset of participants

in which they applied priming rTMS for 10 minutes each day in three training sessions. They found

both improved accuracy and reaction time in a paired- associates translation judgment task with feed-

back during early stages of word learning, and they posit the potential for rTMS for use in aphasia

rehabilitation.

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tES

Perceval et al. (2020) implemented an ambitious atDCS multisession double- blind design in which

younger and older adults were tasked with learning non- word “names” for “space alien” characters

and two semantic attributes for each. Active atDCS was administered for 20 minutes during the

learning phase on each day over the left inferior frontal gyrus to target areas implicated in verbal asso-

ciative learning. Training was administered over five consecutive days with immediate testing and

day- after testing to assess retention, and follow- up testing done one day, one week, and three months

after the last day of training. In both younger and older adults, effects for atDCS were only observed

for those with lower scores of baseline learning ability measured at pretest. This study shows the

potential for individual differences to interact with neurostimulation to show more nuanced effects, in

this case a modest learning “boost” for those participants who were a priori disadvantaged. Learning

facilitated by atDCS was largely immediate and persisted up to three months for the low ability older

adults, contrasting with younger adults for which there were no immediate stimulation effects (pos-

sibly due to ceiling effects) but retention was enhanced at delayed testing up to three months. While

the authors conclude uncertainty around the mechanisms of action to explain their results, they pro-

pose that future research focus on the neural mechanisms underlying their novel finding of differen-

tial effectiveness of atDCS according to baseline ability.

In the minority are non- tDCS tES studies like Pasqualotto et al. (2015) looking at tRNS and

Antonenko et al. (2016) looking at tACS. In comparison to Perceval et al. (2020), Antonenko et al.

(2016) found increased retrieval accuracy in older adults but not younger adults with tACS during

implicit language learning, also finding an advantage for older adults with tES that is worth further

mechanistic exploration. Pasqualotto et al. (2015) was the first to investigate the potential of tRNS on

language learning and found a potential memory consolidation advantage for active tRNS delivered

over posterior parietal areas during learning at a one- week delayed posttest.

PNS

While PNS is a broad category of techniques, there is a rapidly growing L2 literature using a spe-

cific subtype of PNS and VNS: taVNS. Pandža et al. (2020) were the first to directly investigate

taVNS- facilitated language learning, specifically for Mandarin tone. In a double- blind study, they

directly compared priming and peristim taVNS protocols compared to a sham control in a two- day

Mandarin tone word training for tone- na�ve native speakers of English. Active stimulation was

applied before (priming) or during (peristim) learning and testing tasks. They found peristim but

not priming to reliably improve accuracy on lexical recognition while priming but not peristim

improved reaction time on lexical recognition. In an analysis of pupillometry data to assess cog-

nitive effort and ties to the LC- NE system, they found a reduction in sustained effort day- to- day

for the sham group in line with the idea of less effort being applied to better- learned words, and a

significantly stronger effect for peristim. For priming, results were interpreted as possibly in line

with a tonic, rather than phasic (task- evoked) effect on arousal. In total, the authors concluded

that effects are in line with the idea that peristim taVNS supported better encoding of new to- be-

learned information whereas priming taVNS may instead support better lexical access of already-

learned information.

Thakkar et al. (2020) also found positive taVNS results to extend to orthography acquisition.

Separately, Calloway et al. (2020) investigated the potential for another researched mechanism

of action for VNS to affect language learning, the mitigation of anxiety (George et al., 2008),

and found 10 minutes of priming taVNS to reduce negative affect, somatic anxiety, and cognitive

anxiety.

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There is still a dearth of language learning research with TMS, tES, and PNS, so there are numerous

ways to expand the field. Much more research is needed with each of these techniques to form firm

conclusions around their efficacy.

Considerations and Limitations of NIBS Methods

There are a number of pros and cons for each of these NIBS methods. All result in at least some-

what indirect neuromodulation as stimulation is applied on the outside of the body versus directly to

brain tissue. TMS and tES can be combined with structural MRIs to pinpoint where brain areas are

specific to a person and even specific to language processing (e.g., Broca’s Area, Wernicke’s Area),

given that brain morphology varies widely. In this way, TMS and tES can accurately encapsulate

a brain region of interest; however, given the spatial resolution of both methods, they are inexact

and likely to also affect neighboring regions and their associated functions. PNS has both better

and worse spatial resolution: better in that there is a specific brainstem nucleus (e.g., nucleus of

the solitary tract for VNS) that will be stimulated in turn by the stimulated cranial nerve, but worse

in that the effects of interest for PNS interventions are more about what happens downstream from

that initial nucleus, for example stimulation of the LC and the release of neurotransmitters like NE

that are dispersed throughout the cortex. Thus, a major advantage for TMS and tES over PNS is the

inability of PNS to target specific brain structures of interest. However, while TMS and tES can only

target a variety of surface- level cortical structures on the order of centimeters, PNS on the other hand

indirectly stimulates structures in the brainstem leading to connected structures such as the LC and

to the release of myriad neurotransmitters throughout the brain to produce broader (if more domain-

general) impacts to cognition and language learning. Nevertheless, TMS, tES, and PNS can each be

combined with other behavioral, physiological, and neurocognitive methods to more conclusively

pinpoint brain areas and functions and causally associate them with a particular behavior. Again, most

critically, all three techniques have also been associated with LTP under certain conditions and thus

to the causal enhancement of learning and memory processes broadly, making each relevant to the

investigation of language learning.

In terms of practical concerns around data collection, there are a number of considerations. It’s

important to note that TMS has a noise artifact (repetitive clicking noise). This may not be a problem

for NIBS priming methodologies, but may be problematic if concurrent or peristimulus stimulation is

mechanistically desired for a particular research design. Minimally, this could be distracting to a par-

ticipant during a task, and, maximally, could actively interfere with auditory task designs. While tES

and PNS are silent interventions, tES can induce somatosensory artifacts across the scalp, which can

also be distracting to participants. PNS can also produce a physical sensation under the electrodes or

even harmless muscle spasms, depending on the location and stimulation intensity. For instance, the

cervical branch of the vagus nerve is embedded deeper under the skin of the neck than the auricular

branch is under the skin of the ear. Because of this, tcVNS is practically impossible without some

physical sensation (even harmless muscle spasms in the neck if placed incorrectly) which may be

uncomfortable or distracting to individuals. However, multiple studies have now shown that taVNS

can be effectively delivered below a participant’s individual perceptual threshold while still showing

positive language learning outcomes (e.g., Calloway et al., 2020; Pandža et al., 2020).

To show causal evidence of NIBS on behavior, double- blind studies are ideal in which participants

and proctors are blind to whether the participant is receiving active stimulation vs. sham stimula-

tion or belongs to a no- stimulation control. At the minimum, single- blind studies can also be used

in which the participants but not the proctors are blind to participant group. Properly blinding NIBS

methods means replicating the TMS clicking noise for a sham control, using tES over a control

brain region to produce similar somatosensory effects or, if using PNS at a location or intensity that

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Nick B. Pandža

necessarily causes physical sensation, using PNS over a control skin area that is not innervated by the

cranial nerve of interest.

While all three techniques involve potentially costly equipment upfront and require training for

their effective and safe use, relative to other neurocognitive methodologies there are some advantages

to neurostimulation. For example, the equipment for these techniques is cheaper than MRI data

collection, participant setup and takedown are often faster and easier than for EEG, and no specially

shielded room is required for data collection. However, methodologically sound TMS and to a lesser

extent tES studies involve “neuronavigation” such that researchers can be sure they are targeting

brain regions of interest for each individual as precisely as possible (Polan�a et al., 2018), and this

is done in conjunction with a structural MRI, which increases the cost of the research and adds MRI

contraindications back into the research design.

In terms of safety, TMS, tES, and PNS appear to be safe when used properly (Antal et al., 2017;

Redgrave et al., 2018; Rossi et al., 2021). However, rare side effects can occur of which researchers

should be aware. The most serious adverse effect is for TMS, which in extremely rare circumstances,

if proper use guidelines are not followed, may cause acute seizures (Rossi et al., 2021). TMS is also

contraindicated for individuals with metallic or electronic implants near the TMS coil (Rossi et al.,

2021). For tES and PNS protocols, in which electrodes come into physical contact with the skin and

scalp, the most common side effects can include uncomfortable heating or sensation directly under

the electrode (Antal et al., 2017; Redgrave et al., 2018), but mild pain is also common under the

stimulation site for TMS (Rossi et al., 2021). The potential for headaches has also been observed for

all three methods (Antal et al., 2017; Redgrave et al., 2018; Rossi et al., 2021). Because of a remote

possibility of cardiac effects (Farmer et al., 2021), tVNS is usually administered to the left ear or

left side of the neck as right branches of the vagus nerve are more closely connected to the heart.

Because of the still relative novelty of some of these methods and the study of their implementa-

tion, researchers often impose restrictive eligibility criteria out of an abundance of caution where

safety research is still currently lacking, such as administering tVNS to the left side as mentioned or

excluding participants from tVNS if they have a history of fainting spells (vasovagal syncope is a

condition in which an overactive vagus nerve can result in fainting).

TMS, tES, and PNS in the literature currently all suffer from a replication problem due to still

active uncertainty around how different parameters affect a number of factors ranging from not only

variation in sensation artifacts but also, crucially, the neurocognitive mechanisms affected. Indeed,

it has been noted in particular that variation in how TMS and tES are implemented has actively

prevented conclusive findings from meta- analyses (see Polan�a et al., 2018). All three methods can

be implemented in myriad ways, for example, at different levels of intensity, with different stimula-

tion wave forms, or with different electrode placements. The conclusive impact of any modification

of these settings is still under active investigation (e.g., see discussion in Farmer et al., 2021 for

tVNS), but researchers looking to implement these techniques should take particular care in choosing

parameters as changes in any of these could affect not only the strength of any NIBS effects but even

change the underlying mechanism of action (Polan�a et al., 2018).

Innovations and Future Directions

New NIBS parameters and techniques are still being developed for existing methods to overcome

their current limitations or even to target additional neurological mechanisms, such as to improve

both spatial resolution and the ability to target deeper brain structures for tES (Datta et al., 2009).

At the same time, new neurostimulation techniques are also being developed. For example, early

findings for transcranial focused ultrasound stimulation (tFUS) suggest it can successfully overcome

the inherent limitations of existing transcranial methods by being able to easily target deeper brain

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Non-Invasive Brain Stimulation to Investigate Second Language

structures and with a higher spatial resolution, inducing cortical excitation on the order of millimeters

compared to centimeters for TMS and tES (Tufail et al., 2010).

The topic of language learning is still relatively new to exploration with NIBS interventions, but

the cited research provides preliminary evidence that various NIBS methods have the potential to

meaningfully impact language learning outcomes. The effects of NIBS on attention and memory

consolidation could promote more effective language learning, and some of the cited studies suggest

potential for longer- lasting effects. Assessing the benefits of any new intervention on language learning

outcomes is non- trivial. Does the intervention serve to increase phoneme and/ or word recognition

accuracy overall? Increase the overall learning rate? Reduce the mental load associated with learning

an individual item, freeing up mental resources for other aspects of learning? Are there interactions

with NIBS efficacy and learner individual differences? Particularly challenging for NIBS is that,

due to a paucity of established research with comparable implementations and parameterizations,

expected effect sizes have not been firmly established, and thus it is possible that NIBS- induced

changes in neural function might be subtle or very focused, and thus primarily relevant for only

a subset of possible language learning outcomes. Given the range of possibilities, assessments of

NIBS- driven language- learning benefits should encompass multiple outcome measures whenever

possible, to include indices like accuracy, reaction time, pupillometry, EEG, and fMRI. Perhaps most

importantly, rigorous assessments of longer- lasting effects of NIBS in the form of delayed posttests

are critical to evaluate the potential for any truly practical implications of NIBS to support language

learning.

As currently more tDCS and tVNS studies exist in the realm of language learning enhancement, a

promising additional line of research would be to directly pit the two NIBS methods against each other

with the same training paradigm. Given the potential differences in mechanisms of action, it would

be of interest to investigate differences in effect size improvements between the two techniques.

Additionally, there is a dearth of literature on TMS to enhance language learning, and further explor-

ation of that methodology should be made to empirically evaluate its utility versus other alternatives.

Likewise, further exploration of tES techniques in addition to tDCS (including tACS and tRNS) is

warranted given preliminary positive results. While the PNS studies cited here focused on taVNS,

tcVNS also has the potential to accelerate language learning with some incipient research showing

lasting cognitive effects after brief stimulation (e.g., Lewine et al., 2019), as does transcutaneous

trigeminal nerve stimulation (TNS) across the forehead (Colzato & Vonck, 2017).

Contrasting with more correlational neurocognitive techniques, neurostimulation provides the

opportunity to make causal inferences between particular cognitive functions and associated behav-

ioral outcomes, and it can be combined with those correlational methods for a more targeted or mech-

anistically validated impact. Research on NIBS in L2 is clearly still at the initial stages, but it has the

potential to transform the field of SLA.

Further Readings

For a comprehensive overview of NIBS research, covering primarily TMS and tES techniques:

Polan�a, R., Nitsche, M.A., & Ruff, C.C. (2018). Studying and modifying brain function with non- invasive brain

stimulation. Nature Neuroscience, 21(2), 174– 187. https:// doi.org/ 10.1038/ s41 593- 017- 0054- 4

For a systematic review of high- frequency rTMS studies stimulating over prefrontal cortex, many of which found

improvements on verbal measures:

Guse, B., Falkai, P., & Wobrock, T. (2010). Cognitive effects of high- frequency repetitive transcranial magnetic

stimulation: a systematic review. Journal of Neural Transmission, 117(1), 105– 122. https:// doi.org/ 10.1007/

s00 702- 009- 0333- 7

For a meta- analysis of tES effects on second and foreign language learning, which covers multiple tES methods,

identifies gaps in the literature, and makes recommendations for future research:

Page 11

Nick B. Pandža

Balboa- Bandeira, Y., Zubiaurre- Elorza, L., Ibarretxe- Bilbao, N., Ojeda, N., & Pe�a, J. (2021). Effects of

transcranial electrical stimulation techniques on second and foreign language learning enhancement in

healthy adults: A systematic review and meta- analysis. Neuropsychologia, 107985. https:// doi.org/ 10.1016/

j.neuro psyc holo gia.2021.107 985

For an empirical taVNS study of Mandarin tone word learning that analyzed event- related potentials from Pandža

et al. (2020), showing evidence of stronger lexico- semantic encoding via the N400 after taVNS:

Phillips, I., Calloway, R.C., Karuzis, V.P., Pandža, N.B., O’Rourke, P., & Kuchinsky, S.E. (2021). Transcutaneous

auricular vagus nerve stimulation strengthens semantic representations of foreign language tone words during

initial stages of learning. Journal of Cognitive Neuroscience, 1– 26. https:// doi.org/ 10.1162/ jocn_ a_ 01 783

Acknowledgments

This material is based in part upon work supported by the Naval Information Warfare Center and Defense

Advanced Research Projects Agency under Cooperative Agreement No. N66001– 17- 2- 4009. The views,

opinions, and/ or findings contained in this material are those of the author and should not be interpreted as

representing the official views or policies of the Department of Defense or the U.S. Government.

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