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The contribution of transcranial magnetic stimulation (TMS) to our understanding of the

processes underlying empathy.

Transcranial magnetic stimulation (TMS) is a noninvasive technique whereby nerve cells in the

brain are stimulated using electromagnetic fields delivered through a coil held against the head

(Mayo Clinic, 2015). It is primarily used as a method of treating depression, however it is

increasingly being used as a research method in cognitive neuropsychology. Over the last decade,

researchers have been using TMS to study empathy; the process of understanding, and sometimes

experiencing, what others are thinking or feeling (Pijnenborg, Spikman, Jeronimus, Aleman, 2012).

Hetu, Taschereau-Dumouchel, and Jackson (2012) identify three distinct components of empathy;

resonance; a bottom-up process whereby brain activity is influenced by observing the affective state

of someone else, mentalizing; a deliberate top-down process allowing us to understand the mental

states and intentions of others, and self-other discrimination; the ability to attribute the source of an

affective state to oneself or others. This essay will review some of the studies which have used TMS

to study these processes underlying empathy. I will discuss the merits of using TMS to study

empathy, in contrast to other methods, and consider possible directions for future research

throughout.

Resonance has primarily been studied through the phenomena of sensorimotor contagion; the

reduction of corticospinal excitability which occurs when observing somebody else experiencing

pain (Farina, Tinazzi, Le Pera, & Valeriani, 2003). A study by Avenanti, Bueti, Galati and Aglioti

(2005) was one of the first to use TMS to explore this sensorimotor aspect of empathy. Peyron,

Laurent, and Garcia-Larrea (2000) proposed that pain is represented in a corticosubcortical network

called the ‘Pain Matrix’ which comprises sensorimotor (representing location and intensity) and

affective (representing unpleasentness) nodes. Previous research (Singer, Seymour, O’Doherty,

Kaube, Dolan, & Frith, 2004) indicates that only the affective component of the pain network is

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involved in empathy (i.e. the anterior cingulate cortex and the anterior insula). However, using

TMS, Avenanti et al. were able to show that perceiving pain in others was associated with a

significant drop in corticospinal excitability in the sensorimotor system.

Applying a strong magnetic pulse to specific areas of the motor cortex produces motor evoked

potentials (MEPs) in the associated muscles. The strength of the TMS pulse needed to evoke MEPs

is an inidcator of corticospinal excitability. As with real pain, seeing others experience pain prompts

a specific corticospinal inhibition, suggesting the activation of pain representations in the observer’s

sensorimotor system. TMS was used to measure changes in corticospinal motor representations for

the hand muscles of participants who were asked to watch the hands or feet of a human model or

noncorporeal objects being penetrated by a needle. Participants in the experimental condition of this

study watched a video of a needle being pushed into a hand, whilst those in the control conditions

watched a cotton bud pressing the hand or a needle being pushed into a tomato. The results show a

significant drop in the magnitude of MEPs specific to the muscle that participants saw being

pricked (i.e., participants seeing a particular hand muscle being pricked with a needle experienced a

reduction of motor excitability in their own corresponding muscle). No change in corticospinal

excitability occurred in the control conditions, indicating that the reduced excitability was

associated with observing the pain of another person.

MEPs were recorded from the first dorsal interosseous (FDI, in the index finger) and the abductor

digiti minimi (ADM, in the pinky finger) muscles of the right hands of participants. Watching a

needle penetrating a model’s foot did not affect the corticospinal excitability of these muscles.

MEPs measured at the FDI muscle of the participants were prompted by seeing the needle enter the

FDI muscle of the model’s hand but not the ADI muscle. MEPs measured from participants’ ADM

muscle demonstrated the opposite pattern. This inhibition of excitability correlated significantly

with participants’ subjective rating of the model’s pain. It also correlated with measures of sensory,

but not emotional empathy. These results indicate that embodying an understanding of other

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poeple’s pain into the motor system is important for pain-related social learning.

Singer and Frith (2005) provide possible explanations for how Avenanti et al. found a

sensorimotor empathic response when previous studies did not. This may be due to the different

materials used to evoke empathy. Participants in Avenanti et al’s study saw a stranger’s hands being

pricked with a needle likely emphasising the sensory aspect of the pain. Conversely, in Singer et al’s

2004 study participants were shown an arrow to indicate when their partner would experience a

painful stimulus likely emphasising the affective aspect of the pain.

However even functioning magnetic resonance imaging (fMRI) studies (Jackson, Meltzoff, &

Decety, 2005) that use materials highlighting the sensorimotor side of pain found different results.

This may be because TMS can pick up on slight changes in the sensorimotor system that are below

the significance threshold for fMRI, which does not always detect activation in the somatosensory

cortex (SI), even when participants receive painful stimuli. According to one meta-analysis (Peyron

& Laurent, 2000), only 50% of imaging studies examining pain reported activity in the SI. These

discrepancies have also been observed in action observation research. TMS studies have shown that

action observation can change corticospinal excitation and may be mapped directly to the specific

muscles used (Fadiga, Craighero, & Olivier, 2005). Conversly, fMRI studies of action observation

usually show activity in the inferior frontal gyrus and the inferior parietal lobule, moreso than the

primary sensorimotor cortex (Rizzolatti & Craighero, 2004).

However it is likely that there is more to empathy than the neural mapping of the sensory and

affective aspects of others’ pain. Individual differences in emotional and cognitive empathy are

likely to influence reaction to witnessing pain. Lamm, Batson, and Decety (2007) recognised that

empathic reactions to the pain of others can be more other-oriented (expressed through concern for

others) or self-oriented (experiencing distress when witnessing pain). Whilst these two components

may work alongside one another, they have different implications for how the observer responds.

Another important aspect of empathy is the ability to understand the perspectives of others. This is

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known as cognitive empathy and entails thinking about and imagining the feelings of others,

without necessarily having an affective reaction (Davis, 1996). Avenanti et al. (2009) conducted a

study to research how differences in these distinct components of empathy modulate somatomotor

responses. This study followed a procedure similar to Avenanti et al. (2005) with participants

watching a needle penetrating a model’s hand whilst having TMS-prompted MEPs recorded. They

also included self-report measures for the emotional and cognitive aspects of empathy. As with the

previous study, a reduction in corticospinal excitability was observed specific to the muscle

observed (FDI) in the index finger. The somatomotor response was higher for participants who

reported higher levels of cognitive empathy and lower for participants who reported higher levels of

distress. Each of these measures predicted somatomotor pain responses independently, indicating

that the processes underlying empathy are associated in different ways within the sensorimotor

system. The results indicate that an increased tendency to cognitively simulate the affective states of

others strengthens the somatomotor response to pain through a top-down process and operates

independently of the emotional mechanism. An increased tendency to experience distress at the pain

of others seemed to correlate with a facilitation of corticospinal activity, and may lessen or possibly

block mapping the pain of others in the somatomotor system. This is supported by another TMS

study showing that observation of emotional stimuli causes an increase in corticospinal excitability

(Hajcak et al., 2007). The authors suggest that a reduced empathic response caused by personal

distress may be linked with a reduction in mirror-matching the mental states of others. This study

takes account of the variability in empathic reactivity to the pain of others.

We might note an alternative explanation of the observed effect through the activation of the

motor mirror system. Corticospinal inhibition when witnessing pain may indicate a defensive motor

reaction similar to a withdrawal reflex (Farina et al. 2003). However, such motor reactions tend to

involve MEP suppression in all distal hand muscles (Urban et al. 2004) and given the precision of

the effect observed in this experiment (there was no inhibition in the ADM muscle), a mass reflex

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activation is unlikely.

Further research has used TMS to explore how mentalizing activities, in this case racial bias, can

influence pain resonance. Previous research in social psychology has suggested that racial

discrimination often results from a lack of empathy (Feagin, Vera, & Batur, 2001), however little

research had been conducted into differential sensorimotor reactivity to the suffering of someone of

a similar or different racial background until a study by Avenanti, Sirigu, and Aglioti (2010). TMS

was used to explore the sensorimotor empathic brain responses in participants who demonstrated

implicit but not explicit in-group preference. Participants watched a video showing the right hand of

a black or white model being penetrated by a needle or being touched by a cotton bud. As with the

previous studies, the specific location was the first dorsal interosseous (FDI) of the right hand.

MEPs were recorded from the right FDI (target) and ADM (control site) hand muscles of the

participants. It was found that participants' corticospinal motor system was inhibited when

observing the pain of in-group models and of models with violet skin (indicating ethnic neutrality).

While this inhibition was specific to the FDI muscle, no such inhibition was recorded in the ADM.

Corticospinal inhibition was significantly lower when viewing the pain of out-group models, and

this reduction was positively correlated with scores of implicit racial bias (measured using an

adapted version of the Implicit Association Test). These results indicate that while people may have

an automatic empathetic response to other people’s suffering, this reaction can be inhibited by racial

stereotypes and biases resulting in a lower sensorimotor response.

It is possible that the observed effect is merely due to visual unfamiliarity or increased perceived

dissimilarity between participants and the model, as opposed to racial bias. However, given that

corticospinal reactivity to the violet hand, which was rated as the least familiar skin colour among

participants, was significantly stronger than the response to the outgroup hand, this is unlikely to be

the case. These results provide a neural basis for the idea that racial biases can influence social

categorisation, leading to a devalued view of out-group strangers. It also demonstrates an

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interaction between two components of empathy, with the mentalizing effect of racial bias

influencing resonance.

Another form of mentalizing acivity which has been studied with TMS is mental state attribution:

the ability to infer the mental states of others. fMRI studies have shown that there is increased

activity in the right temporoparietal junction (RTPJ) when reading about someone's beliefs in a

moral situation (Young, Cushman, Hauser, & Saxe, 2007). The functional selectivity of the RTPJ

for moral beliefs increases significantly between the ages of six and eleven. Compared to adults,

young children, whose ability to reason about other peoples’ mental states is not fully developed,

place a higher value on the consequences of actions, rather than intentions, in order to make moral

judgments (Karniol, 1978). However, fMRI is unable to say whether activity in this area is a

necessary condition for mental state reasoning or even moral judgment itself. Young, Camprodon,

Hauser, Pascual-Leone, and Saxe (2010) hypothesised that the RTPJ is necessary for making moral

judgments and that disruption in this area would lead participants to rely less on the mental states or

beliefs of an actor and place more importance on outcomes. In this study, TMS was applied to the

RTPJ in the experimental condition, and the right parietal cortex in the control condition. TMS was

applied while participants had to make a moral judgement about a scenario where the agent either

intends to harm somebody or not and either succeeds in harming them or does not (4 conditions).

The results show a significant interaction between TMS site and agent belief-state but not between

TMS site and outcome. Following TMS to the RTPJ, participants relied more on the outcomes of

the action and less on the agent’s mental state to judge the rightness or wrongness of that action.

Participants in the experimental condition rated attempted harms (the intention but failure to harm)

as more or less desirable than participants in the control condition. This implies that disrupting the

activity in the RTPJ interferes with individuals’ capacity to assimilate mental states when making

moral judgments, particularly with regard to attempted harms, but does not disrupt the process of

moral judgment-making altogether.

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It is notable that TMS to the RTPJ did not affect judgments of intentional harms (i.e. a harmful

action was not considered anymore immoral when the protagonist intended to cause harm, than

when the harm was accidental). Based on this, it is likely that moral judgments reflect the weighting

of other factors, such as outcome, when information about the agent's mental state is unavailable.

An alternative hypothesis is that RTPJ disruption impairs the very process of moral judgment,

particularly when there are multiple factors to consider. According to this hypothesis, participants

perform less well when assimilating information about multiple morally relevant considerations

(e.g., someone’s past history, the methods used, and any constraints on them) following TMS to

RTPJ. The authors reject this hypothesis since it would not predict the effects found in this

experiment. If the ability to take account of multiple morally relevant considerations was hindered

by TMS to the RTPJ then we would expect judgments to have been otherwise slower in the

experimental condition. Further, the results showed a systematic bias, in accordance with the

predictions for each condition, not resulting from a mere slowing process. Therefore, the authors

conclude that TMS to the RTPJ disrupts input to moral judgment making, such as information about

mental states, but does not affect the process of moral judgment making itself. However, the

interference of multiple morally relevant features on moral judgment would benefit from further

research.

Another possible explanation is that TMS to the RTPJ interfered with other cognitive processes.

The lateral inferior parietal region, an area of the brain associated with attention shifting, is located

near the RTPJ (Mitchell, 2007). However, a study by Decety and Lamm (2007) found that these two

regions are separated by approximately 10mm, whilst TMS has a spatial resolution of 5 to 10 mm

(Kammer, 1999). The particular region of the RTPJ associated with mental state attribution was

located using image-guided TMS and a functional localizer. Furthermore, the results of this

experiment do not suggest there was any interference with attention or any other effect on task

performance; participants’ judgments were no slower or less reliable (i.e., more or less variable)

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during TMS to the RTPJ or control site in any of the conditions. These results support the initial

hypothesis that the effect of TMS on the RTPJ is due to the stimulus itself (i.e., belief information).

Through the use of action observation and pain perception models, these TMS studies provide

good evidence supporting the contention that the resonance process gives rise to the sesnsori-

affective components of empathy. However, if these resonance responses use similar neural

pathways to the ones used for our own sensor-affective responses then there are likely to be neural

mechanisms which facilitate the identification of the source of our internal state, (Hetu et al. 2012).

The process of determining whether an affective internal state arises from oneself or from

observing others is referred to as self-other discrimination (Hetu et al. 2012). Several studies have

been conducted whereby rTMS is applied during agency attribution or self-other discrimination

tasks to discover which areas of the brain are associated with this process. David et al. (2009)

identified the extrastriate body area (EBA) as a brain region instrumental with this component of

empathy, given that performance on a self-other discrimination task deteriorates following rTMS

over the EBA. Notably, the tasks in this study required participants to judge whether the movements

of visually presented stimuli were caused by their own actions or the actions of somebody else.

Whilst this study may inform us about the self-other distinction, its use of limb movements may

place disproportionate focus on the motor aspects of these stimuli whilst facial expressions and

characteristics are more likely to be better indicators of a person’s affective state.

Uddin et al. (2006) therefore asked participants, following a single session of rTMS to the right

inferior parietal lobule (IPL), to judge whether a face was theirs or that of someone they knew well.

The face of each participants was merged with the face of the other person to varying ratios of self/

other (e.g. 40% self to 60% other). The results indicated that task performance was significantly

impaired following rTMS to the IPL but not the control site (left IPL). In particular, there was a

higher number of “self” responses even when the face was comprised mostly of “other”

characteristics. These results may be interpreted as a less liberal technique in response to features of

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the ‘other’ or the overinclusion of stimuli with features that are associated with the ‘self’ (i.e., a

lower threshold for ‘self’ criteria). Based on these results, Uddin et al. proposed that self-

recognition is part of a larger system that represents the self and serves two roles. They propose that

it establishes representations between the self and others to facilitate communication, and also that

it distinguishes representations of self from others. They cite developmental studies showing a link

between the emergence of self-recognition and an increase in imitation abilities (Asendorpf &

Baudonniere, 1993). However such co-occurring developments ought to be researched further to see

if they are causally linked. One limitation of this study is the use of static faces as stimuli given the

typical association of mirror neutron activation and action observation. However research

demonstrating the activation of mirror neurons in response to static stimuli, whilst minimal, does

imply a similar process may be happening in this study. Given that this study did not include a

control condition involving discriminating between a non-self face and a familiar face, the

possibility that the observed effect might be the result of interference with general face

discrimination abilities, as opposed to self-recognition ought to be considered. Whilst the likelihood

of finding these results from such interference is very unlikely, future studies may wish to include

this control condition.

This finding improves our understanding of the self-other discrimination process but future TMS

research on empathy ought to expand on this to explore more directly the link between this process

and the observation of another individual’s emotional state. Given that the EBA and the parietal

cortex both seem to play a role in self-other discrimination, further research may benefit from

examining how they activate independently during social interactions or connectedly.

There are several general limitations that should be taken into consideration when using TMS.

When using TMS-induced MEPs as a research method it should be remembered that similar

electrophysiological output can result from different neural stimulation, and also that these

measurements are not based on the activity of single neurons but rather on the mass activation of

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neural pathways. These limitations make it difficult to infer whether shared neural activations

indicate shared emotional representations. Future TMS research ought to be used alongside more

precise techniques such as multivariate pattern analysis, which has been used to show that the same

local activation patters are used when both experiencing pain and observing others (Corradi-

Dell’Acqua, Hofstetter, & Vuilleumier, 2011).

A related limitation is the correlational nature of MEP-TMS (Logothetis, 2008), which only shows

the experience of empathy coinciding with neural reactivity. To draw further causal conclusions

about shared activations’ being a necessary condition for empathy, these methods ought to be used

alongside neurostimulation or lesion studies. Neurostimulation is also limited by the unavailability

at this time of any methods that would allow the noninvasive stimulation of regions such as the

insular and cingulate cortices, which have both been identified as being important for mentalizing

(Fan, Duncan, de Greck, & Northoff, 2011), as they are deep under the cerebral mantle. Deeper

TMS techniques are required for a thorough examination of the involvement of these regions. It

should also be noted that each of the components that has been examined is on its own insufficient

to account for empathy. Future studies ought to take into account the role that each of them plays

and study them alongside one another.

The majority of TMS studies examining empathy have only looked at the negative effect on its

underlying components. However it is worth mentioning, briefly, the research examining the

possible application of TMS in order to enhance empathy in clinical populations where it may be

deficient. A small number of studies have shown that it may be possible to use TMS to influence

brain activity to enhance the levels of empathy experienced by individuals. Working with adults

with autism spectrum disorder (ASD), Theoret et al. (2005) first showed that TMS may be used to

increase motor resonance reactivity in populations with neuroatypical levels of empathy. Whilst

impairments usually result from low frequency rTMS, high frequency rTMS causes an increase in

excitation, and may be used as a method of treating deficiencies in empathy. One case study

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(Enticott, Kennedy, Zangen, Fitzgerald, 2011) reported that a woman with ASD, following nine

sessions of high frequency rTMS to the bilateral medial prefrontal cortex, had reported higher levels

of social functioning (measured using the IRI, AQ, and Ritvo Autism-Asperger Diagnostic Scale) as

well as feeling an “increased capacity in empathy and perspective taking” which her family

subsequently corroborated. However, when considering the use of TMS as a form of intervention, it

is important to make certain ethical considerations. Notably, many of the clinical conditions where

TMS may be beneficial are neurodevelopmental disorders meaning young children could benefit

from such interventions. However, little research has been conducted into the safety of TMS in

children (Rossi, 2011) and this is something which should be explored further before popularising

this intervention technique.

To conclude, studies using TMS have advanced out understanding of the processes underlying

empathy. They have shown that observing pain in others provokes the same neural response as if the

observer themselves were experiencing pain. They have shown that this resonance process may be

modulated by top-down mentalising such as racial bias. They have also provided the foundations

for further research into the process of attributing the source of internal affective states to the self or

other. The current literature indicates that TMS is a promising technique to use in understanding

empathy and may provide the area of empathy research with unique contributions, given its

exclusive ability, unlike other imaging techniques, to modulate the neural processes involved in the

different components of this system. Whilst imaging methods cannot examine causality, they have

provided much of the information which has lead to further research with TMS. Furthermore, it

would be simplistic to reduce empathetic experience neural activity alone such as cortical

excitability is a sufficient accurate indicator for empathy. It is therefore important to include

subjective accounts and behavioural measures of empathy alongside TMS, as well as using it in

conjunction with other brain imaging methods. Fortunately, the use of TMS to study empathy and

its underlying processes is becoming increasingly popular (Hetu et al., 2012).

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