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Empathy is central to the human experience and
thus has received much attention in the field of neuroscience. Empathy is generally studied in the context of vicarious reward
and punishment – one person processing someone else’s reward or punishment by
understanding or feeling events from the other person’s perspective. There is currently a debate in neuroscience about whether humans’
capability to learn by observation and experience vicarious rewards and
punishments relies on neural mechanisms similar to those employed when humans
learn based on personal experience. The
“extended common currency schema” school of thought would support this idea,
contending that a neural circuit integrates information from all factors (i.e. social and non-social factors) relevant to a
choice or experience. The
“social-valuation-specific schema” provides an alternative explanation as to
how neural value is computed in social versus non-social contexts, proposing
that social and non-social information are processed via similarly computation
principles but are implemented in distinct regions of the brain, specialized for
each type of information (Ruff & Fehr, 2014).

Several
studies support the idea that certain regions of the brain are involved in the
valuation of both non-social, personal experiences as well as social
experiences concerning the observation of others. In one
such study, while being scanned in an fMRI, participants viewed contestants
play a game in which they had to guess whether an unseen card was higher or
lower than a second unseen card and would win money based on their performance. After viewing the contestants play the game, the participants
played the game for themselves. Results
indicated that there were similar increases in ventral striatum activation for
when participants observed others win rewards and for when participants won
rewards themselves. Additionally, perceived similarity between the
participants and observed contestants modulated this value-related neural
activity in the ventral striatum, with increased similarity eliciting
heightened activity (Mobbs et al., 2009).

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            Other
studies have supported this finding that the ventral striatum integrates social
information and reward information. For
example, researchers conducted a study in which participants played a gambling
task guessing the outcome of a coin flip. The
participants could either win or lose money for themselves, their best friend,
or a disliked other – another “participant” who the researchers manipulated to
play an unfair strategy in an Ultimatum Game conducted prior to the gambling
task. The only brain region that showed an
interaction between the recipient of the gambling task’s outcome and the
outcome itself was the ventral striatum. There
was greater activation in the ventral striatum following gains than losses when
the participants and their friends’ received the outcome. Conversely, the ventral striatum was more active following losses
than gains when the disliked other received the outcome. Notably, the dorsal medial prefrontal cortex (dmPFC) and the
temporal parietal junction (TPJ), two regions of the brain associated with
social cognition, encoded information solely about the recipient of the outcome.  In these regions, there was
greater activity when the friend and the disliked other received the outcome
than when the participants received the outcome, regardless of the outcome
itself (Braams et al., 2014).

            Similar
to how the ventral striatum is associated with processing rewards to self and vicarious
rewards, the anterior cingulate cortex (ACC) and the insula are associated with
processing punishments to self and vicarious punishments. In one study, female participants in fMRI scanners sat next to
their partners and could see their partners’ right hand through a mirror system. Both partners in the couple had an electrode attached to their
hands that would deliver painful stimulations. Random
cues on a screen indicated whether the female (self) or her partner (other)
would receive a stimulation that was either painful or not painful. Experiencing the painful stimulation and observing their partner
experience the painful stimulation both triggered increased neural activity in
the ACC and insula. Importantly, activity the brain regions
associated with the sensory components of pain, such as in the sensorimotor
cortex, only increased when the female participants themselves received the
stimulation. This suggests that only the parts of the “pain
network” associated with affective qualities – the ACC and insula – and not
those associated with sensory qualities mediate the experience of vicarious
punishment (Singer et al., 2004).

This
result regarding vicarious punishment also held up when “punishments” involved
the smelling of disgusting odors. When
participants passively viewed movies of people smelling the content of a glass
and reacting with disgust and when participants inhaled disgusting odors, there
was increased activation in the anterior insula and ACC. This result indicates that at least for the feeling of disgust,
the neural substrate of feeling the emotion and perceiving it in others is
similar (Wicker et al., 2003).

            At
first, these results for the ventral striatum and ACC and insula taken together
may seem to support the idea that certain structures in the brain represent a
common currency that guides decision-making by integrating the motivational
relevance of all possible stimuli or actions present in a situation. This is because these parts of the brain encode both
reward-related and punishment-related information for both the self and others. In the context of non-social decisions, the common neural currency
in the brain is thought to be automatic and context-invariant. For example, in one study examining preference formation, during fMRI
scanning, participants were asked to rate either the pleasantness (explicit
task) or the age (distractive task) of images from various categories, such as
faces, houses, and paintings. After
scanning, participants were presented with the same images in pairs and were
instructed to choose which image they preferred. Results
indicated that the brains regions in the limbic fronto-striatal circuit encoded
participants’ preferences. This
effect was found across all three image types, suggesting context-invariance,
and for both the explicit pleasantness rating task and the distractive age
rating task, suggesting automaticity (Lebreton et al., 2009).
However, in the context of social cognition, context-invariance was not
observed, as value representations of vicarious rewards in the ventral striatum
varied depending on the perceived characteristics of others. Activity in the ventral striatum was modulated by perceived
similarity to oneself (Mobbs et al., 2009), and friendship and prior
fair behavior (Braams et al., 2014). This
finding seems to call into question the validity of a common neural currency
incorporating social factors into reward representation.

            Even
though the ventral striatum, ACC, and insula are active similarly for personal
experiences and vicarious experiences, there are regions of the brain that are
active during vicarious experiences that do not process rewards or punishments. This suggests that observational learning may not solely rely on
neural mechanisms that process the valuation of personal experiences. As previously mentioned, when participants played a coin flip
gambling task and won or lost money for themselves, their friend, or a disliked
other, the dmPFC and TPJ did not encode information about the outcome of task. Instead, there was only increased activity in these two regions
when the recipient of the outcome was the friend or the disliked other,
suggesting that these regions only process the social aspects of vicarious
reward (Braams et al., 2014).

The TPJ
has also been implicated to have a unique role in the processing of
agent-specific information when predicting socially guided decisions. In one study, participants played a simplified poker game against
either human or computer opponents. For
each trial of the game, participants received either a high or low card and
could choose to either bet or fold. If they
chose to bet and their opponent called, the participants would win if they had
the high card and lose if they had the low card. Participants
could also win money if they bet while having the low card and their opponent
folded. The researchers examined the unique
combinatorial performance (UCP) of 55 regions of the brain to determine whether
a particular region of the brain contributed to participants’ future behavior
more than all other regions of the brain. Their
analysis revealed that the TPJ distinctly carried social information compared
to all other regions, as its UCP against the human opponent was much greater
than its UCP against the computer opponent. Thus,
they concluded that the TPJ uniquely preferentially processed social
information, further supporting the idea of social-valuation-specific regions
of brain (Carter et al., 2012).

A recent
meta-analysis of 25 functional neuroimaging studies further supports the idea
that certain brain regions are engaged in the processing of vicarious rewards
due to their involvement in social cognition, but not in the processing of
personal rewards. This study found that regions related to
mentalizing – the act of inferring others’ mental states – are preferentially
associated with vicarious rewards. These
regions include the TPJ and dmPFC. The
primary explanation for this effect is that the vicarious sharing of others’
rewards typically requires someone to be able to understand “the extent to
which others value a particular outcome”. In
order for this understanding to occur, especially when an observer’s
preferences differ from a social target’s, one must project his or herself
outside their present circumstances and infer the preferences of others (Morelli, Sacchet, & Zaki, 2015).

Finally,
recent single-unit neural recoding studies in non-human primates have begun to
identify neurons in the ventral striatum that selectively encode either social or
non-social aspects of rewards. One
study employed a two-alternative forced-choice while recording the firing rates
of individual neurons in macaques’ striatum. The
monkeys indicated their choices with saccades to one of two visual targets. One target yielded a juice reward followed by a picture of a
monkey or a gray square while the other target only yielded a juice reward. Results indicated that certain striatal neurons only fired for
social information – the image of another monkey – while other neurons only
fired for reward information – receiving the juice (Klein & Platt, 2013). While
it is not feasible to conduct similar single-unit recording studies in humans,
it is possible that the ventral striatum in humans has neurons that are also
selective for either reward or social information. This
differential selectivity would not be revealed in fMRI imaging studies due to
their lack of adequate spatial resolution but may explain why the ventral
striatum is active for both social and non-social reward processing.

On a
broader level, this debate about the existence of a common neural mechanism to
understand others’ experience of reward and punishment is fundamentally a
question of what it means when one brain region similarly activates for
different actions. For example, when the same brain activates for
both personal and vicarious experiences, does it mean the ventral striatum is
actually representing the vicarious experience as if it was a personal experience
or is it merely representing what is happening in the given social scenario? In
the context of vicarious punishment, a brain imaging study is hardly needed to
conclude that observing a painful shock is different from actually experiencing
a painful shock, even though both actions may activate the ACC and insula. This is because experiencing pain is an affective experience
involving sensorimotor context. However, for vicarious reward, the answer to
this question is not immediately clear and warrants future research before
wider conclusions can be made about what similar activation patterns in the
ventral striatum represent. A
worthwhile direction for future research would be examining whether the two
types of reward processing actually influence choice behavior and learning in
similar manners. This is because currently, “similarities
between social and non-social reward processing and valuation are often
proposed based exclusively on reverse inference” (Ruff & Fehr, 2014). There
are few insights into how the brain treats rewards involving non-social and
social aspects in the classic biological sense. Based
on this lack of clarity presently, I think it would be misguided to assert that
a common neural currency exists that integrates non-social and social reward
information, especially the given the findings that implicate other brain
regions encoding social information but not reward information.

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