Mirror Neurons
In monkeys, neurons in the rostral (toward the front) part of inferior premotor cortex becomes activated not only when the monkey performs an action, but also when it observes another monkey or a human performing the same action (Pellegrino, et al., 1992). Called mirror neurons, these cells allow one to understand what another is experiencing without actually experiencing it oneself.
Mirror neurons have also been reported in humans
(Fadiga, et al., 1995). It is proposed that the mirror neuron system underlies emotion perception because the inferior frontal gyrus (an area in human brains that corresponds to the monkey ventral premotor cortex) becomes activated during the observation and the imitation of facial expression of emotions (Carr, Iacoboni, Dubeau, Mazziotta, & Lenzi, 2003). Furthermore, the insular cortex appears to be fundamental in the communication between the mirror neuron system and the limbic system (Iacoboni & Lenzi, 2002) which is primarily responsible for emotion processing (Decety, 2011a).
It appears that mirror neurons not only underlie the understanding of action, but also the intention behind action. Mirror neurons are more active when a behaviour is observed being performed in context (such as picking up a cup in an untidy room) than when the behaviour is performed without context (such as picking up a cup in a room without any other objects) (Iacoboni et al., 2005). Due to the increase in mirror neuron activity when context is provided, researchers suggest that both the recognition of action, and interpretation of the reason behind the action, rely on similar neural mechanisms (Iacoboni, et al., 2005).
Research using people who have congenital insensitivity to pain provides evidence against theory theory (Gopnik & Meltzoff, 1997) which proposes that people understand others' by using theories based on previous experience. In this rare syndrome, patients cannot rely on previous experience to understand the pain of others—they have never experienced pain.
Despite never having had the personal experience of pain, when these people observe others in pain, they show similar activity to ‘normal’ people in the anterior cingulate cortex and anterior insular which are believed to be the “shared circuits” for self and other pain (Danziger, et al., 2009, p. 203). Furthermore, activation in these areas is positively correlated with empathy scores (Singer et al., 2004). Thus, the discovery of mirror neurons lends support to simulation theory (Gallese & Goldman, 1998) which proposes that people understand others’ by imagining others' experiences, that is, by putting one's self in the other's shoes.
A recent study, however, failed to find evi¬dence of mirror neurons in humans (Lingnau, et al., 2009). When actions were first observed and then executed, there was activity in the intraparietal sulcus, superior parietal lobe, and dorsal premotor cortex; however, when actions were first executed and then observed, there was no resulting activity in these same areas. It was concluded that this lack of cross-modal activity is not compatible with mirror neuron theory because both action recognition and understanding do not appear to be governed by the same motor areas (Lingnau, et al., 2009). While the results may have been influenced by the authors’ use of context-neutral conditions, other concerns have been raised.
Specifically, the question still remains of the precise location of mirror neurons in the human brain (Decety, 2011a). Two meta-analyses have found that, in humans, the neurons in the ventral precentral gyrus (and not the inferior frontal gyrus as first proposed) correspond to the mirror neurons found in monkeys (Grèzes & Decety, 2001; Morin & Grèzes, 2008).
In addition, a more recent meta-analysis found that the dorsal premotor cortex, and the superior and inferior parietal lobules activate equally, or more, than the inferior frontal gyrus during imitation of hand and finger movements (Molenberghs, Cunnington, & Mattingley, 2009). Furthermore, no dysfunction in empathy has been found when regions believed to include mirror neurons are lesioned (Decety, 2011a).
The lack of empathetic dysfunction found in mirror neuron lesioning studies introduces the important point that mirror neurons cannot produce empathy alone (Hickok, 2009). Complex behaviour relies on networks of neurons that are interconnected, thus, there is no one place in the brain dedicated to empathy (Preston & de Waal, 2002a). In fact, subcortical circuits, including the amygdala, hypothalamus, hippocampus, and orbitofrontal cortex, are the essential to emotional arousal, with the reciprocal connection between the amygdala and orbitofrontal cortex, and the superior temporal sulcus, underlying rapid and prioritized processing of emotion input (Decety, 2011a).
Nonetheless, if an perception-action mechanism is crucial to empathy production, to date, mirror neurons represent the most parsimonious neural system enabling such a mechanism (Gallese, et al., 2002). It needs to be kept in mind; however, that an action-perception mechanism alone is not adequate for understanding another’s intentions, especially in a social context (Jacob, 2008).
Therefore, it is not helped that most neuroimaging studies using action and emotion tasks simply disregard activity in all cortical areas except for those expected to contain mirror neurons (Decety, 2011a). Furthermore, many relevant studies interpret any activity in these regions as due to mirror neurons rather than recognising that mirror neurons may only account for a small minority of neurons in these areas, and that these regions also subserve different abilities (Decety, 2011a). Clearly, additional research is necessary and, as pointed out by Gallese, Ferrari, and Umiltà (2002, p. 36), “the trick here is not to confound the phenomenal aspect of behaviour, its functional level of description, and the neural mechanism at its base”.
