STUDENT PSYCHOLOGY JOURNAL VOLUME II MIRROR NEURONS: OUR CURRENT UNDERSTANDING Georgina Mullen Senior Freshman, Psychology, UCD [email protected] ABSTRACT Mirror neurons are a relatively new phenomena, first observed in the premotor cortex of macaque monkeys when a number of neurons were observed to respond both when a monkey performed a goal orientated task, and when the monkey watched another (human or monkey) perform that task. A number of researchers have suggested that mirror neurons also exist in humans. It is proposed that a human mirror neuron system may contribute to a number of cognitive functions such as action understanding; ‘theory of mind’, humans’ abilities to infer another’s mental state through experiences or others’ behaviour; emotion understanding; imitation; and speech perception. Faulty human mirror neurons have even been suggested to underpin social impairments such as those characteristic of Autistic Spectrum Disorder (ASD). However, there has been much debate regarding the existence and functional roles of mirror neurons in humans. While there is much literature regarding human mirror neurons, the majority consist of reviews while few concern empirical experiments. Additionally concern has been expressed for some of the experimental methods used in empirical studies. A recent experiment from Mukamel et al. (2010) is the first of its kind to directly gather evidence for the existence of mirror neurons in humans and for their function subserving action understanding. The present review critically outlines the growth in this controversial field of research, taking into account the recent direct recording of human mirror neurons, and what implications this may have on our understanding of social cognition. INTRODUCTION Social cognition involves any process among conspecifics, allowing for individuals to understand the actions, intentions and emotions of others. Such social abilities are a crucial aspect of human survival and success (Blakemore et al., 2004). For this reason much research has been devoted to exploring what mechanisms and processes underlie social cognition. REVIEW This expansive research has resulted in what has been suggested by some to be the greatest recent discovery in neuroscience; mirror neurons (Ramachandran, 2000). Ramachandran (2000) suggests that the discovery of mirror neurons will impact the field of psychology as DNA influenced biology; that mirror neurons can provide a unifying framework which can explain a number of humans’ mental abilities. However, such grand claims have not gone undisputed. While it is largely accepted that mirror neurons exist in macaque monkeys where they were originally recorded (Casile et al., 2011), researchers have been skeptical regarding the existence and proposed functions of a human mirror neuron system (Dinstein et al., 2008). MIRROR NEURONS IN MONKEYS Direct recordings in macaque monkeys have found that both observation of an action and performance of that action can lead to the discharge of a subset of neurons, called mirror neurons (Gallese et al., 1996). These mirror neurons were first observed in the F5 area of the premotor cortex (di Pelegrino et al., 1992; Gallese et al., 1996) but were later observed in the inferior parietal lobule (Fogassi et al., 2005; Rozzi et al., 2008) (see Figure 1). Further research investigated mirror neuron activity in two conditions; when a monkey watched a human hand reach and grasp an object (the visible condition), and when the monkey watched a human hand reach and then disappear behind a screen (the hidden condition) (Umilta et al., 2001). Some mirror neuron activation was recorded during the hidden condition, but only when the monkeys had first seen an object at the location behind the screen, suggesting that mirror neurons in monkeys are responsible for action understanding (Umilta et al., 2001). Furthermore Fogassi et al. (2005) suggested that mirror neurons contribute to monkeys’ understanding of intention, based on their experiment in which different populations of parietal neurons fired when a monkey grasped an object which was subsequently eaten, and when the monkey grasped an object which was subsequently placed in a box. A HUMAN MIRROR NEURON MECHANISM Based on the discovery of mirror neurons in monkeys, researchers began to question whether a similar mirror mechanism may exist in humans (Gallese et al., 1996). Experiments that support the existence of a human STUDENT PSYCHOLOGY JOURNAL VOLUME II mirror neuron mechanism have mostly used indirect methods which indicate neural activation. Some of the first evidence was derived from Hari et al.’s (1998) study which recorded neuromagnetic oscillatory activity in participants’ precentral cortex. Hari et al. (1998) measured brain activity using magnetic resonance imaging (MRI) and magnetoencephalography (MEG) while participants manipulated a small object with their hand. They observed a significant modification in neuromagnetic activity when participants’ observed others manipulating objects. This indicated the existence of an action observation/execution matching system in the human brain, similar to the one previously observed in monkeys (Hari et al., 1998). Subsequently a number of empirical studies investigated the presence and possible functions of a human mirror system. These appeared to support a mirroring mechanism within humans located in the frontal and parietal areas of the brain (Iacoboni et al., 1999; Iacoboni et al., 2005), and in other motor regions (Hari et al., 1998; Koski et al., 2003; Gazzola & Keysers, 2009) (see Figure 2). Additionally multisensory mirroring mechanisms have been observed in nonmotor regions such as the amygdala and insula (Hutchison et al., 1999; Calder et al., 2000; Wicker et al., 2003; Keysers et al., 2004). These studies used a number of techniques to measure brain activation, including functional (f)MRI, MEG, electroencephalography (EEG), positron emission tomography (PET), transcranial magnetic stimulation (TMS), and lesion studies (observation of neurological patients). These methods do not allow for a direct and exclusive measure of mirror neuron activity, which has led a number of researchers to question the validity of the assumption that a mirror neuron system exists in humans (Dinstein et al., 2008; Hikock, 2009). While it is clear that these indirect methods cannot provide definite evidence, they are thought to correlate well with direct measures (Iacoboni, 2009). Dinstein et al. (2008) caution the inferences drawn from indirect methods such as fMRI as responses may not be generated by mirror neurons but by other neural populations. These interpretations therefore fail to take into account the fact that mirror neurons in monkeys only make up a small minority of neurons in these areas and thus the activation may not be from mirror neurons but rather from neighbouring visual, motor, and visuomotor neurons (Dinstein et al., 2008). Recent research from Mukamel et al. (2010) recorded single neuron activity in humans, rather than using indirect methods. This recording REVIEW during action observation and execution appears to support previous indirect suggestion that a mirroring mechanism exists in humans. While past research has focused on recording activity in areas of the brain homologous to regions containing mirror neurons in monkeys, Mukamel et al. (2010) detected a mirroring mechanism in areas of the medial frontal and temporal cortices which were not previously suggested to contain mirror neurons. Mukamel et al. (2010) did not record in areas where human mirror neurons had been suggested as placement of electrodes was determined only by clinical considerations (participants consisted of patients with pharmacologically intractable epilepsy). Despite Mukamel et al.’s (2010) influential evidence, some criticisms of a human mirroring mechanism remain valid. For example, the argument that evidence for mirror activation in humans does not have key features in common with the mirror neuron system seen in monkeys (Heyes, 2009). Specifically, human mirror activation occurs in both homologous areas and in areas where mirror neurons have not been reported in monkeys (Dinstein et al., 2008). Additionally the majority of mirror neurons found in monkeys are responsive to actions on objects, while proposed human mirror neurons often respond to gestures as well as actions on objects (Hickok, 2009). POTENTIAL FUNCTIONS OF A HUMAN MIRROR MECHANISM ACTION UNDERSTANDING As mirror neurons in monkeys are thought to be the neural basis of action understanding (Umilta et al., 2001), when researchers began to question the existence mirror neurons in humans, they did so with the assumption that if mirror neurons were observed, their function would involve action perception (Hari et al., 1998; Grezes & Decety, 2001; Gazzola & Keysers, 2009; Mukamel et al., 2010). Further research questioned what other functions mirror neurons might subserve, including higher social cognitive abilities such as communication (Rizzolatti & Craighero, 2004), empathy (Iacoboni, 2009), sensations (Keysers et al., 2004) and emotions (Wicker et al., 2003). Gazzola and Keysers (2009) used fMRI to record shared neural activity in individuals during action observation and execution. The areas of the brain suggested to contain shared voxels include the dorsal premotor cortex, the supplementary and cingulate motor areas, the superior parietal lobe, the somatosensory cortices and the cerebellum (Gazzola & Keysers, 2009) (see Figure 2). While