Mirror neuron system

Abstract

Byline: V. Rajmohan, E. Mohandas Mirror neuron system is a group of specialized neurons that mirrors the actions and behaviour of others. The involvement of mirror neuron system (MNS) is implicated in neurocognitive functions (social cognition, language, empathy, theory of mind) and neuropsychiatric disorders. MNS discovery is considered to be the most important landmark in neuroscience research during the last decade. Mirror neuron system - discovery The mirror neurons were discovered serendipitously by Giacomo Rizzolatti and colleagues while working on the grasp response of macaques. They observed that a group of neurons in the area F5 of the premotor cortex that fires when a macaque performs an action; also discharges when it observes the same action being performed by another animal.[sup] [1] Subsequent research has elucidated the diverse regions involved in the MNS of monkeys. Recently different cortical structures have been described as part of the MNS in humans. MNS in monkeys The area F5 of the premotor cortex (premotor area located.in the posterior bank of the inferior arcuate sulcus and the cortical convexity immediately caudal to it) in monkeys has two sets of visuomotor neurons namely the 'canonical' and the 'mirror neurons'. The 'canonical neurons' (in F5 bank region) respond to presentation of an object while mirror neurons (in F5 convexity) respond to performance of an action and observation of an object directed action.[sup] [2] The mirror neurons are triggered by any action that involves the interaction between a biological effector (mouth, hand etc.) and an object. They are stimulated by the observation of the exact same action involving the effector and object ('strictly congruent' neurons) and also by actions that are similar but not having exact effector-object interaction ('broadly congruent' neurons).[sup] [3] Other areas that form part of the MNS are the superior temporal sulcus (STS) and the area 7b (PF of Von Economo) in the inferior parietal lobule (IPL). The STS codes for a larger number of movements than the F5 neurons but lacks motor properties (i.e., does not discharge while performing the movement). The area 7b neurons in IPL are heterogeneous and have a role in coding sensory stimuli and respond to somatosensory, visual or bimodal stimuli. I in addition to this, they also have motor properties and discharge on action observation and performance. The IPL receives inputs from the STS and sends an important output to the ventral premotor cortex including area F5 [Figure 1].[sup] [3],[4] MNS in humans Brain imaging studies reveal that action observation in humans activates the inferior frontal gyrus (IFG), lower part of the precentral gyrus, the rostral part of the IPL and also the temporal, occipital and parietal visual areas.[sup] [3] The frontal and the parietal mirror neuron regions are somatotopically organized. The activation of pars opercularis of the IFG reflects the observation of distal hand and mouth actions, whereas the activation of the premotor cortex reflects proximal arm and neck movements [Figure 2]. The mirror neurons in humans, unlike those in monkeys fire even while observing meaningless (intransitive) movements. The observation of transitive actions causes the firing of the frontal and the temporal nodes of the MNS while that of intransitive actions result in the firing of the frontal node only.[sup] [5],[6] FUNCTIONS OF MNS Action understanding Action understanding is the fundamental function of the MNS. Each time the animal observes a certain action being performed by another animal, the mirror neurons representing the performance of that action are activated. The mirror neurons transform visual observation into knowledge.[sup] [3] Studies on humans during action observation have shown activation of the IFG, the IPL and a region within the STS. The precentral motor cortex though not activated by action observation is involved indirectly in action understanding as they have a role in motor imagery. …