Mirror Neurons

Basic Findings

Using single cell recording, mirror neurons have, so far, been discovered in two areas of the macaque brain: first in area F5 of the inferior frontal cortex (ventral premotor cortex) and later in the inferior parietal lobule (IPL). These two brain areas work in concert and receive input from areas involved in the perceptual processing of biological motion, such as the superior temporal sulcus (STS). It is important to note that only a small portion of neurons in area F5 and IPL have mirroring properties.

Two types of mirror neurons can be distinguished, strictly congruent and broadly congruent neurons. Strictly congruent neurons (roughly 1/3 of the mirror neurons in area F5) discharge when the monkey performs a particular action, such as grasping an object with the thumb and index finger (precision grip), and when the monkey observes the same movement. In contrast, broadly congruent neurons (roughly 2/3 of the mirror neurons in area F5) discharge to a wider range of movements during observation. For instance, a broadly congruent neuron may only fire during the performance of a precision grip but not during the performance of a power grip, whereas it may fire regardless of grip type during observation.

It has been suggested that mirror neurons provide a simple and direct form of action understanding. The mirroring mechanism they provide could allow monkeys to understand the actions of their conspecifics by simulating the observed actions using their own action repertoire. The finding that mirror neurons do not discharge when the monkey observes hand actions that are not directed at an object (intransitive actions) illustrates that this understanding seems to be limited to object-directed actions. However, the object an action is directed at does not need to be visible in order for mirror neurons to discharge. If the monkey observes someone grasping an object that is hidden from the view of the monkey, the same mirror neurons will discharge as when the object is not hidden. Thus, knowing that an action is directed at a particular object is sufficient to trigger mirror neuron discharge.

It has been suggested that the two different parts of the mirror circuit, area F5 and IPL, support two different aspects of mirroring. Neurons in IPL seem to be involved with coding the goal of an action, such as eating a fruit or placing it into a container. In contrast, mirror neurons in area F5 seem to code the actual movement that is needed to achieve a particular action goal. Although most studies on mirror neurons have focused on action perception in the visual domain, there is also evidence that some mirror neurons respond to sound. These neurons discharge not only when the monkey sees an action, but also when it hears a sound that normally accompanies a particular action, such as the sound of a peanut being cracked.

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