In the late 1990′s neurophysiologists Giacomo Rizzolatti, Luciano Fadiga, Leonardo Fogassi, and Vittorio Gallese began publishing articles about their research and discovery of what they termed mirror neurons.In their study of macaque monkeys, they observed that neurons in the anterior intraparietal area (AIP) and in the ventral part of the frontal premotor area 6 (F5) which typically responded when a monkey performed an action also responded when the monkey observed another monkey performing an action. Upon further investigation the researchers discovered a neurological circuit which was capable of transforming action observation into action execution.  These findings led researchers to examine human participants where they discovered the same neurological system in operation:
Fadiga, Fogassi, Pavesi and Rizzolatti (1995) stimulated the motor cortex of normal human participants (transcranial magnetic stimulation) and simultaneously recorded motor-evoked potentials. They reasoned that, if the observation of a movement activates the premotor cortex, this activation should induce an enhancement of motor-evoked potentials elicited by the magnetic stimulation of the motor cortex. Fadiga et al. found that motor-evoked potentials were selectively enhanced when the participants observed the experimenter grasping objects. Based on this result, they suggested that there is a brain system which is sensitive to both action observation and execution in humans. This issue was also addressed by the use of positron emission tomography (Rizzolatti et al. 1996b). The main finding was the presence of a selective activation in the posterior part of the left inferior frontal gyrus when participants observed the experimenter in the act of grasping objects.
These findings prompted researchers to hypothesize that mirror neurons subserve the capacity of individuals to recognize actions made by others.
In 2001, Functional Magnetic Resonance Imaging (fMRI) was used to locate the specific areas of the brain that were activated during action observation. Results indicated that when object and non-object related actions were observed the AIP and F5 areas of the brain organized the actions somatotopicallywhich means that internal images of the actions observed were generated in the premotor cortex where one might expect to find a mental rehearsal of a an act to be performed. Mirror neurons are therefore neruomechanisms that facilitate a type of inward imitation.
On the basis of this research Vittorio Gallese formulated the shared manifold hypothesis which proposed that the human capacity to understand other human beings as intentional agents constituted an inter-subjectivity that made social relations possible. Gallese theorized that human beings are social animals for whom the ability to identify others of their own kind is vitally important. He explains:
As humans, we implicitly ‘know’ that all human beings have 4 limbs, walk in a certain way, and act in peculiar ways. Identity is articulated on many different levels of complexity. It can be subjected to increasingly complex tests in which different species might score differently, but it is nevertheless the membership fee all individuals have to pay in order to self-guarantee the sense of belonging to a larger community of other organisms. Identity is so important within a group of social individuals because it enables them with the capacity to better predict the consequences of the future behavior of others.
Gallese observes that there are two types of identity: self-identity and social identity. Self-identity allows individuals to individuate themselves and social identity allows individuals to situate themselves within a larger community. These identities are the result of social cognition active within inter-subjective relationships. Gallese points out that infants are capable of imitating the mouth and facial movements of adults within their first 18 hours of life. This innate capacity to mirror the behavior of others points to the neuromechanism that facilitates social cognition and identity.
Gallese theorizes that this shared manifold of human of inter-subjectivity, is the basis for empathy. Given, the discovery of mirror neurons and their activation during action observation he hypothesizes the same neural substrate is operative during expression observation and therefore constitutes a subpersonally instantiated common space in which an individual can understand the emotions of others. He points out that the word empathy originally had an aesthetic connotation and described an imaginative act where an observer located themselves within a work of art. He explains:
Empathy is a later English translation [Titchener, 1909] of the German word ‘Einfühlung’. It is commonly held that Einfühlung was originally introduced by Theodore Lipps [1903a] into the vocabulary of the psychology of aesthetic experience, to denote the relationship between an artwork and the observer, who imaginatively project himself/herself into the contemplated object. But the origin of term is actually older. As pointed out by Prigman , Robert Vischer  introduced the term in 1873 to account for our capacity to symbolize the inanimate objects of nature and art. Vischer was strongly influenced by the ideas of Lotze , who already in 1858 proposed a mechanism by means of which humans are capable of understanding inanimate objects and other species of animals by ‘placing ourselves into them’ (sich mitlebend … versetzen’).
But Gallese does not see empathy as a solely intellectual act. In fact, he argues that empathy is deeply grounded in the experience of a lived body. This insight led Gallese to the phenomenology of Edmund Husserl who observed that perception is predicated on an awareness of the acting body. But, it was the work of Maurice Merleau-Ponty that most intrigued Gallese; specifically, his notion of intercorporeality.
Gallese found in Merleau-Ponty’s Phenomenology of Perception support for his hypothesis that the relationship of self and other is governed by a dynamic system of reversibility. Gallese cites the following passage from Phenomenology of Perception as suport for his shared manifold hypothesis:
The communication and comprehension of gestures comes about through the reciprocity of my intentions and the gestures of others, of my gestures and intentions discernible in the conduct of other people. It is as if the other person’s intention inhabited my body and mine his.
Gallese interprets Merleau-Ponty’s notion of reciprocity as philosophical support for his neurophysiological evidence of mirror matching mechanisms that facilitate empathy. However, Gallese is quick to point out that he is not suggesting that individuals can understand others in the same way as they understand themselves. The shared manifold hypothesis does not reduce alterity to sameness; a common criticism of Merleau-Ponty. On the contrary, the shared manifold simply enables and bootstraps mutual intelligiblility. The social implications of this hypothesis are readily apparent. Mutual intelligibility between human persons allows each person to demarcate themselves from the larger social community in order to develop an individual identity, while simultaneously situating each individual within the social community. This type of social cognition provides a balanced equilibrium between the need to express our individuality and uniqueness, and the necessity to follow the social ‘rules’.
The most observable problem with Gallese’s phenomenological turn is his claim that mirror neurons operate at a subpersonal level. Even more problematic, Gallese appeals to a passage from Phenomenology of Perception in support of his claim seemingly unaware of Merleau-Ponty’s explicit argument in this work against the reduction of perception to third person processes like mirror-neurons. And yet, Gallese is pointed in the right direction. Merleau-Ponty is working with a particular principle, namely intercoporeality, that does serve as a basis for empathy but it is not a neurophysiological mechanism. Instead, it is the structure of a embodied subject living in the world.
 Rizzolatti,G., Fadiga, L.,Gallese, V.,&Fogassi, L. “Premotor Cortex and the Recognition of Motor Actions.” Cognitive Brain Research 3 (1996): 131–141. See also Gallese, V., Fadiga, L., Fogassi, L., & Rizzolatti, G. (1996). Action recognition in the premotorcortex. Brain, 119, 593–609.
 Craighero, Laila, Luciano Fadiga, Giacomo Rizzolatti and Carlo Umiltà. “Visuomotor Priming.” Visual Cognition, 1998, 5 (1/2), 110–111.
 Craighero, et al. “Visuomotor Priming,” 111.
 Buccino, G., et al. “Action Observation Activates Premotor and Parietal Areas in a Somatotopic Manner: an fMRI Study.” European Journal of Neuroscience, Vol. 13 (2001): 400.
 The correspondence between the position of a receptor in part of the body and the corresponding area of the cerebral cortex that is activated by it.
 Buccino, et al. “Action Observation,” 401.
 Gallese, Vittorio. “The Roots of Empathy: The Shared Manifold Hypothesis and the Neural Basis for Intersubjectivity.” Psychopathology 36 (2003): 171.
 Gallese, “The Roots of Empathy,” 171-172.
 Ibid., 172.
 Ibid., 172.
 Ibid., 176.
 Gallese, “The Roots of Empathy,” 175.
 Ibid., 176.
 Ibid., 176.
 Ibid., 176.
 Merleau-Ponty, Maurice. Phenomenology of Perception. Translated by Colin Smith. New York: Routledge Classics, 2002, 215.
 Gallese, “The Roots of Empathy,” 176.
 Gallese, “The Roots of Empathy,” 177.
 Ibid., 177.
 Merleau-Ponty, Maurice. Phenomenology of Perception, 64.