The discovery in the early 1990s of mirror neurons in the brain of macaques (Gallese et al. 1996; Rizzolatti et al. 1996), and the subsequent discovery of mirror mechanisms in the human brain (see Gallese et al. 2004; Rizzolatti & Sinigaglia 2010) suggest that there exists a direct modality of access to the meaning of other people’s behaviours—a modality that can be set aside from the explicit attribution of propositional attitudes. Mirror neurons are motor neurons, originally discovered in macaques’ ventral premotor cortex area F5, later on also found in the reciprocally-connected posterior parietal areas AIP and PFG. Mirror neurons not only respond to the execution of movements and actions, but also respond to the perception of actions executed by others. Mirror neurons map the action of others on the observers’ motor representation of the same action. Further research also demonstrated in the human brain the existence of a mechanism directly mapping action perception and execution, defined as the Mirror Mechanism (MM; for a recent review, see Ammaniti & Gallese 2014; Gallese 2014). In addition, in humans the motor brain is multimodal. Thus, it doesn’t matter whether we see or hear the noise made by someone cracking peanuts, or locking a door. Different—visual and auditory—sensory accounts of the same motor behaviour activate the very motor neurons that normally enable it. The brain circuits showing evidence of the MM, connecting frontal and posterior parietal multimodal motor neurons, most likely analogous to macaques’ mirror neurons, map a given motor content like “reach out” or “grasp” not only during their performance, but also when perceiving the same motor behaviour performed by someone else, when imitating it, or when imagining performing it while remaining perfectly still. The relational character of behaviour as mapped by the cortical motor system enables the appreciation of purpose without relying on explicit propositional inference.
Altogether, these findings led to the formulation of the “Motor Cognition” hypothesis as a crucial element in the emergence of social cognition (Gallese 2009). According to this hypothesis, cognitive abilities like the hierarchical representation of action with respect to a distal goal, the detection of motor goals in others’ behaviour, and action anticipation are possible because of the peculiar functional architecture of the motor system, organized in terms of goal-directed motor acts. Traditionally, the relation between actions and their outcomes is assumed to be largely independent of the motor processes and representations underpinning action execution. Such processes and representations allegedly concern elementary motor features such as joint displacements or muscle contractions only. However, solid empirical evidence challenged this view. Motor processes may involve motor representations of action goals (e.g., to grasp, to place, etc.), and not only kinematic or dynamic components of actions. This suggests that beliefs, desires, and intentions are neither primitive nor the only bearers of intentionality in action. We do not necessarily need to metarepresent in propositional format the intentions of others to understand them. Motor outcomes and motor intentions are part of the “vocabulary” that is spoken by the motor system. On occasion we do not explicitly ascribe intentions to others; we simply detect them. Indeed, we posit that motor representation is enough to ground the directedness of an action to its outcome (Gallese 2000, 2003b; Butterfill & Sinigaglia 2014; compare also Gallagher’s 2005 notion of direct perception).
One of the consequences of the discovery of mirror neurons was the possibility of deriving subjectivity from intersubjectivity at the sub-personal level of description. The sense of self is precociously developed, beginning from a self that is first of all physical and bodily, and which is constituted precisely by the possibility of interacting and acting with the other. Embodied simulation can provide the neurobiological basis for early forms of intersubjectivity, from which the sense of the self is built. The discovery of mirror neurons and the simulation mechanism would therefore seem to further stress that being a self also implies being with the other. The model of intersubjectivity suggested by mirror mechanisms and embodied simulation correlatively sheds new light on the subjective dimension of existence. Let us see first what type of intersubjectivity mirror neurons seem to suggest.
The discovery of mirror neurons gives us a new empirically-grounded notion of intersubjectivity connoted first and foremost as intercorporeality—the mutual resonance of intentionally meaningful sensorimotor behaviours. The ability to understand others as intentional agents does not exclusively depend on propositional competence, but it is highly dependent on the relational nature of action. According to this hypothesis, it is possible to directly understand the meaning of other people’s basic actions thanks to a motor equivalence between what others do and what the observer can do. Intercorporeality thus becomes the main source of knowledge that we have of others. The motor simulation instantiated by neurons endowed with “mirror properties” is probably the neural correlate of this human faculty, describable in functional terms such as “embodied simulation” (Gallese 2003a, 2005, 2011; Gallese & Sinigaglia 2011b).
Action constitutes only one dimension of the rich baggage of experiences involved in interpersonal relations. Every interpersonal relation implies the sharing of a multiplicity of states like, for instance, the experience of emotions and sensations. Today we know that the very nervous structures involved in the subjective experience of emotions and sensations are also active when such emotions and sensations are recognized in others. A multiplicity of “mirroring” mechanisms is present in our brain. It was proposed that these mechanisms, thanks to the “intentional attunement” they generate (Gallese 2006), allow us to recognize others as our fellows, likely making intersubjective communication and mutual implicit understanding possible. The functional architecture of embodied simulation seems to constitute a basic characteristic of our brain, making possible our rich and diversified intersubjective experiences, and lying at the basis of our capacity to empathize with others.