First, we would like to point out that ES is not confined to motor resonance of others’ actions, like that instantiated by macaques’ mirror neurons, as in humans ES also encompasses the activation of somatosensory areas during the observation of others’ tactile experiences, the activation of pain-related areas like the anterior insula and the anterior cingulate cortex during the observation of others’ pain, and the activation of the anterior insula and limbic structures like the amygdala during the observation of others’ emotions like disgust and fear (see our paper, p. 9 and Gallese & Sinigaglia 2011a). Thus, motor resonance only describes one partial aspect of ES.
Two distinct arguments can be used to explain why we do not think that AS constitutes a valid alternative to ES, as argued by Pfeiffer. We certainly agree with Pfeiffer that shared attention, that is, the capacity to direct the gaze to an object gazed by someone else, is a basic ingredient of social cognition. Indeed, as maintained by Colwyn Trevarthen (1977), shared attention marks in human infants around 9 months of age the transition from primary to secondary intersubjectivity. However, shared attention constitutes only one aspect of intersubjectivity and social cognition, thus AS at best only covers a partial aspect of social cognition and therefore appears to be more limited than ES in this respect. Furthermore, and most importantly, shared attention can be linked to motor resonance. Shepherd, Klein, Deaner, and Platt 2009) discovered in macaques a class of mirror neurons in the lateral intraparietal (LIP) area involved in oculomotor control, signaling both when the monkey looked at a given direction in space and when it observed another monkey looking in the same direction. These authors suggested that LIP mirror neurons for gaze might contribute to the sharing of observed attention. This evidence shows that shared attention is not divorced from motor resonance, but actually requires it.
A further argument in our opinion demonstrates that ES and AS should not be seen as alternative solutions to the problem of social cognition. Multisensory integration is a pervasive feature of parieto-frontal centers involved in sensory-motor planning and control. Indeed an influential theory about attention, the “Premotor Theory of Attention” (see Rizzolatti et al. 1987; Rizzolatti et al. 1994) states that spatial attention results from the activation of the same “pragmatic” circuits that program oculomotor behavior and other motor activities, even if such activation does not produce any overt motor behavior, thus qualifying as motor simulation.
We would like to emphasize even more strongly than we did in the paper that a crucial role of the cortical motor system is precisely that of integrating multiple sources of body-related sensory signals, like tactile, visual and auditory stimuli (see our paper, pp. 10–11; see also Gallese & Sinigaglia 2010, 2011b; Gallese 2014). The ventral premotor cortex (vPMC) might represent one of the essential anatomo-functional bases for the motor aspect of bodily selfhood, specifically because of its role in integrating self-related multisensory information. This hypothesis is corroborated by clinical and functional evidence showing the systematic involvement of vPMC with body awareness (Ehrsson et al. 2004; Berti et al. 2005; Arzy et al. 2006). This evidence demonstrates a tight relationship between the bodily self-related multimodal integration carried out by the cortical motor areas specifying the motor potentialities of one’s body and guiding its motor behavior and the implicit awareness one entertains of one’s body as one’s own body and of one’s behavior as one’s own behavior.
The vPMC is anatomically connected to visual and somatosensory areas in the posterior parietal cortex and to frontal motor areas and for this reason it is plausible to assume that vPMC activity reflects the detection of congruent multisensory signals related to one’s own body parts: this mechanism could be responsible for the feeling of body ownership. The motor aspects of the bodily self-enable the integration of self-related multimodal sensory information about the body and about the world with which the body interacts, as epitomized by the properties of macaques’ premotor neurons in area F4 (see Fogassi et al. 1996; Rizzolatti et al. 1997) and the analogous functional properties displayed by the human homologue of area F4 (see Bremmer et al. 2001). The same neurons controlling the movement in space of the head or of the upper limb also respond to tactile, visual, and auditory stimuli, provided they are applied to the same body part, like tactile stimuli, or they occur in the body-part-centered peri-personal space, like visual and auditory stimuli. Thus, we think that ES and multisensory integration should not be seen as alternative solutions to the problem of the neural bases of the bodily self, because multimodal integration carried out by vPMC is an instantiation of ES. We agree with Pfeiffer, however, that other brain areas, like TPJ, might contribute to a coherent sense of one’s own body. It must be added that TPJ is part of a network (including the posterior parietal cortex, and the premotor cortex) implicated in multisensory integration during self-related and other-related events and experiences. Indeed, as shown by Ebisch et al. (2011), the observation of others’ affective tactile experiences leads to the activation of observers’ vPMC and second somatosensory area and to the inactivation of observers’ posterior insula. Functional connectivity revealed a significant interaction between the posterior insula, right TPJ, left pre-central gyrus, and right posterior parietal cortex during the observation of other’s affective touch. These data suggest that TPJ might be involved in mapping the self–other differentiation, by means of lower-level computational mechanisms for generating, testing, and correcting internal predictions about external sensory events.
Last, we agree with Pfeiffer that the vestibular system might contribute to the bodily foundation of both the consciousness of self and others and we thank him for having pointed this out, thus integrating our perspective.