Titolo della tesi: Investigation of the dynamic modulations of peripersonal space: evidence from clinical conditions and mindfulness-based practices on the plasticity of the self
In everyday life, we constantly interact with stimuli in the environment, whether they are objects or people. These interactions may require to protect ourselves, for example, when a bee flies threateningly toward our body. Although such responses appear quickly and effortlessly, the brain coordinates and integrates numerous sources of information in real time, both from the body and from the external environment. In the case of bee, the eyes help track the insect in space; if it comes closer, the upper limbs and hands may be recruited to protect the face; in some cases, the trunk and lower limbs may also be involved to initiate locomotion for escape.
From a spatial perspective, each body part involved contributes to encode the external stimulus, and the distance between the body and the stimulus plays a crucial role. Neurophysiological and behavioral research has demonstrated that the brain builds a specific representation for objects near the body, in a region referred to as peripersonal space (PPS). While the extent of this area varies, it generally corresponds to the space within reach.
From an evolutionary viewpoint, the existence of such a representation is adaptive. Within PPS, neural and behavioral responses are enhanced. Different theoretical frameworks have attributed distinct roles to PPS: some emphasize its role in supporting goal-directed actions and interactions with the environment, while others stress its importance in protecting the body margin. Current evidence suggests that both perspectives capture core aspects of PPS. More recent findings have also proposed that PPS contributes to shaping the phenomenological experience of the self, which is deeply grounded in bodily sensations. In other words, the sense of self may extend beyond the body’s physical boundaries.
Such hypothesis stems from studies on the bodily self, one of the components of self-processing along with the cognitive self, showing that bodily illusions (e.g., changes in body ownership or self-identification) rely on reorganization of multisensory integration between internal and external cues, and are accompanied by modulations in PPS size.
These modulations refer to a key and well-studied property of PPS, that is its plasticity. It is well established in the literature that a boundary separates PPS from far space (also known as extrapersonal space). Both neurophysiological studies, first in animals, then in humans with neglect or crossmodal extinction, and behavioral research show that this boundary is subject to shifts. Specifically, PPS can either shrink (i.e., boundary closer to the body) or expand (i.e., boundary further into space). Such modulations have been observed in response to changes in action capabilities (e.g., temporary limb immobilization or tool use), exposure to threatening stimuli, bodily illusions, and social contexts. For example, the mere presence of another person at about 100 cm shrinks one’s PPS. However, the quality of interpersonal interactions modulates PPS differently, depending on whether the interaction is perceived as positive or negative.
This thesis focuses precisely on the plasticity of PPS. The introduction outlines the mechanisms underlying PPS and its relevance for action, interaction, and self-representation. The first part of the thesis presents a systematic review of studies investigating PPS alterations in individuals with sensorimotor impairments, including stroke, spinal cord injury (SCI), and amputation. These conditions were selected because, although they produce similar sensorimotor outcomes, they have different causes and involve damage to distinct locations (i.e., brain, spinal cord, peripheral body). The inclusion of amputation further allows for examining the role of vision of the limb, which is present in stroke and SCI, but absent in amputations.
Seventeen studies were reviewed and discussed within a framework that views PPS primarily as a space for action. Despite difficulties in disentangling motor from sensory contributions, most studies consistently reported PPS shrinkage or absence in affected individuals. Studies investigating the use of rehabilitative tools or prostheses showed that these promote partial or temporary recovery of PPS representation.
The second part of the thesis is dedicated to experimental research on mindfulness meditation. This section provides a systematic investigation of how three types of mindfulness practice influence PPS: focused-attention meditation (FAM), open monitoring meditation (OMM), and compassion meditation. The rationale for this investigation stems from phenomenological literature on alterations in self-experience, particularly in the sense of boundaries, i.e., the perceived separation between self and non-self or between self and the surrounding environment. These boundaries, which do not coincide with the physical body, have been referred to vanish or expand during meditation, and this may be reflected in changes in PPS.
The first experimental study compared expert meditators and non-meditators in a pre-post design. Using an audio-tactile interaction paradigm to measure multisensory facilitation, the results showed that a short FAM session led to a blurring of PPS boundary in both groups. This effect aligns with reports of a perceived dissolution of self-world boundaries during mindfulness.
The second study compared FAM and OMM in two groups of non-meditators, to assess whether the effect observed in the first study was specific to FAM or generalizable to mindfulness practices. Results indicated that OMM did not alter PPS characteristics, suggesting that the two practices and their differences in attentional focus (object-based vs. metacognitive) have distinct effects. The modulation observed after FAM may be due to its focus on internal sensations, such as breathing.
While the first two studies highlight PPS modulations potentially linked to self-experience, the third experiment explores PPS in relation to social cognition. This study examined the state effects of compassion meditation in non-meditators. Participants completed the same audio-tactile task pre- and post-meditation, and additionally performed two tasks assessing interpersonal distance preferences during social interaction: one required participants to indicate a comfortable distance from a stranger, and the other involved reachability judgments. We found that meditating on compassion toward others and human interdependence reduced preferred interpersonal distances, though PPS was not altered. This may support the hypothesis that meditation leads to increased social openness. In conclusion, the findings of the studies and the review are discussed. While the systematic review frames PPS mainly as a space for acting, it cannot be excluded that sensorimotor impairments also impact bodily self-representation.
Instead, the experimental studies on mindfulness focus more directly on the bodily self, and up to the self-other relationship in the study on compassion. Building on influential theoretical models and evidence linking high-level changes in empathy and altruism with low-level changes in bodily self-experience, this research supports the idea that mindfulness practice may influence both domains. In fact, although meditation is typically an individual practice, it is consistently associated with increases in prosocial behavior.
Understanding prosociality and the low-level mechanisms underlying it is crucial for addressing public health issues recently emphasized by the World Health Organization, particularly those related to loneliness and social isolation. Moreover, it has broader implications for social and environmental challenges, such as climate change and global peace. Thus, the potential of mindfulness- and compassion-based interventions, or training of their core components, needs to be considered to reduce self-centeredness and promote prosocialness.