The human body is virtually surrounded by a peri-personal space (PPS), the portion of space lying in the boundary zone between reachable objects near the body and the body itself (1, 2). The PPS has an essential role distinguishing the self from others (3), and discriminating the location of external stimuli relative to the position of one’s own body. Ultimately, the PPS serves the individual interaction with the surrounding space (3). Defensive peripersonal space (DPPS) has been defined as the part of the PPS where there is increased physiological reactivity to stimuli that are potentially harmful to the body (4, 5). The DPPS can be investigated using the Hand Blink Reflex (HBR) task (6–8) which measures the changes in orbicularis oculi muscles contraction in response to an electrical stimulation of the median nerve when the arm is placed in different positions with respect to the head. In particular, in a static position of the arm, the HBR is modulated by the hand position in space: the response dramatically increases when the stimulated hand is located close to the face, inside the DPPS (8). Further, in dynamic conditions (i.e., when subjects performed upper limb voluntary movements toward and far from the face), the DPPS boundary around the body is continuously shaped by the predictive motor system, resulting in an increased HBR response when the hand moves toward the face, but decreases when the hand moves away from the face (7). This shows that in dynamic conditions HBR modulation depends not only by the actual position of the stimulated hand, but also by the final position where the hand is expected to be at the end of the movement.

HBR is in fact related to the perceived “threat” for the face posed by the electrical stimulation of the wrist, and is therefore modulated by those conditions that modulates the dangerousness of a stimulus. For example, when a wooden screen is placed between the participants’ face and their hand the HBR enhancement by hand-face proximity is suppressed (9), whereas merely limiting the visual component (i.e., keeping the eyes closed) leads to an increase in the intensity of the reflex when the hand is inside the DPPS (8).

Studies of the DPPS in adults show significant interindividual differences, although little is still known about the determinants of such variability. Ronga and colleagues showed that the PPS is already present hours after birth (10), and recently (11) has been demonstrates an association with adult attachment style (attachment anxiety specifically) and the PPS flexibility to adapt situationally. The DPPS may display similar changes across developmental phases. Accordingly, Fossataro and colleagues suggests that early attachment experiences have an impact on how the boundaries of DPPS are encoded, finding a different HBR modulation in a group of participants with organized interpersonal cognitive schemas about the early attachment experiences with respect to a group with disorganized one (12). It has been found that not only early, but also late experiences related to sport activity can modify threat perception, and thus the extension of the DPPS, also measured through HBR recording, which was shaped by expertise in boxing athletes (13). Moreover, studies found that the DPPS displayed inter-individual differences in relation to psychopathology, such as the levels of arousal in phobic subjects (14) and the severity of anxiety (15, 16). Two other studies examined the relationship between specific traits related to interpersonal functioning, namely social cognition (17), and empathy (18). To our knowledge, however, the DPPS has never been investigated in relation to personality. Personality is defined as the “pattern of behaviors, feelings, and thoughts, that shapes specific forms of interactions between the subject and its environment, determined both by genetic and neurobiological systems and by the experiences” (19). Modern accounts of personality conceptualize it as comprising multiple dimensions spanning affective, cognitive, and behavioral domains, shaped by the interaction of innate predisposition and experience. In this framework, sensorimotor, embodied processes related to the self and spatial navigation may be essential components that influence, or are influenced by, the development of personality, especially in interpersonal and social functioning (20).

Given these premises, the aim of this study was to examine the relationship between the variability of the DPPS extension, measured by the HBR response, and personality facets in human subjects. We did not have predefined hypotheses regarding the association between specific personality traits and DPPS, hence we used and explorative approach. We expected, in fact, that parameters of the behavioral task and self-report measures to be highly inter-correlated within each domain, but weakly correlated cross-domain (21). Thus, we used network analyzes, an exploratory multivariate approach, to examine the association between parameters of the peripersonal space and personality scores from the PID-5. Thus, our aim was to investigate the relationship between the DPPS and personality in humans through a network analysis. In particular, we explored whether the extension of the DPPS, measured as the increase of HBR response when the hand is placed near the face, was associated with specific personality traits.

The aim of the study was to explore the relationship between personality facets and DPPS. Using the slope of HBR response intensity as a measure of DPPS extension, we considered two portions of space, one directly surrounding the face and one toward their hand. We found a positive association between MtoN slope, namely the extension of the DPPS near the subject face and the Detachment personality dimension, so that a steeper boundary of the DPPS was related to personality with a higher propensity to detachment. Instead, personality was not connected with the FtoM slope or the intercept parameter, suggesting that it may bear less or no association with basal EMG activity or with the HBR evoked in the far portion of space.

The personality dimension of Detachment corresponds to a stable behavioral pattern with a tendency to avoid emotional experiences. Detachment includes both withdrawing from interpersonal interactions ranging from casual, daily interactions to close friendships or intimate relationships, as well as a restricted experience and expression of affect (39). The domain of Detachment derives from the facets of withdrawal, intimacy avoidance, anhedonia, depressivity, restricted affectivity, and suspiciousness (39). Detachment is intended as the need for greater interpersonal distance and tendency to avoid social contacts (40, 41). Developmental accounts of individuals with detached personality have been associated with a poor sense of security to self and others and insecure attachment styles, possibly related to poor parental bonding in the early phases of life (42). Interestingly, no correlation was found between Detachment and a steeper FtoM slope, thus suggesting a relationship with broader DPPS boundaries rather than a gradual increase of HBR. Our findings suggest that people with more pronounced Detachment attitudes display an over-responsive attitude toward external threatening stimuli, as indicated by a steeper proximal DPPS. Thus, the Detachment personality trait may constitute a learned compensatory mechanism gradually implemented since the early experiences to adult life to minimize the risk of contact with external stimuli with potential threatening valence. Knowledge on the relationship between the PPS and psychopathology is still limited and often contradictory, especially considering the more specific DPPS. To our knowledge, our study is the first to employ a comprehensive, reliable measure of personality, a construct that includes several inter-related features. Our finding is consistent with, and extend the findings of previous studies on specific psychological or psychopathological traits, showing that healthy individuals with high empathy trait (18), disorganized attachment styles (12) high anxiety trait (43, 44) or claustrophobia (43, 45) can modulate the PPS extension. Intriguingly, both a transitory state like anxiety and a chronic condition like claustrophobia were linked to an enlarged PPS size, with reduced plasticity associated to the phobic disorder (28, 43, 45).

The question here is whether certain personality domains can affect the representation of one’s DPPS or if how specific characteristics of the PPS underlie the etiology of maladaptive personality traits (such as Detachment). A first hypothesis is that maladaptive personality traits, intended as pervasive interior modality of thinking, behaving, and relating with others, can lead to a reduced extension of the DPPS, which becomes less reactive to external influences or conditioned by a dysfunctional interpretation of them. As a larger DPPS size is strictly associated with the necessity to avoid interpersonal contact and on defensive purpose, its positive correlation with the detachment dimension results coherent with the underpinned personality profiles and their defensive mechanisms.

On the other hand, PPS directly contributes to the experience of self and self-boundaries, through tactile processing and multisensory interactions, encoding the experience of body integrity (46), and of the body ownership (body self-consciousness) (47), corroborating the stability (or instability) of personal identity over time. We may hypothesize that early experiences could shape DPPS extension, that could become stable associated with hypervigilance and attentional bias characteristics. The Detachment, intended as the need of greater interpersonal distance (40, 41), would be a defensive compensation for the feeling of invasion of one’s intimacy. Interestingly, von Mohr and colleagues (11) explored the extension on PPS and interpersonal space (48) in subject with attachment anxiety, which itself is characterized by negative biases in the interpretation of social cues and worry about emotional closeness (49). In particular they found that in subjects reporting high levels of attachment anxiety, PPS boundaries are always very sharp, whereas in subjects with low scores, PPS boundaries are modulated situationally: sharper in the presence of strangers and less defined when subjects are alone. Authors suggests that the “hypervigilant strategy” adopted by individuals more anxiously attached may be an adaptive response developed in response to an inadequate childhood environment and then consolidated over time. Although this study focused on a more social aspect, the results seem in line with our association between detachment and more defined DPPS boundaries.

While the Detachment is a stable personality trait, a crucial characteristic of the PPS is represented by its plasticity and dynamicity: plasticity refers to the flexibility elicited through training or learning, while dynamic changes are elicited in response to modifications in the environment or to the internal state changes of an individual, including emotional state changes (46). Therefore, PPS is continuously remapped by both top-down and bottom-up interoceptive and exteroceptive stimuli (50). As a safety boundary around the body, DPPS is dynamically shaped through predictive motor mechanisms (7), with an increased HBR when the stimulus is near to the face (7, 8). Given this premise, it can be assumed that the body-space interactions vary depending on individual’s purpose (reaching or defensive), despite an identical spatial encoding of the incoming stimulus (51). Therefore, DPPS is modulated by several factors: interpersonal interaction and levels of empathy can affect subcortical defensive responses as measured by the HBR (18).

A seminal work of Sambo & Iannetti (28) found a correlation between a larger DPPS extension and higher trait anxiety, measured with STAI (Sate Trait Anxiety Inventory) (52), classifying subjects depending on the size of their DPPS with fixed hand-face distances (e.g., “ultra-far,” 60 cm; “far,” 40 cm; “near,” 20 cm; “ultra-near,” 4 cm.). On the contrary, we did not find any significant correlation between DPPS extension and the PID-5 negative affect dimension, which comprises the anxiety facet. Inconsistency between these findings may depend on classification of subjects and on assessment instruments. We choose to consider a continuous indicator of DPPS extension, i.e., the two FtoM and MtoN slopes, to continuously order the responses of our subjects. On the other hand, Sambo and colleagues tried to define 4 limited classes of behavior based on HBR responses. Further, the personality assessment chosen in the two works are very different. In particular, the STAI anxiety trait encompasses different anxiety symptoms, including anticipatory, somatic, affective and cognitive symptoms (52). On the contrary, the anxiety facet of the PID-5 focuses on anticipatory anxiety (e.g., “I worry a lot about terrible things that might happen,” “I get very nervous when I think about the future,” “I always expect the worst to happen”) (53). Future studies might clarify whether enlargement of the DPPS extension is specifically associated with a specific tendency to experience somatic symptoms of anxiety, in particular hyperarousal, which has been associated with an increased functioning of the human defensive system (54). In fact, the personality domain of Detachment has been associated with avoidant, obsessive-compulsive, and schizotypal personality disorders (55) and with PTSD, particularly the internalizing subtype (56). These disorders share a tendency to develop symptoms of hyperarousal in response to various perceived threats.

This study is strengthened by a relatively large sample size, as regard as HBR topic, a rigorous modeling of DPPS data and a reliable assessment of personality. However, we cannot draw inferences on the causal association between DPPS extension and psychopatological traits, due to the cross-sectional design. While personality domains are hypothesized to be stable in the long-term, the PPS may be plastic, thus possibly reflecting training or learning from adaptively responding to the environment (46). In this framework, the PPS three-dimensional space modulable by environmental or organic stimuli (50). Further longitudinal studies are needed to disentangle the relationship between DPPS and personality. If the DPPS was confirmed as a determinant of maladaptive personality traits, it may constitute a biomarker or a target for rehabilitative interventions. This work presents, however, some limitations. While this can be considered a rather large study in the specific HBR literature, the sample size may have been insufficient to capture small effects with sufficient statistical power. Further studies are still required to estimate what size of effects would correspond to significance in this field. Even if this is, to our knowledge, one of the largest studies regarding the analysis of HBR data, the sample size is relatively small for the investigation of the personality dimensions and is represented by a non-clinical population. Another critical point may regard the questionnaire chosen for the personality assessment. Despite the PID-5 being a validated tool for scientific research and the assessment of personality traits, dimensions, and disorders, its relatively recent introduction may potentially limit the reliability of results.

This study also presents an exploratory approach, since we did not have a priori hypotheses, and further investigations may be necessary to investigate more precisely the correlation between the detachment, its features and DPPS.

In conclusion, we suggest that inter-individual differences in DPPS extension may be partially explained by personality trait. This suggest that maladaptive aspect of personality can affect the experience we have of the world, conditioning how we learn to which stimuli react, thus modulating the expression of our “safety margin” (57).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Regional Ethics Committee - CER Liguria: 633/2020–DB id 11064. The patients/participants provided their written informed consent to participate in this study.

MoB, AE, MaB, and MA contributed to conception and design of the study. MoB, AE, AT, FD, and BS recruited the subjects and performed research. AB developed the analysis software. MM performed the statistical analysis. AT wrote the first draft of the manuscript. MM and GS wrote sections of the manuscript. All authors contributed to the article and approved the submitted version.