What do ‘peripersonal space measures’ really reflect? The action field perspective

The magnitude of a large number of behavioural and neurophysiological measures depends on the proximity between an individual and environmental objects. This relationship has led to the concept of peripersonal space (PPS). Here we argue that the proximity-dependence of such PPS measures could arise as a result of calculating the relevance of actions that aim to create or avoid contact with objects in the world. This perspective, supported by the interactive behaviour framework of systems-level brain function, allows us to describe PPS as a set of continuous fields reflecting contact-related action relevance. The action relevance perspective gets rid of incorrect notions about PPS, such as it being a single in-or-out zone that mainly reflects the spatial distance between objects and the body. This reconceptualization incorporates PPS into mainstream theories of action selection and behaviour. Furthermore, the formal comparison of this framework to others shows that contact-action value allows for a more complete description of PPS measures than proximity coding, impact prediction, and multisensory integration do, while simultaneously explaining the relationship between those concepts and PPS measures.

Peripersonal space

Research in cognitive neuroscience indicates that we process the space surrounding our body in a specific way, both for protecting our body from immediate danger and for interacting with the environment. This research has direct implications for philosophical issues as diverse as self-location, sensorimotor theories of perception, and affective perception. This chapter briefly describes the overall directions that some of these discussions might take. But, beforehand, it is important to fully grasp what the notion of peripersonal space involves. One of the most difficult questions that the field has had to face these past 30 years is to define peripersonal space. Although it bears some relations to the social notion of personal space, to the sensorimotor notion of reaching space and to the spatial notion of egocentric space, there is something unique about peripersonal space and the special way we represent it. One of the main challenges is thus to offer a satisfactory definition of peripersonal space that is specific enough to account for its peculiar spatial, multisensory, plastic, and motor properties. Emphasis can be put on perception or on action, but also on impact prediction or defence preparation. However, each new definition brings with it new methods to experimentally investigate peripersonal space. There is then the risk of losing the unity of the notion of peripersonal space within this multiplicity of conceptions and methods. This chapter offers an overview of the key notions in the field, the way they have been operationalized, and the questions they leave open.

Peri-personal space as an interface for self-environment interaction

The brain has developed a specific system to encode the space closely surrounding our body, our peri-personal space (PPS). This space is the theatre where all physical interactions with objects in the environment occur, and thus is postulated to play a critical role in both approaching and defensive behaviour. Here, we first describe the classic neurophysiological findings that have led researchers to conceive of PPS as a multisensory-motor interface. This historical perspective is given to clarify what properties are strictly related to PPS encoding, and what characteristics bear out or are related to PPS. Then, in an effort to uncover gaps in knowledge that often go unnoticed, we critically examine the association between PPS and i) multisensory processing, and ii) the motor system—its strongest allies. We do not mean to say that PPS isn’t multisensory-motor, simply to pinpoint current research shortcomings. Subsequently, we detail more recent psychophysical studies, highlighting the extreme plasticity of PPS, and its putative role in bodily self-consciousness and social cognition. Lastly, we briefly discuss computational models of PPS. Throughout the chapter, we particularly attempt to emphasize open areas of investigation. By critically evaluating past findings, many of them our own, we hope to provide a forward-looking perspective on the study of PPS.

Functional actions of hands and tools influence attention in peripersonal space

Human perceptual and attentional systems operate to help us perform functional and adaptive actions in the world around us. In this review, we consider different regions of peripersonal space—peri-hand space, reachable space, and tool space when used in both peri- and extrapersonal space. Focusing on behavioural and electrophysiology/event-related potentials (EEG/ERP) studies using comparable target detection paradigms, we examine how visuospatial attention is facilitated or differentiated due to the current proximity and functional capabilities of our hands and the tools we hold in them. The functionality of the hand and tool is defined by the action goals of the user and the available functional affordances or parts available to achieve the goals. Finally, we report recent tool-use studies examining how the distribution of attention to tool space can change as a result of tool functionality and directional action crossing peripersonal and extrapersonal space boundaries. We propose that the functional capabilities of the hand and tools direct attention to action-relevant regions of peripersonal space. Although neural mechanisms such as bimodal neurons may enhance the processing of visual information presented in near-hand regions of peripersonal space, functional experience and the relevance of the space for upcoming actions more strongly direct attention within regions of peripersonal space. And, while some aspects of functionality can be extended into extrapersonal space, the multimodal nature of peripersonal space allows it to be more modifiable in the service of action.

Human emotional expression and the peripersonal margin of safety

The brain evolved to give special representation to the space immediately around the body. One of the most obvious adaptive uses of that peripersonal space is self-protection. It is a safety buffer zone, and intrusions can trigger a suite of protective behaviours. Perhaps less obvious is the possible relationship between that complex protective mechanism and social signalling. Standing tall, cringing, power poses and handshakes, even coquettish tilts of the head that expose the neck, may all relate in some manner to that safety buffer, signalling to others that one’s protective mechanisms are heightened (when anxious) or reduced (when confident). Here I propose that some of our most fundamental human emotional expressions such as smiling, laughing, and crying may also have a specific evolutionary relationship to the buffer zone around the body, deriving ultimately from the reflexive actions that protect us.

Peripersonal space, bodily self-awareness, and the integrated self

Our experience of self seems to be of a single, coherent entity. This experience seems to conflict with the empirical literature, which suggests a different story: that the self is actually composed of many distinct and often conflicting elements. How do we get from the one story—a story of fragmentation, conflict, and dissociation—to the felt experience of ourselves as unified, coherent beings? This is a difficult problem that has vexed much recent work on the self. In this chapter, I want to highlight the pervasive nature of such integration problems at all levels of sensory, emotional, and cognitive processing, and argue that self-awareness can be best understood as arising from the very same principles that operate in these other domains. My central focus will be on the integration processes that function to generate peripersonal space. This focus is ideal, because peripersonal space itself seems to be one of the integrated bundles subserving our sense of bodily self-awareness, which in turn plays an essential role in our overall sense of self. By examining the processes of integration that generate peripersonal space, we will better understand the processes by which the self (itself) is constructed.

Close is better

The neuroscientific approach to peripersonal space (PPS) stems directly from electrophysiological studies assessing the response properties of multisensory neurons in behaving non-human primates. This multisensory context fostered frameworks which i) stress the PPS role in actions (including defensive reactions) and affordances, which are optimally performed through multiple sensory convergence; and ii) largely make use of tasks that are multisensory in nature. Concurrently, however, studies on spatial attention reported proximity-related advantages in purely unisensory tasks. These advantages appear to share some key PPS features. Activations in brain areas reported to be multisensory, indeed, can also be found using unimodal (visual) paradigms. Overall, these findings point to the possibility that closer objects may benefit from being processed as events occurring in PPS. The dominant multisensory view of PPS should therefore be expanded accordingly, as perceptual advantages in PPS may be broader than previously thought.

The structure of egocentric space

This chapter offers an indirect defence of the Evansian conception of egocentric space by showing how it resolves a puzzle concerning the unity of egocentric spatial perception. The chapter outlines several common assumptions about egocentric perspectival structure and argues that a subject’s experience, both within and across her sensory modalities, may involve multiple structures of this kind. This raises the question of how perspectival unity is achieved, such that these perspectival structures form a complex whole, rather than merely a disunified set of individually, distinctively structured experiences. The shortcomings of a variety of accounts are considered: switch accounts; sensory accounts; transformation accounts; and ultimate accounts. These shortcomings are addressed by a further kind of account provided by the Evansian conception—an agentive account—according to which egocentrically structured experiences present the world in relation to parts of a single thing, the body as a dynamic unity.

Risk-taking behaviour as a central concept in evolutionary biology

Predation is a common cause of mortality, having resulted in the evolution of a diverse kind of anti-predator behaviour across the animal kingdom. One such key behaviour is flight initiation distance (FID), defined as the distance at which animals take flight, when approached by a potential predator such as a human. Extensive research during the past two decades has revealed that optimal anti-predator behaviour is adjusted to life history (the combination of timing of reproduction, fecundity, survivorship, and others). FID is heritable, responds to natural selection and hence shows rapid micro-evolutionary change when animals are exposed to domestication, climate warming, or when introduced to novel environments. Peri-personal space (PPS) and inter-personal space (IPS) may be linked to FID, opening up the possibility of studying these disparate components of behaviour in a common context. Here, I provide a brief review of the extensive literature on FID, but much less well-studied PPS and IPS, and suggest ways in which such behaviour can provide insights into the evolution of anti-predator behaviour and life history. Such knowledge may help us resolve problems in conservation, effects of human disturbance on wild animals, problems of anti-predator behaviour for animal welfare, and potentially even maladaptive anti-predator behaviour and PPS and IPS in humans.

Do we represent peripersonal space?

Work in both animals and humans has demonstrated that the brain specifically tracks the space near the body—the so-called ‘peripersonal space’ (PPS). These representations appear to be multimodal and expressed in body-centred coordinates. They also play an important role in defence of the body from threat, manual action within PPS, and the use of tools—the latter, notably, ‘extending’ PPS to encompass the tool itself. Yet different authors disagree about important aspects of these representations, including how many there are. I suggest that the questions about the nature and number of PPS representations cannot be separated from the question of the mathematical basis of the corresponding representational spaces. I distinguish cartographic from functional bases for representation, suggesting that the latter provides both a plausible account and support a single-representation view. I conclude with reflections on functional bases and what they show about representation in cognitive science.