Prof. Giandomenico Iannetti | An Action Field Theory of Peripersonal Space

Interactions occurring within the space near the body are studied in a wide range of disciplines, from ethology to philosophy. Indeed, many behavioural responses are enhanced when stimuli occur near certain body parts. This makes evolutionary sense: a predator within striking distance is more relevant than one farther away. Neuroscientific studies in primates have suggested a physiological foundation for such behavioural modulations, leading to the concept of peripersonal space (PPS). But what is precisely meant when referring to PPS? Predominant conceptual frameworks describe PPS as a single, distance-based, in-or-out zone within which stimuli elicit enhanced neural and behavioural responses. In this talk I will first show that this intuitive framework is contradicted by neurophysiological and behavioural data. I will then argue that the so-called PPS measures do not represent stimulus proximity, but rather the value of actions aiming to create or avoid contact between objects and the body – and that for this reason they should be referred to as bodypart-centred response fields. This reconceptualisation of PPS as a set of graded egocentric fields describing the value of contact actions takes into account mainstream theories of action selection and behaviour. I will finally demonstrate, using reinforcement learning in artificial neural networks, that bodypart-centred response fields arise naturally from two simple and plausible assumptions about living agents: 1) they experience reward when they contact objects in the environment, and 2) they act to maximise reward. This perspective reproduces multiple foundational findings in the peripersonal space literature (seamlessly reconciling a number of contradicting empirical observations), provides testable predictions, and subsumes existing formal models of the so-called peripersonal space (PPS).