| Summary: | The spatial location of environmental events with respect to one’s body largely dictates their behavioural relevance. Given that stimuli occurring near the body have a greater potential to cause harm, even the phylogenetically-old defensive hand-blink reflex (HBR) increases in magnitude with stimulus proximity. The HBR has allowed for a preliminary characterisation of a defensive peripersonal space (DPPS). The work described here provides a full spatial characterization of DPPS using formal geometrical modelling of HBR data, and highlights the functional significance of DPPS through its dependence on various contextual factors. Modelling and empirical results indicate that (1) the shape of the body area defended by this DPPS can be approximated as a half-ellipsoid centred on the face. (2) The DPPS extending from this to-be-defended area has the shape of a bubble elongated along the vertical axis. (3) This DPPS is malleable: its shape is continuously updated based on gravitational cues. The DPPS also changes in disease: while blind individuals do also display a HBR, (4) the nervous system only develops the ability to modulate HBR magnitude if vision is present during early childhood. (6) In trigeminal neuralgia (TN), a condition in which innocuous trigeminal stimulation triggers paroxysmal unilateral facial pain, DPPS is larger on the side of space ipsilateral to TN. This reflects an increased estimated potential of sensory events to cause harm on that side of space. Finally, (7) DPPS expands when the HBR-eliciting stimulus is moving towards the face. These findings show that the brain purposefully modulates the defensive HBR with proximity in a context-dependent manner, in order to ensure optimal behavior and protection from estimated threats. At a more theoretical level this work also critically discusses ambiguities in the terminology used to report empirical results about peripersonal space, which have generated much confusion in the field.
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