Unveiling top-down modulation of Hand blink reflex: The frontoparietal network of defensive peripersonal space.

The Hand Blink Reflex (HBR) is a subcortical reflex response influenced by limb position in relation to the Defensive Peripersonal Space (DPPS) of the face. We investigated its cortical correlates with functional Near-Infrared Spectroscopy (fNIRS) by measuring cortical hemodynamic activity before and after evoking the HBR in NEAR (hand close to the face) and FAR (hand far from the face) positions, with the aim to explore whether cortical activity associated with arm positioning predicted HBR modulation. We hypothesized that HBR would elicit activation of the frontoparietal network involved in DPPS encoding. PRE_STIMULUS analysis revealed significant activity in primary somatosensory cortex (Brodmann area (BA) 3) and posterior parietal cortex (BA40) modulated by arm position. Enhanced activity in NEAR position might suggest increased salience of potential threats based on the integration of body-and-spatial information. BA3 and BA40 activity showed a significant correlation (r = 0.60; p = 0.002), indicating a functional relationship in encoding body representation with respect to the spatial context. POST_STIMULUS analysis revealed sustained BA3 activation and a decreased activity in the right inferior frontal cortex (BA44). Further, a negative correlation was found between BA44 activity and HBR duration (r = -0.36; p = 0.033), supporting a mechanism whereby the threat evaluation affects inhibitory control, facilitating defensive responses. These findings show that top-down modulation of HBR begins before stimulus onset with the activation of a frontoparietal network, integrating sensory and spatial cues and influencing subcortical circuits, adapting the defensive behavior as a function of the body segments position.