Approaching Threats: The interplay between Corticospinal Excitability and Peripersonal Space

Abstract

Background: The peripersonal space (PPS) is a network that integrates multisensory information to guide interactions with environmental threats. Studies such as Gerits (2023) suggest a relationship between threat proximity and corticospinal excitability (CE). Our research extends this exploration, considering how threat direction and proximity affect CE, specifically in relation to individuals' fear of spiders. Aim: This study investigates how the perception of a spider threat affects corticospinal excitability (CE) and motor processing in the motor cortex, with a focus on the direction of the threat's movement and its precise trajectory relative to the participant's hand. Methods: Participants were divided into fear and no-fear groups based on their self-reported fear of spiders. They were exposed to animations of spiders approaching and receding from their PPS. Motor Evoked Potentials (MEPs) were evoked using Transcranial Magnetic Stimulation (TMS) and measured via Electromyography (EMG). Results: Consistent with our hypothesis about threat direction, approaching threats led to a reduction in MEP amplitude, suggesting a proactive inhibition response. As threats receded, an increase in MEP amplitude was observed, indicative of a release from inhibition. Contrary to our hypothesis about influence of individual fear levels, no significant differences were found between the fear and no-fear groups. Supporting our Location Hypothesis, a greater reduction in MEP amplitude was noted when the TMS pulse was closer to the hand, underscoring the significance of threat proximity in motor modulation. Discussion: These results suggest an interaction between threat proximity and motor response, not fully explained by the fear level. The findings challenge the traditional view that fear significantly enhances CE changes in response to perceived threats within the PPS.The confirmed impact of threat direction and proximity provides valuable insights into the spatial determinants of motor preparedness and defensive behavior. Conclusion: Our findings emphasize the complex and dynamic nature of the PPS in motor response modulation to threats. Future research, utilizing virtual reality to simulate more ecologically valid scenarios with dynamic and unpredictable threat trajectories, will be crucial for a deeper understanding of these mechanisms in real-world contexts.