T128. Modulation of exteroceptive electromyographic responses in defensive peripersonal space

Abstract

Introduction The cutaneous silent period (CSP) is a spinal nociceptive inhibitory reflex, that briefly interrupts voluntary contraction of limb muscles involved in reaching and grasping movements after noxious electrical stimulation of a cutaneous nerve. It is mediated by A-delta afferents. CSPs act in conjunction with flexor withdrawal reflexes to constitute a sophisticated defensive system for protecting a limb from harmful stimuli. CSP is functionally rather than anatomically organized. I1 and I2 components are considered spinal, E2 is regarded transcortical in origin. Defending the body surface from environmental threats requires sensorimotor integration between multimodal sensory afferents and motor planning of defensive movements. Cortical areas involved in this integration are the poly-sensory zone in the precentral gyrus and the ventral intraparietal area. The defensive peri-personal space (DPPS), immediately surrounding the body, was first described in macaque monkeys. Neurophysiological studies of the human DPPS demonstrated larger somatosensory blink reflexes with the stimulated hand located near the face. We investigated the effect of hand position on CSP in DPPS vs. extrapersonal space (EPS). Methods Fifteen healthy volunteers underwent serial CSP testing with the arm resting on an elevated table, elbow flexed, and the hand held either within 5 cm from the nose (NEAR) or away from the body (FAR). Electrical stimuli were applied to either digit 2 or 5 (D2, D5). Sensory thresholds (ST) were established for each subject and digit. EMG recordings were obtained from first dorsal interosseous (FDI) muscle during 20% maximal voluntary contraction, following random stimulation of either D2 or D5 in both NEAR and FAR conditions, with 5, 10, and 15X ST intensity. CSPs were identified by a decrease in mean rectified EMG activity below 80% of prestimulus baseline. CSPs were divided into I1, E2, and I2. Results Following D2 stimulation, CSPs consisted of I1, E2, and I2. With increasing stimulus intensities, CSP duration and I2 duration increased, suppression index (SI) of CSP and I2 decreased, and E2 amplitude decreased progressively, both in DPPS and EPS. E2 amplitude was significantly larger in DPPS vs. EPS, while no other parameter differed between DPPS and EPS. Following D5 stimulation, CSP equaled I2, and there was no clear E2. With increasing stimulus intensities I2 duration increased and SI decreased, both in DPPS and EPS. No parameter differed between DPPS and EPS. Conclusion Concurring with previous literature, E2 was present in FDI only when stimulating homotopic D2 and not heterotopic D5. E2 was larger in DPPS vs. EPS, representing direct neurophysiological evidence of transcortical sensorimotor processing related to DPPS, while inhibitory CSP parameters did not differ between DPPS and EPS, concurring with a spinal organization of its circuitry.