Distance based spatial summation of noxious stimuli in spatially adjacent but somatotopically distinct body regions

Jack Beard, Mark J. Catley, Danny Camfferman, G. Lorimer Moseley

Enquiries: Jack Beard, email: beaja005@mymail.unisa.edu.au

To date, distance-based spatial summation has only been studied in the somatotopic domain. However, as distance-based spatial summation is centrally driven, it is plausible that the phenomenon is not restricted to somatotopically adjacent body regions but can occur across somatotopically distinct body regions that occupy the same space. For example, if a person were to sit too close to a fire such that the skin of their forearm and adjacent leg (i.e. two body limbs occupying a similar space yet somatotopically distinct) were simultaneously exposed to the threatening stimulus, it could be advantageous from a biological perspective for it to hurt more than if only the leg were threatened.

This study aims to determine whether distance-based spatial summation occurs when paired noxious stimuli are applied to spatially adjacent but somatotopically distinct body regions. This study has been approved by the University of South Australia’s human ethics committee.

Design: Randomised and counter-balanced four by two repeated measures design.

Participants: Healthy participants will be sought via social media and flyers placed throughout the University of South Australia. Data collection will be undertaken at the Body in Mind laboratories at the University of South Australia’s City East Campus.

Methods: Participants will provide basic demographic information (e.g. age, gender) and complete the Edinburgh Handedness Inventory. The participant will be seated at a table with pre-determined markings on the table such that the participant can easily move his/her hand to a position instructed by the examiner. To determine the perceptual intensity of the stimuli to be delivered during the experiment, the participant’s pain threshold will be determined according to the method of limits. Noxious electrical stimuli will be delivered simultaneously to the dorsal aspects of both hands via intracutaneous electrodes. The participants will be instructed that stimulus intensities will be randomly delivered at 15-second intervals. The stimuli will be delivered in eight experimental blocks, each consisting of 24 randomised and counter-balanced stimuli (i.e. 3 stimuli at 4 separate distances for 2 temperatures). Participants will be asked to close their eyes and focus on the midpoint between the two hands and to rate the intensity of the stimuli following each trial using the computer generated VAS.

Analysis: A repeated measures four (distance) x two (stimulus intensity) analysis of variance with significance set at α=0.05 will be used to assess whether distance-based spatial summation occurs over two somatotopically distinct body regions.