Peripheral and Central Mechanisms of Limb Position Sense and Body Representation

Each of us holds a unique and specific view of our own body. This arises from sensations, such as the sense of limb position and interoception (awareness of internal sensations of the body), allowing us to make conscious appraisals about our bodies. For instance, when we feel hungry, where our mouth is relative to our hands, and how to get food from the plate into the mouth with our fingers.
       Little is known in regards to how the brain processes sensory information in order to build a coherent central representation of the body. Muscle spindles, considered the main receptor in signalling position and movement sense, are one contributor to the body representation. The aim of the thesis was to explore the mechanisms of position sense and, more broadly, to investigate the sensory information involved in generating the central representation of the body.
       The findings in this thesis provided evidence to support the view that the brain is concerned with the signal difference coming from muscle spindles of antagonist muscles. Further, the brain likely compares this difference between limbs when matching the position of two limbs to determine their relative position (Chapter two, three). The studies utilise a history-dependent property of muscle fibres, thixotropy, to produce a directional bias in perceived limb position, referred to as position error.
       In Chapters four and five, position sense was examined using a two-arm matching task and a single-arm pointing task. It was found that manipulating muscle spindle signals through thixotropic muscle conditioning and vibration did not elicit the same effect on position sense between tasks. Hence, the existence of two distinct position senses was proposed. One sense was concerned with the position of a body party relative to the other, while the other sense was used to determine the location of a body part in extrapersonal space.
       Position sense tasks were also performed by individuals with chronic pain to examine the role of spindle signals in proprioception (Chapter seven) and body representation (Chapter six, eight). While it was shown that position sense appeared unaltered in chronic pain, the participants, who reported severe body image distortions, were more likely to have poor interoceptive awareness compared to those with no or minor distortions. This suggests that pain-related disturbance in body image was associated with awareness of the internal sensations of the body.
       Taken together, this thesis supports the existence of multiple body representations that derive and weigh information from various sensory sources to generate the sense of the body.