The mechanisms underlying altered somatoperception and somatosensation in healthy and subclinical populations

Manipulating somatic representations has been found to alter somatic experiences; however, the precise mechanisms underlying these altered somatic experiences are as yet unclear. This thesis primarily investigated the mechanisms underlying altered somatic experiences following illusions that manipulated perception of the body representation. The current thesis also addressed individual differences in somatic perception across individuals with propensities towards various clinical conditions, including amplified somatosensory sensitivity and medically unexplained symptoms (MUS). The pilot investigation in Chapter 3 provided evidence for susceptibility and ownership towards somatic illusions generated using the MIRAGE mediated-reality system, thus validating manipulations induced using this system. In Chapter 4, longer and shorter body representations were judged as veridical (or normal) following stretched and shrunken illusions respectively, while in contrast to early studies ownership was not lost as a result of the illusory manipulations. An association between self-reported somatic sensitivity and illusion strength was also observed for females, with females reporting increased somatic sensitivity being more susceptible to the illusion. Chapter 5 demonstrated that illusory alterations of body shape and size improved perception of near threshold tactile stimuli. However, changes in tactile perception were driven by differing mechanisms when body size at the site of stimulation was altered, whilst similar mechanisms drove this change when body size away from the site of stimulation was altered. Interestingly, a detached condition (in which the finger-tip and stump were disconnected) resulted in a significant reduction in overall positive reports of feeling tactile stimuli. Finally, overall false-touch reports and reduced sensitivity (i.e., the inability to discern between touch present and absent trials) were found to be characteristic of those with propensities towards MUS. Chapter 6 demonstrated that a purely visual illusion, in the absence of any real somatic input, did not interfere with external tactile perception or lead to different response patterns between individuals with increased or decreased tendencies towards MUS. The thesis provides evidence for the dynamic and bidirectional flexibility of the body representation by providing direct evidence for the immediate updating of the body representation following size-altering illusory manipulations. These illusions also altered external somatic sensations via different underlying mechanisms and reflected individual differences in response patterns between healthy and sub-clinical populations, thus suggesting that susceptibility to such illusions may be clinically relevant, and useful in identifying the nature various psychological pathologies