Putting touch into context: investigating tactile perception in body-based and object-based contexts

Abstract

The present thesis consists of two themes. The first theme aims to expand our understanding of how the visual body context influences visuo-tactile interactions for nonspatial perception and the potential role of body identity in this process. A growing body of research has demonstrated that interactions between external visual events and touch on the body is augmented when the visual events arise on or within reach of the body. In addition, viewing a body part, such as a hand, can amplify the sensitivity of tactile processing. These findings (reviewed in Chapter 1) may reflect a ?contextual rule? of multisensory processing and are linked to the broader multisensory representations of the body and the space within reach of the body. However, few studies to date have focused on if and to what extent the visual body context modulates temporal visuo-tactile interactions, particularly for multisensory numerosity judgements. A series of psychophysical experiments were carried out using a nonspatial cross-modal congruency task (CCT; introduced in Chapter 2); participants were tasked with discriminating how many vibrations they felt on their hand whilst ignoring numerically compatible or incompatible visual distractors superimposed onto an image of a hand or an object. CCT work (Chapters 3 ? 6) confirmed that the cross-modal congruency effect (the difference in error rates between incongruent and congruent visuo-tactile trials) was not modulated by the visual body context, regardless of whether the hand was presented from ?mirrored? allocentric (i.e. third person) or egocentric (first person) views or when viewing one?s own or another?s hand. When the image of a hand was presented from the egocentric view, overall tactile discrimination errors were significantly decreased compared to the object context. This also emerged when viewing a hand image from the ?mirrored? allocentric view with salient identity information (Chapter 3). The effect was replicated when comparing error rates in response to a hand, object or baseline condition (i.e. no stimulus) and was unaffected when participants viewed the same digit being stimulated or a different digit on the hand (Chapter 4). This context-induced difference in error rates did not appear when viewing a hand or object from the ?mirrored? allocentric view without salient identity information. Further experiments confirmed that this was not due to the visibility of a participant?s physical hand/arm during the ?mirrored? allocentric set-up or the visual similarity of the object to their hand. Increasing the visual saliency of the object stimulus once again resulted in a significant reduction in performance accuracy. These findings suggest that, relative to viewing an object, viewing a hand is associated with a more accurate discrimination of tactile sequences of events and a concomitant reduction in cross-modal interference, and are discussed in relation to the combined role of visual context, tactile sensitivity, task difficulty and selective attention on cross-modal interactions. Chapter 5 investigated the developmental trajectory of these effects in children aged 5 ? 12 years and adults. There was no effect of visual context found on error rates for any age group, possibly due to reduced task difficulty. However, children aged 5 ? 9 years showed larger interference from visual distractors on tactile targets compared with tactile distractors on visual targets as well as evidence for optimal visuo-tactile processing. The findings suggest a protracted development of temporal tactile perception and the emergence of visuo-tactile integration for numerical judgements in early childhood. The second theme examines individual differences in multisensory processing with respect to body plasticity, synaesthesia and mental imagery (reviewed in Chapter 6). Chapter 7 investigated the role of the perceptual body image on responses to the rubber hand illusion, a measure of the malleability of body ownership. A group of individuals who recognised their own hand in a visual array showed reduced plasticity of the body representation in response to spatiotemporally synchronous visuo-tactile input compared to a group who did not, showing a role of the perceptual body image in influencing the plasticity of the bodily self. Chapter 8 investigated tactile mirroring and tactile mental imagery in the general population and in a sample of synaesthetes. Heightened tactile mirroring in mirror-touch synaesthetes (compared to non-mirror-touch synaesthetes or non-synaesthetes) was observed, and was not affected by the type of tactile inducer, viewing perspective or the compatibility of the sex of the viewed body with that of the observer. Overall, visuo-tactile mirroring was most vivid when viewing touch to the back of the neck. Mental imagery for object-based haptic and body-based somatosensory sensations was also enhanced in mirror-touch synaesthetes. Nonetheless, strong mental imagery for haptic and somatosensory sensations was found in the general population. Although the vividness of imagery varied across respondents, mental imagery ratings across tasks were positively correlated. The significance of these findings and directions for future research are discussed in Chapter 9.