You can only perceive the location of a touch when you know where your hands are in space. Locating a touch to the body requires the integration of internal (somatotopic) and external (spatial) reference frames. In order to explore the relative contribution of internal versus external information, this thesis employed a crossed-hands tactile temporal order judgment (TOJ) task. This task requires participants to indicate which of two vibrations, one to each hand, occurred first. The magnitude of the deficit observed when the hands are crossed over the midline provides an index into how internal and external reference frames are integrated. This thesis first showed that the crossed-hands tactile TOJ task is a reliable measure, supporting its use as a measure of reference frame integration. Next, this thesis applied a probabilistic model to theoretically estimate the weights placed on the internal and external reference frames. We showed that a bias towards external information results in a larger external weight and vice versa for internal information. Finally, using the model we showed that the crossed-hands deficit is reduced while lying down, supporting an influence of vestibular information on the external reference frame. Taken together, this thesis highlights that we are able to flexibly adapt the weighting of different spatial representations of touch. / Thesis / Doctor of Science (PhD) / Determining the boundary of our body requires we localize the touches to our body. When the body moves and interacts with the world this determination becomes more difficult. Integrating information from other senses can support the localization of touch, and thus knowledge of our body. For example, to locate a touch to your right hand, you must feel the touch on your right hand, but also determine where your right hand is located in space. This thesis shows that the contributions of each sense to locate a touch is consistent within an individual and remains consistent over time. Interestingly, based on the availability of each sense, we flexibly adapt their contributions to ensure that our ability to locate the touch remains unchanged. What we define as our body is constructed based on the information available in the present moment.

Attended information is perceived quicker than unattended information. This is known as prior entry. When making judgments on the temporal order of two successive stimuli, performance is influenced based on attention. We were interested in whether this same attentional shift would occur when we adopt a crossed hands posture. Typically when making these tactile temporal order judgments, performance declines when the hands are crossed. This may be due to a greater influence of the external environment in the crossed posture. We investigated this by providing an exogenous visual cue at one or both of the hands prior to making judgments about the temporal order of two successive vibrations. This was completed with the hands crossed and uncrossed. In Experiment 1 responses were to which stimulus occurred first. In Experiment 2 participants responded to which stimulus occurred second. Changing the response requirement did not influence overall performance. In both experiments we observed prior entry that was in the same direction for both crossed and uncrossed postures. The size of the prior entry effect was larger when the hands were crossed. We remap tactile information quickly to external coordinates, however we are less certain of the hand’s location. / Thesis / Master of Science (MSc)

<p>Multiple reference frames are used to interact with our surroundings. When these reference frames are in conflict, processing errors can occur. For tactile stimuli, this conflict is highlighted when the hands are crossed over the midline of the body. In this posture, vibrotactile temporal order judgments (TOJs) presented to the hands are impaired compared to an uncrossed posture. This decrease in temporal processing is known as the crossed-hands deficit. The deficit was explored in depth throughout this thesis. In Chapters 2, 3 and 4 different elements of the crossed-hands deficit were evaluated including its connections to the rod and frame test, individual and sex differences within the TOJ task, as well as the influence of vision and body position. These elements were framed with underlying goal of investigating the root cause of the deficit. The data presented here provided evidence for a conflict model of crossed hands processing. A conflict between the internal and external reference frames produced the deficit in temporal processing when the hands were crossed. The role of the body’s midline in understanding multisensory integration was further considered in Chapter 5 through the rubber hand illusion, which is a visuotactile phenomenon whereby an unseen real hand is mislocalized towards a seen rubber hand. When the real hand, rubber hand, or both were crossed over the midline the illusion did not occur. It was hypothesized that a failure to integrate the tactile information presented to the real hand with the visual rubber hand was responsible for the absence of the illusion. Taken together, the data presented in this thesis contribute to the greater understanding of how reference frame conflicts are resolved, particularly when the conflict occurs across the body’s midline.</p> / Doctor of Philosophy (PhD)