Localizing touch on the skin surface and in external space: revisiting the serial model

Medina, JaredUpon receiving a tactile stimulus, the brain represents its location on the skin surface (e.g. on the index fingertip) as well as in external space (e.g. 10 cm to the right of my body midline). Prior literature proposed a serial model for these processes: The brain first localizes touch on the skin surface, then localizes touch in external space by combining the stimulus location on the skin surface with the current body position. Given that information regarding body position is utilized after a touch has been localized on the skin surface, the model predicts that perceived stimulus location on the body is not affected by body position. However, some studies provide evidence for the effect of body position on localizing touch on the skin surface, challenging the serial model. The central aim of this dissertation is to further test if information from external space influences tactile localization on the skin surface, using experimental investigations with both neurologically intact and impaired individuals. Evidence from these experimental investigations will provide a more complete understanding of how functional systems involved in tactile processing are organized. ☐ Previous studies have reported brain-damaged individuals who substantially mislocalized touch on the body. However, whether tactile localization errors vary with body posture has not been systematically examined. Chapter 2 of this dissertation reports performance of a brain-damaged individual on tactile localization. The individual systematically and consistently mislocalized touch on the hand towards the left side of space relative to the hand long axis. Importantly, judgments on the skin surface varied with hand posture, biased towards the pinky when the contralesional left hand was palm facing down, and towards the thumb when the hand was palm up. The data thus provide strong evidence that hand posture influences tactile localization on the skin surface. These findings also provide evidence that touch is represented in an external reference frame specifically centered on the long axis of the hand. ☐ Motivated by evidence from the brain-damaged individual, additional investigations were performed on neurologically intact individuals to further examine if tactile localization is influenced by external information. The experiments in Chapter 3 investigated tactile localization on the hand surface when there was a spatial conflict between visual and proprioceptive estimates of hand position in external space. The results showed that tactile localization judgments were consistently biased towards the direction of the proprioceptive estimate, providing evidence that information from external space influences where touch is perceived on the body. Potential mechanisms for the tactile localization biases were discussed. ☐ The experiment in Chapter 4 examined tactile localization across body parts, testing tactile localization among fingers while manipulating the finger posture. The results did not provide strong evidence that the distance between fingers influences tactile localization. However, the distribution of localization error varied depending on whether all the fingers were on the same plane, suggesting influences of relative finger elevation on tactile localization. Finally, using multi-voxel pattern analysis, the neuroimaging study in Chapter 5 demonstrated distinct neural correlates for tactile location representations in external and somatotopic reference frames. Overall, this dissertation provides strong evidence that information from external space does influence tactile localization on the skin surface, indicating that tactile localization is not a strictly serial process by which information flows from somatotopic representation to external representation in one direction. Rather, information from external space feeds back to somatotopic representation, influencing tactile location judgments on the skin surface.University of Delaware, Department of Psychological and Brain SciencesPh.D