Prismatic adaptation effects on non-spatial and spatial processes in the healthy adult

Abstract

The structure of the thesis is broadly based on the hierarchical division of attention into two domains – intensity and selectivity components of attention (Sturm, 1996; Van Zomeren & Brouwer, 1994). The intensity components of attention (arousal and vigilance) are a requisite for the more complex aspects of attention selectivity (Sturm et al., 2004). Spatial neglect is characterized by impairment in both the intensity (Heilman et al., 1978; Husain & Rorden, 2003; Robertson et al., 1996; Samuelsson et al., 1998) and the selectivity components of attention (Bartolomeo & Chokron, 2002; Posner & Peterson, 1990). In the context of understanding how prism adaptation rehabilitates spatial neglect, researchers have attempted to produce (Michel et al., 2003; Redding & Wallace, 2005) mild neglect-like patterns of behaviour in healthy individuals following prism adaptation. The aim of this thesis was to induce ‘neglect-like’ patterns of performance in both non-spatial and spatial attentional tasks following left prism adaptation (Berberovic & Mattingley, 2003; Colent et al., 2000; Michel et al., 2003). Chapter 2 examined the adaptation process and aftereffects in sensory motor function. A prismatic aftereffect in both visual and proprioceptive spatial maps was observed post adaptation to either left or right shifting prisms. Adaptation caused re-alignment in the visual (eye) and proprioceptive (hand) systems. Aftereffects were direction specific. Left shifting prisms produced a leftward shift in a visual indication of subjective straightahead (SSA) and an opposite rightward shift in a proprioceptive indication of subjective straight-ahead. Right shifting prisms produced a rightward shift in visual SSA and a leftward shift in proprioceptive SSA. The mechanisms underpinning these prismatic effects are unclear. The thesis investigated chapter by chapter, the effects of adapting to either prism shift on non-spatial and spatial function. Chapters 3 and 4 explored the effects of left prism adaptation (LPA) on the intensity aspects of attention – arousal and vigilance respectively. Adapting to left shifting prisms significantly impaired right hemisphere arousal and vigilance. Adapting to right shifting prisms (RPA) had no effect on the intensity component of attention. The argument was advanced that LPA impaired arousal and vigilance by depressing right parietal function, and hence ‘simulating’ neglect-like impairment in non-spatial attention. Chapter 5 examined the effects of adaptation on non-spatial selective attention within hierarchical stimuli. Adaptation to right shifting prisms (RPA) facilitated processing on the global level, indicative of left parietal suppression. LPA did not have any effect on local global processing. Chapters 6 and 7 examined the spatial effects of adaptation – attentional orienting and reorienting across space. Both prism shifts produced a rightward shift in lateral bias as measured by the greyscales task. This finding was interpreted in terms of right parietal suppression and prismatic effects in the allocentric spatial frame of reference. However, both prism shifts differentially influenced spatial orienting. LPA made it easier to reorient from right space, and RPA made it easier to reorient from left space. The spatial effects of adaptation to either prism shift was interpreted as depression of function in the right and left parietal area following left and right adaptation respectively. The effects of LPA on non-spatial and spatial functions provide insight into the possible mechanisms that underpin visuo-motor adaptation and neglect rehabilitation. These findings were discussed in terms of influencing the fronto-parietal-cerebellar system.