Abstract/Summary

The Superior Parietal Lobule is a region of the brain that has been implicated in a number of high-level cognitive functions, including shifting spatial attention between locations, the perception of heading direction and path of travel during locomotion, and motion tracking under attentional load. The wide range of cognitive functions linked to this region does not align with the high specificity normally seen in the brain, and therefore it might be possible that the SPL supports a lower-level function that is engaged in a wide range of cognitive tasks. Here we investigate the proposal that the lower-level function involves the perception of the 2D visual distances between objects. To test this hypothesis, we targeted the SPL with both high-frequency online repetitive Transcranial Magnetic Stimulation (rTMS) and offline continuous theta burst stimulation (cTBS) in order to explore the role played by this region in the perception of 2D visual space. We achieved this over three studies. In the first, we delivered online rTMS over the left SPL while participants performed a psychophysical task measuring the precision of their ability to judge the distance between objects. In the second, we explored the effect of delivering offline cTBS over the left SPL in relation to the effect of the Muller-Lyer illusion on the perception of the length of a line, as well as its effect on saccade amplitudes. In the final study, we broadened the investigation to also explore a possible role of the frontal eye field (FEF) in the perception of 2D visual distances. We explored the effect of delivering offline cTBS over the right SPL or the right Frontal Eye Field (FEF) on a range of tasks thought to rely on processing of visual distance including interception of a moving target, the magnitude of the Muller-Lyer illusion, amplitudes for reflexive, voluntary, and memory-guided saccades made to Muller-Lyer and control stimuli, and a reaction time control task. Overall, we observed no effect of rTMS and cTBS on the experimental tasks in Study 1 and Study 2, respectively, although rTMS did unexpectedly have a significant effect on the control task in Study 1. In Study 3, while we observed an effect of cTBS over the SPL for a subset of the experimental tasks, the same effect was observed when cTBS was delivered over the right FEF, which left us unable to rule out the possibility that the effects were a result of testing order rather than cTBS. Overall, the results obtained in this thesis do not fully elucidate the roles of the SPL or the FEF in the perception of 2D visual space, and therefore further research is suggested.