Spatial attention asymmetries: behavioral and neural evidence

Healthy individuals show a leftward attentional bias in the orienting of spatial attention, a phenomenon known as pseudoneglect. Spatial asymmetries have been demonstrated also in different planes (vertical and radial) and in the absence of any visual input (representational pseudoneglect). However, the relationship between spatial asymmetries across orientations and the underlying mechanisms of representational biases are still unclear and require further investigations. Study 1 describes a psychophysical investigation on the effect of cognitive load on horizontal and vertical spatial asymmetries. Indeed, increasing cognitive load by introducing a secondary task, reduces the leftward bias in the horizontal plane. We investigated whether cognitive load has a similar effect on horizontal and vertical attentional asymmetries. Healthy participants were asked to carry out a line bisection and a landmark task in single and dual-task (while concurrently performing an auditory working memory task) conditions, in both horizontal and vertical orientations. We demonstrated that increasing cognitive load reduced the typical leftward/upward bias in the line bisection task, whereas it increased attentional biases in the landmark task. Interestingly, no differences were found between horizontal and vertical orientations, thus suggesting these biases may result from a common mechanism. However, the disparate effect of load across tasks suggests that these paradigms may engage different mechanisms. In Study 2, we examined the role of a normal binocular vision in visual and haptic pseudoneglect. Indeed, strabismic amblyopes do not exhibit pseudoneglect in visual line bisection, suggesting that the right-hemisphere dominance in the control of spatial attention may depend on a normal binocular vision. We aimed to investigate whether an abnormal binocular experience also affects spatial attention in the haptic modality. Hence, we compared normally sighted, strabismic and early monocular blind participants in a visual and a haptic line bisection task. In visual line bisection, strabismic individuals tended to err to the right of the veridical centre, in contrast with normally sighted participants who showed pseudoneglect. Monocular blind participants exhibited high variability in their visual performance, with a tendency to bisect toward the direction of the functioning eye. In haptic bisection, all participants consistently erred towards the left of the veridical centre. Our findings support the view that pseudoneglect in the visual and haptic modality relies on different functional and neural mechanisms. In Study 3, we explored the neural correlates of both perceptual and representational spatial asymmetries. Indeed, prior studies suggested a possible role of cerebellar regions in mediating spatial attention mechanisms; however, the extent and significance of the cerebellar contribution are not clear. Here, we aimed to shed light on this issue by means of two TMS experiments. In Experiment 3a participants completed a landmark task in two orientations (horizontal and vertical) while receiving single-pulse TMS over the cerebellar vermis, the visual cortex and the vertex (control sites). TMS over the vermis did not modulate participants' bias or response times, whereas TMS over the visual cortex delayed response latencies. In Experiment 3b, participants completed a landmark task and a number bisection task while TMS was delivered over the cerebellar vermis, the left cerebellar hemisphere and the vertex. TMS over the left cerebellar hemisphere delayed participants' response latencies in the number bisection task. Our data suggest that the left cerebellar hemisphere (but not the vermis) is causally implied in spatial attentional mechanisms in numeric intervals, but not physical lines, bisection. Finally, the last chapter of the dissertation discusses and integrates the main findings of the studies illustrated in the experimental chapters.