A Bridge Between Body and Environment in Egocentric-Allocentric Switching Processes: Evidence from Behavioural and Neuroimaging Studies

In daily life activities like working on the computer or eating a meal at the lunch time, we encode the position of the objects around us according to an egocentric (subject-to-object) or allocentric (object-to-object) frame of reference. To give an example, while eating we encode the position of the fork with respect to the hand (i.e., egocentrically), but also with respect to the plate where the food is (i.e., allocentrically). It should be noted that while eating, rather than representing the position of the fork exclusively with respect to the hand or to the plate, we constantly switch between these two kinds of spatial representations. This example describes how we combine egocentric- and allocentric-based spatial representations in our daily life, translating the encoding of spatial information from-egocentric-to-allocentric reference frames and, vice versa, from-allocentric-to-egocentric ones. Although some evidence already exists in support of such translational processes between spatial representations, the literature on visuo-spatial switching processes is still in its infancy. Indeed, there are still open research questions regarding how egocentric and allocentric reference frames combine. Notably, it is still to be investigated (i) if switching and non-switching processes between reference systems are influenced by the characteristics of the surrounding environment, (ii) if typical ageing influences these processes and how, (iii) whether these visuospatial processes also produce a measurable cognitive load and finally (iv) which cortical regions underlie these visuo-spatial processes. For this reason, the present thesis aimed to investigate visuo-spatial switching processes between reference frames through an ad hoc devised visuo-spatial memory task (Ego-Allo Switching Task); participants were required to memorize triads of geometrical objects, then to provide two subsequent spatial judgments about the memorized stimuli in both non-switching and switching conditions. In the non-switching condition, the two spatial judgments regarded the same reference frame that could be egocentric (Ego-Ego) or allocentric (Allo-Allo). In the switching conditions instead, the two spatial judgments regarded two different reference frames, so participants were asked to provide egocentric-than-allocentric (Ego-Allo) or allocentric-than-egocentric (Allo-Ego) spatial judgments. Research has shown that an enduring environmental frame can exert an influence on visuo-spatial memory components. Here, to investigate the effects of the environmental characteristics on egocentric-allocentric switching/non-switching processes, two studies were carried out. Specifically, in Ex- periment 1 participants were immersed in a virtual squared room with the room walls parallel (aligned condition) or rotated by 45° degrees (misaligned condition) with respect to the egocentric perspective. In Experiment 2, instead, participants were immersed in a squared (orthogonal condition) or circular (non-orthogonal condition) virtual room. The results of the experiments showed that in presence of a misaligned or non-orthogonal environmental layout switching between reference frames was more difficult, in particular when the first reference frame was allocentric. The second research question regarded the effect of typical aging on visuo-spatial switching processes between reference frames. Typical aging impacts on the capacity to effectively adopt both reference frames, especially the allocentric one. To this aim, in the Experiment 3 participants were required to provide switching judgments and non-switching judgments between egocentric and allocentric reference frames. Instead, in the Experiment 4 participants were required to provide switching and non-switching judgments between egocentric and allocentric reference frames combined with coordinate (e.g., “was the object X the closest to you?” – “was the object X the closest to the object Y?”) and categorical (e.g., “was the object X on the right of the object Y?” – “was the object X to your left?”) spatial relations. Overall, the results showed that elderly participants were less accurate than young participants in allocentric-based spatial judgments in non-switching (Allo-Allo) and switching (Allo-Ego) conditions. Notably, the results also demonstrated that elderly participants were impaired in switching between spatial representations, particularly when allocentric-coordinate (based on metric properties) spatial judgments were required first. The third research question regarded the cognitive load behind switching processes between reference frames. Neurofunctional and behavioural evidence has shown that translational processes between visuospatial components can absorb cognitive resources asymmetrically. To this end, in the Experiment 5 participants underwent the Ego-Allo Switching task while the task-evoked pupil dilation variation was measured through an eye-tracking system. The measure of the cognitive pupillometry during visuo-spatial switching processes showed that switching from allocentric-to-egocentric reference frames causes a greater cognitive load than switching from egocentric-to-allocentric ones. Finally, the last research question involved the attempt to identify the cortical correlates of the egocentric-allocentric switching processes. While the literature is clearer on the cortical activities associated with egocentric and allocentric encodings, it is still completely unexplored what happens when the two reference systems combine. To this aim, in the Experiment 6, a group of young adult participants underwent the Ego-Allo Switching task while the cortical hemodynamic task-related responses were measured by means of the fNIRS neuroimaging technique. The results demonstrated that, visuo-spatial switching processes between reference frames are supported rostro-caudally by fronto-parieto-temporal regions, involving also the temporo-parietal junction. In sum, the results of this thesis project provided (i) new insights relative to how egocentric and allocentric reference frames combine showing that environmental geometry and typical ageing could exert a negative effect on spatial switching processes between reference frames, (ii) demonstrated that switching between reference frames produces a measurable cognitive load, and that (iii) fronto-parieto-temporal cortical regions underlie such visuo-spatial switching processes. Overall, these findings could be of relevant interest among various fields of research and application, from clinical investigations to user-centred design, especially for the elderly population.