Action-shaped perception. Towards a reunification of two systems.
In this dissertation I investigated the relationship between action and perception in the processing of Space, Time and Numerosity. In the first chapter the most prominent literature on the topic is reviewed to introduce the conceptual framework in which the experimental paradigms were developed. In 2003 Vincent Walsh proposed A Theory of Magnitude (ATOM) which posits that space, time and numerosity share a common processing mechanism rooted in our need for information about the spatial and temporal structure of the external world. According to Walsh, we could learn about the association between the fundamental magnitudes through our interaction with the environment, as in real-life settings they often correlate with each other (for instance, a higher number of items takes up a larger space and requires a longer time to be retrieved). One of the functional consequences of the ATOM theory would be a perceptual interference across magnitudes, which would result in judgments regarding one magnitude being biased by another irrelevant one. In the second chapter of this thesis, I demonstrate that the interaction between duration and numerosity is task-dependent: participants’ judgments about stimulus duration were influenced by stimulus numerosity only when tested with a discrimination task. This suggests that the cross-magnitude interaction predicted by the ATOM Theory does not occur at the processing level, but it is dependent by the kind of task (ie. comparisons) the observer is required to perform. This, in turn, suggests that the interplay between time and numerosity occurs at a later stage after perceptual processing as for example, at the decisional level. In chapter 3 I report evidence for a new cross-modal after-effect, revealing that the metric with which the visual system computes the relative spatial position of objects is shared with the motor system. A few seconds of mid-air self-produced tapping movements (adaptation) yielded a robust compression of the apparent separation of dot pairs subsequently displayed around the tapping region. As the influence of tapping on numerosity and duration perception had been previously demonstrated, these results offer clear evidence for a generalized interaction between the motor system and the processing of perceptual magnitude information in line with the ATOM Theory predictions (see above). After demonstrating the influence of upper-body movements on magnitude perception, Chapter 4 is dedicated to the investigation of the influence of lower-body movements on the perception of duration and numerosity. As already reported in previous literature, I found that running systematically interferes with duration perception as it causes an overestimation of perceived time. However, in order to overcome some of the discrepancies in the existing literature that might be caused by methodological differences, I applied a standardized motor paradigm to different time ranges and sensory modalities. This allowed me to generalize the effect induced by running on duration perception across visual and auditory modalities and across sub-second and supra-second duration ranges. On the other hand, I found no distortion of numerosity judgments because of running, suggesting that this effect does not generalize across magnitudes. Lastly, I focused on the importance of Peripersonal (PPS) and Extrapersonal (EPS) space in the investigation of the relationship between action and perception, which is often overlooked, but remains a vital component in the theoretical framework of ATOM theory. Indeed, action needs proximity with its target, and there is evidence of perceptual networks dedicated exclusively to PPS. As is often the case with magnitude perception, time has been the first domain investigated while taking into account the influence of stimulus distance. One of the research lines carried out during the PhD was aimed at generalizing the effects of PPS and EPS on time to numerosity perception. My results clearly indicate numerosity perception relies less on stimulus distance than time does, with participants showing the same perceptual precision and accuracy in both EPS and PPS. In the last chapter I applied the psychophysical methodologies I got familiar to during the PHD to study perception in a virtual reality (VR) environment that allow to test perceptual processes in highly ecological settings to make VR always more important in future research of perceptual neuroscience. This study was conducted in collaboration with the Center for Applied Neuroscience of the University of Cyprus where I spent 7 months during my period abroad. My aim was to validate in VR a paradigm that has been replicated multiple times in real world settings to measure the size of participants’ PPS. Following this, I also experimented a tool-training method aimed at reshaping participants’ PPS. My results show that, similarly to real world settings, a short period of tool-training is sufficient to cause a significant enlargement of the PPS. This result is of primary importance, as it shows for the first time that PPS in Virtual Reality has similar characteristics of Peripersonal space as measured in real life settings, thus offering evidence towards a valuable validation of VR as a tool to study the characteristics of Peripersonal space.