Sensory mechanisms for the processing of spatial, temporal and numerical information

Despite decades of research concerning the sensory mechanisms for the processing of spatial, temporal and numerical information, several points still remain subject of debate. In this work, we will report a series of studies aimed at providing new evidence regarding the sensory mechanisms specific for the processing of space, time and number, and also to investigate the possibility that a common magnitude system might play a role in their processing. In the first part of the work, we examine the disruptive perceptual effects during eye movements (“saccades”), affecting the representation of space. Such distortion of space, thought to be related to the ocular-motor parameters and linked to visual stability processes, is not usually observed under normal viewing conditions – which give rise to the possibility that it could be due at least in part to the saccade automaticity and stereotyping reached in the typical experimental paradigms. Our results, however, showed that the pattern of saccadic effects was only marginally affected by practice over the course of the experiment and that performance of experts remained similar when tested in a condition leading to less stereotypical saccades. These results indicate that perisaccadic compression is a robust behavior, insensitive to the specific paradigm and to the level of practice with the saccade task. In the second part, we will report two studies concerning the perception of time. In the first study we investigated the effect of motion adaptation on apparent time (i.e. the observation that adapting to fast motion causes a reduction in perceived duration of the subsequent stimuli), which has been previously tested using only simple translational motion. Our results showed that the adaptation-induced compression of time is specific for translational motion, while adaptation to complex motion, either circular or radial, did not affect perceived duration of subsequently viewed stimuli. These results show that such effect occurs only for uni-directional translational motion, ruling out the possibility that the neural mechanisms of the adaptation occur at early levels of visual processing. In the second study, we investigated the predictions of a recent model concerning time perception (the State-Dependent Network model), to test whether it could be extended to different sensory modalities. Our results showed that, while some of the constrains might be variable according to the specific sensory modality tested, the general predictions of the model hold under different circumstances. In the third part, we will present a study concerning the perception of numerosity, and the idea of number as a primary perceptual feature. Recent works showed that like other perceptual attributes, numerosity is susceptible to adaptation, but this idea has been challenged claiming that adaptation may operate via related mechanisms, such as texture-density. To disentangle this issue we measured the effect of adaptation on clouds of connected-dots (creating a robust underestimation of numerosity) and unconnected dots. We showed that adaptation to the same number of dots as the test causes robust adaptation of the connected, but not of the unconnected dot-pattern, suggesting that adaptation occurs at neural levels encoding perceived numerosity, rather than at lower levels responding to the number of elements in the scene. Finally, in the fourth part, we investigated the possibility of a generalized magnitude system. To find further evidence for such system, we tested the effect of motion adaptation on perceived numerosity, as it as been previously tested on perceived time. Our results showed a partially similar pattern of results, suggesting a common general system, but showing also that mechanisms for time and numerosity could partially different. In the second study, we investigated the interplay between space and time – namely, the possibility to exploit spatial information to improve temporal judgments. Our results showed that such interplay is actually possible: providing additional information about where an event is bound to happen, improve the temporal resolution in judging when the event will happen – which supports the idea that such magnitudes might be encoded within a common metric. We also tested the possibility of exploiting timing information (audio-visual asynchrony) to perform spatial judgments concerning size and distance in depth, but without finding any influence of such cues on visual size and distance judgments. Although we could not completely rule out this possibility, our results suggest that the influence of timing cues would not be as strong as other visual cues, and so it might be limited to a small range of circumstances. We conclude that: (1) saccadic distortions and are not a by-product of specific methodologies, but are strictly related to saccade ocular-motor parameters; (2) time perception is supported by a distributed mechanism, deeply rooted into the sensory streams; (3) numerosity is a primary perceptual attribute, and numerosity adaptation acts via number-specific mechanisms; (4) space, time and number might be processed by a generalized magnitude system with a common metric, but their processing could exploit partially different mechanisms.