Relativistic Compression and Expansion of Experiential Time in the Left and Right Space Carmelo Mario Vicario2, Patrizia Pecoraro1, Patrizia Turriziani1, Giacomo Koch3, Carlo Caltagirone3, Massimiliano Oliveri1,3* 1 Dipartimento di Psicologia, Universita` di Palermo, Palermo, Italy, 2 Dipartimento di Psicologia, Universita` di Roma La Sapienza, Rome, Italy, 3 Fondazione ‘‘Santa Lucia’’ Istituti di Ricovero e Cura a Carattere Scientifico (IRCCS), Roma, Italy Abstract Time, space and numbers are closely linked in the physical world. However, the relativistic-like effects on time perception of spatial and magnitude factors remain poorly investigated. Here we wanted to investigate whether duration judgments of digit visual stimuli are biased depending on the side of space where the stimuli are presented and on the magnitude of the stimulus itself. Different groups of healthy subjects performed duration judgment tasks on various types of visual stimuli. In the first two experiments visual stimuli were constituted by digit pairs (1 and 9), presented in the centre of the screen or in the right and left space. In a third experiment visual stimuli were constituted by black circles. The duration of the reference stimulus was fixed at 300 ms. Subjects had to indicate the relative duration of the test stimulus compared with the reference one. The main results showed that, regardless of digit magnitude, duration of stimuli presented in the left hemispace is underestimated and that of stimuli presented in the right hemispace is overestimated. On the other hand, in midline position, duration judgments are affected by the numerical magnitude of the presented stimulus, with time underestimation of stimuli of low magnitude and time overestimation of stimuli of high magnitude. These results argue for the presence of strict interactions between space, time and magnitude representation on the human brain. Citation: Vicario CM, Pecoraro P, Turriziani P, Koch G, Caltagirone C, et al (2008) Relativistic Compression and Expansion of Experiential Time in the Left and Right Space. PLoS ONE 3(3): e1716. doi:10.1371/journal.pone.0001716 Editor: Chris Miall, University of Birmingham, United Kingdom Received October 11, 2007; Accepted February 1, 2008; Published March 5, 2008 Copyright: 2008 Vicario et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: maxoliveri@unipa.it Introduction Time perception is fundamental to many aspects of our lives. Most scientists agree that the explicit study of time falls in the purview of physics, yet time is an integral part of virtually all psychological phenomena. A century ago Einstein’s insight revolutionised physics, postulating the relativity of time in the physical world and its strict correlation with space. A similar conceptual approach is likely needed to understand the timing of visual events in the human brain. In fact, a wealth of human and animal research has supported common processing of temporal information with other magni- tudes [1,2,3,4,5]. Classical models [1,6] suggest that using multiple switches and accumulators an organism could quantify time and number simultaneously. More recently, these models have been revised to postulate that numerical and temporal integration may be carried out by a distributed neural circuit that includes cortical areas activated by both timing and counting tasks [7]. As to the relations linking time with spatial factors, previous evidence [8] has documented an experiential relativity of perceived time in humans according to the environmental scale. Indeed, much work has shown that for the brain time and space are not processed separately but can influence each other strongly [9,10]. All these findings are unified in a theory stating that time, space and quantity are part of a generalized magnitude system in the primates’ brains, where specific cortical areas process these elements of the environment [11]. However, in spite of this growing interest in the functional interactions between time and other magnitudes, there is relatively little in the literature on how spatial, numerical and temporal dimensions interact in the cognitive system and on the parallel or hierarchical nature of such interactions. Here, we used different ordered materials (temporal and numerical) in the same task to test whether relativistic-like effects could compress and expand perceptual time according to two factors: the magnitude of the stimulus; the side of space where a visual stimulus is presented. Results When stimulus digits were presented in midline position, time perception was biased according to the magnitude of the digit: the duration of test low digits was underestimated compared with that of reference high digits and vice-versa, as revealed by the significant difference of bisection points in the two conditions (unpaired t-test: t = 12.2; p = 0.0001). When stimulus digits were presented lateralised in the right and left visual hemifields, time perception was biased according to their spatial lateralisation: the ‘‘bisection’’ point was significantly longer in the left vs. right space, either when the digit 9 (paired t-test = 20.9; p = 0.0001) or the digit 1 (paired t-test = 8.8; p = 0.0001) was the test stimulus, as well as in the case of stimuli constituted by circles (paired t-test = 13.9; p = 0.0001) (Figure 1 A,B,C,D). PLoS ONE | www.plosone.org 1 March 2008 | Volume 3 | Issue 3 | e1716