Mental Time Number Line Figure 1. Duration distortions in the right and left space. (a) Experiment 1: average bisection points when digit 9 was the test stimulus. (b) Experiment 2: average bisection points when digit 1 was the test stimulus. (c) Experiment 3: average bisection points when circles were used as reference and test stimuli. (d) Representative data from one participant in Experiment 1: Black continuous line and black triangles: test stimulus in right space; black dashed line and black circles: test stimulus in central position; grey line and white squares: test stimulus in left space. doi:10.1371/journal.pone.0001716.g001 Duration perception of spatially lateralized stimuli was still influenced by numerical magnitude: in fact, bisection points were significantly shorter when the test stimulus was the digit 9 as compared with digit 1, either in the right space (unpaired t-test: t = 10.9; p = 0.0001) or in the left space (unpaired t-test: t = 7.7; p = 0.0001). Discussion The current findings suggest that, as in the physical world, psychological time is relative and elastic. Time can be compressed and expanded by a number of environmental factors such as the position of a stimulus, on a left-to-right vectorial distribution, or its magnitude. The relation between time and numbers seems to be influenced by numerical quantity: small numbers bias estimation towards short durations whereas large numbers bias estimation towards long durations in a behavioral standard time comparison task. These findings argue for a functional interaction between number magnitude processing and time estimation. The implicit influence of quantity on time estimation implies that subjects keep track of magnitude when cued to time, consistently with previous findings reporting that the numerical information interferes with subjects’ ability to discriminate duration [4,5,12]. Thus, analogical quantity and duration may share information processes resources, behav- ioral goals and common neural circuits oriented to action, presumably selected through development. In addition to duration distortions induced by number magnitude, the present findings indicate a strong influence of spatial lateralisation on time judgments: left space biases estimation towards short durations whereas right space biases estimation towards long durations, irrespective on the magnitude of the stimulus timed. This pattern suggests that elapsing time is internally mapped onto spatial representations. We suggest that even the influence of number magnitude on duration judgments could be mediated by spatial attentional factors. In fact, the mere presentation of numbers induces a spatial attentional bias depending on the magnitude of the number: low numbers shifting attention to the left and high numbers to the right space [13]. One could hypothesize that by shifting the focus of attention within a mental representation numbers could indirectly influence time judgments towards underestimation or overestimation. This hypothesis implies a central role of spatial coordinates in the relations linking time with numbers. This issue needs to be investigated more thoroughly at the neural level, to test whether a common foundation for these relationships may be neural processing in posterior parietal cortex, dorsolateral prefrontal cortex and cerebellum, a set of regions involved in cognitively controlled timing [14] as well as in the computation of space and other magnitudes [11]. Overall, these findings fit with the prediction that time could be cognitively represented by means of spatial coordinates, with a left- to-right oriented linear representation, in analogy with numbers and other types of ordered material, such as numbers [15], sound pitches [16], months and letters [17]. In fact, recent evidence has provided direct support to the hypothesis of a left-to-right directionality in time representation. Manipulation of spatial attention by means of optokinetic stimula- tion biases temporal judgments in healthy subjects according to the side of space where attention is oriented: rightward attentional shifts PLoS ONE | www.plosone.org 2 March 2008 | Volume 3 | Issue 3 | e1716