Mental Time Number Line Figure 2. Experimental procedures. doi:10.1371/journal.pone.0001716.g002 induce temporal overestimation and leftward attentional shifts induce underestimation of temporal judgments [18]. Moreover, healthy subjects are faster in judging short temporal durations in the left space and long temporal durations in the right space [19]. These evidences challenge traditional views of temporal perception mechanisms, positing that perceptual events are timed by a centralized supramodal clock [6]. The spatial-specific effects of time modulation would be more in accord with the finding that visual events of subsecond duration are timed by visual neural mechanisms with spatially circumscribed receptive fields, localized in real-world, rather than retinal, coordinates [20]. The combination of these findings fit with the proposal that relative time between events may be transformed into specific patterning in neural maps, interpretable with the same mecha- nisms used to decode cortical representation of spatial images. Methods Twenty-eight right-handed healthy subjects (15 male, age range: 27–39 years), with normal or corrected vision, participated in the experiments after providing written informed consent. Experimental procedures Experiment 1. Ten subjects participated in this experiment. They were positioned 60 centimeters from a P791 Della computer monitor configured to a refresh rate of 100 Hz. Visual stimuli constituted by digit pairs (1 and 9, size: 0.8u60.1u) were presented in the centre of the screen or in the right and left space (5u eccentricity). The duration of the reference stimulus was fixed at 300 ms. The test stimulus was presented after an interval of 1000 ms from the reference one and its duration varied randomly from 150 to 450 ms (excepted the interval of 300 ms) in steps of 20 ms. Subjects had to indicate the relative duration of the test stimulus compared with the reference one, by pressing either of two response keys using their right hand. In this experiment, digit 1 was the reference stimulus and digit 9 the test stimulus. Each stimulus pair (reference and test cue) was presented lateralized in the right and left space or in central position in a randomized order. The corresponding conditions were: digit 1 in the left and digit 9 in the right space; digit 1 and digit 9 in central position; digit 1 in the right and digit 9 in the left space (Figure 2). Each condition consisted of 160 trials of reference and test cues (10 repetitions616 time intervals). Experiment 2. Ten subjects participated in this experiment. The task and the adopted procedure were identical to that of experiment 1, with the exception that digit 9 was the reference stimulus and digit 1 the test stimulus (Figure 2). Experiment 3. Eight subjects participated in this control experiment, performed to test whether duration judgments are influenced by spatial position of the stimuli irrespective on their numerical magnitude. Reference and test cues were constituted by identical stimuli (black circles, 1u width). Time intervals, number of trials and experimental procedure were identical to those of the first two experiments. Spatial position of reference and test cues was varied, such that they were presented in the left and right space or in central position in an equal number of trials. Therefore, the correspond- ing conditions were: reference cue in the left and test cue in the right space; reference and test cue in central position; reference cue in the right and test cue in the left space (Figure 2). Each condition consisted of 160 trials of reference and test cues (10 repetitions616 time intervals). PLoS ONE | www.plosone.org 3 March 2008 | Volume 3 | Issue 3 | e1716