emotions have the opposite effect. Contrary to this idea, however, experimental research indicates that positive and negative emotions have a similar impact on interval timing. Both apparently lead individuals to overestimate the passing of time. When participants were asked to judge or reproduce the duration of a stimulus, the reported or produced durations were longer when the stimulus was positive or negative as compared to neutral [15–19]. Explanations of the effect of emotion on interval timing in these studies rely on pacemaker-accumulator timing models and their presumed clock components. Moreover, they draw on the idea that pacemaker rate is influenced by bodily arousal as reflected by peripheral markers such as heart rate or skin conductance [20]. Accordingly, positive and negative stimuli are believed to increase arousal thus speeding up the pacemaker and causing a greater number of pulses to be accumulated. Therefore, positive and negative stimuli are experienced as longer than equivalent duration neutral stimuli. While pacemaker rate [21] is a possible mechanism through which emotions can influence timing, it is not the only one. Some researchers propose that emotion effects are mediated by attention [22–24]. This proposal rests on evidence that temporal estimates vary as a function of attention [25–27]. For example, temporal estimates are shorter if attention is distracted by a concurrent verbal task [28]. The information-processing version of SET accommodates these and similar findings [29–30]. It holds attention responsible for whether the mode switch allows transfer of pulses from the pacemaker to the accumulator [29]. This switch may ‘‘flicker’’ or oscillate between the open and closed states during the timing signal [31–33]. Hence, when more attention is allocated to the timing signal, a larger number of pulses are accumulated and the duration seems longer as compared to a situation in which less attention is allocated to the timing signal. Thus, if emotional events enhance attention to the timing signal above baseline levels, they may inadvertently contribute to a larger number of pulses being collected and lead to event duration being perceived as longer than the duration of an equally long, but emotionally neutral, event. Evidence for the later proposition comes from research that highlights the capacity of emotional stimuli to capture and hold attention. For example, visual search studies indicate that participants are faster at detecting an emotional target among neutral distracters and slower at detecting a neutral target among emotional distracters relative to an all neutral baseline [34]. Likewise, experiments using the Posner spatial cueing paradigm [35] revealed that participants are more likely to benefit from valid cues if they are emotional as compared to neutral [36]. Additionally, they are less likely to disengage from invalid emotional as compared to neutral cues [37]. Thus, one may speculate that the attentional advantage gained by encoding emotional as compared to neutral stimuli modulates the accumulation of pacemaker pulses and perceived stimulus duration. If true, timing an emotional event should produce a longer duration estimate because more pulses accumulate than when timing a neutral event. Moreover, the opposite effect should emerge when individuals time a neutral event in the context of an emotional or neutral distracter. The emotional distracter should be more successful in diverting attention away from the target event resulting in shorter duration estimates than in the context of a neutral distracter. While there is ample evidence of participants overestimating the duration of emotional relative to neutral events [15–19], the role of emotional and neutral distracters for the timing of neutral events has not yet been explored. To fill this gap, we employed an S1/S2 temporal discrimination paradigm. In this paradigm, a stimulus Emotion Effects on Timing (S1) was presented and the participant was asked to encode its duration. After a short retention interval, a second stimulus (S2) appeared and the participant indicated whether it was shorter or longer than the first one. During the retention interval, a picture with emotional or neutral content was presented to manipulate the participant’s emotion independently of the timing task. If, as proposed previously, emotions affect time perception by modulat- ing pacemaker rate, this paradigm should reveal an overestimation of S2 duration on trials with emotional as compared to neutral task-irrelevant images. Heightened arousal on these trials should cause a higher pacemaker rate and more pulses to be accumulated [38]. If, however, emotions affect time perception by modulating attention, we should observe an underestimation of S2 duration on emotional as compared to neutral trials. A diversion of attentional resources from the monitoring of time to the processing of emotionally relevant information should cause participants to miss pacemaker pulses and thus to perceive the interval as shorter. Experiment 1 Method Participants. Fourteen female college students participated in the present study in fulfillment of an introductory psychology course requirement. Their mean age was 19.6 years (SD = 0.65). They were all right-handed [39] and reported normal or corrected to normal vision. Materials. A filled circle 1.7 cm in diameter was used as the timing stimulus (i.e., S1 and S2). Sixty-four task-irrelevant images were selected from the International Affective Picture System (IAPS [40]). Half the images were negative with a mean arousal rating of 6.15 (range 5.25–7.35) and a mean valence rating of 2.22 (range 1.51–2.95). The remaining pictures were neutral with a mean arousal rating of 2.70 (range 1.76–3.29) and a mean valence rating of 5.10 (range 4.45–6.28). Examples of negative pictures include mutilated body parts, assault, and toilet scenes. Examples of neutral pictures include an elderly man, plants, and building scenes. All stimuli were presented on a black background in the center of a computer screen. Procedure. Participants were seated in front of a computer screen, instructed on their task and subsequently commenced the experiment. Each trial in the experiment began with the presentation of a fixation cross at screen center for 500 ms followed by a 500 ms blank screen. Then, a circle (S1) appeared for 1200 ms followed by another blank screen of 1400, 1600, 1800, or 2000 ms duration. Next, the participant saw a negative or a neutral picture for 800 ms, followed by a 500 ms blank screen and another circle (S2). S2 was either shorter (i.e., 1040 or 1120 ms) or longer (i.e., 1280 or 1360 ms) than S1. Participants were informed that distracter pictures interleaved every S1/S2 pair and were asked to simply keep looking at the screen. The task instruction was to compare the duration of the two circles (S1 and S2) and to indicate whether S2 was shorter or longer than S1 by pressing one of two buttons on a response box following S2 offset. A new trial started 6300, 6600, 6900, or 7200 ms after the offset of S2 (Fig. 1). There were 64 trials in the experiment. Each S2 duration was presented 8 times in the negative condition and 8 times in the neutral emotion condition. Trial order was randomized and the assignment of response buttons to response alternatives was counterbalanced across participants. Results Probability of responding ‘‘shorter’’ was subjected to an ANOVA with Emotion (negative, neutral) and S2 Duration (4 PLoS ONE | www.plosone.org 2 July 2011 | Volume 6 | Issue 7 | e21829