1–4 were not merely an artifact of the S1/S2 discrimination paradigm. Moreover, Experiment 5 eliminates the possibility that they were due to emotional events lengthening duration representations that are concurrently maintained in working memory. Rather, emotional events seem to affect temporal processing prospectively. Compared to neutral events, emotional events shorten the perception of subsequently presented timing signals. General Discussion The present study set out to investigate the influence of emotions on time perception. Of particular interest was whether pacemaker rate or attention represents a more viable mechanism for emotional effects on timing. Across five experiments, we found that participants perceived a timing stimulus as shorter when it was preceded by an emotional as compared to a neutral distracter. Moreover, this effect was observed for within modality and across modality manipulations, for positively and negatively valenced emotional distracters, for 500 ms and 2000 ms intervals between the distracter and the timing stimulus, and was independent of the type of timing response (i.e., temporal discrimination or repro- duction) participants made. Together these results challenge the idea that emotions influence time perception merely by influencing pacemaker rate. If that were the case, then the present study should have replicated prior work indicating that participants overestimate the duration of emotional as compared to neutral events [15–19]. That is to say, the emotional distracters in the present study should have increased pacemaker rate and allowed more pulses to be accumulated. Compared to the condition with neutral distracters, this should have reduced the proportion of ‘‘shorter’’ responses to the S2. As we found the opposite pattern of results, a mechanism other than pacemaker rate likely accounts for our effects. We propose attention to be this mechanism and in the following paragraphs suggest some possible ways by which emotions might alter attention to affect changes in time perception. First, it is possible that participants directed greater attention to the encoding of emotional as compared to neutral pictures and thus tended to miss the onset of the timing stimulus. In support of this, we found the emotion effect to be statistically independent of timing stimulus duration. However, the durations used here may not have been different enough and/or the statistical power too small to detect duration related changes in the emotion effect. In support of this, we found that emotion induced underestimation was unaffected by an increase in the interval between the emotion stimulus and the timing stimulus in Experiment 2. This increase should have reduced an underestimation effect due to a delayed latency to begin timing because participants had more time to recover their attention. Moreover, in Experiment 5 participants reproduced shorter durations in the emotion condition despite the fact that they were prompted about the impending reproduction of the timing stimulus. Finally, the condition means in the latter experiment suggest that the temporal shortening effect increases from the short (56 ms), to the intermediate (61 ms) to the long duration condition (91 ms). Thus, there are likely other attentional mechanisms contributing to the present results besides a mere onset effect. One such mechanism could involve the effect of non-specific bodily arousal on attention. Previous research has indicated a relationship between bodily arousal and cognitive performance, which is commonly known as the Yerkes-Dodson law [51]. According to this law, cognitive performance increases with increasing arousal up to an optimal level. Once this arousal level is Emotion Effects on Timing passed, cognitive performance declines. Although the Yerkes- Dodson law has been criticized [52], past findings were generally supportive making it a candidate explanation for emotion effects on timing. Accordingly, the mix of moderately and highly arousing images used here may have led to a general cognitive impairment that included attention and thus impacted participants’ ability to monitor time. This assertion is supported by the fact that temporal judgments were related to arousal. However, as they were equally strongly related to emotion intensity, bodily arousal may not or not solely account for the observed effects. A final mechanism we would like to propose involves attention capture by appraisal processes and/or emotion regulation attempts [53–54]. Preoccupation with such processes could deter participants from focusing on the timing task. Neuroimaging research revealed an involvement of lateral and medial prefrontal cortex in both emotion appraisal [55] and emotion regulation [56]. Given that similar substrates have been implicated in the perception of time [57–58], emotion regulation might compete with time perception for these neural resources. Although our data support an attention account, one may object that a pacemaker rate account cannot be fully ruled out. This is because participants may have used the emotional/neutral picture as a reference for judging or reproducing the target duration. As emotional stimuli were found to be overestimated in previous studies, a longer timing reference may have led to an underestimation of the target. However, the following consider- ation makes this possibility implausible. Changes in arousal after an emotional challenge develop in time and increases in arousal markers such as heart rate and skin conductance have been noted to peak between 3 to 6 seconds following stimulus onset [59–60]. Given the interval between picture onset and the imperative timing signal varied from 1.3 to 2.8 seconds across the present experiments, an arousal effect should have been smaller during picture processing than during S2 processing and thus increased the probability of ‘longer’ responses. However, we observed the opposite result indicating that pacemaker rate is ill-suited to explain the relationship between emotion and time. Instead, attention capture by the emotional distracter appears more appropriate. Although the pacemaker account has been very popular as an explanation of the relationship between emotion and time, the present work is not the first to suggest a relationship between emotion, attention, and time. For example, Meck and MacDonald [24] observed that lesions to the amygdala, a structure known to mediate the attentional bias towards emotional events, caused a deficit in the temporal estimation of threat stimuli. This deficit was comparable to that caused by lesions to the prefrontal cortex, a structure implicated in attentional control [24]. Furthermore, a study by Gil and colleagues [23] found time estimates to be shorter for disgusting food pictures as compared to neutral pictures. Unlike threat-relevant stimuli, disgusting stimuli are disease salient and only harmful if we approach them. Thus, rather than eliciting a fight-or-flight response, they simply warn us to stay away [61]. Therefore, Gil and colleagues [23] argued that the perception of disgusting stimuli causes attentional diversion and temporal pulses to be lost. Although prior work cited above raises the possibility that attention capture or aversion modulate the relationship between emotion and time, it does not exclude a pacemaker account. Apart from modulating attention, lesions to the amygdala are known to impair arousal responses to emotional stimuli [62–63], which offers an alternative explanation for the timing impairments seen by Meck and MacDonald [24]. Moreover, in the study by Gil and colleagues [23], disgust was likely associated with a reduction in PLoS ONE | www.plosone.org 7 July 2011 | Volume 6 | Issue 7 | e21829