Figure 4. The group mean probability of responding ‘‘shorter’’ and standard error of the mean are plotted for each S2 duration in Experiment 3. doi:10.1371/journal.pone.0021829.g004 valence scores were converted to emotion intensity scores by subtracting 5 (the rating scale midpoint) from each value and taking the absolute value of the result. A correlation analysis for these and the IAPS arousal scores revealed that both measures were highly correlated (r = .94, p,.0001). We subsequently tested two ANOVA models to determine whether and in what way each measure predicted the probability of responding ‘‘shorter’’ in Experiments 1 and 3. The first model included Emotional Intensity and Experiment as factors and returned a significant effect of Emotional Intensity (F(1,124) = 5.6, p,.05) with all other effects being non-significant (p..1). The second model included Arousal and Experiment as factors and returned a significant effect of Arousal (F(1,124) = 5.2, p,.05) with all other effects being non- significant (p..1). Discussion Participants perceived the duration of a stimulus to be shorter when it was preceded by a positive as compared to a neutral picture. Given that this effect was comparable to that observed for negative pictures, one may conclude that positive and negative stimuli similarly affect S2 duration estimation. S2 is perceived to be shorter when immediately preceded by emotional as compared to neutral stimuli. A discussion of non-specific emotion mecha- nisms by which this effect may occur is presented in the General Discussion. Below we report a fourth experiment aimed at addressing the question of whether emotion induced changes in attention to time are limited to within modality processing or persist when emotion and timing information come from different modalities. This question is important as attention is a limited resource [35] and Emotion Effects on Timing emotion effects may spread more readily within than across modalities. Research investigating cross-modal integration found attention to be guided by cross-modal signals [47–48]. Cross- modal signals, even if task-irrelevant and uninformative, appear to facilitate the processing of spatially overlapping targets. Existing research further implies that cross-modal influences on attention are stronger for emotional as compared to neutral stimuli [49–50]. For example, Brosch and colleagues presented angry and neutral vocalizations dichotically to the left and right ears of participants who were engaged in a visual target detection task. Target detection was enhanced when angry vocalizations were played at the same side as a visual target relative to when they were played at the opposite side. While revealing with respect to cross-modal influences of emotion on attention, this evidence is limited to spatial processing. Moreover, it is unclear what, if any, cross- modal influence would be observed on temporal processing. To address this question, we conducted a fourth experiment in which participants timed the duration of auditory stimuli before and after viewing neutral and negative pictures. Experiment 4 Method Participants. Eighteen female college students participated in the present study in fulfillment of an introductory psychology course requirement. Their mean age was 19.9 years (SD = 0.77). They were all right-handed [39] and reported normal or corrected to normal vision. Procedures. Experiment 4 was equivalent to Experiment 1, with the exception that the timing stimulus (i.e., S1/S2) was a 500 Hz pure tone rather than a circle. Results Analysis of the probability of responding ‘‘shorter’’ revealed significant main effects of S2 Duration (F(3, 51) = 71.01, p,.001, 2 2 g partial = .807) and of Emotion (F(1, 17) = 6.75, p,.05, g partial = .284). Participants were more likely to respond ‘‘shorter’’ for shorter S2 Durations and when S2 was preceded by a negative as compared to a neutral picture (Fig. 5). The interaction effect was non-significant (p..1). Discussion In line with Experiments 1–3, participants underestimated the duration of S2 when it was preceded by an emotional as compared to a neutral picture. Given that Experiment 4 used an auditory timing stimulus and manipulated participant emotions through visual stimuli, these results suggest that the effects of emotion on time perception are not limited to within-modality processes, but extend to across modality processes. Together the current results are opposite to those typically reported in the literature. Emotions here led to temporal underestimation rather than overestimations thus making an attention account a viable competitor of the existing pacemaker account. Nevertheless, one may object that the divergence in results stems from a divergence in methodology, which employed an S1–S2 paradigm in the present case as opposed to the bisection or temporal reproduction paradigms in past research. Therefore, emotion effects on responses may not reflect a shortening of S2 but a lengthening of S1, which was being encoded and maintained in working memory during the emotional challenge. To address this possibility we conducted a final experiment in which each trial comprised the presentation of an emotional or a neutral picture followed by a sample duration, which participants were required to reproduce. If emotions increase the sense of passed and passing PLoS ONE | www.plosone.org 5 July 2011 | Volume 6 | Issue 7 | e21829