for the observed effects. Specifically, emotional, but not neutral pictures may have induced an attentional blink causing partici- pants to miss the onset of S2 and thus temporal pulses. To test this possibility, we conducted a second experiment in which the interval between picture offset and S2 onset was 2000 ms. All other parameters were as in Experiment 1. Experiment 2 Method Participants. Fifteen female college students participated in the present study in fulfillment of an introductory psychology course requirement. Their mean age was 18.9 years (SD = 0.64). They were all right-handed [39] and reported normal or corrected to normal vision. Materials and Procedure. In Experiment 2, a blank computer screen was presented for 2000 ms between the offset of the picture and the onset of S2. Nevertheless, the average trial duration was comparable to Experiment 1 as the intervals between trials were shortened to 5300 and 6200 ms. All other parameters were left unchanged. Results Analysis of the probability of responding ‘‘shorter’’ revealed significant main effects of S2 Duration (F(3, 42) = 59.07, p,.001, g2partial = .808) and Emotion (F(1, 14) = 5.15, p,.05, g2par- tial = .269). Participants were more likely to respond ‘‘shorter’’ with decreasing S2 duration and when S2 was preceded by a negative as compared to a neutral picture (Fig. 3). S2 Duration and Emotion failed to interact (p..1). Figure 3. The group mean probability of responding ‘‘shorter’’ and standard error of the mean are plotted for each S2 duration in Experiment 2. doi:10.1371/journal.pone.0021829.g003 Emotion Effects on Timing Discussion The results of Experiment 2 are similar to those of Experiment 1. Thus, we can exclude the possibility that the effects elicited in Experiment 1 reflect an attentional blink following negative pictures. Instead, we argue that the negative pictures triggered emotional processes that distracted individuals from monitoring the S2 duration, leading to a loss of pacemaker pulses and a temporal underestimation relative to the neutral condition. In a third experiment, we sought to replicate this observation and to determine whether it is specific to negative emotions. Past research revealed consistent evidence for attention capture and distraction by negative emotional events [44–45]. However, evidence for a similar role of positive emotions remains rare. Moreover, compared to positive stimuli, potentially threatening stimuli are more readily detected in visual search paradigms and more easily conditioned [34,43,46]. Thus, some researchers argue that negative events are more imperative for survival and thus more successful in modulating attention [34,43,46]. Given that the emotion effect on timing observed here is postulated to arise from attentional modulation, it may be specific to negative emotions. We tested this idea by conducting a third experiment using positive pictures to elicit an emotional state. Experiment 3 Method Participants. Thirteen female college students participated in the present study in fulfillment of an introductory psychology course requirement. Their mean age was 20.2 years (SD = 1.1). They were all right-handed [39] and reported normal or corrected to normal vision. Materials and Procedure. Experiment 3 was equivalent to Experiment 1 with the exception that the negative IAPS pictures were replaced by positive IAPS pictures. Examples of positive pictures are erotic couple, fireworks, and roller coaster scenes. Positive pictures had a mean arousal rating of 5.72 (range 5.08– 6.99) and a mean valence rating of 7.29 (range 6.56–8.22). The mean arousal rating of the positive pictures used in Experiment 3 was significantly lower (t(1,62) = 3.25, p = .002) than that of the negative pictures used in Experiment 1. Additionally, the absolute differences of the positive and negative pictures’ valence ratings from the mean valence rating of the neutral items were compared. The negative pictures were more different from the neutral pictures than were the positive pictures (t(1,62) = 7.14, p,.001). Results Analysis of the probability of responding ‘‘shorter’’ revealed significant main effects of S2 Duration (F(3, 36) = 54.86, p,.001, g2partial = .821) and Emotion (F(1, 12) = 5.28, p,.05, g2par- tial = .305). Participants were more likely to respond ‘‘shorter’’ with decreasing S2 duration and when S2 was preceded by a positive as compared to a neutral picture (Fig. 4). S2 Duration and Emotion failed to interact (p..1). A direct comparison between positive and negative conditions was conducted by subjecting the data from Experiments 1 and 3 to a 3-way Experiment (1 vs. 3)6Emotion (emotional vs. neutral)6S2 Duration (4 levels) mixed design ANOVA with probability of responding ‘‘shorter’’ as the dependent variable. There was a 2 significant S2 Duration effect (F(3, 75) = 126.32, p,.001, g par- tial = .835) and a significant Emotion effect (F(1, 25) = 14.53, p,.01, 2 g partial = .368). Importantly, neither the Experiment main effect nor an interaction with Experiment reached significance (ps..1). Finally, we explored the contribution of picture valence and arousal to the observed effects in Experiment 1 and 3. IAPS PLoS ONE | www.plosone.org 4 July 2011 | Volume 6 | Issue 7 | e21829