Interval timing is a kind of time perception, perceiving time ranging from hundreds of milliseconds to several minutes, even to hours, a basic perception for survival. While Pain defined by International association for the study of pain(IASP) as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage. Pain contains not only sensory components, but also emotional components. Former researches testified that there were mutual effects of pain and interval timing, however different pain would lead to different influences on interval timing and the underlying mechanism were not clearly studied. In this study, we will look into how electrical stimulation evoked pain and pain expression impact on interval timing, and to investigate if pain perception and pain emotion would be the possible factors to affect interval timing.In experiment 1, we studied the effect of electrical stimulation evoked pain on interval timing by using an electrical stimulation. In a temporal bisection task of which interval ranged from 400ms to 1600ms, subjects received not painful stimulus and painful stimulus in the last 100ms or 300ms of presenting interval or subjects received no stimulus at all. To all the subjects, we found proportion of “long” judgment of 1200ms was significantly lowered when subjects received 300ms painful stimulus. There were no significant differences of PSEs of different stimulus, and only difference founded in WFs was between 300ms painful stimulus trials and no stimulus trials. However, further analysis found that the temporal task results were different while differentiating male and female. The main effects of gender were significant whether for PSE or WF. The sensitivity of timing seemed to be weakened whenever female subjects received stimulus, which however was not the same case in male subjects. The scores of the Fear of Pain Questionnaire-III (FPQ)of female were significantly higher than those of males. Correlation analysis showed that the scores of FPQ were related to PSE change rates under painful stimulus.In experiment 2, we investigated the effect of pain expression pictures on interval timing. The subjects were asked to judge whether the pictures presented were "long" or "short" in a bisection task as the same as that in the experiment 1. The subjects recruited in this study included non-religious and Christians. It was found that pain expression had no effect on the PSEs of non-religious while significantly decreased PSEs of Christian subjects, indicating the interval timing getting quicker in Christians. The Weber coefficients of the two groups were not affected. It was showed that the total scores of empathy ability of Christian subjects were not different from that of non-religious. Only empathy concern scores were significantly higher than that of non-religious. Through correlation analysis, it was found that there was no significant correlation between empathy scores and PSEs, but there was a marginal correlation between empathy scores and Weber coefficient (P = 0.06). In addition, it was found that there was a significant negative correlation between the pain evaluation scores of pain expression pictures and the change rates of PSEs in Christian subjects.To sum up, our results confirmed that electrical stimulation evoked pain can affect the interval timing of subjects, mainly in the effect on time sensitivity. The fear of pain evoked by electrical stimulation may play an important role in this process; Pain expression pictures can also result in the change of temporal perception, but this phenomenon may be different among different cultural background groups. The differences may be attributed to the understanding, empathy concern or the pain evaluation of pain expression pictures.
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