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I'm planning an event-related fMRI-experiment to investigate the difference in processing natural and computer-generated expressions. For this, we recorded videos of 4 different actors posing fearful and neutral expressions which last 3 seconds each and will compare them to computer-generated videos of fearful and neutral expressions. This was planned as 4 different conditions.

We noted now that the videos/expressions we recorded are of different emotional intensity (preliminary subjective ratings). Meaning some natural fearful expressions seem to be less emotionally intense than other natural fearful expressions and some artifical fearful expressions seem to be less emotionally intense than other artificial expressions. In general, artificial fearful expressions seem to be less emotionally intense than natural fearful expressions.

If we run the fMRI experiment as planned and contrast for example BOLD response to natural fearful expressions with BOLD response to artificial fearful expressions, how would the different emotional intensities of the videos affect the results?

We do not have enough videos to form a further "emotional intensity condition" and model the influence of the emotional intensity ratings in subsequent analyses.

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  • $\begingroup$ Just to be sure I understand your question (I'm from a basic psychophysics background, mind): with 'intense' you refer to 'emotionally more intense' and not to simply to 'that image is brighter'. In my discipline, intense means a higher intensity level, i.e. a higher stimulus level,i.e. 'that real scary face is brighter than that artificial one'. Perhaps it would clarify the question when you refer to 'emotionally more intense' or something alike - whatever milks your Jersey. $\endgroup$ – AliceD Jan 24 '18 at 23:43
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    $\begingroup$ "We do not have enough videos to form a further "intensity condition" and model the influence of the intensity ratings in subsequent analyses." - What do you mean you do not have enough videos? If you had just 2 you could test an intensity condition: the "higher" versus the "lower" intensity. You actually might be better off with fewer videos, because it's unlikely that "intensity" is easily coded as a continuous variable. $\endgroup$ – Bryan Krause Jan 24 '18 at 23:52
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    $\begingroup$ @AliceD: Thanks for pointing this out. I was inaccurate about the intensity - meant was the intensity with which the emotional expression was displayed. I'll edit this in the post. $\endgroup$ – Anna Jan 25 '18 at 7:45
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    $\begingroup$ @BryanKrause: Thanks for your answer. I assumed, that there must be an equal number of trials per condition if I want to compare them (e.g. "4 videos with low emotional intensity" vs. "4 videos with high emotional intensity"). With our video set this isn't possible because the number of emotionally low and high intense videos is not balanced. For this reason I wrote that we do not have enough videos. $\endgroup$ – Anna Jan 25 '18 at 7:45
  • $\begingroup$ The different emotional intensities of the videos may be the cause of the greatest change in variance. Try to reformulate the question. $\endgroup$ – hexadecimal Jan 25 '18 at 14:39

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