Dear Astrid,
Are you sure your TR is 1980 ms? The difference betwenn subsequent pulses seems to be 2000 ms. Also note that the unit in Presentation is 10ths of ms, so you would have to divide by 19800 or, assuming a TR of 2000 ms, by 20000. Personally I prefer to specify everything in seconds and not in volumes (sometimes people forget that the first volume is number 0 and not number 1). You would go with something like (F6-F$6)/10000 then.
0 means 0 s, not the TR. Durations of 0 s are common because the stimuli are often presented for a short period only, and in these cases it doesn't really matter whether you specify a duration of 200 ms or go with 0 s. If the pictures are presented for a longer period, and 4 s are already rather long I'd say, then I'd go with 4 s of course. You might also want to use the trial-specific reaction times as a duration vector, depending on what you're interested in (mere presentation of the stimulus or rather the perceptual and cognitive processes till the subject responds).
Best,
Helmut
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