For our fMRI study we want to present videos of 3 seconds duration. The videos have different frame rates:
48 videos were recorded with 50 fps
16 videos were recorded with 25fps
- 64 videos were recorded with 30 fps
I'm now faced with the problem that the videos are not presented for 3 seconds. The videos are either extended or shortened:
48 videos (50 fps): extended (maximum extension between +6ms to +10ms)
16 videos (25 fps): shortened (maximung shortening between -17ms and -20ms)
64 videos (30 fps): shortened (maximung shortening between -17ms and -20ms)
For this reason I assumed that the problem is caused by the different frame rates of the videos. But then I discovered that the extent of the extended or shortened presentation is changing from presentation to presentation and independent of the videos. So for example: Video1 is presented for 3006ms instead of 3000ms during the first presentation. During the next presentation Video1 is presented for 3008ms instead of 3000ms and so on.
Another problem is the presentation of a blank screen between each stimulus. It is set to range between 3000ms and 5000ms in varying steps of 250ms. Here I noticed an extended presentation ranging from +16ms up to 30 ms.
- Any ideas what might cause this timing deviation?
- Does this have to do with the hardware of the stimulus presentation computer?
The refresh rate of the stimulus presentation computer is set to 60Hz. The computer is connected to a projector for the fMRI measurements (but the timing deviations do also occur when I present the stimuli full screen on the stimulus presentation laptop).