Hello Yael
well, your visual stimulus presentation times will always be a multiple of the refresh rate. So, on there can’t be a 100 ms interval on a computer running with a refresh rate of 60 Hz. In addition, when using a simple keyboard as response device, you should keep in mind that it is not scanned on a millisecond basis. If you are not interested in RT or your effect is large, you don’t need to care about that.
In an fMRI-experiment it is better to wait for the sync-pulse from the scanner than to adjust the trial duration to the scanner.
Best wishes Jens