Timing issue in online stop task
I have programmed a stop task with 25% stop-signal trials among 75% go trials. The stop signal delay (SSD) is adjusted after each stop trial (-50 ms if the subject responds; +50 if the subject withholds the response). The ideal OS solution for stop trials would be to work with coroutines, but this option is not yet available on OSweb, on which this experiment should ultimately run. So instead, I programmed stop trials by first presenting a sketchpad with the target stimulus, with duration [SSD], followed by a sketchpad of the stop stimulus. As it turns out, the difference between the time stamps of these stimuli is longer than the intended SSD by 8.2 ms (sd 1.3 ms) on average. My questions:
- Which value gives the best estimate of the real SSD? The intended value or the value calculated from the time stamps?
- What is the best estimate of the reaction times on stop-signal respond trials? In the current implementation a keyboard response object follows the stop-signal with delay = 0, with the idea to calculate the stop-signal response RT off line by adding SSD and the keyboard response.
I add the OS implementation and my pilot data (Excel last columns) as attachment.