Setting duration based on a calculation with variables
I'm creating a test on inhibition of responses. In the first part the participant completes a number of trials responding as quickly as possible to say whether an arrow is pointing left or right.
In the next block of trials I introduce a 'stop signal' in half of the trials. When they hear the stop signal the participant shouldn't respond. The problem I'm having is that I want to set the timing of the stop signal based on the participants reaction time in the first block of trials - so for example, if their reaction time had been 850ms, then I'd like the stop signal to be given 400ms before that. I've tried defining this as [avg_rt] - 400 but that doesn't seem to work - do you have any suggestions?