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Setting duration based on a calculation with variables

Hi,

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?

Thanks,

Jonathan

• Hi Jonathan,

What part does not work? Please provide more details, then we can help you better. So, what did you try, what do you see, are there error messages, etc.

Thanks,

eduard

• Hi Eduard,

I have set up the trial as follows:

• Sketchpad showing an arrow pointing to the left or right
• In half the trials this is then followed by a sound which signals that the participant shouldn't respond
• At the moment they hear the stop sound 100ms after they see the arrow - I have set this using the duration for the sketchpad
• However I would like to set the duration based on the participants reaction time in a previous block of trials, and tried to set the duration as [[avg_rt] - 200]
• When I run the experiment I get the message - "Invalid duration: [493.8 - 200]" - the 493.8 is clearly the avg_rt, but it's not preforming the calculation that I would like it to!

I hope that makes sense, just in case I've also attached the version of the experiment that isn't working:

Thanks,

Jonathan

• Hi Jonathan,

There is probably a way to do it like you intend to, but I don't remember exactly what the syntax is supposed to be. A way to get around it, is to add an inline script before that sketchpad, in whose prepare phase you define the duration first. (e.g. var.dur = var.avg_rt- 200), then you can add [dur] as duration in the sketchpad and it should work. If you want the avg_rt to keep on updating during the experiment, you do it on every trial (so in the block loop), or if it should stay constant for a block, you do it before the loop.

Good luck,

Eduard

• Thanks - I'll try that!

• That seems to be working - thank you!