Variable won't register negative values (difference between stimuli and response time)

Charlotte_ · July 2016

Hi!

I'm doing an experiment where a cue is presented, and then the subject should press a key (space) as soon as possible. A circle will flash before or after the key press, and the subject should determine whether the flash occurred before or after the key press. I use a script to count the mean response time of the key press, to be able to sometimes present the flash before the key press.

I have a variable that registers the response time for the key press, and a variable that registers the time delay before the circle is presented. I also want to know the time difference between these two events (circle_vs_response), but when I subtract the time delay before the circle (flash_delay) from the response time (trial_response_time), the variable circle_vs_response never shows any negative values, whether I press the key before the circle or not. The lowest value it shows is 24. When I look at the specific values in flash_delay and trial_response_time and count the difference the values in circle_vs_response are correct, but obviously there is something wrong with the code, because the difference should sometimes be negative. I guess the problem could be in registering response time or time delay of the circle?

Here is my overall design of the experiment (it continues after the practice block but I will use the same script later in the experiment):

Here is my prepare phase of the script:

Here is the run phase of the script:

Josh · July 2016

Hi Charlotte, if the specific values for flash_delay and trial_response_time are correct, your desired variable should be flash_delay - trial_response_time. In your script it seems that you have switched these two around, is that right?

Cheers

Josh

Charlotte_ · July 2016

Hi! Well, first I tried defining the variable just like that (flash_delay - trial_response_time), but then it was the other way around, the variable only gave me negative values, never positive ones. So while I thought the problem was in defining that variable, I start to think that it is the response time and/or the time delay of the circle that are not correct (even though the times look plausible when I print them).

/Charlotte

Josh · July 2016

Hi Charlotte, it should be easy to check if those individual values make sense or not, as you can calculate some time differences manually.

Actually, I think the problem is that you're always collecting your keyboard response after the flash canvas in your run-phase. This is bound to always yield positive/negative values depending on which one comes first.

A better setup may be to start timing with t0 = self.time(). Then you enter a while-loop, where you continuously collect a keyboard response with timeout = 1. You break from the while-loop when a response is given (response != None) or when a general timeout is reached. Before entering the loop, you determine at which point your flash should appear. You can display the flash during the loop by means of:

     current_time = self.time() - t0
     if current_time >= flash_time:
          flash_canvas.show()

Does this make sense?

Cheers

Josh

Charlotte_ · July 2016

Thank you for your help! It all seems to work fine now, and circle_vs_response registers both positive and negative values.

/Charlotte

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Variable won't register negative values (difference between stimuli and response time)

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Variable won't register negative values (difference between stimuli and response time)

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