fixation point stimulus trigger_flag updating

swegger's Avatar


28 Jul, 2014 03:02 PM

Hi Chris,

I'm starting to play with the fixation point stimulus to make a simple saccade program. I've noticed that, as expected, the trigger_flag variable of the stimulus will be updated when I put the trigger watch variables in the window or move them out of the window. However, when I move the fixation point stimulus to a new location, the trigger_flag variable remains unchanged (so that, if it was 1 before changing locations, it will remain 1 even though I don't change the watch variables). This seems like undesirable behavior, particularly if one wants to design a smooth pursuit experiment. How is the updating of the trigger_flag handled?

I've attached a file of what I'm working with if that helps clarify the issue. Thanks,

  1. Support Staff 1 Posted by Christopher Sta... on 28 Jul, 2014 04:00 PM

    Christopher Stawarz's Avatar

    Hi Seth,

    Updates to trigger_flag are driven solely by changes in the eye position variables (eye_x and eye_y in your experiment). This means that changes to the position of the fixation window won't be taken into account until the next eye position update occurs. While you wouldn't see this problem as much with real eye data (which is updated continuously), it'd still be possible for you to test fixOn before the next eye sample comes in.

    I think the right solution here is for your experiment to have two fixation windows: one for the fixation stimulus and one for the target stimulus, each with a different variable for its trigger_flag. That way, you won't have to worry about the timing issues surrounding moving the fixation window.

    Also worth noting: I recently added a circular fixation point stimulus, which works identically to the existing fixation point, except that the visible stimulus is an ellipse, and the associated electronic window is a circle. So, in your experiment, you could eliminate fix_win and turn fix and target into circular fixation points, which would simplify things a bit.


  2. 2 Posted by swegger on 28 Jul, 2014 04:31 PM

    swegger's Avatar

    Thanks Chris. I suspected the updates where contingent upon changes in eye position variables, just wanted to confirm. Good tips on simplifying the design.


  3. Christopher Stawarz closed this discussion on 28 Jul, 2014 04:44 PM.

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