display refreshing in tobii eyetracking extensions for eprime?
Adam Osth
I'm currently running EPrime 2.0 with extensions for Tobii for eyetracking. I have this problem where in the data, every trial seems to have a couple of points in which "-1" is returned for the fixation location. I've heard that this is caused by the display refreshing after a certain amount of time. Is there any way to control when the display refreshes so it won't do that when a stimulus is being presented?
It should be noted that my stimuli only display for 4500 ms, so it doesn't have to refresh while the stimulus is up.
Adam
Michiel Spape
Hi Adam,
It’s been a while since I last worked with the Tobii, so I might be wrong here, but I think it’s unlikely it has anything to do with the monitor’s refreshing. First, the monitor always refreshes in a fixed number of hz, I’ve never heard of anything you can do to stop this updating entirely, even though you can typically change the number of refreshes in system>control panel>display options>hardware (or something like that) – but less than 0.3 Hz (given your stimuli’s duration): forget it. Obviously, you normally wish to sync your stimulus presentation with refreshes (that’s the common problem), which you can set in E-Prime (onset sync, etc). Second, the Tobii output when it doesn’t know where a subject is looking (either because of the subject moving a lot, the subject blinking, or hardware problems) is also -1. Ask your subject not to blink too much might be the best course of action. You might also want to change the sample-rate of the Tobii – ours could go to 120 Hz, but its output at higher sample-rates (>80 Hz) wasn’t too brilliant.
Anyway, Tobii gives validity markers for time-points in which the output can or cannot be trusted – see documentation – I expect your “-1” output is probably also marked as invalid. Just delete the invalid data.
Hope this helps.
Mich
Michiel Spapé
Research Fellow
Perception & Action group
University of Nottingham
School of Psychology
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