[open] Invisible boundary trigger for eye tracking

JohnLJones

Hi,

Is there a way to create invisible boundary triggers with open sesame? The effect that I want to achieve is to tabulate how many trials participants move their gaze from a central fixation before prompted to do so.

Thanks!
John

Edwin

Hi John,

There isn't a ready-made plug-in, but you could use an inline_script to do so using exp.eyelink.sample() (see here). An example script that waits until the gaze position has been moved too far from a central fixation point:

(place the following in the Run phase of an inline_script item)

# import relevant stuff
from openexp.canvas import canvas

# max deviation
maxdev = 60 # pixels from fixation dot

# fixation position at half the screen width and half the screen height (i.e.: the screen center)
fixpos = (self.get("width")/2, self.get("height")/2)

# draw and show the fixation canvas
cv = canvas(exp)
cv.fixdot(x=fixpos[0], y=fixpos[1])

# show canvas and wait for movement
moved = False
t0 = cv.show()
while not moved:
    x, y = exp.eyelink.sample() # the sample method returns the most recent x and y gaze coordinates
    if (x-fixpos[0])**2 + (y-fixpos[1])**2 > maxdev**2:
        t1 = self.time()
        moved = True

# save some variables, to use in the OpenSesame GUI
exp.set("fixation_duration", t1-t0)

Does this help?