The overall goal of this procedure is to achieve binocular eye tracking during dichoptic visual stimulus presentation. In the study of vision, it's often important to show each eye, a different image. For instance, using mirrors.
This is called, dichoptic stimulation. The main advantage of this technique is that it allows eye tracking during dichoptic viewing in a set up that is very compatible with standard eye trackers and has low cost and poses very few limits on the size and color of the visual stimuli you use. This method can help answer key questions in the study of human vision, like, how do the eyes move in 3D scenes and how do the pupils respond to visual input that's presented subconsciously using a technique called, interocular suppression.
Generally, individuals attempting to do eye tracking in normal dichoptic set ups will struggle because the mirrors used for dichoptic presentation, block the sight of video based eye trackers. To build the fiberboard framework, begin by putting together one central component, which will hold the mirrors and eye tracker and two reference boards painted in matte black to reduce light scatter. Place the top element of this component, such that it leaves eight centimeters in depth in front of the desk, to allow enough room for the participant's face when stabilized on the headrest and avoid condensation on the mirrors during exploration.
Next, position the two reference boards at the desired locations of the monitors. Mount two flat screen monitors on standard monitor arms, clamped to the side of the desk to allow translation in three dimensions, as well as rotation in the plane of the screen. Then, connect the mirror mounts to the fiberboard and stabilize the mirrors on the mirror mounts.
Position the mirrors to touch at a 90 degree angle in the center, in order to reflect stimuli from the two screens that are positioned at opposite ends of the table. Align the headrest with the center of mirrors. Finally, create blinders made of black cardboard and foam padded whole straps painted in black and attach them to the posts of the headrest.
Position the eye tracker behind the mirrors and the whole set up is ready for experiments. Begin by seating a participant in front of the mirrors and have them view a different screen reflected via a different mirror with each eye. For best results, use a headrest for stabilizing the participant's head.
Position an infrared sensitive video based eye tracker, including a camera and an illuminator in front of the participant but behind the mirrors. If the desired monitor position is lost, retrieve this position by using the markings on the calibration board, to put the board back in the same place, relative to the reference board that has a fixed position on the desk. Move the monitor again to line up with the appropriate markings.
When using blinders, move them to the participant's eye level and rotate them slightly toward the midline to minimize participant's exposure to other visual input. Ensure that each eye can see the whole visual stimulus in the mirror without seeing any of it directly. For the software calibration, present a dot on each of the two screens in alternation.
Instruct the participant to eliminate the perceived position change, by moving the dot on one of the screens. Alternatively, instruct the participant to align the frames of experimental stimuli instead of two dots, so that both visual fields critical to the particular experiment are aligned. Finally, after applying either method, center the stimuli in the experiment on the resulting onscreen positions to begin the experiment.
During the experiment, the image shown in the eye tracker software will just look as if the mirrors are not there. A short experiment was conducted with only one mirror in place to compare the results with and without the mirror. The participant made to different locations on the screen with one eye tracked without the mirror and the other eye tracked with the mirror.
The eye tracker did not miss any samples for either eye. The average correlations in the horizontal gaze angle and vertical gaze angle were 0.99. While building this set up, it is important to purchase code mirrors that feature near complete reflectance of visible wavelength and near complete transmission of near infrared wavelength.
Once mastered, this set up and calibration can be completed in 10 minutes for each participant if performed properly. After watching this video, you should have a good understanding of how to build and use this dichoptic presentation system with an eye tracker. After this development, this technique paved the way for researchers in the field of vision science to explore a gaze and pupil dynamics with a system that allows dichoptic visual presentation.
