Eye tracking is ideal for studying how babies learn about the environment around them. And how they might be using those environmental cues to learn language. Here we study those perceptual cues that might be guiding babies language learning.
The main advantage of eye tracking and our methods described here is that it is a safe, reliable, and non-invasive way to collect copious amounts of gaze data in a short amount of time. Begin by designing the looking preference paradigm, where two different video stimuli are shown simultaneously, each on one half of the screen. Ensure both stimuli differ by one feature, and are otherwise identical for all other visual elements.
Insert attention-grabber images before each trial to maintain and redirect infants'attention to the center of the screen immediately before the trial begins. At the beginning and end end of the experimental sequence, insert a three-point calibration check procedure consisting of three slides, each with one target that appears in the upper left corner, screen center, and lower right corner. Next, position the eye tracker directly under the stimuli monitor, and at a low angle, so that it will face the infant's face as directly head-on as possible.
Use rulers and a digital angle gauge to measure the placement, and angle of the eye tracker, and the monitor. Position a separate web camera often called a user, or scene camera above the stimulus monitor to record the participant's full face during the experiment. Finally set up the experimental presentation software, usually commercially available with the eye tracker, to present the stimuli, record the eye movements, record the user or scene camera, display gaze points during the experiment, and optionally perform gaze data analysis.
Begin by escorting the parent and infant into the testing room. Explain the procedure, and obtain signed consent to proceed with the experiment. Collect standard demographic and medical information, and information about the infant's language and technology environment.
Then, dim the lights in the experimental room, and ensure there are no other obvious visual distractors in the room. Use curtains to occlude the infant's field of view from all distractors in the room. Invite the parent to sit in the chair with the infant sitting on their lap.
To provide more stability the parent may strap the infant in a soft booster seat placed on the parent's lap. Use the eye tracker software to confirm that the infant's eyes are visible to the eye tracker. Next, perform a five-point calibration procedure according to the eye tracker instructions.
After calibration has been successfully verified, begin the experiment with the three-point calibration check. Manually control the duration of each target, when the infant fixates on the target in one slide, immediately proceed to the next target. Then continue with the experiment, beginning with the attention-grabber before the first trial.
Manually control how long the attention-grabber is displayed, and begin the trial when the infant fixates on the attention-grabber. After all trials have been shown, perform the same three-point calibration check procedure again to test for possible signal drift, or calibration changes during the experiment. After the experiment is complete, provide compensation and, if consented to share additional flyers, or materials for the parent to distribute among their peers to assist in the recruitment.
Finally, analyze the data by drawing two areas of interest, AOIs, one for each side of the screen. Ensure the AOIs are slightly larger than the visual elements themselves, to accommodate any minor calibration inaccuracies, or standard instrument error. Maintain a gap of about 25 pixels or larger in between the two AOIs in the center of the screen.
Then, calculate the total looking times for each AOI for each trial, by summing up all gaze points falling within the AOI, and multiplying this count by the sampling interval. Results indicate that younger infants, but not older infants are sensitive to sonority-based phonological restrictions in sign language, despite having never been exposed to sign language before. Further, younger and older infants have different viewing preferences for upright, and inverted signing stimuli.
Specifically, younger infants looked longer at the upright stimuli, while older infants looked longer at the inverted stimuli. Some important things to remember. First, make sure that there are no major changes during recording like in the infant's position.
You don't want large body movements, or changes in lighting. Also, it is important that the two stimuli presented to infants differ only in one way, so that if the infants do prefer to look at one of them, then you will know why, and results can be interpreted with respect to that one visual feature. Eye tracking technology allows scientists to better understand about how infants perceive and organize cues in their world.
Integrated tracking software, and equipment have become more advanced and versatile, allowing us to test babies more easily and address questions, in ways that we couldn't 10 years ago to better understand infant development.
