In the Switch procedure we measure the eye movements of young children while they are looking at a pictures or some other visual display on a large TV screen. While they are looking at the screen, they hear various sounds, words or languages. The child sits on the parent’s lap, while the parent listens to music via headphones, so that he or she doesn’t inadvertently influence the child’s behaviour. Children can often hold a plastic toy during the experiment. The majority of experiments last for five minutes. The idea behind this procedure is for the child to listen to a word, for example, long enough that he or she no longer finds it interesting and looks away from the screen more and more frequently. This is also known as “habituation”. We determine the child’s interest based on how long he or she continues to look at the screen. When the child’s interest has decreased considerably, we let him or her listen to another word (that is sometimes very similar to the previous one). If children notice the difference between the new word and the previous one, then their interest should increase and they should once again spend more time looking at the screen.
Our research using this procedure has shown that 10-month-olds are able to discern small sound changes at the beginning of words. For example, infants continuously heard the fictitious word “tos” and after a while gradually lost interest and spent less time looking at the screen. When they subsequently heard a new fictitious word “pos”, where the beginning of the word changed from “t” to “p”, the time they spent looking at the screen increased again.