Habituation: The Switch Procedure
The foundational idea for discrimination studies is that once infants have been become so thoroughly familiar with a stimulus that they no longer pay attention to it, their attention will recover if a stimulus that they recognize as different is presented. This is exploited in the particular form of this type of study called the Switch procedure.
In this procedure, during the initial, Habituation phase, the infant sits on the parent’s lap facing a monitor on which he/she sees a visual image of some sort while listening to a sound stimuli. The experimenter records on an external computer how long the infant looks at the monitor while listening to the stimuli. When the looking time drops to a pre-established criteria (a set percentage of initial looking times), Test trials begin.
During the Test trials, infants are presented with either the same (“Same”) or different (“Switch”) sound, visual, or paired sound-and-visual stimulus. Recovery of attention to the monitor during the Switch trial is interpreted as evidence that the infant notices the change in stimulus, and thus is capable of making the relevant discrimination.
Newborn infants can also be tested with this procedure using an identical design. In this case, the dependent variable is not looking time but high amplitude sucks. Infants supported in a carrier seat are given a pacifier to suck on that is connected to a computer that monitors frequency and strength of sucks. Newborn infants indicate that they have become habituated to a stimulus by sucking less and less frequently and strongly, and show discrimination by recovering frequent and stronger sucks to the presentation of the Switch stimulus.