| Babies are very hard to conduct psychological experiments on. They get bored, they can't understand instructions, they cry, they fall asleep, and they can't press buttons. However, one of the few relatively sophisticated abilities that babies have are their visual abilities. A baby will pay attention and look at anything that is new and interesting. This is the root of preferential looking tasks, as pioneered by Fantz.
Preferential looking tasks will present two stimuli to a baby and the length of time the infant looks at each is measured. Now, the stimulus that the baby looks at for the longest amount of time can be inferred to be the one that the baby finds the most interesting. For instance, visual acuity can be tested for by presenting various stimuli consisting of black and white stripes of varying widths, against a control stimulus that is uniform grey and of the same luminance. There comes a point when the time a baby looks at each is split 50/50. At this point, we conclude that they both look the same to the baby, and hence have found the limit to the infant's visual acuity.
Habituation is a technique developed from the ideas of preferential looking. In such experiments, a baby is shown a stimulus until they are bored of it, and look at it no longer. Then this habituated stimulus is presented alongside a test stimulus. The preferential looking technique is then applied. If the child now looks at the novel stimulus more than the habituated one, we can conclude that they can discern the difference between them. Using this technique, Slater et al. (1983) showed that even 36-hour-old neonates could discriminate between simple shapes.
Preferential listening was developed from preferential looking by Eimas et al. They came up with the "high amplitude sucking technique" where babies were presented with repeated auditory stimulus which was the same syllable repeated over and over. The repeated syllable was then the suddenly changed. Babies increase their rate of sucking on a dummy when they hear something novel, and their response would increase if they could tell the difference between the syllables. It was found that babies can discern many more phonemes than adults, as they have not yet attuned to specialise in the sounds of their native tongue. | 
