Average children spend about a year in the world before producing their first word and it will take them around six months more to start learning words at a fast pace. Babies do not spend this time in idle contemplation of the world; they spend it learning about how their own body works, and more relevant to our current argument, about how the world works. Very young infants display a knowledge of what happens and what doesn't happen in the world; they can predict the consequences of actions and be surprised when their expectations are violated.
For example, babies know about how objects behave in support or occlusion events. Babies as young as 4.5 months realize that objects which are not supported will fall [Needham and Baillargeon, 1993] and 8.5-month-old infants can judge whether an object is being sufficiently supported and be surprised when an insufficiently supported object fails to fall [Baillargeon and Hanko-Summers, 1990]. Young infants also know about the impenetrability of objects and the parts of objects that should still be visible given the shape of the occluder [Baillargeon, 1992]. Other results show that this knowledge develops. For example, very young babies are not surprised by seemingly unsupported stable objects if there is an occlusion event between the habituation and the test [Spelke et al., 1992]. Also, very young babies are not surprised when unsupported objects fail to fall if there is no motion involved in the event [Spelke and Kyeong, 1992].
Babies also have the more abstract notion of ``objectness''. Young infants are able to use some of the cues adults use to segregate objects, such as relative motion and textural cues [Needham and Baillargeon, 1997]. This knowledge also develops: motion comes first, then textural information, and finally gestalt properties [Spelke et al., 1993]. Four-month-old infants expect objects to retain characteristics such as size and trajectory even though they are not visible [Baillargeon, 1991].
Infants also seem to be able to categorize spatial relations. Three- to 4-month-old babies categorize LEFT-RIGHT [Behl-Chadha and Eimas, 1995] and ON-UNDER [Quinn, 1994] relations, generalizing over the orientation, the size, and the absolute location of the objects involved in the relation. Seven-month-old infants are able to generalize over different kinds of objects involved in the relation. Again, this knowledge develops; younger infants are not able to abstract over different objects involved in the relation, while older infants can [Quinn et al., 1996].
In sum, before they have learned any words, children seem to be forming categories that are useful in representing what the world is like. We think that the learning that occurs during this period is important in setting the basis on which linguistic concepts will be formed; hence our model has a pre-linguistic learning period.