Critical Times

As others have noted, one of the main issues disagreed upon in the field is whether infants are born with some innate knowledge, or merely the hardwiring to develop knowledge based on experience. All agree that babies learn very rapidly within their first year. Though Piaget, the "father of cognitive development," thought that infants have no real physical knowledge before 4 months of age, research has since supported the theory that infants as young as 2.5 months have expectations about how objects should interact in the physical world.
In Spelke's theory of innate knowledge, she suggests that infants are born with knowledge of 3 core principles of "naive physics": cohesion, continuity, and contact. As they experience the world, infants then expand on these basics with additional knowledge. In the continuity experiment (Kellman & Kestenbaum, 1986) which we looked at in class, infants as young as 2-3 months were shown to be surprised by violations of the continuity principle (the ball disappearing behind one occluder and reappearing behind a separate occluder). Several studies (many by Baillargeon) on infants 2-3 months old have also measured their expectations of the principles of occlusion, containment, and covering, and have found that infants of this age do indeed have expectations about these events. Does this give support to the theory of innate knowledge, or do these other measured competencies at such a young age suggest that all knowledge is learned? What is it about this age of about 2.5 months that allows infants to suddenly exhibit their competence in these principles? If they had competency prior to this age, could it be accurately tested?
In 1990, Slater et. al. used a study design previously used by Kellman and Spelke (1983) to test whether infants perceive a center-occluded object, which moves as a whole, as one object or two (infants at around 4 months had shown surprise when the occluder was removed to reveal two objects relative to one object). Using this design, Slater and colleagues altered the display in such a way that a newborn, with their limited visual abilities, would be able to perceive it. They then tested two different age groups, newborns and 2 months olds with the display. The results showed that newborns looked longer at the single object (in contrast with the 4 month olds), and the two month olds showed no preference. This suggests that these younger infants do not have the same conception of object unity, and therefore this knowledge must be learned. But could there be other explanations for the younger infants' performance on this task? I suggest that there is, and that it involves not their knowledge of the physical world, but simply their perceptual abilities and the systems by which they process that information.
Two very important things are changing in visual perception abilities for infants younger than 2.5 months, and I don't mean quantitative improvements in vision. First of all, we've learned that eye tracking changes in nature as infants progress through this stage. They begin by tracking only one edge of one object at a time, progressing to several edges and between objects. Secondly, infants are transitioning between their reliance on subcortical pathways and cortical pathways for visual information between birth and two months. So, looking at the "failure" of young infants on the object unity task, we could suggest that young infants may not be able to perceive and examine more than one object at a time at that stage, since their tracking is limited. Additionally, brought in on the "quick and dirty" subcortical pathways could be processed in some qualitatively different way than information that is later brought in through cortical pathways (after about 2 months).
Whatever the explanation, innate knowledge or learned, 2.5 months of age seems to be an important stage after which we know a great deal about cognitive development, and before which we know little. What other methods of experimentation might psychologists tap into in order to study this enigmatic time?