Assimilation is how we are able to perceive the world as determined by our current level of neurological organization.
The infant’s network of basic reflexes determines both motor behavior and sensory input. A touch on the cheek for the infant, for example, is perceived as a food stimulus, and the rooting reflex will be initiated in response.
It makes no difference what the source of the touch is--a bottle, a finger or a tongue blade etc. To a baby, it is all food (assimilation).
Two things will change that perception. They both contribute to the growth of the neural infra-structure.
One is the normal growth due to natural maturational development.
The other involves connections developed as a consequence of experiences.
These experiences center around a need that arises to change a behavior to return the body system to a state of balance. The state of imbalance may be that the baby is hungry.