Kiernon asked:

When do infants know about objects?

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This is an extremely difficult question to answer, because of a problem with the term "object". Just
what is an object? A block of wood? A circle against a background? One's own hand? A splotch of
color? A sound? Something that stays the same when it disappears behind something else? There is
a sense in which infants are aware of objects several weeks before they are born. But that is a very
primitive sense, which is constantly being refined. I simply cannot answer your question without more
specification. Check out these for a very sparse introduction:

Cowan, N., J.S. Saults, L.D. Nugent, and E.M. Elliott. "The Microanalysis of Memory Span and Its
Development in Childhood." International Journal of Psychology34, no. 516 (1999): 353-358.

Gopnik, A., and A. Meltzoff. "The Development of Categorization in the Second Year and Its Relation
to Other Cognitive and Linguistic Developments." Child Development58 (1987): 1523-1531.

Gopnik, A., and D.M. Sobel. "Detecting Blickets: How Young Children Use Information About Novel
Causal Powers in Categorization and Induction." Child Development71, no. 5 (2000): 1205-1222.

Piaget, J. The Construction of Reality in the Child.2nd ed. New York, NY: Ballantine Books, Inc.,
1971.

Piaget, J. The Grasp of Consciousness.3rd ed. Cambridge, MA: Harvard University Press, 1978.

Stern, D. N. The Interpersonal World of the Infant.New York, NY: Basic Books, 1985.

Zheng, M., and S. Goldin-Meadow. "Thought before Language: How Deaf and Hearing Children
Express Motion Events across Cultures." Cognition85 (2002): 145-175.

Steven Ravett Brown