At around 8 months, there is a change in the way that infants remember. Before, they could recognize objects and people that they had seen before, but now they can begin to “recall” them on their own. Starting around 8 months, infants not only recognize familiar things but also notice and have an emotional reaction when someone or something is new, missing, or out of sight. This is related to the cognitive skill called “object permanence” and is why infants this age begin to experience “stranger anxiety.”
Fox, N.A. & Bell, M.A. (1990). Electrophysiological indices of frontal lobe development: Relations to cognitive and affective behavior in human infants over the first year of life. Annals of the New York Academy of Sciences, 608, 677
Konner, M. (1991). Universals of behavioral development in relation to brain myelination. In Gibson, K. R., & Petersen, A. C. (Eds.). Brain maturation and cognitive development: Comparative and cross-cultural perspectives. New York: Aldine de Gruyter.