The study of the maternal/infant system is accomplished in this playpen situation. Small doors between living units and the central play area permit passage into the play area by infants, but restrict mothers to the living area. Hansen Playpen, 1963. Maternal deprivation experiments performed by Harry Harlow of the University of Wisconsin in the 1950's involved separating infant Rhesus Macaques (Macaca mulatta) from their mothers and rearing them with surrogate mothers made of wire or cloth. The monkeys were kept in partial or total isolation, in wire cages or in "pits" or "wells of despair." These experiments found that comfort, security and affection are necessary for a monkey's healthy psychosocial development.