ABSTRACT
Pacifiers were used routinely in many laboratories in the 1960s and 1970s in order to keep the infants in a more stable state with fewer limb movements. Infants who remained awake responded with heart rate deceleration to the first presentation of a tone, but the group who fell asleep within 5 minutes responded with acceleration on the first trial. The auditory system had been explored by looking at infants' discrimination of location, pitch, timbre, and so on. Association learning and habituation had been explored in the 1960s and 1970s. One of the puzzles in infant data is the presumed discrepancy between reaching and looking data with regard to infants' representation of unseen objects. When infants as young as 3-4 months view objects and events, they appear to appreciate the solidity and position of unseen objects, yet they will not reliably search for a hidden toy until 8-9 months.
