Three-month-old infants learned to activate a crib mobile by means of operant footkicks. Retention of the conditioned response was assessed during a cued recall test with the nonmoving mobile. Although forgetting is typically complete after an 8-day retention interval, infants who received a reactivation treatment--a brief exposure to the reinforcer 24 hours before retention testing--showed no forgetting after retention intervals of either 2 or 4 weeks. Further, the forgetting function after a reactivation treatment did not differ from the original forgetting function. These experiments demonstrate that (i) "reactivation" or "reinstatement" is an effective mechanism by which early experiences can continue to influence behavior over lengthy intervals and (ii) memory deficits in young infants are best viewed as retrieval deficits.
Three-month-old infants learned to activate an overhead crib mobile by operant footkicking and received a visual reminder of the event (a "reactivation treatment") 2 weeks later, after forgetting had occurred. Subsequent manifestation of the association was a monotonic increasing function of time since the reactivation treatment, and performance of infants tested 8 hours after the remainder was related to the time spent sleeping in the interim (r = 0.75). These data demonstrate that normal retrieval is time-dependent. Moreover, individual data suggest that retrieval may be continuous rather than discontinuous.
The ability of 3-month-olds to acquire generalized expectancies of reward and the role of these expectancies in memory retrieval was assessed in 2 experiments. In both, infants learned to activate the components of a crib mobile by kicking and were trained with mobiles containing either the same (invariant) or different (variable) components in successive daily sessions. In Experiment 1, infants exhibited positive transfer over both the invariant and variable stimulus series. However, there was a trend toward disruption of retrieval when infants were tested with a familiar set of components following training on a variable stimulus series. In Experiment 2, infants were trained for 2 sessions with mobile components that either did or did not change between sessions. In a 24-hour retention test, mobile components that either continued or violated the serial order established during the preceding sessions were presented as retrieval cues. Infants again exhibited positive transfer over both variable and invariant series when the test stimulus was predicted by the serial order of training stimuli, but violations of either expected order produced a retention deficit. These results were interpreted as suggesting that very young infants develop generalized expectancies of reward that are based on the serial pattern in which events occur.
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