Fifty-five 6- to 7-month-old human infants were trained in an operant conditioning procedure, adapted from a procedure developed for 3-month-olds, in which kicks were reinforced by conjugate movement of a mobile. Retention was assessed in a simple forgetting paradigm (Expt. 1) or in a reactivation paradigm (Expt. 2) with either the training mobile or a different one serving as the retrieval cue. In Experiment 1, retention was tested 1, 7, 14, or 21 days after training. When the training and test mobiles were the same, infants exhibited virtually no forgetting for 14 days, but forgetting was complete by 21. When the training and test mobiles were different, infants exhibited no retention, discriminating the novel mobile for as long as they could remember the contingency. In Experiment 2, when the training mobile was presented as a reminder, the forgetting previously seen after 21 days was alleviated; when a different mobile was the reminder, it was not. These findings reveal that the efficacy of a reminder is predicted by the efficacy of that same stimulus in cuing the original memory 24 hr following training. Although the 6-month-olds learned more rapidly and remembered longer than infants half their age, their memory processing was described by the same basic principles.

In 3 experiments, 6-month-old infants learned to move a mobile by kicking and were tested 1 to 21 days later for retention of the newly acquired memory as a function of the training and testing contexts. In Experiment 1, decreasing the relative distinctiveness of the training and testing context did not impair retrieval of the newly acquired memory. In Experiment 2, however, testing in a different context completely eliminated retention after delays of 1 and 3 days, when retention was otherwise perfect; after progressively longer delays, retention improved paradoxically. The familiarity or novelty of the test context was not a factor in the failure of infants to recognize the mobile in the altered context after 1 day. In Experiment 3, the effect of an altered context was assessed in a reactivation paradigm. After the training memory was forgotten, infants were presented with the original mobile as a reminder and were tested for retention of the training memory 1 day later. When either the reminding context or the testing context was different, they exhibited no retention. These findings reveal that memory retrieval at 6 months is highly specific to the setting in which the memory is acquired. We propose that infants learn what specific events are associated with what specific places prior to the age when they can locomote independently and acquire a spatiotemporal map of the relations between those places.

Six-month-old infants trained in an operant conditioning procedure were allowed to forget the contingency and were presented with a reminder in a memory-reactivation paradigm. The time course of memory retrieval after the reminder, the relation between the forgetting functions of the newly acquired and the reactivated memory, and the potential contribution of the context to retention after long delays were investigated. Memory retrieval was found to be a time-locked process at 6 months, as at 3 months. Although retrieval was more rapid at the older age, the reactivated memory was more transient than the newly acquired memory at 6 months and remained accessible for a briefer period than at 3 months. A distinctive context was requisite for memory reactivation at 6 months but did not insure it. These studies reveal that the temporal parameters of memory processing change with age.Research on learning and memory development over the first half-year of life has shown that older infants learn faster and remember longer (Gekoski, 1977;Hill, Borovsky, & Rovee-Collier, 1988). Hill et al, for example, found that 6-month-olds acquired an operant contingency in one third of the time required by 3-month-olds and remembered it after a retention interval twice as long, despite the fact that 6-month-olds had less total training time. Two-month-olds, on the other hand, require at least twice as long as 3-month-olds to acquire the same contingency, and they forget after retention intervals only half as long (Greco, Rovee-Collier, Hayne, Griesler, & Earley, 1986). Similar findings have been obtained in studies of learning and memory development in animal infants (Campbell & Campbell, 1962). Findings such as these have been taken as evidence that memories of older infants, both animal and human, are mediated by a fundamentally different system than memories of younger infants (Bachvalier & Mishkin, 1984;Schacter & Moscovitch, 1984).In both animal and human infants of all ages, however, memories are forgotten gradually, can be recovered by a reactivation procedure, and are context-specific (

In 3 experiments, 6-month-old infants learned to move a mobile by kicking and were tested 1 to 21 days later for retention of the newly acquired memory as a function of the training and testing contexts. In Experiment 1, decreasing the relative distinctiveness of the training and testing context did not impair retrieval of the newly acquired memory. In Experiment 2, however, testing in a different context completely eliminated retention after delays of 1 and 3 days, when retention was otherwise perfect; after progressively longer delays, retention improved paradoxically. The familiarity or novelty of the test context was not a factor in the failure of infants to recognize the mobile in the altered context after 1 day. In Experiment 3, the effect of an altered context was assessed in a reactivation paradigm. After the training memory was forgotten, infants were presented with the original mobile as a reminder and were tested for retention of the training memory 1 day later. When either the reminding context or the testing context was different, they exhibited no retention. These findings reveal that memory retrieval at 6 months is highly specific to the setting in which the memory is acquired. We propose that infants learn what specific events are associated with what specific places prior to the age when they can locomote independently and acquire a spatiotemporal map of the relations between those places.

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