Kristin Hartshorn

The ontogeny of long-term memory over the first year-and-a-half of life

This research documents the development of long-term memory in human infants from 2 months through the end of the first year-and-a-half of life. In the initial study phase, we trained 6- to 18-month-old human infants in an operant task and tested them after increasing delays until they exhibited no retention for 2 successive weeks. In the second phase, their data were combined with data previously obtained from 2- to 6-month-olds in an equivalent task. The resulting function revealed that the duration of retention increases monotonically between 2 and 18 months of age. This increase was not due to age differences in original learning. This is the first systematic analysis of the course of long-term memory across an extended period of infant development that is based on standardized parameters of training and testing. It provides a reference function against which measures of retention from infants of different ages that are obtained in different memory tasks with different parameters can be meaningfully compared.

Infant learning and long-term memory at 6 months: a confirming analysis.

In three experiments, we tested the generality and validity of prior evidence of delayed recognition, memory reactivation, and retrieval specificity at 6 months of age using a new operant task. In Experiment 1, the forgetting function was found to be 2 weeks but not 3, the same as previously obtained using the mobile conjugate reinforcement task. In Experiment 2, a reactivation treatment with the original cue (train set) in the original context (room in the home) recovered the memory but was ineffective when either was changed. These results also confirmed prior findings but expanded the manipulation of context to include the place where training occurred. Following a reactivation treatment, infants in Experiment 3a failed to recognize the original cue in a different context, as before, but generalized to a different cue in the original context. Because the latter test condition was novel, a comparison group was trained in the mobile task and tested with a different mobile in the original context 1 day after a reactivation treatment (Experiment 3b); this group failed to recognize the mobile. The disparity in these outcomes was attributed to the salience of features common to the two train sets. Taken together, these findings demonstrate that, for the same parameters of training and testing, memory performance at 6 months is task-independent.

An expanding training series protracts retention for 3‐month‐old infants

This research examined whether an expanding training series protracts retention for infants as it does for children and adults. In three sessions spanning an 8-day period, 3-month-olds learned to move a crib mobile by kicking. Intersession intervals were either constant (1 or 4 days) or progressively expanding (average ISI = 4 days). The expanding-series group exhibited significant retention on a delayed recognition test 3 weeks after training was over, but the two constant-series groups exhibited none. Although the 1-day constant-series group remembered after 1 week, the 4-day constant-series group did not. Surprisingly, a reactivation treatment administered 4 weeks after training was over was ineffective whether infants were trained, reminded, and tested in a distinctive context or not. These results demonstrate that the retention advantage afforded by programming training sessions in an expanding series extends to infants and suggest that the upper limit on reactivation is timed from initial encoding and not from the point of forgetting.