In the first of three studies, children (aged 8 to 14 years) were found to perform worse than young and middle-aged adults in unprompted identification of doors, with average performance much like that of elderly adults. Comparisons on other tasks, specifically odor threshold, prompted odor identification, and object naming (Boston Naming Test), across the life span (five groups) revealed that children have the same excellent olfactory sensitivity as young adults and merely lack odor-specific knowledge that accumulates slowly through life. Such knowledge apparently accumulates so slowly that age-associated discriminative losses, measurable by early middle age, begin to wear away gains obtained through experience before odors can become overlearned. In the second study, a novel adaptive psychophysical method, the step procedure, confirmed the equivalent sensitivity of children and young adults. In the third study, a paired-associate task illustrated the sluggish course of odor learning. Young adults outperformed children, though the youngest group, first graders, made up ground relatively fast. For children and adults, common odors facilitated performance relative to novel odors. The outcome highlighted the relevance of semantic factors in odor learning irrespective of age.
The characteristics of memory in infants and adults seem vastly different. The neuromaturational model attributes these differences to an ontogenetic change in the basic memory process, namely, to the hierarchical maturation of two distinct memory systems. The early-maturing (implicit) system is functional during the first third of infancy and supports the gradual learning of perceptual and motor skills; the late-maturing (explicit) system supports representations of contextually specific events, relationships, and associations. An alternative model holds that the basic memory process does not change, but what infants and adults select to encode for learning does. This ontogenetic change in selective attention has been mistaken for an ontogenetic shift in the basic memory process. Over the last 25 years, evidence from transfer studies with developing rats and human infants has revealed that the first third of infancy is actually a period of exuberant learning that ends, not coincidentally, at the same age that the late-maturing memory system presumably emerges. This article reviews data from recent studies of sensory preconditioning, potentiation, associative chains, and transitive inference with human infants that support this conclusion-data for which the neuromaturational model cannot account. Fast mapping is a general learning mechanism that accounts for this evidence.
We previously found that young infants spontaneously associate stimuli that they merely see together. Using a sensory preconditioning paradigm with 6-and 9-month-olds, we asked how long such associations remain latent before being forgotten and what exposure conditions affect their persistence. Groups were preexposed to two puppets for 1 hr/day for 2 days, 1 hr on 1 day, or 1 hr on 1 day in two sessions; 1 to 27 days later, target actions were modeled on one puppet, and infants were tested with the other puppet 1 day later. The longest delay after which infants imitated the actions on the other puppet defined how long they remembered the association. The data revealed that the preexposure regimen determined retention. Regardless of exposure time, both ages remembered the association longer after two sessions, and younger infants remembered longer than older infants--for 4 weeks--after two 30-min sessions on 1 day.
Sensory preconditioning (SPC) is a form of latent learning in which preexposure to co-occurring neutral stimuli (S1-S2) permits subsequent learning to be transferred from one stimulus (S1) to the other (S2). We examined whether human infants exhibit developmental transitions in the temporal parameters of SPC by manipulating the preexposure regimen. Infants received simultaneous or sequential preexposure to puppets S1 and S2 (Days 1–2); saw target actions modeled on S1 (Day 3); and were tested for deferred imitation with S2 (Day 4). Although 6-, 9-, and 12-month-olds associated the puppets, there was a shift in the effective regimen from simultaneous to sequential preexposure—similar to prior findings with rat pups (Experiment 1). Experiment 2 revealed that human infants potentially exhibit another transition in SPC at 15 and 18 months of age. We consider the roles of ontogenetic shifts in infants’ ecological niche, selective attention, and unitization in developmental transitions in SPC.
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