The development of declarative memory in human infants: age-related changes in deferred imitation.
In 2 experiments, deferred imitation procedures were used to trace age-related changes in declarative memory by human infants over the first 2 years of life. An adult modeled 3 actions with an object, and infants' ability to reproduce those actions was assessed 24 hr later. Some infants were tested with a new object or in a new context relative to the original demonstration. Changes in the context disrupted the performance of 6-month-olds but had no effect on the performance of 12- and 18-month-olds. Changes in the object disrupted the performance of 6and 12-month-olds but had no effect on the performance of 18-month-olds . This age-related increase in representatio nal flexibility may account for the decline of childhood amnesia during the 3rd year of life. The notion that there are two or more functionally distinct memory systems began with the hallmark studies of one severely amnestic patient, H.M. (Corkin, 1968; Milner, Corkin, & Teuber, 1968; Scoville & Milner, 1957). H.M. suffered a profound impairment in forming new memories after surgical removal of most of the hippocampal formation and its associated medial temporal lobe structures. Despite the severity of his amnesia, H.M. showed almost normal retention when tested on tasks of motor skill learning or perceptual priming. This finding gave rise to the notion that memory is not a unitary process, but rather is composed of two (or more) dissociable systems that serve different functions and operate according to different principles (for reviews, see Schacter & Tulving, 1994). A large number of dichotomous terms have been generated to describe these memory systems. Those most commonly cited are declarative versus nondeclarative memory (Cohen & Squire, 1980; Squire, 1987, 1992a, 1992b, 1994) and explicit versus implicit memory (Graf & Schacter, 1985; Schacter, 1987, 1994). Although these dichotomies are not identical, they share two common claims. The first claim is that one memory system supports conscious recollection of episodic facts and events and allows for rapid, one-trial learning (declarative or explicit memory), while the other memory system supports the retention of habits, learned skills, and priming, and is made possible through gradual or incremental learning (nondeclarative or implicit memory).