The memory stores described above are assumed to be universal and present throughout life. Indeed, there is evidence that the capacity to store information in long-term memory begins even before birth. In a well-controlled investigation conducted by Anthony DeCasper and Melanie Spence, the researchers asked pregnant women to read aloud a Dr. Seuss story during the last six weeks of their pregnancies, a point in prenatal development at which fetuses can hear. Shortly after birth, the newborns' recognition memory was tested by comparing their reactions to the familiar passages versus similar but new story excerpts, both of which were read by the babies' mothers. The assessment built on the knowledge that babies can learn to modify the time between bursts of sucking when a change in sucking is followed by the presentation of a stimulus that serves as a reinforcer. The new-borns wore headphones and were given a pacifier that recorded their sucking bursts. They indeed modified the way they sucked when the change in sucking was followed by the familiar passage, but they did not do so with the unfamiliar passage. The fact that the old, but not the new, passages served as reinforcers demonstrated that the babies could recognize the stimuli to which they were exposed before birth.
Although even very young infants can recognize sights, sounds, and smells they have previously encountered, the ability to recall an object or an experience develops later. Recall differs from recognition in that it requires coming up with a response as well as determining that it is correct. Some simple recall is present in the second half of the first year. As every babysitter knows, very young infants remain calm when their parents go out; by around seven months of age, however, separation protest is apparent. By about nine months of age, babies can imitate an action after a twenty-four-hour delay. Note that early recall is heavily dependent on cues and is limited to relatively brief time intervals. Recall continues to develop over the second year of life, corresponding to the development of the prefrontal cortex and other brain structures associated with explicit memory. Between age two and two and a half, toddlers can be expected to remember to stay away from common hazards, provide their first and last names when asked, repeat parts of nursery rhymes, and possess simple event schemas (also called scripts) for everyday events.
By age two and a half, as is well documented in the work of Robyn Fivush and her colleagues, children describe specific past experiences such as a trip to an amusement park. Such early memories, however, do not generally become a permanent part of autobiographical memory, the subset of episodic memory that represents individuals' own life histories. Most people do not recall anything that happened before they were three years of age. This phenomenon is described as infantile amnesia. Although the reasons for infantile amnesia are not completely understood, several factors appear to be important in explaining the developmental emergence of autobiographical memory. One of these is the maturation of the frontal lobes of the brain, which continues throughout early childhood. A second factor appears to be the emergence of the self-concept, which serves as a conceptual framework for the organization of memories. Another is the role of social interactions in maintaining early memories. Katherine Nelson has emphasized the importance of the child's participation in family discussions about past events for keeping early memories alive, a process described as reinstatement. Children are about three years old before they can actively participate in conversations about past events. Finally, some early memories may not be retrieved at later points in development because they are not effectively cued. Because the typical everyday environments of older individuals differ greatly from those of very young children, some potential memories may not be accessed.
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