Naps may help infants form abstract memories

Frequent napping is apparently not wasted on the very young. By age 15 months, naps contribute significantly to infants’ ability to learn and remember general rules about the welter of information they encounter, a new study suggests.

In particular, brief bouts of somnolence assist infants in grasping the core structure of speech, propose psychologist Almut Hupbach of the University of Arizona in Tucson and her colleagues. Fifteen-month-olds display day-long memory for the underlying format of a made-up language only if they nap within four hours of hearing that language, Hupbach’s team reports in a paper published online April 6 and in an upcoming Developmental Science.

How sleep affects memory and learning is much better understood in adults (SN: 8/9/03, p. 93) than it is in infants.

“We suspect that napping plays a role in many kinds of learning during infancy, one of which is extracting and remembering regularities from incoming information,” Hupbach says.

Her new findings tie in with evidence of a similar sleep effect in adults and school-age children, remarks neuroscientist Jan Born of the University of Lübeck, Germany. In several studies, Born’s team had volunteers press a sequence of buttons repeatedly without knowing that the button presses followed a pattern. “When training was followed by sleep, compared with staying awake, adults and children even more so were better able to generate the correct sequence of button presses” eight hours later, Born says.

In Hupbach’s study, an experimenter visited the homes of 24 healthy 15-month-olds. Visits occurred shortly before times of day when children typically took naps. The experimenter quietly interacted with each child while a 15-minute tape recording of a woman reading phrases from an artificial language played in the background.

Recordings consisted of 48 three-word strings presented in two forms, each featuring the same first and last words separated by a middle word that varied. Some infants heard strings that included pel-wadim-jic, vot-kicey-rud, pel-deecha-jic and vot-loga-rud – where “pel” always occurred before “jic” and “vot” always occurred before “rud.”

Similar patterns occur in the English language, Hupbach says. Consider three-part verb forms that assign certain first and last elements to a variety of stem words, such as is playing.

All youngsters napped within four hours of the experimenter’s home visit.

The next day, parents brought their kids to a laboratory and sat with them in front of two loudspeakers. In alternating trials, one speaker played three-word strings that had been heard at home  and the other played three-word strings in two new forms, with unfamiliar first and last words.

Infants showed signs of remembering the structure of word strings from the previous day’s session, rather than of remembering specific words from the artificial language. The evidence for this memory was in the way infants listened to both the old and new word strings. 

If a trial started with a familiar word string, infants listened longer to familiar than to unfamiliar strings for the remainder of that trial. If a trial began with an unfamiliar word string, infants listened longer to unfamiliar than to familiar strings. Had the infants just remembered the words, they would likely have preferred the familiar word strings both times. Instead, the structure of the language was what grabbed their attention. Hupbach’s group treated the amount of time children looked toward each speaker as an indicator of listening time.

In a second experiment, another set of 24 healthy 15-month-olds heard a recording of the artificial language at home but did not nap within the next four hours. When tested the next day after having slept overnight, these infants showed no pattern in how they listened. This randomness suggested the infants had no memory of either the structure of the word strings or of the actual word strings.

Infants forgot specific pairs of first and third words within 24 hours of hearing them, regardless of when they napped, the researchers propose. But infants who napped within four hours of language exposure remembered the rule that first and third words in a string always go together, the team suggests. Thus, on the second day, those kids monitored word strings that matched the structure of whatever they had heard on the first day.