Evidence of robust recognition memory early in life even when assessed by reaching behavior.

Infants of only 5-6 months prefer to look at something new when given the choice of looking at a stimulus shown earlier or something new, even after a long delay (the visual paired comparison task). However, if infants must reach and displace a stimulus to retrieve a reward, even 18-month-olds respond randomly when given the choice of reaching to the stimulus shown earlier or to something new, even after a brief delay (the delayed nonmatching to sample task). To investigate this paradox we modified the delayed nonmatching to sample task to make it more similar to visual paired comparison. Each stimulus served as its own reward; no rewards were hidden under any stimuli. Infants were habituated to a sample object, a delay was imposed, and then the sample and a new object were presented. Infants could choose to look at (in visual paired comparison) or reach for (in delayed nonmatching to sample (stimulus = reward)) either object. One hundred twenty infants were tested: 60 (20 each at 4, 6, and 9 months) on visual paired comparison and 60 (20 each at 6, 9, and 12 months) on delayed nonmatching to sample (stimulus = reward). The same 10 pairs of stimuli were used on both tasks. Each subject was tested twice at all five delays (10, 15, 60, 180, and 600 s). At even the youngest age that reaching was tested (6 months), infants showed evidence of recognition memory on the reaching task at delays at least as long as those at which they demonstrated recognition memory on the looking task. Indeed, when subjects reached, not in order to obtain something else, but to obtain the stimulus itself, they succeeded on a recognition memory task even at delays 10 min long very early in life.