Task-specific size invariance in 3-month-old infants

Purpose: Adults are sensitive to size changes when asked to perform an old/new recognition task but show strong size invariance in naming latency tasks (Biederman & Cooper, 1992; Cooper, Schacter Ballesteros, & Moore, 1992). These two tasks have been equated with separate memory systems in adults; recognition is an explicit memory task, while naming latency is a priming or implicit memory. Our question here was whether infants demonstrate the same dichotomy of sensitivity/invariance in analogous tasks thought to require explicit and implicit memory, respectively. Methods: In the mobile conjugate reinforcement paradigm, used in both tasks, an infant learns to kick to move an overhead crib mobile displaying seven 3° disks, each with a character (+) on each side. After 2 consecutive days of training, one group of 3-month-olds received a 24-hour recognition test with a stimulus (+) of a different size. This is equivalent to an old/new recognition test and was the explicit memory task. Another group was identically trained for 2 days but allowed to forget the training memory over a 14-day delay. Its memory was then reactivated by a brief exposure to a mobile displaying differently-sized +'s. The effectiveness of the reactivation procedure was detemined 1 day later during a retention test. A reactivation treatment primes the inaccessible memory and is thus an implicit manipulation. Results: The 24-hour test group failed to recognize the differently-sized test stimulus. The memory of the 14-day reactivation group, however, was successfully primed by the differently-sized stimulus. This group exhibited near-perfect retention 24 hours after priming, despite previous evidence that the training stimulus is forgotten 7 days earlier at 3 months (without priming, infants show no evidence of retention after 7 days). Conclusions: 3-month-old infants display the same task-specific size sensitivity and size invariance as adults. This finding suggests that similar systems mediate the performance of both adults and infants.