Author: Winni Ye
Editors: Yanxi Chen and Jasleen Matharu
Artist: Lalita Ma
Infantile amnesia refers to the inability to remember early childhood memories, typically those formed within the first three years of life. Most people struggle to remember episodic memories up until the age of ten. However, it is incorrect to assume that this means infants cannot form memories at all. A typical example of a memory an infant has is when the infant knows to smile when they see someone they recognize, such as their mother. This is categorized as an implicit (or declarative) memory, which includes people's skills, procedures, and conditioned responses. Countless studies have shown that infants can form implicit memories using simple procedures and conditioning. Still, recent studies have shown that infants can also form explicit (or non-declarative) memories, including the facts and experiences that people consciously recall.
Psychologist Carolyn Rovee-Collier is known for her groundbreaking work on infant memory. In 1987, she designed and proved in an experiment that, while infants are unable to form declarative memories, they are capable of forming procedural memories. In her first experiment, Rovee-Collier put infants between the age of 2 and 6 months in a crib with an overhead mobile. She first measured how often the babies naturally kicked and then tied a string to their legs so that the mobile would move every time they kicked. She observed that the babies started to kick more after quickly learning that the more they kicked, the more the mobile would move. In a second experiment for older infants aged 6 to 18 months, Rovee-Collier sat infants on their parents' laps near a lever that would move a train around a train track. At first, the lever didn't work, and she measured how much the babies pressed on the lever when there was no incentive. Then, they turned the lever on, and the babies pressed it significantly more. After the initial demonstrations, Rovee-Collier later tested the infants on how well they remembered those actions; at 6 months, infants demonstrated retention for over 21 days, and the older the infant, the longer they remembered. Rovee-Collier's experiment was the first that proved infants could form memories, although only procedural, and retain those memories.
In 1995, psychologist Andrew N. Meltzoff demonstrated that infants are capable of deferred imitation or reenacting a past event from memory. This is significant because deferred imitation does not fall into the category of procedural memory, which was believed to be the only type of memory infants could form. In the first experiment, he invited 96 infants, 48 were 14 months old, and 48 were 16 months old. They were randomly assigned to three groups: Control, Model (obs only), and Model (obs+immed). In the first visit, infants in both the Model (obs only) group and Model (obs+immed) were individually brought into the lab and shown four target acts: 1) Taking a dumbbell-shaped toy made of a tube of plastic and two wooden cubes and pulling the cubes off; 2) Leaning forward and touching the top orange panel on a flat rectangular box with the forehead, after which the panel lights up; 3) Flattening a collapsible plastic cup by using the palm to push down on the cup; 4) Using the index finger to push a button in a small circular hole in a small black box, after which a buzzer sounds. The Model (obs only) group only saw the demonstrations of the target acts, which were repeated three times. They were not allowed to touch or play with the objects. The Model (obs+immed) group was allowed to play with the objects immediately after the demonstrations. The Control group was brought into the lab room and acclimated to the surroundings, but they were not shown the demonstrations. The purpose of the Control group was to measure, in the second visit, how often infants would spontaneously produce the same acts on the toys. After two months, the 96 infants were brought back, and the toys were presented to each infant in the order it was presented originally. The results showed no significant difference between the Model (obs only) and Model (obs+immed) groups when looking at the mean number of target acts produced by those infants.
He repeated the experiment with infants who were 14 months old and 16 months old and imposed a delay of 4 months instead of 2. The results were the same as before: no significant difference between the Model (obs only) and Model (obs+immed) groups. Because the experiments showed that the Model(obs only) infants were able to remember and reproduce the target actions just as well as the Model(obs+immed) infants, Meltzoff's experiment indicated that what the infants formed a memory of was not only the action, the procedural memory, but the demonstration itself, which the infant had to recall during the second visit to perform the action without prior practice. The deferred imitation Meltzoff demonstrated in his experiment is what he calls "a form of nonverbal declarative memory" because the infant is unable to describe what they remember, as people typically can with explicit memories. Still, the infant is undeniably recalling a memory from the past.
However, the question remains: why can’t infants form explicit, or specifically episodic, memories? The most widely accepted theory is that the hippocampus, the area of the brain responsible for the formation of memories, is not fully developed in infancy, preventing infants from forming full memories that they can remember in adulthood. Another theory is that to make autobiographical memories, infants must first have a sense of self, which they develop at 18-24 months. In the mirror recognition test, also known as the "rouge test", a dot of lipstick or blush is put on a baby's nose, and the baby is then placed in front of a mirror. Infants younger than 18 months will typically just smile at the mirror, unable to tell that there is a red dot on their nose. From 18 to 24 months, babies begin to touch their noses and even show embarrassment, showing that they've developed a sense of self. Finally, a theory states that infants can't form episodic memories because they don't have the language to form narratives that they can remember in the future.
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