Memory of what happened

     A aspect of memory believed to depend on the prefrontal cortex is what’s sometimes called autobiographical, or episodic, memory-the kind of memory that lets you consciously recall where you were and what you were doing while you were, say, on your last vacation. If a four-year-old  sees you put a marble in a box, or hears you tell him you’ve put a marble in a box, he’ll remember not just that there’s a marble in the box but also how he knows – whether he saw it or was told. A three-year-old, on the other hand, may remember there’s a marble in the box but not the circumstances of learning it, just as you probably don’t remember how or when you learned that Paris is the capital of France. This kind of memory, called source memory, doesn’t develop until about age four.

     That’s why a three-year-old isn’t lying if he says he saw Rudolf on the roof. He won’t remember whether he actually saw something, or heard it in a story, or even just imagined it. All he’ll remember is the fact of having gotten the idea in his head. It’s no coincidence that it’s also not until about age four that a child can explicitly relate the past to the present to understand, say, that the reason the cat ran away is that he left the door open.

Memory in infants

     It’s not until about the latter half of the first year of life that infants begin to show what might be called explicit, or conscious, memory. During the first half year or so, and even in the womb, infants have various kinds of memory, shared by new animals. They display a primitive kind of learning known as habituation, in which they stop responding to a stimulus after they’ve encountered it a few times. Fruit files and sea slugs can do this, too. Newborns can also be conditioned as when the Russian scientist Pavlov taught dogs to salivate at the sound of a bell by ringing the bell every time he fed them. Even fruit flies can be conditioned.

     Young infants also demonstrate a simple kind of recognition memory when they show interest in things they haven’t seen or heard before, and when they recognize their mother’s voice and even their mother tongue.

     All these kinds of memory are automatic, and none are unique to humans. They correspond to what in an adult would be kinds of implicit memory, which means they happen independently of conscious awareness.