In a 1951 paper, psychologist Neal Miller, PhD, wrote that although people cannot remember their very early childhood, the events that happen then still influence them years later.

"The young child does not notice or label the experiences which it is having at this time," Miller wrote. "Nevertheless, the behavioral record survives."

In the annual Neal Miller lecture at APA's 2005 Annual Convention, University of Arizona psychology professor Lynn Nadel, PhD, described how his and others' research has begun to back up Miller's observation.

"Fifty years ago we didn't have the evidence," Nadel said, "but now we're in a position to talk about this in an empirical way."

Children and memory

That evidence begins, Nadel said, with the fact that there are really two kinds of memory: memory for episodes, events and facts, and memory for skills and habits. The first kind of memory, Nadel and others have found, mainly involves the hippocampus. In a 2001 study, for example, Nadel and his colleagues used functional magnetic resonance imaging to examine the brains of seven older adults as they recalled specific memories from their recent past (such as giving a talk at a conference) and from their long-ago past (such as learning to drive). Recalling all of these episodic memories activated the participants' hippocampi.

However, implicit memory--the skills and habits we acquire without even trying--involves many other brain structures. Research points to the cerebellum, basal ganglia and amygdala, which is important for acquiring emotional memories, Nadel said.

He and others have found evidence that the amygdala system is present at birth, but the hippocampus matures later. So the events of our early childhood might leave an emotional memory imprint and influence our habits and other implicit learning without leaving any trace of an episodic memory via the still-immature hippocampus.

"This helps us understand why we can't remember when and how we learned things in infancy, but these things exert a strong influence on our subsequent behavior," Nadel said.

The stress connection

The hippocampus also plays another critical role: It helps regulate the body's stress response.

"The hippocampus is loaded with stress hormone receptors," said Nadel. He and others have found that, because of this, stress hormones can affect the hippocampus's other functions, including episodic memory.

In a 2002 study, for example, Nadel and his colleagues found that stress increased false-memory rates. They asked participants to give a 10-minute speech to a panel of judges. Then, those stressed participants and some unstressed control participants listened to a long word list. Later, the stressed participants were more likely to say that they remembered hearing words that were not really on the list.

However, Nadel has found that stress is less likely to affect the types of memories that involve the amygdala rather than the hippocampus--such as emotional memories. In a study currently in press, participants who were either stressed or unstressed viewed a series of slides that told a story--either an emotional story or a neutral one. When the researchers asked the participants to recall the story one week later, they found that stressing the participant before the emotional story actually increased memory, but stressing the participant before the neutral story decreased memory.

Overall, Nadel said, research shows that "Aversive events that happen early in life can leave powerful traces behind, but with no record of where or when these experiences occurred."

This could, he said, help explain the root of such phenomena as strong phobias that have no known cause, or even post-traumatic stress disorder.

"And later in life," he continued, "stress can accomplish the same thing."