Anatomy of Memory
People are rarely able to recall memories of events that happened earlier than age three, a phenomenon referred to as childhood amnesia. Sigmund Freud, the Austrian neurologist considered the father of psychology, theorized that childhood amnesia was caused by repression of memories involving traumatic events or sexually threatening desires that occurred early in the child's psychosexual development. Neuroscientists, however, argue that the key to childhood amnesia lies in the development of the hippocampus.
The implicit memory system is functional at birth, but the hippocampal (explicit) memory system takes about three years to become functional. It takes another two years to develop full connections to the rest of the brain. During the initial phase of development, explicit memories can be acquired and maintained over the short term. However, these memories decay rapidly, resulting in amnesia for events during this developmental period. Adults who have sustained damage to the hippocampus or have had their hippocampus surgically removed experience profound anterograde amnesia.
The most famous case is that of American patient Henry Gustav Molaison (1926–2008), referred to in textbooks as H.M. At age seven, Molaison was knocked down by a bicycle and sustained a head injury. He began having epileptic seizures at age 16, which eventually became uncontrollable and life-threatening. When he was 27 years old, Molaison had brain surgery aimed at relieving the epileptic fits. The surgery involved removing about two-thirds of his hippocampus. After the surgery, he had normal working memory, normal retrograde memory (memories of his past), and normal implicit memory. However, he could no longer form new conscious (explicit) memories. Brenda Milner, a researcher who worked with him for more than 50 years, reported that Molaison never recognized her.
Researchers have also studied three young adults who suffered damage to the hippocampus during difficult births that interrupted their oxygen supplies. Like Molaison, the children could not form new episodic memories, such as remembering what activities they had engaged in during the day. But they learned to read, write, and spell, developed normal vocabularies, and performed well in school. This suggests that the hippocampus is crucial for forming and maintaining memories of personal experiences (episodic memory) but is less involved in the formation and maintenance of more general memories (semantic memory).
The Cortex, Cerebellum, and Basal Ganglia
In 2005, neuroscientists Yoko Okada and Craig Stark repeated the experiment with a variety of video clips while people underwent functional magnetic resonance imaging (fMRI), a brain imaging technique that allowed them to view which areas of the brain were active during processing. They found that true memories involved a high level of activity in both the prefrontal cortex and hippocampus. False memories involved low activity in the prefrontal cortex. They concluded that activity in the prefrontal cortex encodes the source, or context, of a newly formed memory. If the prefrontal cortex is not actively engaged during exposure to misinformation, that misinformation is more easily embedded in the context of the first event, creating false memories. Researchers Scott Slotnick and Daniel Schacter reached similar conclusions using different methodology. They claimed that activity in the prefrontal cortex is a neural signature that distinguishes true from false memories. However, this kind of neural activity may not be accessible to conscious awareness. Activity in the prefrontal cortex is therefore crucial for "tagging" misinformation as inconsistent with original memory.