Biological Perspectives on Memory

In the first section of this chapter (From Ebbinghaus to Encoding) we saw that memory research was influenced by the advent of computers and cognitive psychology. Advances in neuroscience have also influenced memory research. Beginning with the observation of some brain-damaged patients in the 1960s, biological research on memory flourished.

The story of H.M.

A patient known as H.M. has been called the most influential in the history of neuropsychology. This patient suffered from severe seizures caused by abnormal tissue deep within the brain. During a brain operation, a surgeon cut areas near the temporal lobe and hippocampus, two areas near the center of the brain.

The operation was a success, in one sense: it halted H.M.'s seizures. Before the operation, H.M. had at least one severe seizure per week. After the operation, he was nearly seizure-free. But after the operation, H.M. could no longer transfer memories from primary (short-term) to secondary (long-term) memory. . This finding was very influential in the field of psychology, because it showed that primary and secondary memory were due to biologically distinct systems.

What was H.M.'s problem, after the operation? What is anterograde amnesia? What did the Kennedy half dollar reveal?

H.M.'s primary memory was normal. He could hold a normal conversation, and his memory span (primary memory capacity) was normal. However, when his attention was distracted, H.M. forgot everything that was in his mind a moment before. This condition is called anterograde amnesia. The word amnesia refers to any forgetting syndrome. Anterograde amnesia refers specifically to a loss of memory for events after the traumatic event causing the amnesia.

H.M. could remember most of what happened in his life before the operation. However, his life after the operation was a complete blank to him, except for a few emotion-laden events. When shown a Kennedy half dollar, H.M. identified Kennedy and was able to say at once that Kennedy had been assassinated. After his father died, H.M. remembered this fact, although only vaguely.

The extent of H.M.'s problem was apparent during his return visits to the medical center.

What "stereotyped description" of his mental state did H.M. volunteer?

During three of the nights at the Clinical Research Center, the patient (H.M) rang for the night nurse, asking her, with many apologies, if she would tell him where he was and how he came to be there. He clearly realized he was in a hospital but seemed unable to reconstruct any of the events of the previous day (when he had been driven to Boston for the check-up). On another occasion he remarked, "Every day is alone in itself, whatever enjoyment I've had, and whatever sorrow I've had." Our own impression is that many events fade for him long before the day is over. He often volunteers stereotyped descriptions of his own state, by saying that it is "like waking from a dream." His experience seems to be that of a person who is just becoming aware of his surroundings without fully comprehending the situation, because he does not remember what went before. (Milner, Corkin and Teuber, 1968)

Where is the "gatekeeper" for event memory?

Where is the "gatekeeper" for event memory?

Larry Squire of the San Diego VA Medical Center found a stroke victim with anterograde amnesia similar to that of H.M. After the man died of natural causes, an autopsy showed that he had a normal brain except for one tiny area of degenerated cells in the hippocampus. This tiny group of cells was in the middle of a circuit known as the trisynaptic circuit of the hippocampus. Squire theorized that this circuit pulls together information from various areas of the cerebral cortex. Squire said in this way the hippocampus acts as a "gatekeeper of memory" (Hostetler, 1988). Specifically, the hippocampal circuit (which also involves parts of the temporal lobe and other structures) seems to be crucial for episodic or autobiographical memory.

The event memory circuit identified by Squire uses a distinctive transmitter substance in its neurons: NMDA, which resembles the transmitter glutamate. Once memory gets through the gatekeeper circuit it is stored in durable form. People like H.M. who lose this ability usually remain able to form new declarative memories, showing that different mechanisms are involved.

How are episodic and procedural memories stored differently?

Other biological studies show that procedural memory is a distinct type. Procedural memory (memory for sequences, skills, actions, or procedures) involves circuits passing through the cerebellum, not the hippocampus, and they do not use NMDA.

How did H.M. provide evidence that procedural memory is a distinct type?

The case history of H.M., who had damage to the hippocampus and surrounding areas, helped to show researchers in the 1960s that event memory and procedural memory were distinct. Although he could form no new event memories, H.M. learned to trace a star while looking at his hand in a mirror. "Mirror tracing" is difficult at first because the mirror reverses normal eye-hand relationships. Normal people improve at mirror tracing with practice, and so did H.M. However, he did not remember each practice session.

Clive Waring, who lost all memory for events both before and after his illness (and was thus more impaired than H.M.) nevertheless learned his way around a new environment, an assisted care home in the English countryside. If he wanted to make a cup of coffee, he knew where to get the ingredients and how to put them together (showing he formed new declarative and procedural memories). He remained in love with his wife and held good conversations with her. She reported that his personality and intelligence remained intact, and he kept his skill at playing piano. All he lost was access (through the gatekeeper circuit) to memory for the events of his life.


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Copyright © 2007 Russ Dewey