Let's look more closely at the case of H. M., or as we now know him, Henry Gustav Molaison (see Section 6-7). In 1953, the hippocampus, the amygdala, and some neighboring structures in the cortex on each side of Henry's brain were removed because of severe epilepsy. As you've already learned, when he woke up after the operation, Henry exhibited severe anterograde amnesia: he could remember recent events for only about 15-30 seconds, which resulted in problems such as the following:

• He was unable to learn his way around the hospital after the operation.
• Soon after reading something, he could remember neither what he had read nor that he had read it.
• After moving to a new house, it took him eight years to learn how to get from one room to another.
• He never learned to find his way back to the house from a distance of more than two blocks.
• When he was moved into a nursing home in 1980, he never learned where he lived or who cared for him.

Henry’s memory problems involved mainly the ability to form new explicit memories, especially those involving life events. On the other hand, he seemed to have little or no trouble forming new implicit memories, especially those involving new behaviors and skills. For example, Henry learned to read words written backwards, to solve particular puzzles, and to walk to the room in which he was tested each year at MIT. Nevertheless, he never remembered that he knew how to perform these tasks because he had not formed explicit memories of them. For example, when walking to the testing room, he would state that he did not know where he was going or why he was walking in that direction (Hilts, 1995).

Although Henry's anterograde amnesia seemed complete, research in later years showed that he was able to recall explicit memories of some events that had occurred since 1953:

For example, Henry seems to remember something about a President Kennedy and an assassination, even though the killing occurred ten years after Henry’s surgery. But ... you couldn’t say he remembers the assassination of John Kennedy exactly. ... If asked about an assassinated president he will remember McKinley [assassinated in 1901], and recall an attempt on Franklin Roosevelt’s life [in 1933]. If Dallas is mentioned, he may say, “Kennedy!” Asked for a first name, he may say Ted. Then, asked to reconstruct the Kennedy assassination, he will speak of an assassin who was trying to shoot the president but someone else got in the way; this is a description of the killing of Chicago mayor Anton Cermak, who was killed in 1933 by a bullet intended for Franklin Roosevelt. ... It seems as if there is some leakage from the world into his memory, some vague impressions which have become something like memories. (Hilts, 1995, p. 113)

To a small degree, then, Henry was able to develop semantic memories, which are explicit memories that consist of general knowledge about an object, event, activity, or situation. Semantic memories may be thought of as facts about the world. For example, fill in the following blanks: (a) Abraham Lincoln was the ___th president of the United States; (b) ______ was the first person to step on the moon; (c) Robert Kennedy, the brother of John F. Kennedy, was assassinated in the city of ______. When you study for a test, you are trying to develop semantic memories. You probably also have semantic memories of your social security number, your telephone number, your birth date, and your street address.

When you remember events from your own life, on the other hand, you are retrieving episodic memories, which are explicit memories of events that include the memory of one's self participating in them. In other words, episodic memories are memories of life events. For example, memories of what you did ten minutes ago or what happened on your first day of kindergarten are episodic memories. When you tell someone what you had for dinner last night, who was there, and what time it started, you are recalling an episodic memory (as long as you remember your own participation in the dinner).

People with damage to their hippocampi (and other related structures) have more trouble storing and retrieving episodic memories than they do storing and retrieving semantic memories. A good example of this can be found in Oliver Sack's (1995) case study of Greg, first described in Section 4-2. In 1991, Sacks took Greg to a concert featuring Greg's favorite band, the Grateful Dead. Greg’s amnesia extended back to the late 1960s, and he had no semantic or episodic memories of any music performed by the group since that time. At the concert, Greg seemed to enjoy himself but was confused when the "new" music was performed:

The first half of the concert had many earlier pieces, songs from the sixties, and Greg knew them, loved them, joined in. ... But the second half of the concert was somewhat strange for Greg: more of the songs dated from the mid- or late seventies and had lyrics that were unknown to him, though they were familiar in style.... The newer songs [from 1980 and after] went far beyond any development that he could have imagined, were so beyond (and in some ways so unlike) what he associated with the Dead, that it “blew his mind.” It was, he could not doubt, “their” music he was hearing, but it gave him an almost unbearable sense of hearing the future. (Sacks, 1995, pp. 75-76)

By the next morning, Greg remembered neither being at the concert nor having heard any of the newer songs: he had no episodic memories of the night before. Nevertheless, he started singing some of the new songs that he had heard for the first time the night before! He had, to some extent, developed semantic memories for them. This suggests that the hippocampi are more involved with the encoding and storing of new episodic memories, and less involved with the formation of new semantic memories. (If you want to learn more about how the hippocampi are involved in the formation of long-term memories, please see Section 6-12 A).

Figure 1 presents a summary of what you have learned so far about the components of the long-term memory subsystem.

Figure 1. The components of long-term memory

Figure 2 presents a summary of what you have learned about sensory memory, working memory, and long-term memory. The major theories of forgetting from the long-term store are listed at the bottom of Figure 2. These will be discussed in Section 6-14.

Figure 2. The components of the memory system

Study Questions for Section 6-12

1. What kind of amnesia was H. M. suffering from?
2. Which structure in the limbic system is of central importance for forming and retrieving long-term memories?
3. Was Henry's anterograde amnesia complete?
4. How would you define semantic memories in your own words?
5. What are some examples of semantic memories from your own life?
6. How would you define episodic memories in your own words?
7. What are some examples of episodic memories from your own life?
8. How are semantic memories similar to and different from episodic memories?
9. Which type of explicit memory do people with damage to their hippocampi have the most difficulty storing and retrieving?
10. What are the major components of the long-term memory subsystem (see Figure 1)?
11. What are the major components of the entire memory system (see Figure 2)?

Go to Quiz 6-12 questions

Go to Readings Section 6-13