Organic Amnesia

The most well-studied case of organic amnesia in the literature is that of HM, who became amnesic in his mid-twenties after an operation to relieve the symptoms of severe epilepsy. HM’s operation involved surgery to the hippocampal complex and parts of the temporal lobe in both brain hemispheres.

Although HMs epilepsy improved following surgery, he became profoundly amnesic, unable to remember events just after they had happened and many that had occurred prior to his operation. For example, he was severely impaired at recalling lists of words that had just been read to him and at recognizing those words as the ones he had heard before when they were presented to him again.

This was also the case when he was asked to remember other types of stimuli, such as faces, numbers, sounds, or shapes. There are now over 50 reported patients who, like HM, show difficulties with learning after damage to the hippocampal complex. This region in the human brain is, therefore, considered critical for the acquisition of new episodic and semantic memories.

As well as being unable to remember events from moment to moment, patients with damage similar to HM often have difficulty remembering events that occurred a number of years previously. This pattern of memory loss, in which memories from the distant past (e.g., childhood or early adulthood) are typically remembered better than more recently experienced events (e.g., from the day, week, or year before), is a phenomenon that has fascinated memory researchers for over 100 years.

The fact that memory loss can be so dramatically affected by the age of the memory has led some researchers to conclude that the system involved in acquiring new human memories, the hippocampal complex, may not necessarily be involved in the permanent storage of all our episodic and semantic memories. Instead, other areas of the brain, such as the anterior temporal lobes, may be the location for our enduring stores of human memory, especially semantic knowledge about the world.

Studies of patients with damage that affects the temporal neocortex, but less so the hippocampal complex, highlight the complementary roles played by these regions in long-term memory. These patients often show the opposite pattern of memory impairment to HM and other patients with damage to the hippocampal complex, that is, better recall of recent memories than those from childhood or early adulthood.

Although it is currently unclear how the different temporallobe regions interact in the acquisition and storage of episodic and semantic memory and whether the effects of memory age evident in patients reflect a true transfer of information between brain structures, most current theories of memory concur that memory consolidation, perhaps occurring during sleep, is a critical aspect of memory storage.

In recent years, the important role in memory played by another brain area, the frontal lobe, has become clearer. Patients with specific damage to this region often exhibit symptoms of impulsiveness and lack of inhibition that affect personality and behavior, but careful testing can reveal impairments in certain aspects of memory function. For example, such patients often have difficulty remembering details of the context in which previous events were experienced, such as where or when the event occurred.

Moreover, they are typically impaired, like patients with hippocampal complex damage such as HM, at recalling lists of words that were previously presented. In contrast to hippocampal complex dysfunction, however, frontal lobe damage may spare the ability to recognize information as having been seen before. Such evidence suggests that the frontal lobe and hippocampal complex work together to perform relatively complex, effortful memory tasks such as recalling words, that require the strategic search of the memory store, as well as organization and manipulation of retrieved material.

The idea that interactions between the frontal and temporal lobe areas are important for memory function is supported by data from a patient with damage to the uncinate fascicle, a bundle of fibers that provides an important connection between these brain regions. This patient was impaired at recalling autobiographical events from his past, but performed normally at recognizing and discriminating events that he had experienced from those he had not.