The Long and Short of Memory
H.M. is, arguably, the most famous patient in the history of psychology and neuroscience. He was studied intensively for more than fifty years by hundreds of scientists (including, briefly, the author of this review); he died in 2008, and his brain is still being analyzed. Permanent Present Tense, by Suzanne Corkin, is the story of how these investigations led to a fundamental revolution in our understanding of the human brain and, particularly, of the organization and varieties of memory. Her accessible book places his story in the context of past and present research on memory and describes many of the questions initiated by research on H.M. It is a scientifically exciting and personally moving portrait of a man whose life and brain ended up being devoted to the science of memory.
By the time he was 24, in 1950, H.M. (a k a Henry) had developed severe epilepsy, perhaps from a bicycle accident years earlier, and was referred to the neurosurgeon William Beecher Scoville, who had performed many frontal lobotomies on patients diagnosed as “psychotic.” Scoville had been unsatisfied with the results of frontal lobotomies and was trying a new surgery, bilateral medial temporal lobotomy, in another attempt to treat psychosis. Two of his psychotic patients happened by chance to suffer from epilepsy, and temporal lobotomies had the unexpected effect of reducing their seizures. By 1953, Henry’s epileptic episodes had become more frequent and incapacitating, and Scoville decided to perform a medial temporal lobotomy on him to treat it. He removed, from both sides of H.M.’s brain, a large portion of the medial temporal lobes, including portions of the hippocampus, most of the amygdala, and parts of the adjacent cerebral cortex such as the perirhinal and parahippocampal cortex. After the surgery, H.M.’s seizures diminished significantly, yet the patient could not remember anything that had happened since the procedure, nor could he recognize anybody new or recall a conversation that had transpired a few minutes earlier. He left the hospital with devastating amnesia and could no longer form new memories.
The pioneering neurosurgeon Wilder Penfield, founder of the Montreal Neurological Institute, had been successfully performing unilateral temporal lobe surgery for the treatment of temporal lobe epilepsy. Two of his many patients had developed memory problems after the surgery; it turned out that the unoperated-upon parts of the temporal lobes were abnormal, giving them bilateral temporal lobe damage, as was later the case for H.M. These two patients had their memory examined by Brenda Milner, a McGill University graduate student collaborating with Penfield. When Milner presented their research at the 1954 meeting of the American Neurological Association, Scoville realized the similarity to H.M.’s case and called Penfield, who then sent Milner to examine him. Thus began the modern science of memory.
H.M. turned out to be an ideal subject for the study of memory. Milner found him to be highly intelligent and willing to sit for hours of testing. His perceptual and cognitive abilities, other than memory, were intact. He was able to focus his attention on the task immediately at hand as well as anyone. He was usually very good-tempered and eager to cooperate. He never got bored or restless, perhaps because everything was continually new to him.
Although memory has been a central topic since the founding of experimental psychology in the middle of the nineteenth century, and the effect of brain injury on memory had been studied before Milner’s work on H.M., little was known about the role of different brain areas in memory. The hippocampus was thought to be involved in smell because of its connections with the olfactory system. One widespread view in the early twentieth century, based on the research of Karl Lashley, was that complex memories were distributed throughout the cerebral cortex and therefore could not be localized.
In her initial studies with Henry, published in 1957, Milner established that the hippocampus or adjacent structures were crucial for the long-term storage of information in memory. Milner had discovered that Henry’s digit span (the ability to repeat a gradually increasing series of numbers) and immediate memory were essentially normal, but after about thirty seconds, unless he rehearsed it continually, all record of conscious experience had vanished—nothing new was stored in long-term memory. Furthermore, Henry seemed to have memories intact from several years before the surgery. Milner’s carefully documented experiments revealed several characteristics of the memory mechanism: a specific part of the brain, namely the hippocampus or adjacent tissue, is necessary for the storage of facts and experience; immediate memory is a different process from long-term memory; long-term memory is not permanently stored in the medial temporal region; and severe amnesia can exist in the presence of normal perceptual, motor and language functions.
Milner made another extraordinary discovery when she tested H.M. on a motor skill task called mirror drawing. In this task, the subject looks in a mirror, where he sees both his hand holding a pencil and a piece of paper with two concentric outlines of a five-pointed star. The task is for the subject to draw a line between the outlines on paper while only looking in the mirror. Because the star and the hand can be seen only in the mirror, the image is reversed: for example, to draw a leftward-moving line requires moving the hand to the right. Normal subjects require several trials before they can draw a line between the concentric outlines. H.M. could learn and remember this task as well as normal people. He improved his performance in the first day of trials, and by the third had virtually perfect performance. Yet he had no memory of ever having seen the star or the task before. Milner concluded that “motor skill” learning was an exception to H.M.’s amnesia.
Milner then discovered that H.M. could perform a particular test of perceptual learning that researchers call the Gollin incomplete picture test. In it, subjects are shown drawings of objects in a sequence, from incomplete to complete, and asked to identify the image. The object is too inchoate in the first drawing to be identified, and most people need to see several of the drawings before they can identify it. An hour after seeing the drawings, Henry recognized the objects in far fewer trials than originally, demonstrating perceptual learning and memory. Yet he had no conscious memory of having done the task previously. Milner also showed that H.M. could remember how to do mirror tracing and Gollin figures in spite of his total inability to remember new facts or events.
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