Milner examined Henry for the first time in April 1955, twenty months after his operation. She gave him every cognitive test she could lay her hands on, and her findings launched a new era in the science of memory. Her formal testing showed that Henry’s overall intelligence was above average and that his capacities for perception, abstract thinking, and reasoning were normal. But when she probed his ability to remember information beyond the immediate present—his long-term memory—his deficit was obvious, despite his excellent motivation and cooperation. Henry performed the same memory tests that F.C. and P.B. had taken, but his scores were even worse. When asked to recall brief stories and geometric drawings, his scores were far below average, and in some cases zero. Throughout testing, Milner was struck that once Henry switched to a new task, he could no longer recall the preceding one or recognize it when it was repeated. Any distraction immediately put him at a total loss.15
Henry’s amnesia was more profound than that of F.C. and P.B., likely due to greater damage to his medial temporal-lobe structures, and he gradually became the yardstick against which other amnesic patients were judged. In the scientific literature, they were deemed “as bad as H.M.” or “not as bad as H.M.” Because his amnesia was not intertwined with a psychiatric disorder, Henry was a more straightforward case than D.C. Because his operation did not cause any cognitive deficits other than amnesia, his performance on memory tests was a pure measure of his memory capacities. Henry became the gold standard for the study of amnesia.
Scoville and Milner concluded their celebrated paper by identifying the hippocampus and adjacent hippocampal gyrus as the substrate for remembering new information. The severity of memory loss in all ten cases was related to the size of the hippocampal removal—the larger the removal, the greater the memory impairment. What distinguished Henry from patients with amnesia due to other causes, such as Alzheimer disease or head injury, was that his memory impairment was amazingly specific. The purity of his disorder made him a perfect focus for the investigation of memory mechanisms in the human brain.16
As Milner delved deeper into the study of memory loss, and Henry’s in particular, Scoville moved on. He maintained an active neurosurgical practice and published more than fifty papers in medical journals, but did not continue to see Henry. I know firsthand, however, that Scoville was still interested in Henry’s case. In the late 1970s, when I was visiting my parents, who lived across the street from him, he invited me to his house to get an update on Henry and our research with him.
In his writings and lectures, Scoville shared Henry and D.C.’s catastrophic losses with the medical community for their scientific value. In the interest of a larger cause, he warned other neurosurgeons against damaging the hippocampal area on both sides of the brain, and they took his warnings to heart. In a 1974 lecture, he called Henry’s operation “a tragic mistake.” According to his wife, he “deeply regretted” what he had done to Henry. In 2010, Scoville’s grandson, Luke Dittrich, wrote an article for Esquire magazine, in which he gave a colorful account of his grandfather’s life and career.17
In 1961, I joined Milner’s laboratory at the Neuro as a McGill University graduate student. This institution was renowned for the treatment of epilepsy patients, using the surgical procedures Penfield had developed. In Milner’s lab, our research focused on these patients. Milner was especially skilled at designing tests that could be given before and after an operation to tease apart a patient’s performance on different cognitive tasks—sensory perception, reasoning, memory, and problem solving—to discover any changes in brain function caused by the surgery. We communicated closely with the surgeons and knew after each procedure what part of the patient’s brain had been removed and the size of the excision.18
In addition to conducting preoperative and postoperative testing, I had the opportunity to witness the operations on my patients’ brains. From behind a glass window in the viewing gallery in the main operating amphitheater, I could look over the surgeon’s shoulder at the patient’s exposed brain and watch the surgeon stimulate the brain to map out landmarks before removing any tissue. To guard against damaging areas specialized for language and movement, the surgeons identified these regions by electrically stimulating the outer layers of the brain while the patients were awake. When this stimulation interrupted the patients’ speech, caused a spontaneous movement, or made them think of a particular object, face, sound, or touch, the surgeons placed a small letter on the stimulated brain area. A stenographer seated next to the operating table noted the behavior associated with each letter. Photographs of the brain showed the letters and later gave clues about localization of functions in the cortex. The electrical stimulation also helped identify where the epileptic seizures originated, and that area would be excised. I could see which parts of the brain were removed, and the extent of each procedure was later spelled out in a report with photographs and the surgeon’s drawing of the location and size of the removal.
This documentation was crucial for making sense of the behavioral test scores that we collected in the lab. By combining test results with the surgeons’ reports, we could link any cognitive deficits in our patients with the brain areas that were lost, and their normal performance to the areas of the brain that remained intact. With this collaborative approach, Milner and her colleagues made important discoveries about the organization of the left and right cerebral hemispheres in humans, based on the actual necessity of each brain region for a particular cognitive process.19
My PhD thesis project studied how operations to alleviate epilepsy affected the somatosensory system—the sense of touch. To do this, I devised and constructed memory tests that required patients to rely on touch rather than vision or hearing. I tested many patients who had brain tissue removed from either the left or right frontal, temporal, or parietal lobe. I was particularly eager to test the three amnesic patients who had lesions in both hippocampi—patients about whom I had read earlier in the papers that Milner had coauthored with Penfield and Scoville: F.C., P.B., and Henry. Epilepsy surgery did not typically result in amnesia; these three cases were rare.20
I first met Henry in May 1962, when Milner arranged for him to visit us at the Neuro for testing. This was his first and only trip to Montreal, and it was momentous. He and his mother came by train, which is how they traveled long distances. Mrs. Molaison feared air travel, and the train was less costly. They stayed in a nearby rooming house, and every morning for a week, the two of them arrived at the Neuro and made their way to the Neurology waiting room.
During that week, my colleagues and I took turns testing Henry. Each day, I picked him up in the waiting room and guided him to my testing room, and when we finished, I escorted him back. He was a cooperative research participant, as he would be for the rest of his life, and we completed all the tasks I had planned for him. Even then, I felt privileged to work with Henry, along with F.C. and P.B.—a rare trio of amnesic patients. But in 1962, I had no idea how famous Henry would become.
At the time of his visit to Montreal, Henry was in his thirties, in the prime of his life, but completely dependent on his mother. Mrs. Molaison, a housewife and Henry’s constant caretaker, was a pleasant, sweet woman. During the entire visit, she sat patiently in the dreary waiting room while researchers took her son to various testing rooms. She was terrified of the big city where people spoke French, a language she did not understand, and preferred to stay within the safe walls of the Neuro rather than explore on her own.
Henry’s week at the Neuro was busy. We had prepared an extensive series of tests for him, designed to measure various facets of his memory and other cognitive functions. Although we did not know it then, the results of our studies would reveal the scope and limits of his amnesia, and in doing so would foreshadow new ways of exploring how memory is organized in the human brain. His memory loss, while having a devastating impact on his daily life, proved a priceless gain in the quest for the underpinnings of learning and memory.
Four
Thirty Seconds
From the beginning, one of the most striking aspects of Henry’s memory loss was how remarkably specific it was. He forgot all of his experiences after his 1953 operation, but retained much of what he had learned before that. He knew his parents and other relatives, recalled historical facts he had learned in school, had a good vocabulary, and could perform routine daily tasks, such as brushing his teeth, shaving, and eating. Studying Henry’s remaining capacities proved just as instructive as studying those he had lost. One important lesson scientists have learned from people with selective memory loss such as Henry’s is that memory is not a single process but a collection of many different processes. Our brains are like hotels with eclectic arrays of guests—homes to different kinds of memory, each of which occupies its own suite of rooms.
Henry’s case shed light on a longstanding controversy about whether brief memory mechanisms are distinct from lasting ones. The basic question was whether the processes that support short-term memory, which holds a limited amount of information temporarily, differ from those that support long-term memory, which hangs on to vast amounts of information for minutes, days, months, or years.
Most of us use the term short-term memory incorrectly. Short-term memory, as defined by memory researchers, does not refer to recalling what we did yesterday, this morning, or even twenty minutes ago. That sort of recollection is recent, long-term memory. Short-term memory is the immediate present, the information on our radar screens at this very moment; it expires within about thirty seconds or less, depending on the task. Its capacity is limited, and it fades immediately if we do not rehearse it or convert it into a form that can be retained in long-term memory. When I tell a friend my phone number, the digits will remain in her short-term store briefly, and she will quickly forget them unless she mentally processes them or writes them down. The short-term store is not a warehouse in the brain; instead, it is a series of processes that keep bits of information, such as my phone number, active for a brief period of time. Long-term memory, on the other hand, is anything we remember after just seconds have elapsed.
Was the formation of short- and long-term memory part of a single process, or instead, governed by wholly separate processes? Those who supported the dual-process theory sought convincing evidence that a particular patient was impaired on tests of long-term but not short-term memory, and that another patient was impaired on short-term memory tasks but not long-term. These two results, taken together, would indicate that the two kinds of memory were independent. Studies of patients with selective damage to their brains sharpened the debate over whether memory is a single or dual process, and Henry played a starring part in this research.
Henry’s role as a research participant began in 1953, just prior to his operation. Scoville ordered a complete psychological evaluation to establish a preoperative baseline against which to measure any changes resulting from the procedure. The day before his surgery, clinical psychologist Liselotte K. Fischer sat down with Henry at the Hartford Hospital and conducted a series of tests, including an IQ test, a memory test, and several others designed to reveal his personality and psychological status. One task is a common measure of short-term memory called digit span, in which an examiner asks a patient to repeat a gradually increasing series of numbers. For example, if she says, “Three, six, nine, eight,” the patient immediately repeats, “Three, six, nine, eight.” The researcher then gives five digits, then six, then seven, eight, and so on; if the patient repeats eight digits but fails at nine, then the patient’s digit span is eight. Fischer administered this test to Henry, and then asked him to repeat strings of digits in reverse order, a much harder task. If she said, “Three, six, nine,” then the correct answer would be “Nine, six, three.” His combined score for both tests was six—well below the normal range.
Two years after Henry’s operation, when Milner gave him a similar test, his digit span had improved, putting him in the normal range. His ability to remember more digits postoperatively, however, does not mean that the surgery improved his memory. Multiple factors may have contributed to his weak preoperative performance. During testing, Fischer witnessed several petit mal seizures, which were not unexpected since Henry had been taken off his medication in preparation for surgery. In addition, he was anxious about the upcoming operation, triggering stress-related mechanisms in his brain that may have interfered with his test performance and masked his true abilities. His deficits before the big event were likely a combined result of seizures and nerves.
Over the decades when my colleagues and I studied Henry, he maintained a normal level of performance when we tested his digit span. This finding presented a sharp contrast: Henry suffered a catastrophic memory loss, yet he could briefly remember and repeat a string of digits. This suggested that Henry’s short-term memory was intact; where he failed was in converting short-term memories into long-term memories. For instance, during the course of a fifteen-minute conversation, he would tell me three times the same story about the Molaison family’s origins without knowing that he was repeating himself. The information could be collected in the hotel lobby of Henry’s brain, but it could not check into the rooms.
William James, a brilliant psychologist and philosopher, was the first to make a distinction between two kinds of memory. In 1890, he produced an often quoted, two-volume tour de force, The Principles of Psychology, in which he described primary and secondary memory. Primary memory, he said, makes us aware of “the just past.” The content of primary memory has not yet had a chance to leave consciousness; primary memory covers such a short span of time that it is considered “right now.” Reading these sentences, we are simply carrying all the words in our minds at the present moment, not actively dredging them up from the past.
By contrast, secondary memory, in James’s scheme, is “the knowledge of an event, or fact, of which meantime we have not been thinking, with the additional consciousness that we have thought or experienced it before.” This type of memory “is brought back, recalled, fished up, so to speak, from a reservoir in which, with countless other objects, it lay buried and lost from view.” With secondary memory, the information is no longer milling around the hotel lobby, but is instead resting upstairs and must be found and retrieved.
Remarkably, James’s categorization of memory appears to have come solely from his own introspection. He did not conduct memory experiments on himself or others, although he may have talked with colleagues who did. After he proposed this scheme, however, scientists went to their laboratories to devise behavioral experiments to tease apart these two memory processes. Their work resulted in the concepts of what is now called short-term memory—James’s primary memory—and long-term memory—James’s secondary memory.
If short- and long-term memory represent two distinct kinds of cognitive processing, then their biological foundations should also differ. In addressing this issue, scientists have asked two basic questions: do separate neural circuits support short- and long-term memory, and can we identify in the respective brain circuits structural changes that contribute to memory storage? Researchers have pursued these fundamental questions broadly, with insights from theoretical, cellular, and molecular levels of analysis.
One early advance in examining the dual-process theory of memory came from Penfield’s colleague, neuroscientist Donald Hebb. For some time, scientists had known that functions of the brain—remembering, thinking, or controlling body movements—depended on communication among brain cells, neurons. A major function of neurons is to send electrical and chemical messages across a synapse, a miniscule space between two neurons, to other neurons waiting to receive the message. Understanding how to link a complex process such as memory to some measurable activity in neurons was difficult—and still is.
In 1949, Hebb speculated that the central difference between the two types of memory is that long-term memory is accompanied by a physical change in the connections between neurons, whereas short-term memory is no
t. He proposed that short-term memory is made possible when neurons in a particular circuit talk continuously to one another in a closed loop, like a conversation kept alive by a group of people standing in a circle. Long-term memory, in contrast, comes from lasting new growth on the neuron at the synapse. If short-term memory is like an oral conversation, long-term memory is like a written transcript of past communication that can be taken out and reread at will.1
In developing his theory, Hebb was likely inspired by the famous Spanish anatomist Santiago Ramón y Cajal, who spent his career observing nerve cells through a microscope. In the late 1800s, Ramón y Cajal had proposed that learning was linked to a physical outgrowth of a nerve cell at the synapse. Hebb likewise believed that the structural connections between two neurons change physically and grow stronger as learning progresses. The ability of synapses to change structurally offers a way to record information permanently for later use.2
Hebb’s model was enormously influential. It bridged the wide gap between psychology and biology, linking the seemingly elusive process of memory with a tangible change in the brain. It also gave scientists a way to frame further experiments, and set the stage for major breakthroughs in memory research. Hebb’s postulate lives on in academia today: every student of neuroscience can recite Hebb’s rule, “Cells that fire together wire together.”3
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