When he walked into the room, he looked a decade younger than 71; his face handsome, with few wrinkles. I pulled out a chair and asked him to sit in front of a robotic contraption that earlier that morning we had set up in an examination room at the Clinical Research Center at MIT. He sat calmly and avoided touching the contraption. I pulled the robotic arm toward him and asked him to hold its handle. He grabbed the handle and started moving it around, keeping his gaze on the handle. I asked him to look up at a monitor, where he saw a little cursor moving around as he moved the robot’s handle. The computer displayed a target box, and he moved the cursor into the box, at which point the computer animated it, producing an explosion.
A smile came to his face. He said: “You know, when I was a kid I liked to go bird hunting.” Exuberantly, he described the birds that he hunted, the guns that he owned, and the woods around his childhood home. He continued doing the task, reaching while holding the robotic arm and making little explosions. About five minutes later he said: “You know, when I was a kid I liked to go bird hunting.” The exuberance was unabated. He had no idea that a few minutes earlier he had told me that same story.
Memory without awareness
The day before, with my two graduate students Kurt Thoroughman and Maurice Smith, I had packed the robot and computers in the back of my wife’s station wagon and drove up from Baltimore to meet and examine Henry Molaison. Henry, or as he is known to the scientific world ‘H.M.’, had suffered from debilitating seizures. When he was 27, desperate for something that might help, he had agreed to an experimental procedure that surgically removed the hippocampus and amygdala from both sides of his cerebral cortex. The surgery was successful, greatly reducing his seizures, but left him with a staggering deficit: an inability to form certain long-term memories.
Now, in the examination room, the robotic arm that Henry was moving started to produce forces, pushing his hand as he approached the target, making it so that he would miss-reach and not get those explosions. But he kept practicing, and after a few minutes, a part of his brain that was not damaged learned how to generate the right motor commands so that his arm could compensate for those unusual forces. Once again he got the explosions, and once again he excitedly told me of his childhood bird hunting days.
After about an hour of playing with the robotic arm, Henry left for lunch and an afternoon nap. He returned about 4 hours later. I said hello and ask him whether he remembered meeting me and playing with the robotic arm. He said no. I pushed the robot aside, showed him the exam chair, and asked him to sit down. He sat down, but then something interesting happened: rather than avoiding the machine, the behavior of someone who has never done the task before, he grabbed its handle, brought it toward him, looked at the video monitor, and started to move the cursor toward the target.
He had no awareness that he had seen me before, or that he had played with this robotic arm only hours earlier. Yet, that experience had left two kinds of memories in his brain: the memory of how to use the tool, and the memory that associated the sight of the tool, and the act of moving it, with a rewarding outcome.
He was not aware of it, but the sight of the strange tool, the robotic arm, was sufficient for him to want to hold it and move it around so that he could chase targets and get explosions. Would he have voluntarily reached for the robot if, while playing with it, he had not had a pleasurable experience, recalling those childhood memories? Probably not. In 1911, Edouard Claparede, a physician in Geneva, described an amnesic woman much like Henry. Claparede had wanted to test her memory, so he played a small joke on her: when he reached his hand out to shake hers, he had hidden a small tack in his palm. When the patient shook his hand, she felt the sharp tack. The next day when Claparede had approached her, she could not recall having seen him before. However, when he reached out to shake her hand, she pulled away, despite being unable to say why she did not want to shake his hand.
Henry could not remember the episode of having played with the robotic arm, but that act left a memory, associating the robot with a rewarding outcome. The part of his brain that learned this value association was exhibiting its knowledge by reaching out to the robot and moving it in search of a target to explode.
In addition to this value-action association, he also had a memory of how to use the contraption. Those forces that he had practiced to overcome had left a different kind of memory: much like picking up a coke can that you expect to be full but is empty, Henry’s movements on this revisit had the ‘after-effects’ of the earlier experience. The robot was not producing any forces, but he moved it as if he was expecting it to be producing those earlier forces. When the forces were re-introduced, he moved the robot skillfully.
He did not have the ability to form memories of episodes of his life, but these two other intact forms of memory served him well. For example, as he aged, he developed osteoporosis and required a walker to keep physically active. With practice, he learned to use the walker skillfully. Importantly, he would use the walker without being told to do so. That is, he ‘knew’ that the walker was a useful tool that helped him get around.
Permanent present tense
What is it like to live a life with such a disability? In a recent book titled “Permanent present tense”, Suzzane Corkin, a scientist who studied and cared for Henry for 46 years, describes his life in loving, exquisite detail.
In a photograph showing Henry with his mother at his 50th birthday, he recognized his mother, but not himself. While attending his 35th high school reunion, he did not recognize anyone by sight or by name. He could not recall any specific event in his life, even events from before his operation. For example, he could not recall a single specific Christmas gift that his father had given him. He remembered some of the facts that he had learned from the time before his operation, and the gist of the experienced events, but no recollection of any specific episodes.
Henry rarely spoke of being hungry or thirsty. He never sought out food for himself; it was simply given to him by his caregivers. In 1981, Corkin asked him to rate his hunger from 0 (famished) to 100 (absolutely full). He consistently gave a rating of 50, whether he had just finished eating, or was about to eat. One evening, Corkin played a small trick on him. After Henry had finished eating and his tray had been taken away, the kitchen staff brought him another tray, with exactly the same meal. Henry ate the second dinner, cleaning the plates, except for the salad. He seemed unable to express a feeling of satiety.
When we were examining him, a caregiver mentioned that Henry rarely verbalized that anything might be wrong. For example, if he had a tooth ache, he would rarely mention it. Only by observing that he was deviating from his normal behavior during the day would the caregiver suspect that something was wrong. The caregiver would then go through a list of things to see if they could find out what may be the problem.
Corkin tested Henry’s ability to perceive pain by using a hairdryer to project a spot of heat onto his skin. The heat was not intense enough to burn the skin, but the idea was to test whether Henry could feel pain. Corkin’s results showed that Henry not only could not discriminate normally between various levels of pain, he did not report any of the stimuli as painful, and never withdrew his arm. It is possible that the inability to normally perceive pain, to know hunger or thirst, was related to his operation; perhaps it was associated with removal of the amygdala, as Corkin suggests.
Henry lived a life without keeping memories of the events, and without pain. His father had passed away, and his mother, who had taken care of him for much of his life, was in a nursing home. He kept two notes in his wallet that he had written to himself: “Dad’s gone”, “Mom’s in nursing home—is in good health.”
Corkin S. (2013) Permanent present tense: the unforgettable life of the amnesic patient H.M., Basic Books.
Shadmehr R, Brandt J, Corkin S (1998) Time dependent motor memory processes in amnesic subjects. Journal of Neurophysiology 80:1590-1597.