This form of memory, too, is not unique to humans. The third category, first recognized by Endel Tulving, is called episodic memory, memories for specific events, such as your prom night, or the day you broke your ankle playing basketball, or as the psycholinguist Steve Pinker puts it, “When and where who did what to whom.” Semantic memories are like a dictionary whereas episodic ones are like a diary. Psychologists also refer to them as “knowing” versus “remembering” only humans are capable of the latter.
Harvard psychologist Dan Schacter has made the ingenious suggestion that episodic memories may be intimately linked to your sense of self: you need a self to which you attach the memories, and the memories in turn enrich your self. In addition to this we tend to organize episodic memories in approximately the correct sequence and can engage in a sort of mental time travel, conjuring them up in order to “visit” or “relive” episodes in our lives in vivid nostalgic detail. These abilities are almost certainly unique to humans. More paradoxical is our ability to engage in more open-ended forward time travel to anticipate and plan the future. This ability is probably also unique to us (and may require well-developed frontal lobes). Without such planning, our ancestors couldn’t have made stone tools in advance of a hunt or sown seeds for the next harvest. Chimpanzees and orangutans engage in opportunistic tool making and tool use (stripping leaves from twigs in order to fish termites from their mounds) but they cannot make tools with the intent to store them for future use.
DOCTOR, WHEN AND WHERE DID MY MOTHER DIE?
All of this makes intuitive sense but there is also evidence from brain disorders—some common, others rare—in which the different components of memory are selectively compromised. These syndromes vividly illustrate the different subsystems of memory, including ones that have evolved only in humans. Almost everyone has heard of amnesia following head trauma: The patient has difficulty recollecting specific incidents that took place during the weeks or months preceding the injury, even though he is smart, recognizes people and is able to acquire new episodic memories. This syndrome—retrograde amnesia—is quite common, seen as often in real life as in Hollywood.
Far rarer is a syndrome described by Endel Tulving, whose patient Jake had damage to parts of both his frontal and temporal lobes. As a result Jake had no episodic memories of any kind, whether from childhood or from the recent past. Nor could he form new episodic memories. However, his semantic memories about the world remained intact; he knew about cabbages, kings, love, hate, and infinity. It is very hard for us to imagine Jake’s inner mental world. Yet despite what you would expect from Schacter’s theory, there was no denying that he had a sense of self. The various attributes of self, it would seem, are like arrows pointing toward an imaginary point: the mental “center of gravity” of the self that I mentioned earlier. Losing any one arrow might impoverish the self but does not destroy it; the self valiantly defies the slings and arrows of outrageous fortune. Even so, I would agree with Schacter that the autobiography we each carry around in our minds based on a lifetime of episodic memories is intimately linked to our sense of self.
Tucked away in the lower, inner portion of the temporal lobes is the hippocampus, a structure required for the acquisition of new episodes. When it is damaged on both sides of the brain, the result is a striking memory disorder called anterograde amnesia. Such patients are mentally alert, talkative, and intelligent but cannot acquire any new episodic memories. If you were introduced to such a patient for the first time, walked out, and returned after five minutes, there would be no glimmer of recognition on her part; it’s as if she had never seen you before. She could read the same detective novel again and again and never get bored. Yet, unlike Tulving’s patient, her old memories, acquired prior to the damage, are for the most part intact: she remembers the boy she was dating in the year of her accident, her fortieth birthday party, and so on. So you need your hippocampus to create new memories, but not to retrieve old memories. This suggests that memories are not actually stored in the hippocampus. Furthermore, the patient’s semantic memories are unaffected. She still knows facts about people, history, word meanings and so forth. A great deal of pioneering work has been done on these disorders by my colleagues Larry Squire and John Wixted at UC San Diego and by Brenda Milner at McGill University, Montreal.
What would happen if someone were to lose both his semantic and episodic memories, so that he had neither factual knowledge of the world nor episodic memories of a lifetime? No such patient exists, and even if you were to stumble on one who had the right combination of brain lesions, what would you expect him to say about his sense of self? In fact, if he really had neither factual nor episodic memories, it is unlikely that he could even talk to you or understand your question, let alone understand the meaning of “I.” However, his motor skills would be unaffected; he might surprise you by cycling home.
Free Will
One attribute of the self is your sense of “being in charge” of your actions and, as a corollary, of your belief that you could have acted otherwise if you had chosen to. This may seem like an abstract philosophical issue but it plays an important role in the criminal justice system. You can deem someone guilty only if he (1) could fully envisage alternate courses of action available to him; (2) he was fully aware of the potential consequences of his actions, both short-and long-term; (3) he could have chosen to withhold the action; and (4) he wanted the result that ensued.
The upper gyrus branching from the left inferior parietal lobule, which I earlier referred to as the supramarginal gyrus, is very much involved in this ability to create a dynamic internal image of anticipated actions. This structure is highly evolved in humans; damage to it results in a curious disorder called apraxia, defined as an inability to carry out skilled actions. For example, if you ask an apraxic patient to wave goodbye, she will simply stare at her hand and start wiggling her fingers. But if you ask her, “What does ‘goodbye’ mean?” she will reply, “Well, you wave your hand when parting company.” Furthermore, her hand and arm muscles are fine; she can untie a knot. Her thinking and language are unaffected and so is her motor coordination, but she cannot translate thought into action. I have often wondered whether this gyrus, which exists only in humans, evolved initially for the manufacture and deployment of multicomponent tools, such as hafting an axe head on a suitably carved handle.
All of this is only part of the story. We usually think of free will as the drive to perform that is linked to your sense of being a purposeful agent with multiple choice options. We have only a few clues as to where this sense of agency—your desire to act, and belief in your ability—emerges from. Strong hints come from studying patients with damage to the anterior cingulate in the frontal lobes, which in turn gets a major input from the parietal lobes, including supramarginal gyrus. Damage here can result in the akinetic mutism, or vigilant coma, we saw in Jason at the beginning of this chapter. A few patients recover after some weeks and say things like, “I was fully conscious and aware of what was going on, Doctor. I understood all your questions but I simply didn’t want to reply or do anything.” Wanting, it turns out, is crucially dependent on the anterior cingulate.
Another consequence of damage to the anterior cingulate is the alien-hand syndrome, in which the person’s hand does something he doesn’t “will” it to do. I saw a woman with this disorder in Oxford (together with Peter Halligan). The patient’s left hand would reach out and grab objects without her intending to, and she had to use her right hand to pry loose her fingers to let go of the object. (Some of the male graduate students in my lab have dubbed this the “third-date syndrome.”) Alien-hand syndrome underscores the important role of the anterior cingulate in free will, transforming a philosophical problem into a neurological one.
Philosophy has set up a way of looking at the consciousness problem by considering abstract questions such as qualia and their relationship to the self. Psychoanalysis, while able to frame the problem in terms of conscious and unconscious brain processes, hasn’t formulated clearly testable theories nor do they have the tools to test them. My goal in this chapter has been to demonstrate that neuroscience and neurology provide us with a new and unique opportunity to understand the structure and function of the self, not only from the outside by observing behavior, but also from studying the inner workings of the brain.17 By studying patients such as those in this
