The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

Implicit in the idea of the self is the notion of sequentially organized memories accumulated over a lifetime. There are syndromes that can profoundly affect different aspects of memory formation and retrieval. Psychologists classify memory (the word is used loosely synonymous with learning) into three distinct types that might have separate neural substrates. The first of these, called procedural memory, allows you to acquire new skills, such as riding a bicycle or brushing your teeth. Such memories are summoned up instantly when the occasion demands; no conscious recollection is involved. This type of memory is universal to all vertebrates and some invertebrates; it certainly isn’t unique to humans. Second, there are memories that comprise your semantic memory, your factual knowledge of objects and events in the world. For example, you know that winter is cold and bananas are yellow. 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.