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

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.