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

But seeing David convinced me that the Capgras syndrome is bona fide. What could be causing such a bizarre delusion? One interpretation that can still be found in older psychiatry textbooks is a Freudian one. The explanation would run like this: Maybe David, like all men, had a strong sexual attraction to his mother when he was a baby—the so-called Oedipus complex. Fortunately, when he grew up his cortex became more dominant over his primitive emotional structures and began repressing or inhibiting these forbidden sexual impulses toward mom. But maybe the blow to David’s head damaged his cortex, thereby removing the inhibition and allowing his dormant sexual urges to emerge into consciousness. Suddenly and inexplicably, David found himself being sexually turned on by his mother. Perhaps the only way he could “rationalize” this away was to assume she wasn’t really his mother. Hence the delusion.

This explanation is ingenious but it never made much sense to me. For example, soon after I had seen David, I encountered another patient, Steve, who had the same delusion about his pet poodle! “This dog looks just like Fifi,” he would say “but it really isn’t. It just looks like Fifi.” Now how can the Freudian theory account for this? You would have to posit latent bestial tendencies lurking in the subconscious minds of all men, or something equally absurd.

The correct explanation, it turns out, is anatomical. (Ironically Freud himself famously said, “Anatomy is destiny.”) As noted previously, visual information is initially sent to the fusiform gyrus, where objects, including faces, are first discriminated. The output from the fusiform is relayed via pathway 3 to the amygdala, which performs an emotional surveillance of the object or face and generates the appropriate emotional response. What about David, though? It occurred to me that the car accident might have selectively damaged the fibers in pathway 3 that connect his fusiform gyrus, partly via the STS, to his amygdala while leaving both those structures, as well as pathway 2, completely intact. Because pathway 2 (meaning and language) is unaffected, he still knows his mother’s face by sight and remembers everything about her. And because his amygdala and the rest of his limbic system are unaffected, he can still feel laughter and loss like any normal person. But the link between perception and emotion has been severed, so his mother’s face doesn’t evoke the expected feelings of warmth. In other words, there is recognition but without the expected emotional jolt. Perhaps the only way David’s brain can cope with this dilemma is to rationalize it away by concluding that she is an imposter.⁶ This seems an extreme rationalization, but as we shall see in the final chapter the brain abhors discrepancies of any kind and an absurdly far-fetched delusion is sometimes the only way out.

The advantage of our neurological theory over the Freudian view is that it can be tested experimentally. As we saw earlier, when you look at something that’s emotionally evocative—a tiger, your lover, or indeed, your mother—your amygdala signals your hypothalamus to prepare your body for action. This fight-or-flight reaction is not all or nothing; it operates on a continuum. A mildly, moderately, or profoundly emotional experience elicits a mild, moderate, or profound autonomic reaction, respectively. And part of these continuous autonomic reactions to experience is microsweating: Your whole body, including your palms, becomes damper or dryer in proportion to any upticks or downticks in your level of emotional arousal at any given moment.

This is good news for us scientists because it means we can measure your emotional reaction to the things you see by simply monitoring the degree of your microsweating. This can be done simply by taping two passive electrodes to your skin and routing them through a device called an ohmmeter to monitor your galvanic skin response (GSR), the moment-to-moment fluctuations in the electrical resistance of your skin. (GSR is also called the skin conductance response, or SCR.) Thus when you see a foxy pinup or a gruesome medical picture, your body sweats, your skin resistance drops, and you get a big GSR. On the other hand, if you see something completely neutral, like a doorknob or an unfamiliar face, you get no GSR (although the doorknob may very well produce a GSR in a Freudian psychoanalyst).