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

But as pathway 2 proceeds past the fusiform to other parts of the temporal lobes, it evokes not only the name of a thing but a penumbra of associated memories and facts about it—broadly speaking the semantics, or meaning, of an object. You not only recognize Joe’s face as being “Joe,” but you remember all sorts of things about him: He is married to Jane, has a warped sense of humor, is allergic to cats, and is on your bowling team. This semantic retrieval process involves widespread activation of the temporal lobes, but it seems to center on a handful of “bottlenecks” that include Wernicke’s language area and the inferior parietal lobule (IPL), which is involved in quintessentially human abilities as such as naming, reading, writing, and arithmetic. Once meaning is extracted in these bottleneck regions, the messages are relayed to the amygdala, which lies embedded in the front tip of the temporal lobes, to evoke feelings about what (or whom) you are seeing.

In addition to pathways 1 and 2⁴ there seems to be an alternate, somewhat more reflexive pathway for emotional response to objects that I call pathway 3. If the first two were the “how” and “what” streams, this one could be thought of as the “so what” stream. In this pathway, biologically salient stimuli such as eyes, food, facial expressions, and animate motion (such as someone’s gait and gesturing) pass from the fusiform gyrus through an area in the temporal lobe called the superior temporal sulcus (STS) and then straight to the amygdala.⁵ In other words, pathway 3 bypasses high-level object perception—and the whole rich penumbra of associations evoked through pathway 2—and shunts quickly to the amygdala, the gateway to the emotional core of the brain, the limbic system. This shortcut probably evolved to promote fast reaction to high-value situations, whether innate or learned.

The amygdala works in conjunction with past stored memories and other structures in the limbic system to gauge the emotional significance of whatever you are looking at: Is it friend, foe, mate? Food, water, danger? Or is it just something mundane? If it’s insignificant—just a log, a piece of lint, the trees rustling in the wind—you feel nothing toward it and most likely will ignore it. But if it’s important, you instantly feel something. If it is an intense feeling, the signals from the amygdala also cascade into your hypothalamus (see Figure Int.3), which not only orchestrates the release of hormones but also activates the autonomic nervous system to prepare you to take appropriate action, whether it’s feeding, fighting, fleeing, or wooing. (Medical students use the mnemonic of the “four Fs” to remember these.) These autonomic responses include all the physiological signs of strong emotion such as increased heart rate, rapid shallow breathing, and sweating. The human amygdala is also connected with the frontal lobes, which add subtle flavors to this “four F” cocktail of primal emotions, so that you have not just anger, lust, and fear, but also arrogance, pride, caution, admiration, magnanimity, and the like.

LET US NOW return to John, our stroke patient from earlier in the chapter. Can we explain at least some of his symptoms based on the broad-brushstrokes layout of the visual system I have just painted? John was definitely not blind. Remember, he could almost perfectly copy an engraving of St. Paul’s Cathedral even though he did not recognize what he was drawing. The earlier stages of visual processing were intact, so John’s brain could extract lines and shapes and even discern relationships between them. But the crucial next link in the “what” stream—the fusiform gyrus—from which visual information could trigger recognition, memory, and feelings—had been cut off. This disorder is called agnosia, a term coined by Sigmund Freud meaning that the patient sees but doesn’t know. (It would have been interesting to see if John had the right emotional response to a lion even while being unable to distinguish it consciously from a goat, but the researchers didn’t try that. It would have implied a selective sparing of pathway 3.)

John could still “see” objects, could reach out and grab them, and walk around the room dodging obstacles because his “how” stream was largely intact. Indeed, anyone watching him walk around wouldn’t even suspect that his perception had been profoundly deranged. Remember, when he returned home from the hospital, he could trim hedges with shears or pull out a plant from the soil. And yet he could not tell weeds from flowers, or for that matter recognize faces or cars or tell salad dressing from cream. Thus symptoms that would otherwise seem bizarre and incomprehensible begin to make sense in terms of the anatomical scheme with it’s the multiple visual pathways that I’ve just outlined.