Intelligence in Nature - Jeremy Narby [51]
Brain imaging shows that most of the brain works at one time or another during the day. Though some functions require the activity of only small parts of the brain, most complex behaviors or thought patterns use many different brain areas. Thinking of an alpine landscape activates one network of brain areas, while thinking of cats lights up an entirely different network. Once the thought is over, all the activated neurons fall silent. Brain imaging reveals that each different thought lights up neurons in its own specific combination.
However stunning these results may be, pictures showing splotches of light on a screen do not explain how the brain works. Just because certain neurons are correlated with a behavior does not mean they cause it. Increased blood flow in a specific part of the brain as revealed by a magnetic scan merely indicates that active neurons, which require extra energy to do their jobs, are sucking in glucose and oxygen from the blood. This does not say much about how we experience what we experience. The fact that your neurons are using glucose and oxygen does not explain how you see an image of the words on this page.
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BY OBSERVING PEOPLE with localized brain damage, scientists have long known that the human brain is divided into modules that perform separate tasks. The part of the mind that sees, hears, and thinks is often associated with the top layer of the brain called the cerebral cortex; this includes the frontal lobes, which are involved in making plans and assessing risks, and the visual cortex at the back of the head, which processes visual information. Recent research using brain imaging has confirmed this modular understanding, showing that precise, and sometimes surprisingly small, groups of brain cells work in concert to carry out highly specialized functions.
Brain imaging has also revealed the importance of the brainâs deeper layers, known as the âemotionalâ brain. Most in-coming information, including what we see with our eyes, is initially processed by the deeper parts of the brain before being relayed to the upper levels. For example, visual information first goes to a small cluster of neurons in the center of the brain known as the thalamus, then down to the amygdala, a small almond-shaped structure that mediates instinctive fear. The information is also relayed from the thalamus up to the cortex, but by a longer, slower route. This arrangement explains how we sometimes respond to potential dangers before we become fully conscious of what they are. For example, we recoil from a snake on a forest path before we have an awareness of seeing it, because our emotional brains jolt our bodies into action. This capacity for rapid response may not be very preciseâsometimes the snake is only a stickâbut it provides obvious survival benefits. We are wired for survival, to a certain extent.
Magnetic brain imaging also reveals that our minds are on a kind of neural tape delay. For example, the areas of our brains involved in recognizing objects show peak activity before we ourselves recognize objects. The human brain appears to construct conscious awareness in an after-the-fact fashion. People perceive events about eighty milliseconds after they have occurred, just a bit longer than the blink of an eye. The brain appears to use this time lag to carry out fancy editing tricks. For example, when I snap my fingers, the sight and sound of the snap are processed in different parts of the brain and at widely different speeds, yet they seem simultaneous to me. I am never aware of what is happening now in my brain, but only of a small part of what has just happened there.
Furthermore, the brain is not limited to the skull. My gut alone contains about one hundred million