God Soul Mind Brain

diagram is not known—nobody can build a working model on a computer yet—but the essential pieces, from the light detectors in the eye to the color-sensitive neurons in visual cortex, have been mapped. There is no similar map for social perception. It is much less well understood, but is a hot current topic, perhaps the fastest-growing area of study in neuroscience.

    The present chapter provides a brief tutorial on the basics of the brain. Much of the tutorial is on the monkey brain, and specifically on the monkey visual system, since this is where the research on social neuroscience began.

    I teach a class at Princeton University on the fundamentals of neuroscience. The class is meant for students of all backgrounds. Some are intent on a career in neuroscience and come to the class with a sophisticated knowledge. Some are aspiring artists who want to know more about the perception of color and light, and come to the class with almost no neuroscientific knowledge. Some are politicians in the making who are curious about how brains react to other brains. My goal in the lectures is always to present the information clearly and accurately, so that students of all backgrounds can gain something. I will try to apply the same approach here. I will use as little technical jargon as possible. The goal of the chapter is to emphasize basic concepts, not details.

Neurons make decisions

    The brain, like any other part of the body, is made of a great variety of cell types. Neurons are the main cells that process information. The human brain contains about one hundred billion neurons. (Ten years ago that number was a hazy abstraction. Now we all understand it as the number of dollars it takes to bail out a bank.)

    Each neuron has a cell body, sometimes just large enough to be seen with the naked eye as a speck. Extending from the cell body are long thin strands called axons and dendrites, the wires by which neurons communicate to each other. Some of these strands are microscopically short. Some are extremely long. For example, one type of neuron has its cell body in your back, just beside your spine. The cell has one long strand that extends to your foot and another strand that extends up to your brain. When you bang your toe, a signal is generated at the foot end of the strand and travels up to your brain along a single unbroken cellular wire. (In a giraffe, this type of cell is correspondingly gigantic.)

    Scientists can measure the activity of individual neurons in the brain. This is usually done by inserting a very fine wire that is coated in a plastic insulation. Only a small bit of metal (about twenty microns) is exposed at the tip of the wire. The back end of the wire sticks out of the brain and plugs into an amplifier. If there are any electrical signals in the brain near the tip of the probe, the signals will be measured and amplified, and you will hear them over a loudspeaker as crackles and pops. The method is used, for example, in people undergoing brain surgery when the surgeon wants to map out the brain before removing anything. The person’s head is stabilized in a special holder, the scalp and skull are opened under local anesthetic, and the electrode is inserted into the brain. Even though the person is wide awake, the electrode doesn’t hurt going into the brain—in fact the person can’t feel it at all—because there are no pain or touch receptors in the brain itself. The same general method is also used in animal experiments.

    When you do this type of experiment, you can pick up signals that come from individual neurons. These signals are brief pulses of electrical and chemical activity, usually generated in a neuron’s cell body, that are fired down the axon at a speed that can be as fast as 200 meters per second. The signals reach other neurons and other parts of the brain in a matter of milliseconds.

    Suppose that neuron 1 sends a signal to neuron 2. The signal travels down the long axon of neuron 1. At the last step, however, the neurons are separated by a small gap called a synapse. (The synapse is so small that it can be seen only with an electron microscope. It is about 20 nanometers wide.) The signal, reaching the end of neuron 1, triggers a puff of chemical that crosses the synapse and affects neuron 2. This chemical is called a neurotransmitter. There are dozens of different kinds of neurotransmitters in the brain, but they all serve this basic function of diffusing across the synapse