God Soul Mind Brain

from one neuron to another.

    If all neurons were connected in a simple way, then one signal in one neuron would soon spread, activating every other neuron in the brain in a mass spasm of signal something like an epileptic fit. Neurons are designed, however, to be extremely choosey about whether to send a signal or withhold a signal.

    The job of each neuron, in a sense, is to make a decision. Each neuron might receive chemical signals from thousands of other neurons around the brain. Some of the incoming signals are positive: they say, in effect, “Yes, generate your own signal.” Some are negative: they indicate, “No, don’t generate your own signal.” If the incoming signals are predominantly positive ones, then “yes” has it, and the neuron fires off its own signal. If the incoming signals are predominantly negative, then “no” has it, and the neuron withholds sending its own signal. The neuron essentially adds together its incoming signals as if they were yes and no votes, and based on the tally it decides what to do.

    A neuron can make this decision hundreds of times a second, firing off signals at a high rate like a machine gun if its inputs are consistently positive, or sending signals at a low rate, perhaps one signal every few seconds, if its positive and negative inputs are fluctuating around a balance point.

    There you have the essential story of the brain. Each neuron decides moment-by-moment whether to send an output signal by adding together its input signals. Complex computations are performed by neurons linked together into elaborate circuits. Most neuroscientists accept the hypothesis that the whole range of human thought, emotion, perception, memory, and action emerges from the interaction of these absurdly simple neuronal elements. (It is possible that other elements, such as glia cells, also play a role in information processing, but the role of these non-neuronal elements is not fully understood.)

Divide and conquer in the visual system: Object, motion, space, action

    The human brain is organized very roughly into an outer shell called the cerebral cortex and a central, subcortical mass. Both are dense with neurons. The space between them is filled with fibers, cables connecting the subcortical brain to the cortex and connecting one part of the cortex with another. The subcortical brain was once thought to be more “primitive,” performing simple operations, whereas the cortex was thought to perform advanced or complex operations. That traditional view is still alive and kicking, even among neuroscientists. However, the cortex and the subcortical brain interact constantly in a dance that is largely not understood.

    Most sensory information—vision, touch, hearing, taste—arrives at the subcortical brain first, where it is processed to some degree and then passed up to the cortex. Information is also passed down from the cortex to the subcortex, but the purpose of this backflow is not well understood (a frequent refrain).

    The organization of the visual system is sketched in Diagram 6-1.

    Light enters the eye and is focused by a lens onto the back of the eyeball, on the retina. The light stimulates receptor cells in the retina, producing neuronal signals. These signals pass through a rich network of millions of neurons all within the retina, a circuit that computes some initial properties of wavelength and contrast. After this preprocessing, the retina sends its information via a bundle of nerve fibers to the brain. This cable, the optic nerve, connects mainly to a specific subcortical station.

    The main subcortical station for vision goes by the long jawbreaker name of the Dorsal Lateral Geniculate Nucleus. Although it has an impressive name, nobody knows what it does. It must be doing something. It can’t be just a relay station—like the place where you plug two extension cords together. It has a high density of neurons that must be computing something, but the exact functions are still unknown. This subcortical station sends its output signals to a specific area of the cortex at the back of the brain, the primary visual cortex.

    Some parts of the visual system in the human brain. In this drawing, the brain has been split from front to back and the cut surface is shown.

    Diagram 6-1

    Diagram 6-2 shows a drawing of the side view of a monkey brain. Much of what is known about the visual system was first discovered in the monkey brain and turns out to be similar in