It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind by David A. Rosenbaum Read Free Book Online

scenes to come. You might see mice munched by minks, goats gulped by gators, doves downed by dogs. If your nerves jangle at such sights, the jungle might not be the place for you.
    Of course, I’m speaking only metaphorically here, stretching the jungle metaphor to the point of silliness. But if you come along with me in pursuit of the metaphor, you’ll recall that jungles aren’t the only kind of ecosystem. Others are deserts, tundra, swamps, forests, cliffs, and plains. All these niches afford different perils and potentials. A bluff is a good place for beasts that fly. A cove is a good place for beasts that swim. Species that thrive in different niches are well adapted for them. Those that aren’t tend to die.
    Recognizing that different places afford different adaptations can help you approach the functioning of the brain with positive expectations. You can expect the internal environment of the brain to have different ecosystems, with different cranial creatures thriving within it. In the brain, as in the outer world, different functions flourish depending on the niches they occupy.
    This chapter is about the brain. A wonderful aphorism about the brain by a cognitive neuroscientist at the University of Toronto captures a profound truth about how the brain is organized. According to this cognitive neuroscientist, Geoffrey Hinton, “The brain is locally global and globally local.” What did he mean? 1
    The first part of the sentence, “The brain is locally global,” means that on a local scale, the components of the brain are alike. If you look at the brain with a microscope, figuratively or literally, you can see the elements all doing the same thing more or less: sucking up nutrients, expending energy, and so on. The second part of the sentence, “The brain is…globally local,” means that on a larger scale, the components of the brain do
different
things; they have their own special
local
features. Different regions act differently. Onearea leans toward language, another veers toward vision, another heads toward hearing, and so on. In general, the farther apart two areas of the brain happen to be, the less similar their functions. This isn’t because some brain supervisor declares that there should be a language zone, a vision zone, and a hearing zone. Rather, different regions tend to have different functional properties because of where they happen to reside or, more importantly, with whom they happen to interact most closely. The structuring of mental function follows this fellowship.
A Little about Neurons
    To pursue this idea, it will help to say more about the elements of the nervous system. The nervous system, as you probably already know, is a complex web made of the central nervous system (including the brain, retina, and spinal cord) and peripheral nervous system (sensory receptors and muscle-activating fibers).
    The main building blocks of the nervous system are
neurons
. These come in a variety of shapes and sizes depending on where they’re situated. Neurons specialized for detecting mechanical pressure occupy the skin, neurons specialized for detecting airborne chemicals occupy the nose, and so on. Within the brain, there are also different kinds of neurons. 2
    Despite structural differences among neurons, prototypical neurons have three main sections.
Dendrites
receive incoming signals. The
cell body
integrates incoming signals and metabolically sustains the cell as a whole.
Axons
serve as transoms for the release of signals to other neurons or muscles.
    Prototypical neurons integrate incoming signals over space and time. The space over which the integration occurs covers the dendritic inputs to the neurons. The time over which the integration occurs is the period over which the inputs sum.
    If the integrated inputs to a prototypical neuron exceed a threshold, the neuron can generate a burst of activity called an
action potential
. An action potential races down the axon at a speed that is