If two neurons that are hooked up in an inhibitory way both fire, then neither neuron can fire as vigorously as it would otherwise. When one neuron fires, it suppresses the firing of other nearby neurons. We know, for instance, that many neurons in the visual cortex (the part of the brain responsible for processing visual information) are hooked up in such a way as to inhibit each other ( Beck & Kastner, 2009). If we have an abundance of brain fuel and neurons, how can we explain our limited cognitive abilities? Why can’t we do more at once? The most likely explanation is the way these neurons are wired up. Modern neuroimaging (see “Studying the Human Brain”) has shown that we use all parts of brain, just at different times, and certainly more than 10% at any one time. It is also not the case that we use only 10% of our brain, a myth that was likely started to imply we had untapped potential. The average human brain contains 86 billion neurons. Nor is it likely that our limitations reflect too few neurons. Thus, insufficient “brain fuel” cannot explain our limited capacity. However, as long as we are not oxygen-deprived or malnourished, we have more than enough oxygen and glucose to fuel the brain. The brain is a large consumer of these metabolites, using 20% of the oxygen and calories we consume despite being only 2% of our total weight. The brain uses oxygen and glucose, delivered via the blood. An MRI of the human brain delineating three major structures: the cerebral hemispheres, brain stem, and cerebellum. But what might this limited resource be in the brain? Figure 1.
Why is this? Many people have suggested that such limitations reflect the fact that the behaviors draw on the same resource if one behavior uses up most of the resource there is not enough resource left for the other. We cannot even pat our head and rub our stomach at the same time (with exceptions, see “A Brain Divided”). We cannot do two complex tasks at once: We cannot read and carry on a conversation at the same time, text and drive, or surf the Internet while listening to a lecture, at least not successfully or safely. For example, one well-established fact about human cognition is that it is limited. A detailed understanding of the human brain can help us make sense of human experience and behavior. Every behavior, thought, or experience described in the other modules must be implemented in the brain.
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