6.2The Cerebral Cortex
There are a hundred billion neurons in the brain. These neurons form circuits with each other, and their state of sustained excitation is called consciousness. Different states of consciousness involve different areas of excitation (Fig. 6-2). Generally, the size of the excited area expressed by each neuron is approximately constant, but the frequency is different. However, it remains unclear how each circuit is distinguished and recognized in our brain.
The entire cerebrum is divided into four lobes: frontal lobe, temporal lobe, parietal lobe, and occipital lobe (Fig. 6-3). The cerebral cortex also regulates several functions. The brain allocates these various functions to each brain area. With reference to a brain map created by Korbinian Brodmann (Fig. 6-4), let us now take a look at several historical discoveries.
Fig. 6-4. Brodmann's Map of the Cerebral Cortex
Essentials of Neural Science and Behavior, E. R. Kandel, et al. Appleton & Lange. Reprint from 1995.
Studies of aphasia patients by Paul Broca and Carl Wernicke during the last half of the 19th century revealed that different areas of the brain have different functions. In 1861, Broca reported a patient who after suffering from a stroke, could understand what people were saying, but could say nothing but "tan, tan" for many years. After the patient died, Broca performed an autopsy of his brain and found damage (traces of a stroke) in the left frontal lobe (Brodmann's areas 44 and 45). This area is now called Broca's speech area.
In 1876, Carl Wernicke studied aphasia patients. One patient (Patient A) suffered from auditory aphasia, the inability to understand the meaning of spoken words. Another patient (Patient B) suffered from visual aphasia, the inability to understand the meaning of written words. Wernicke found damage in Brodmann's Area 40 in the left brain of Patient A and damage in Brodmann's Area 39 in the left brain of Patient B. The area that Wernicke found is called the sensory speech center. These two studies showed that speech, the most important ability of humans, is processed in a specific area in the left brain. For this reason, the left brain is called the dominant hemisphere. Other than these abovementioned differences, there is almost no difference between the left and right brain functions.
Phrenology was popularized by the German physician Franz Joseph Gall during the 19th century. It has been hypothesized that personalities and behavioral characteristics, such as aggressiveness and prudence, are manifested in the shape of the brain, and thus, can be determined from measurements of people's skulls. Phrenology is based on the assumption that increased brain use is associated with larger brain size. Column Figure 6-1 shows the results of phrenology. A map depicting the relationships between head shape and personality was developed. According to this map, people whose heads are prominent at the top are obstinate, the area behind the top of the head is associated with pride, the sides of the eyelids are associated with calculation ability, and so on. Gall was the first person to hypothesize that psychological characteristics are caused not by the entire brain but by parts of it. Gall's phrenology was very popular among the upper classes of Europe, but was gradually forgotten as the field of medicine progressed. Today, blood flow and energy consumption are measured using modern instruments, such as functional nuclear magnetic resonance imaging (fMRI) and positron (positive electron) emission tomography (PET) (described in this chapter). However, it is uncertain whether people actually "think" in areas of blood flow. There is a view that they "think" in areas without blood flow. There is skepticism that fMRI and PET may be just today's phrenology.