Intelligence and the Brain
There is certainly some connection between the brain and intelligent behavior. While the spinal cord and brain stem vary according to the size of the body, and the cerebellum with the motility of the species of animal, the size of the cerebrum varies more or less closely with the intelligence of the species. It does vary also with bodily size, as illustrated by the whale and elephant, which have the largest cerebrum of all animals, including man. But the monkey, which shows more intelligence than most animals, has also a very large cerebrum for his size of body; and the chimpanzee and gorilla, considerably surpassing the ordinary monkeys in intelligence, have also a much larger cerebrum. The cerebrum of man, in proportion to the size of his body, far surpasses that of the chimpanzee or gorilla.
The cerebrum varies considerably in size from one human individual to another. In some adults it is twice as large as in others, and the question arises whether greater intelligence goes with a larger brain. Now, it appears that an extremely small cerebrum spells idiocy; not all idiots have small brains, but all men with extremely small brains are idiots. The brain weight of quite a number of highly gifted men has been measured in post-mortem examination, and many of these gifted men have had a very large cerebrum. On the whole, the gifted individual seems to have a large brain, but there are exceptions, and the relationship between brain size and intelligence cannot be very close. Other factors must enter, one factor being undoubtedly the fineness [{293}] of the internal structure of the cortex. Brain function depends on dendrites and end-brushes, forming synapses in the cortex, and such minute structures make little impression on the total brain weight.
While intelligence is related to the cerebrum as a whole, rather than to any particular "intelligence center", there is some likelihood that the special aptitudes are related to special parts of the cortex, though it must be admitted that few aptitudes have as yet been localized. The pretended localizations of phrenology are all wrong. But we do know that each sense has its special cortical area, and that adjacent to these sensory areas are portions of the cortex intimately concerned in response to different classes of complex stimuli. Near the auditory center the cortex is concerned in recognizing spoken words, and in following music; near the visual center it is concerned in recognizing printed words, in recognizing seen objects, in finding one's way by the sense of sight, etc. These special aptitudes thus have a fairly definite cortical localization, and possibly others have also.
Examined microscopically, the cortex shows differences of structure in different parts, and to the structural differences probably correspond differences of function. Now it is practically impossible that such a function as attention or memory should have any localized cortical center, for these are general functions. The instincts are specialized enough to have local centers, but none have so far been localized. What has been localized is of the nature of special aptitudes.
EXERCISES
1. Outline the chapter.
2. Pick out the true statements from the following list:
(a) Man is the most intelligent of animals.
(b) Intelligence depends on the development of the cerebellum.
(c) It has not been found possible to use any single performance as a reliable index of intelligence.
(d) Children of different mental ages may have the same IQ.
(e) A child with a mental age of 10 years can do all the tests for 10 years and below, but none of those for the higher ages.
(f) The intelligence tests depend wholly on accurate response and not at all on speed of reaction.
(g) If intelligence tests depended upon previous training, they could not be measures of native intelligence.
(h) High correlation between the test scores of brothers and sisters is a fact that tends to indicate the importance of heredity in determining intelligence.
(i) The "general factors" in intelligence are the same as the instincts.
(j) Feeble-minded individuals include all those who are below the average intelligence.
3. It is found that eminent men very often have eminent brothers, uncles and cousins. How would this fact be explained?
4. It is also found that the wives of eminent men often have eminent relatives. How would this fact be explained?
5. How could it happen that a boy of 9, in the third school grade, with an IQ of 140, should be mischievous and inattentive? What should be done with him?
6. If a boy of 12, by industrious work, does pretty well in the fourth grade, why should we not accept the teacher's estimate of him as a "fairly bright boy"?
7. How might the brain of an idiot be underdeveloped, aside from the matter of the number of nerve cells in the cortex?
8. Can it be that high intelligence is a disadvantage in any form of industrial work, and, if so, how?
9. Show how "general intelligence" and "special aptitudes" may work together to give success in some special line of work.