III. THEORY OF CONGENITAL LESION OR ATROPHY CRITICIZED
Reasoning from analogy with cases where a function of language is lost selectively, through organic disease or impairment of brain structure, it was thought by those who first described innate special disabilities, as in reading or spelling, that such defects must be due to congenital brain lesions or atrophies. Neurological research has never verified this supposition. No cases showing innate disability to be correlated with any peculiarities of restricted areas in the cortex have ever been recorded. Tilney and Riley, summarizing critically the data of neurology in 1921, cite no cases considered to afford authentic evidence of localized lesions or defects, as the basis of congenital difficulty in reading, spelling, music, or other functions with which the present treatise is concerned.
The theory of innate lesion or atrophy of a cortical area, to account for disability in a special mental function, seems unscientific for other reasons, aside from the fact that it has never been objectively verified by actual observation of a structural defect. One of these reasons is that a theory, formulated to take care of the neural basis of specialized disabilities, must take care of specialized gifts, as well. Cases where a generally stupid child is innately gifted with special ability to master the mechanics of reading, for example, are no doubt as frequent as cases where a generally capable child learns them with difficulty. The theory of specialized lesions or other faults of structure might cover disabilities, but would it cover special talents as well?
Still another consideration prevents us from regarding the theory of localized brain defects as masterly. This is the fact referred to in our preliminary discussion, that every single mental function, which yields to measurement, is found to be distributed among human beings according to a probability curve. (See Figure 1, page [8].) The functions which we herein consider are not exceptions to this principle. Performance in reading, spelling, arithmetic, drawing, music, and so forth, shows children or adults, chosen at random, to be distributed in the given form. Those who have exceptional talents or defects in the function fall within the symmetrical surface of this curve, at its opposite extremes. Nowhere is there a point of demarkation, denoting absolute lack of the trait in a group falling below that point, as there would be if a number of individuals suffering from lesions were introduced into the distribution. We may fairly demand of a theory which undertakes the explanation of the most extreme deviations, the explanation of the deviations of lesser magnitude, as well. The curve obtained by test approximates that form which mathematicians tell us appears when an infinite number of factors act together in an infinite number of ways, the extreme deviations occurring inevitably, by chance. A theory introducing the adventitious circumstance of lesion or atrophy is thus superfluous to the explanation of the extreme unfortunate deviations. To admit it would violate the rule of scientific method known as the law of parsimony, for we do not need it in order to explain the facts.