(5) Relative Size of the Brain
There has been much discussion about man’s relatively large brain and its specific relationship to language. Although the thesis of this book would be much strengthened if we could demonstrate a necessary and sufficient connection between these typically human structural and behavioral developments, a closer examination of the various aspects of brain size reveals several unsolved problems.
First, there are problems of measurement. The relevance of average body weight in relation to brain weight is not obvious. For instance, the variance of body weights is much greater than that of brain weights. Even in a single, mature individual, the body weight may fluctuate considerably, whereas the brain weight tends to be quite constant. When different species are compared, additional problems emerge. The weight and volume of the body may vary with density of tissues (especially bones), and this can interfere with the commensurability of the brain-weight/body-weight ratios. For some animals it is advantageous to carry around a great deal of dead or energy-storing tissue, whereas others must travel as lightly as possible. Starck (1965) who has reviewed the recent literature on the encephalization problem has expressed similar criticisms and suggests that intra-cerebral proportions of tissues might provide more important and interesting quotients for taxonomic purposes than the brain-weight/body-weight ratio.
It may also be well to remember that data may be plotted in many different ways. The direct comparison of measurements may often obscure lawful relationships, particularly when these are nonlinear. Dodgson (1962), for instance, has plotted brain-weight/body-weight relationship on double-logarithmic scales (Fig. 2.24) in which an allometric connection seems to emerge (cf. Chapter six) between brain and body weights that holds for many primates including man. According to this representation, it would seem as if man had kept up with a trend common for the entire order, whereas the great apes were the deviants. However, we should not be so misled by this graph to think that there is nothing peculiar about the dimensions of man’s brain. It is of an extraordinary size and is quite obviously capable of functions that differ qualitatively and quantitatively from that of other animals.
FIG. 2.24. Weight ratios of brain and body of selected primates. (After Dodgson, 1962)A further reason what a simple comparison of a few selected weights is not very revealing is due to the changing proportions of organ weights, including the brain, through growth and development (for details, see Chapter four). The brain-body weight ratios are, for all mammals, different at birth than in maturity. Pertinent data for the primate order have been collected by Schultz (1941) who has shown that it is possible to match a quotient that is typical for mature man by a similar quotient of a subhuman primate but at a more primitive stage of development. Thus the growth histories peculiar to a species are more interesting than the comparison of any single absolute measurement. The growth history of the human brain is quite different from that of other primates.
A second realm of problems concerns the interpretation of the significance of relative and absolute increase in brain size. Man does not only differ from animals in his capacity for language but also in his general cognitive capacities. It may well be that the large-sized brain and the absolute increase in cell number and axodendritic density have increased man’s psychological storage capacity, the capacity for simultaneous processing of input and output, and the combinatorial possibilities among specific processes. In modern man, a failure of brain growth, such as in microcepyhaly, apparently results in lowering of these functions if the condition is severe enough. But it is interesting that language function is comparatively independent (in modern man—the argument must not be extended indiscriminately to fossil forms) from both brain size and variations in cognitive capacities. These phenomena are discussed in greater detail elsewhere in the book. Here we are merely interested in the relationship between brain size and language capacity.
Would it be possible at least to learn to understand a natural language such as English with a brain of markedly different weight and brain-body weight ratio? The answer is yes: as far as modern man is concerned, neither the absolute nor relative weight of the brain is the necessary factor for language-learning potentiality. There is a clinical condition, first described and named nanocephalic dwarfism (bird-headed dwarfs, as they are sometimes called), by the German pathologist, Virchow, in which man appears reduced to fairy-tale size.
FIG. 2.25. Brain weights determined at autopsy plotted as function of patients' chronological age; data from Coppoletta and Wolbach (1993). Bottom plot: various measurements of head-circumference of patient described by Seckel (1960), converted to estimates of brain weight.
Seckel (1960) has recently described two such dwarfs and has reviewed the scientific literature on thirteen others. He ascribes the condition to a single-locus recessive gene for dwarfish stature without affecting endocrine organs and function. Adult individuals attain a maximum height of three feet, and about half of the described patients stand not much higher than two and a half feet at adult age; the shortest adult mentioned measured 23 inches.
Nanocephalic dwarfs differ from other dwarfs in that they preserve the skeletal proportions of normal adults. The fully mature have a brain-body weight ratio well within the limits of a young teen-ager. Yet their head circumference and estimated brain weight barely exceeds those of a newborn infant as shown in Fig. 2.25. on microscopic examination, these brains have an unremarkable histological appearance; both the size of individual nerve cells and the density of their distribution is thought to be within normal limits. However, these brains probably differ substantially from normal adult ones in the absolute number of cells. Intellectually, these dwarfs show some retardation for the most part, often not surpassing a mental age level of five to six years. All of them acquire the rudiments of language including speaking and understanding, and the majority master the verbal skills at least as well as a normal five-year-old child. From table 2.2 it is apparent that neither the absolute nor relative weights of brains and bodies reveal the nature of the relationship between speech and its neurological correlates.
Table 2.2.
Figures in last four lines are estimates based on :Apparently, the organization of the brain is more important for language than its mass, and the entire matter must be discussed in the light of developmental processes and growth. Perhaps growth brings about organization within the brain which does have structural correlates on a molecular level, and in this sense speech and language may have a concrete structural basis. But at the present time, we have no techniques which could demonstrate the structural characteristics of a brain whose owner learned to speak at a normal age to distinguish it from certain brains whose owner had a congenital language disability without other neurological abnormalities.
