integrating all of the disparate facts revealed by research on individual differences in information processing.
However, it is important to distinguish between the explanation of intelligence and the explanation of g. The explanation of intelligence calls for the description of the operating principles of the nervous system that make the functions of intelligence possible in all normal members of the same species. Individual differences in the efficiency, capacity, and power of the nervous system with respect to its information-processing functions are most strongly reflected by the g factor. But the explanation of g per se is an essentially different task from that of explaining intelligence, in that it calls for the discovery specifically of those features of the nervous system that are associated with individual differences in the effectiveness of the organism's neural information processes, particularly those feature(s) of the nervous system that cause positive covariance (or correlation) among virtually all cognitive abilities, which is what g is all about.  A theory that integrates these empirical discoveries would explain the biological basis of psychometric g.
As a possible heuristic for research on the neurophysiological basis of g, therefore, I propose consideration of the following working hypothesis: Individual differences in behavioral capacities do not result from intraspecies differences in the brain's structural operating mechanisms per se, but result entirely from other aspects of cerebral physiology that modify the sensitivity, efficiency, and effectiveness of the basic information processes that mediate the individual's responses to certain aspects of the environment. Thus research on the neurophysiology of mental ability has two aspects, the first dealing with the brain structures and neural processes that make possible intelligent behavior, the second dealing with the physical conditions that produce individual differences in intelligent behavior. The first aspect will probably be more difficult to resolve than the second, but investigation of the second need not depend upon a prior resolution of the first. Investigation can be directed at discovering the relationship between g and the neural conditions that affect a number of different elementary cognitive processes or behavioral capacities which, though served by different brain modules, are nevertheless correlated for individuals.
The highest priority in g research, therefore, is to discover how certain anatomical structures and physiological processes of the brain cause individual differences in g. Advanced technology of brain research has brought g research to the threshold presaged by Spearman himself over seventy years ago, that the final understanding of g "must needs come from the most profound and detailed direct study of the human brain in its purely physical and chemical aspects."