Mental chronometry is the measurement of cognitive speed. It is the actual time taken to process information of different types and degrees of complexity. The basic measurement is an individual's response time (RT) to a visual or auditory stimulus that calls for a particular response, choice, or decision. The elementary cognitive tasks used in chronometric research are typically very simple, seldom eliciting RTs greater than one or two seconds in the normal population.
Just another methodology for psychology? Mental chronometry undeniably has its own methodology. But it is also much more than just another method of what psychologists know as data analysis. Chronometric methods generate a generically different order of measurement than do any of our psychometric tests.
Scientific research and analysis are rightfully more than just data, numbers, mathematics, and statistical hypothesis testing, yet contemporary psychology has more than its share of these appurtenances. More than in most other scientific fields, research psychologists, especially differential psychologists, confront a plethora of journals, textbooks, and specialized courses on innumerable quantitative methods and statistical techniques. These all are offered wholly without specific reference to any of the field's substantive topics, empirical or theoretical. In the physical and biological sciences typically the methodological aspects and analytic methods are more intrinsically inseparable from the particular phenomena and the theoretical questions that are more or less unique to the special field of study. Why?
A likely explanation of this condition can be stated as a general rule: The lower the grade of measurement used to represent the variables of interest, the more their quantitative description and analysis must depend upon complex statistical methods. Even then, the kinds of questions that can be answered by applying the most sophisticated statistical methods to lower grades of measurement are importantly limited. Quantitative research in virtually all of the behavioral and social sciences is based almost entirely on the lowest grade of measurement that can still qualify as being quantitative, that is, ordinal or rankorder scales. Chronometry, on the other hand, allows us to jump up to the highest grade of measurement, that is, a true ratio scale. Its crucial advantages, though mainly scientific, also have aesthetic appeal. The discovery of the form of a functional relationship between variables when measured on a ratio scale represents a scientific truth, a physical reality, not just an artifice of merely ordinal measurement or of any possible mathematical transformation to which an ordinal scale may be subjected. Measurements can always be transformed from a ratio scale to an ordinal scale, but never the reverse. One derives something akin to aesthetic pleasure in discovering a fact of nature that is afforded only by true measurement. Many relationships between different behavioral phenomena that we may accept as true by casual observation or from only ordinal measurement could, in principle, be illusory. Chronometric measurement, however, can in principle confirm or disconfirm beyond question their validity as scientific fact, of course, always within certain explicit boundary conditions. Hence, a graphic presentation of chronometric data represents a true relationship, a fact of nature.
An example of this is demonstrated by what I have called the first two laws of individual differences. They just happen to have important implications for interpreting the effects of education: (1) individual differences in learning and performance increase as task complexity increases; (2) individual differences in performance increase with continuing practice and experience, unless the task itself imposes an artificially low ceiling on proficiency. These lawful phenomena are amply demonstrated by a variety of reaction time paradigms. But they could not be demonstrated definitively with any form of measurement lower than a ratio scale. Because it has a true or natural zero point, it is the only type of scale that permits fully meaningful comparison of the relative differences between means and standard deviations, or the variance ratio, alp..
It is also of significant importance that chronometric variables are related to psychometric measures of cognitive ability and especially to psychometric g. There is also a chronometric g factor, derived from a battery of various chronometric tasks, which is closely related to psychometric g. But chronometric data also reflect a more profoundly biological basis than do psychometric test scores. For example, it is a fact that IQ, and psychometric g even more, measured relatively early in the life span are positively correlated with individual differences in longevity. It was recently shown in a convincingly large sample that persons' age of death is even more closely related to quite elementary chronometric measures, such as simple and four-choice RT, than it is to IQ (Deary & Der, 2005b). In fact, the RT measures are the primary basis of the psychometric correlations, as there was no significant relationship between IQ and longevity when the effect of RT was statistically removed. Also, statistically controlling several social background variables showed no significant effect on this result.
Individual differences in cognitive abilities must ultimately be explained in terms of the physical structures and functions of the brain. Much of the research and theorizing in the field of cognitive psychology has helped in describing more precisely the mental characteristics of variation in psychometric g and other psychometric factors than has been possible within the field of psychometrics. But the theoretical formulations of cognitive psychology are strictly mentalistic, and as such they afford no hooks or handles or leverage of any kind for discovering the physical brain processes that accomplish cognitive behavior or cause individual differences therein. The purely cognitive theories in vogue today posit an increasing number of metaphorical homunculi each performing some specific functions in a fictional mental realm that has no points of contact with the actual physical activities of the brain. It is psychology's modern version of Cartesian mind-body dualism. Individual differences in the speed of information processing, for example, are either overlooked or attributed to the individual's control of "attention," which along with "cognitive resources" is one of the hard-worked homunculi in the metaphorical system. But this approach will not advance us to the promised land, as described by Charles
Spearman in his major work, The Abilities of Man (1927), in which he concluded that the final understanding of variation in cognitive abilities "... must come from the most profound and detailed direct study of the human brain in its purely physical and chemical aspects" (p. 403).
At present, explanations of individual differences in terms of actual brain processes are largely confined to studies of the anatomical localization of specific functions. This is now possible by brain-scan techniques, such as functional magnetic resonance imaging (fMRI). It is a most important step in the right direction. As this research advances to more finegrained analyses of individual differences, it will become increasingly necessary to use simpler but more precise tests of some particular cognitive behavior in order to discover the brain processes involved . This is where chronometry is best suited to make its contribution to brain-behavior research. Chronometry provides the closest noninvasive interface between brain and behavior. Neither the brain nor the behavior can be properly studied in isolation from each other. The common coin for the measurement of both brain activity and overt behavior is real time itself, ergo the importance of mental chronometry for research aimed at understanding the brain-behavior relationship. In a few years perhaps enough research of this kind will have been published to warrant a full-scale review, possibly another book.
Before closing this Preface some readers might wish to know of any important published criticisms of my previous work in mental chronometry. The three most competent critiques, as well as the sharpest and the most thoughtful, are provided by Professors John B. Carroll (1987), H. J. Eysenck (1987), and Ian Deary (2000a, 2003).
Finally, I hope that both professionals and graduate students who read this book will readily find many chronometric phenomena that are potential grist for empirically testable hypotheses, and that they will tackle some of the many technical and theoretical problems that are either explicit or implicit in this book.
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