In the quarter century since Nettelbeck's (1980) selective review of the literature on pathological conditions that affect reaction time (RT), there has been a virtual explosion of the uses of RT measurements in this sphere. A comprehensive bibliography of medical research studies since 1980 utilizing chronometric methods would overwhelm the entire bibliography of this book, and an exhaustive search of the medical literature might well turn up more references to RT than that now exist in the psychological literature per se. For the most part, however, the many reports of the varied clinical uses of RT tests contribute little to our understanding the basis of RT correlations with other psychometric variables, except to underline the fact that RT generally is highly sensitive to many physiological conditions that affect brain functions, which are also reflected in behavior.
Because the medical literature involving RT is so surprisingly vast, yet so specialized and heterogeneous, no attempt is made to review it in detail or to provide specific references. Those wishing to delve into these specialized uses of RT will find most of the literature referenced and abstracted, with an option to obtain the full articles in the Internet website http://www. MEDLINE. com/ (also medscape.com), by entering the key words with the name of the medical condition of interest + reaction time (e.g., Alzheimer disease + reaction time). Prominent among the highly diverse medical topics involving research with RT tests are:
cognitive effects of normal aging, mild cognitive impairment, senile dementia, traumatic brain and closed head injuries, mortality, under-nutrition and malnutrition in children, eating disorders, parasitic infections, neurological effects of HIV and AIDS, drug effects and addictions, multiple sclerosis, sleep disorders, diabetes, attention deficit and hyperactivity disorder (ADHD), stroke, vascular dementia, degenerative brain diseases associated with aging (Huntington, Alzheimer, Parkinson), epilepsy, chronic fatigue syndrome, hypoxia, post-traumatic stress disorder (PTSD), psychiatric disorders (anxiety, schizophrenia, depression, bipolar), yoga and meditation, chemical, pharmaceutical, and nutriceutical (e.g., Gingko biloba) agents.
The most general conclusion that can be drawn from these studies is that in practically all of the above-mentioned conditions, the proband group when compared with a control group or a placebo control, showed statistically significant effects on RT and RTSD as measured by the diverse RT paradigms used in these studies.
It is hardly feasible to review this vast medical literature in the present book. RT itself seldom holds center stage in the overwhelming majority of these studies, as RT is usually included in a battery along with various psychometric tests, while the main focus is on the particular medical condition. Theoretical interpretations of the relationship between the particular medical condition and RT is typically nil or unsystematic. This does not imply that the study itself is not of considerable importance for reasons quite aside from its findings on RT.
After my perusal of over 100 recent articles in this medical literature, it appears that the main obstacle to a systematic review lies in the extreme heterogeneity of the chronometric methods used. Although they report statistically significant RT effects for a particular condition, some findings fail replication, or there are marked differences in effect size. However, true replications across different laboratories are seldom seen. The heterogeneity of studies includes a lack of uniformity in the paradigms, apparatuses, and procedures for measuring RT, such that method variance alone is inevitably huge, consisting of the direct effects of these varied testing conditions and also their statistical interactions with the great variability among studies in the age and sex of the probands. Another uncontrolled source of variance is the stage of development of the particular medical condition at the time RT is measured. The many combinations of all these variables are unique to virtually every study. Although the methodological uniqueness of a given study does not necessarily preclude its unique validity, it makes a meta-analysis highly problematic, if not impossible. This high degree of heterogeneity and methodological uniqueness of the vast majority of studies clearly hinders the development of a systematic and cumulative applied science of mental chronometry. Yet the development of such a systematic applied science could prove highly useful in medical and pharmaceutical research, and in monitoring the effects of treatment.
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