The study of behavioral abnormalities induced by exogenous agents such as drugs, chemicals in the general environment, and chemicals encountered in the workplace. Just as some substances are hazardous to the skin or liver, some are hazardous to the function of the nervous system. In the case of permanent effects, changes in sensation, mood, intellectual function, or motor coordination would obviously be undesirable, but even transient alterations of behavior are considered toxic in some situations. For example, operating room personnel accidentally exposed to very small doses of anesthetic do not exhibit altered performance on an intelligence test, a dexterity test, or a vigilance task. However, significant decrements in performance occur in recognizing and recording visual displays, detecting changes in audiovisual displays, and recalling series of digits. By comparing the behavior of exposed subjects and control subjects, behavioral toxicologists seek to identify agents capable of altering behavior and to determine the level of exposure at which undesirable effects occur. When the agent under study is one in common use, and there is no evidence of its being hazardous to health, experiments may be carried out on human volunteers, or comparisons may be made from epidemiologic data. More frequently, safety considerations dictate the use of laboratory animals in toxicology research. Perhaps the best-known example of toxicity in humans is methyl mercury poisoning (Minimata disease), which occurred in epidemic proportions in a Japanese coastal town where the inhabitants ate fish contaminated with mercury from industrial pollution. Although mercury affects a variety of behaviors, the most obvious symptoms are tremors and involuntary movements. A different set of functional problems is exemplified by the effects of ethyl alcohol, a single agent with direct and indirect, short- and long-term consequences. The short-term, low-dose effects of alcohol include sensory disturbances, motor problems, and difficulties with processing information. Neurologically, alcohol is usually described as a central nervous system depressant which is general, in the sense that it disrupts many functions. In some individuals, large quantities of alcohol consumed over a long period lead to permanent damage to the nervous system. Behaviorally, individuals with Korsakoff's syndrome exhibit severe memory deficits. Anatomically, their brains are found to have degenerative changes in the thalamus. This syndrome is not just an extension of the short-term effects. In fact, it is thought to arise from alcohol-induced malnutrition rather than as a direct effect of alcohol itself. Lasting injuries to the nervous system have been reported to occur in children exposed to alcohol before birth. The behavioral problems associated with fetal alcohol syndrome do not appear to be related to either Korsakoff's syndrome or the immediate effects of alcohol. Rather, they constitute a third set of effects, including learning deficits, problems with inhibiting inappropriate behavior, and fine motor dysfunction, along with some visible physical abnormalities. While malnutrition may play a role in this congenital syndrome, the mechanism and locus of damage are not known. When toxicity is considered only in terms of direct risk to survival, behavioral toxicity may seem to be of minor importance. However, survival is not the only criterion of good health. In a complex society that places heavy demands on an individual's educability, alertness, and emotional stability, even small deviations in behavior are potentially hazardous. Severe disabilities, as in the gross motor malfunctions of Minimata disease, have drawn attention to behavioral toxicology. Such incidents represent failures of control of toxic substances. Successes are difficult to measure, for they can be seen only in reduction of risk—the ultimate goal of toxicology. Toxicology |
