Appraisal of neurobehavioral methods in environmental health research: the developing brain as a target for neurotoxic chemicals.

Psychological tests as developed and validated in the field of differential psychology have a longstanding tradition as tools to study individual differences. In clinical neuropsychology, global or more specific tests are used as neuropsychological tools in the differential diagnosis of various forms of brain damage or neurobehavioral dysfunction following chemical insults, such as mental sequelae of prenatal alcohol consumption by pregnant mothers (fetal alcohol syndrome) or of maternal thyroid deficiency during pregnancy. Psychometric tests are constructed to fulfill basic quality criteria, namely objectivity, reliability and validity. For strictly diagnostic purposes in individual cases they must also possess normative values based on representative reference groups. Intelligence tests or their developmental variants are often used as endpoints in environmental health research for studying neurodevelopmental adversity due to early exposure to neurotoxic chemicals in the environment. Intelligence as treated in psychology is a complex construct made up of specific cognitive functions which usually cover verbal, numerical and spatial skills, as well as perceptual speed, memory and reasoning. In this paper, case studies covering neurodevelopmental adversity of inorganic lead, of methylmercury and of polychlorinated biphenyls (PCBs) are reviewed, and the issue of postnatal behavioral sequelae of prenatal exposure is covered. In such observational studies precautions must be taken in order to avoid pitfalls of causative interpretation of associations between exposure and neurobehavioral outcome. This requires consideration of co-exposure and confounding. Important confounders considered in most modern developmental cohort studies are maternal intelligence and quality of the home environment.