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Toxicological Effects of Methylmercury
used in the studies to assess each domain to be equivalent either in terms of the specific skills assessed or the test sensitivity.
Although the Bender-Gestalt Test was administered in the Faroe study and SCDS as a measure of visual-spatial abilities, different scoring systems were used (the Gottingen system in the Faroe Islands and the Koppitz system in the Seychelles). The finding of a significant association with Hg in the former but not the latter study is similar to the finding reported by Trillingsgaard et al. (1985) that scores derived using the more-detailed Gottingen system were significantly associated with low-dose lead exposure, and scores on the Koppitz system were not. Thus, the Gottingen system used in the Faroe Islands might be more sensitive. Although the Seychelles data could be rescored using the Gottingen system, the committee was told that the data might still not be comparable, because the more sensitive memory for design conditions was not administered in the Seychelles study.
To help determine the degree to which the discrepant results from the Faroe study and SCDS are attributable to differences in the neurobehavioral tests used, the Seychelles group administered several of the more domain-specific tests from the Faroe battery in their 8-year followup. The results of those assessments, however, are not yet available.
A second important difference in the assessment batteries used in the Faroe study and SCDS relates to the age of assessment — 7 years in the Faroe Islands and 5.5 years of age in the SCDS. The final assessment in the New Zealand cohort was at 6 years of age. Generally speaking, developmental assessments are likely to be less sensitive in detecting subtle neurotoxic effects when they are administered during a period of rapid developmental change. The period covering ages 60-72 months, when the SCDS and New Zealand cohorts were evaluated, is one such period during which marked individual differences in the rate of cognitive maturation are likely to eclipse subtle differences in function attributable to a teratogenic exposure (Jacobson and Jacobson 1991). The assessments performed in the SCDS during infancy, particularly the 19-and 29-month Bayley scales, were also not administered at optimal age points. Studies of prenatal exposure to alcohol and other substances that have administered the Bayley scales at multiple ages have repeatedly failed to detect effects at 18 months, probably because it too is a period of rapid cognitive maturation, involving the emergence of spoken language. Twenty-nine months is likely to be an insensitive testing