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Toxicological Effects of Methylmercury
DIFFERENCES IN THE NEUROBEHAVIORAL END POINTS ASSESSED AND THE CHILDREN'SAGES AT ASSESSMENT
The Faroe and Seychelles studies used very different neurobehavioral test batteries. For the most part, the tests selected for the SCDS are considered apical or omnibus tests (e.g., the McCarthy Scales of Children's Abilities), which yield global scores that integrate performance over many separate neuropsychological domains. In contrast, because the Faroe investigators hypothesized multifocal domain-specific neuropsychological effects, their test battery largely consisted of highly focused tests selected from those commonly used in clinical neuropsychology (e.g., California Verbal Learning Test — Children and Boston Naming Test). The Faroe test battery does not include an apical test of global function.
The subscales from the McCarthy test (verbal, perceptual-performance, quantitative, memory, and motor) that assess specific domains of function might be expected to be more directly comparable to the tests administered in the Faroe Islands. For instance, given the finding in the Faroe study that memory, as assessed by the California Verbal Learning Test, was significantly associated with prenatal Hg exposure, it would be expected that children's scores on the McCarthy memory scale in the SCDS would be associated with Hg exposure. However, they were not. In fact, prenatal Hg exposure was not associated with scores on any of the McCarthy subscales. It is important to examine in detail the extent to which the individual McCarthy subscales are comparable to the domain-specific tests selected for the Faroe study. Psychometrically, they are different. The California Verbal Learning Test, for example, involves five learning trials of a 12-word list, with free- and cued-recall trials following short and long delays, and a recognition trial. None of the 18 tests that contribute to scores on the McCarthy scales examine rate of learning, and the memory scale combines scores on four tests that involve recall of differing types of information: pictorial (six common objects arrayed on a page), auditory sequence (xylophone notes), word list (ranging from 3 words in a specified sequence to a 13-word sentence with 9 key words that are scored), connected discourse (recall of individual story elements), and numbers (forward and backward recall of strings of numbers up to seven digits long.) Clearly, a child's score on