excluded that the neurobehavioral deficits of the adult subjects were due to increased prenatal, rather than ongoing, MeHg exposure.

Beuter and Edwards (1998) investigated the prevalence and severity of three types of subtle motor deficits in a cohort of 36 adult Cree (mean age 56 years), comparing them with patients with Parkinson 's disease (PD) (21 subjects), cerebellar deficit (6 subjects), essential tremor (3 subjects), or controls (30 subjects). The mean of the annual maximum hair Hg concentration over a period of 25 years varied from 2.2 to 31.1 ppm. Ten of 14 static tremor end points (with visual feedback) and 5 of 8 kinetic tremor end points (during voluntary finger movements) assessed were significantly related to group (i.e. Cree versus PD versus control). Nested analyses were carried out in which the six Cree with the highest hair Hg concentrations (mean of annual maximum hair concentrations greater than 24 ppm; range, 24.34-31.10 ppm) were matched to six Cree with low hair Hg concentrations (mean of annual maximum hair concentrations 6.02-11.89 ppm) and six controls to get a better idea of whether the group differences were likely to be due to Hg or some confounding factor associated with group membership. Despite the reduced number, significant group differences were still found on several end points. Overall, the performance characteristics that best discriminated groups were drift (static tremor), event index (static tremor), mean tracking error (kinetic tremor), and the center of mass harmonicity (static tremor).

The same groups of subjects were administered a test of rapid, precise promixo-distal movements (i.e. eye-hand coordination) (Beuter et al. 1999a). A eurythmkinesiometer recorded subjects' efforts to strike targets with a stylus, yielding measures of precision, imprecision, tremor, Fitts' constant (an index of the trade-off between speed and accuracy), and irregularity. The Cree subjects' performance was more than 1 standard deviation worse than the controls' performance on tremor, Fitts' constant, and irregularity. In the same type of nested analyses carried out in the study of tremor described above, the order of group scores, from best to worst, was control better than low Hg better than high Hg for all three end points, the group differences on Fitts' constant and irregularity being significant.

Rapid alternating movements (diadochokinesia) were assessed in those groups of subjects by asking them to rotate two foam spheres under three conditions: (1) both hands, natural cadence; (2) right and left

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement