Boucher et al. total area of all fatty acid peaks from C14:0 to C24:1 (percent weight). Control variables. The following con- trol variables were included: age and sex of child; whether or not the child was adopted; whether the child was transported by plane from a small, more remote village to a larger village for the assessment; time at ERP assess- ment (morning vs. afternoon); maternal age at delivery; socioeconomic status (Hollingshead 1975) of the primary caregiver; maternal non- verbal reasoning abilities (Raven Progressive Matrices; Raven et al. 1992); breast-feeding duration (number of months); maternal tobacco smoking (yes/no), marijuana con- sumption (yes/no), and binge drinking (at least one episode of ≥ 5 standard alcohol drinks; yes/no) during pregnancy; and DHA concentrations in cord and child plasma sam- ples. In addition, in the analysis of each con- taminant, the other contaminants were also treated as control variables. Table 1. Descriptive statistics for the study sample (n = 196). Variables Child age at assessment (years) Child sex (% girls) Adoption status (% adopted) Transportation by plane (% yes) Time at ERP assessment (% morning) Maternal characteristics Age at delivery (years) Marital status (% single) Education (years) Hollingshead score Raven score Breast-feeding durationa (months) Maternal use or consumption during pregnancy Cigarettes (% yes) Marijuana (% yes) Binge drinking (≥ 5 standard drinks of alcohol per occasion; % yes) Contaminants and other biological samples Cord blood Pb (μg/dL) Current blood Pb (μg/dL) Cord plasma PCB-153 (μg/kg fat) Current plasma PCB-153 (μg/kg fat) Cord blood Hg (μg/L) Current blood Hg (μg/L) Cord plasma DHA (% phospholipids) Current plasma DHA (% phospholipids) Go/no-go behavioral performance Mean RT, correct go trials (msec) Mean RT, incorrect no-go trials (msec) Correct go trials (%) Correct no-go trials (%) aFor women who breast-fed only. n 196 196 196 196 196 196 195 196 196 196 190 189 160 160 193 193 191 192 193 193 189 191 196 196 196 196 Percent 55.1 15.8 45.4 53.1 25.1 68.4 82.0 21.3 31.3 Current 0.18** 0.23* 0.25* 0.55* 0.46* 1.00 being examined had been entered at the first step. The control variables were then entered individually using a forward selection approach (Greenland and Rothman 1998) with order of entry determined by the strength of the correlation of the control variable to the end point in question. Control variables were retained when their entry in the model altered the standardized regression coefficient for the contaminant variable by ≥ 10%. To explore possible interaction effects, all regression analy- ses were rerun to add the interaction terms: first for cord Pb by cord Hg, then for cord Pb by cord PCB, and then for cord Hg by cord PCB. Similar analyses with interaction terms for the three childhood contaminant measures were also run. Because such interaction terms often lack the power to detect interactions, the regression analyses for cord Pb were also rerun separately for high and low cord Hg expo- sure (split at the median) and then for cord PCB split at the median; the cord Hg regres- sions were rerun split by cord Pb and then by cord PCB; and the cord PCB regressions were rerun and split by cord Pb and then by cord Hg. Similar stratified regressions were also run for the child contaminant measures. Results Descriptive statistics. A total of 196 participants were included in the final analyses. Reasons for exclusion were technical problems during recording (n = 4), too much noise in the EEG signal to produce a reliable ERP waveform (n = 3), and random responding on the task (n = 3). Compared with the retained partici- pants, these 10 excluded participants had higher cord PCB-153 concentrations [t(199) = 2.57, p = 0.01] and tended to have higher current PCB-153 [t(200) = 1.83, p = 0.07] and Pb [t(201) = 1.84, p = 0.07] concentrations but did not differ on any of the other exposure or con- trol variables (all p-values > 0.10). When the data from the children with too much noise in the EEG signal were included in the behavioral analyses and those with random responding were included, the results remained essentially unchanged. An additional three children were excluded from analyses involving stimulus- locked components because of an insufficient number of trials in their ERP averaged wave- form, and 30 were excluded from analyses involving response-locked components for the same reason. Those 33 participants excluded from either ERP analysis had higher PCB-153 [t(189) = 3.16; p = 0.002] and Hg [t(191) = 2.10; p = 0.04] concentrations in cord blood and higher blood Hg concentrations at testing [t(191) = –2.42; p = 0.02]. Descriptive statistics of the final study sample are summarized in Table 1. Pb levels at time of testing are similar to those reported in other epidemiological studies of low-level postnatal Pb exposure at school age (e.g., Statistical analyses. Normality of distri- bution was inspected visually for each vari- able and checked for skewness (normality range, –3.0 to 3.0). Log transformations were performed on cord and current blood Pb, PCB-153, and Hg concentrations, breast- feeding duration, and maternal tobacco con- sumption during pregnancy because these variables followed log-normal distributions. Extreme values (> 3 SDs from the mean) for normally distributed independent and con- founding variables were recoded to one point greater than the highest observed nonoutlying value (Winer 1971). The relation of each behavioral and ERP outcome to Pb, PCB-153, and Hg concentra- tions in cord and current blood samples was examined in a series of hierarchical multiple regression analyses. Each of the control vari- ables related at p < 0.20 to the outcome meas- ure in question was entered hierarchically into a regression analysis, in which the contaminant Mean ± SD 11.3 ± 0.6 Median 11.4 Range 9.8–12.9 23.9 ± 5.8 8.3 ± 2.6 28.9 ± 12.4 35.1 ± 10.1 16.0 ± 17.5 4.8 ± 3.4 2.6 ± 2.2 117.7 ± 90.9 72.0 ± 70.7 21.2 ± 17.6 4.6 ± 5.1 3.7 ± 1.3 2.4 ± 1.0 479.2 ± 75.8 419.5 ± 87.2 90.1 ± 9.7 64.0 ± 14.9 Cord 0.28* 0.04 1.00 22.0 8.5 28.5 37.0 11.5 3.7 2.0 93.3 45.7 16.6 2.8 3.5 2.2 472.0 403.0 93.2 66.7 Current 0.29* 0.25* 0.45* 1.00 15–42 0–16 8.0–66.0 4–56 0.1–108.0 0.8–20.9 0.4–12.8 9.7–653.6 3.5–431.4 1.0–99.3 0.1–34.1 1.1–7.7 0.1–5.5 322.0–720.0 271.0–718.0 52–100 22–94 Cord 0.32* 0.12 0.42* 0.41* 1.00 Cord 1.00 Pb Current 0.17** 1.00 Table 2. Intercorrelations among contaminants (Pearson correlation coefficients, r ). PCB-153 Hg Chemical PCB-153 Pb Hg 610 *p < 0.01. **p < 0.05. Sample Cord Current Cord Current Cord Current ovlume 120 | number 4 | April 2012 • Environmental Health Perspectives