Pb, PCB, and MeHg effects on an ERP go/no-go task The ERN is maximal over frontocentral elec- trodes approximately 80 msec after an incor- rect response and is thought to reflect the dynamics of response selection and conflict (Hughes and Yeung 2011). The Pe peaks 200–400 msec after an incorrect response and is more centrally distributed; it is thought to reflect the conscious recognition, or motiva- tional significance, of an error (Hughes and Yeung 2011; Overbeek et al. 2005). A few reports mention a similar component elic- ited by correct responses: the “correct response positivity” (Pc) (Burgio-Murphy et al. 2007; Laurens et al. 2010). We hypothesized that postnatal Pb and prenatal PCB exposures would both be asso- ciated with increased impulsivity as reflected by behavioral performance in the go/no-go task and that these effects would be accompa- nied by alterations in five ERP components: N2, P3, ERN, Pe, and Pc. Because this is, to our knowledge, the first study to use ERPs to assess the effects of Pb and PCBs on response inhibition, no hypothesis was made concern- ing which components would be affected by each contaminant. Because response inhibition was not consistently associated with MeHg exposure in previous studies with children (e.g., Debes et al. 2006; Stewart et al. 2005; cf. Stewart et al. 2006), no hypothesis was put for- ward concerning MeHg effects in this study. Materials and Methods Participants. The study participants were 212 school-age Inuit children from Nunavik. The Nunavik region is located north of the 55th parallel, about 1,500 km from Montreal. These children were originally recruited in the Cord Blood Monitoring Program (CBMP; 1993–1998), which was designed to docu- ment the levels of environmental contami- nants and nutrients in newborns in Arctic Québec (Dallaire et al. 2001), except for one child who was recruited for the Environmental Contaminants and Child Development Study (1996–2000; Jacobson et al. 2008; Muckle et al. 2001), which was initiated during the latter part of the CBMP. Mothers were con- tacted by telephone, provided with informa- tion about the study protocol, and invited to participate with their children. Assessments were conducted between September 2005 and April 2007 in the three largest Nunavik villages. Participants who resided in other communities were transported by plane to one of the larger vil- lages for testing. A maternal interview was conducted to document information on demographic background, smoking, alcohol and drug use during pregnancy, and other maternal characteristics. The following inclu- sion criteria were used: age between 9.0 and 13.0 years, birth weight ≥ 2,500 g, gestation duration ≥ 35 weeks, no known neurological or clinically significant developmental disor- der, and no medication for attention prob- lems. Of the 212 participating children, 2 with a history of epilepsy, 2 with a history of head trauma associated with loss of con- sciousness and/or requiring hospitalization, 1 with multiple sclerosis, and 1 with a history of meningitis were excluded after data collec- tion. Written informed consent was obtained from a parent of each participant; oral assent was obtained from each child. The research was approved by the Laval University and Wayne State University ethics committees and was performed in accordance with ethical standards of the 1983 Helsinki Declaration. Go/no-go protocol. Each participant was seated 57 cm from a 43-cm flat-panel monitor on which letters were displayed centrally within a 7 × 7-cm space. The child held a button box in his or her hand and was instructed to press the button as quickly and accurately as possible with the index finger for all individually pre- sented letters (the “go” trials) except the target “X” (the “no-go” trials). The stimuli were pre- sented for 500 msec, with random interstimu- lus intervals ranging from 1,200 to 1,400 msec. The first block consisted of 40 go trials and served to prime go responses within the second block of trials. This second block consisted of 126 go trials (70%) randomly intermixed with 54 no-go trials (30%). Correct and incorrect responses were tabulated. Mean reaction time (RT; time between stimulus onset and button press) and response accuracy (percent correct) for go and no-go trials during the second block of trials were computed. Data from the initial block of 40 go trials were not analyzed (for a more detailed description of this protocol, see Burden et al. 2011). Electroencephalogram recording and analyses. The electroencephalogram (EEG) was recorded with 30 Ag-AgCl electrodes placed according to the international 10–20 system (Jasper 1958) referenced online to the vertex (Cz) electrode, with forehead ground. The electro-oculogram (EOG) was recorded from bipolar miniature electrodes placed vertically above and below the right eye. Impedance was kept < 10 kΩ. EOG and EEG gain were amplified with gains of 5,000 and 50,000, respectively. The bandpass filter was 0.1–30 Hz, and a 60-Hz notch filter was engaged. The digitization rate was 200 Hz. ERPs were derived and analyzed using Brain Vision Analyzer software (version 2.0; Brain Products, Munich, Germany). EEG channels were re-referenced offline to linked earlobes. EOG correction (Gratton et al. 1983), artifact rejection (± 100 Vμ) , and base- line correction (100 msec) were applied. All responses occurring 200–1,600 msec after stimulus onset were considered valid. ERPs were averaged for correct go, correct no-go, and incorrect no-go responses separately. The Environmental Health Perspectives • volume 120 | number 4 | April 2012 “stimulus-locked” components, which are measured in relation to when the stimulus first appeared on the screen, were segmented 100 msec before and 1,000 msec after stimu- lus onset; the “response-locked” components, which are measured in relation to when the child pressed the button, were segmented 300 msec before and 500 msec after but- ton press. Peak amplitude (microvolts) and latency to peak (milliseconds) of the stimulus- locked N2 (250–500 msec) component were identified using automatic detection. Mean amplitude values were computed for the stim- ulus-locked P3 (400–700) and the response- locked ERN (incorrect no-go, 0–125 msec), Pe (incorrect no-go, 100–500 msec), and Pc (correct go, 100–300 msec) components. The electrode site at which each ERP com- ponent reached its maximal amplitude was used in analyses. Participants were retained in the analyses if their behavioral performance exceeded chance level on the task and if they had a sufficient number of acceptable trials in their ERP average (≥ 12 correct no-go trials for the stimulus-locked ERPs and ≥ 8 incorrect no-go trials for the response-locked ERPs). Biological samples. Umbilical cord and child blood samples were analyzed for con- centrations of Pb, PCBs, mercury (Hg), and polyunsaturated fatty acids (for a detailed description of the analytical procedures, see Boucher et al. 2011). Cord blood Pb levels were determined by graphite furnace atomic absorption with Zeeman background correc- tion. Selected PCB congeners were measured in purified cord plasma extracts using high- resolution gas chromatography with electron capture detection, and in purified child plasma extracts by high-resolution gas chromatog- raphy/mass spectrometry. PCB congener 153, expressed on a lipid basis, was used as an indicator of total PCB exposure because it is highly correlated with other PCB con- geners and is considered an adequate marker of exposure to environmental PCB mixtures in the Arctic (Ayotte et al. 2003). Total Hg concentrations were determined in cord blood using cold-vapor atomic absorption spectrom- etry. Total Pb and Hg concentrations in child blood samples were determined by inductively coupled plasma-mass spectrometry. The limits of detection (LODs) for cord sample analyses were 0.2 gμ/dL for Pb and Hg, and 0.02 gμ/L for all PCB congeners in plasma. LODs for child blood sample analyses were 0.002 gμ/dL for Pb, 0.1 gμ/L for Hg, and < 0.05 gμ/L for most PCB congeners. A value equal to half the LOD was entered in the database when- ever a substance was not detected. Omega-3 fatty acid composition of plasma phospho- lipids was analyzed using capillary gas-liquid chromatography with flame ionization detec- tion. Concentrations of docosahexaenoic acid (DHA) were expressed as percentages of the 609