used to assess the effects of PCB exposure on child development (Daniels et al. 2003; Gray et al. 2005). More than 1,000 children were selected among those born singleton and for whom PCB levels in maternal blood were doc­ umented. Cognitive outcomes have been stud­ ied in relation to PCB exposure at 8 months and at 4 and 7 years of age. Analyses at 7 years of age included nearly 900 participants. Hokkaido. The Hokkaido Study on Environment and Children’s Health evalu­ ated the effects of prenatal PCB and dioxins exposure on child development (Nakajima et al. 2006). Pregnant women were recruited between July 2002 and July 2004 in a Sapporo hospital in Japan. Maternal blood samples were analyzed for PCB exposure arising mainly from fish consumption. To date, 135 infants have been tested at 6 months of age. Results General cognitive abilities. Infant develop- ment. Table 2 summarizes the results for the six studies in which the Bayley Scales of Infant Development (Bayley 1969, 1993) was used. This examination provides two main scores: the Mental Development Index (MDI) and the Psychomotor Development Index (PDI). The items of the MDI com­ prise memory, habituation, problem solv­ ing, first number concepts, generalization, classification, vocalizations, language, and social skills (Bayley 1993). Only the German study reported a significant decrease in MDI score as a function of maternal breast milk levels of PCBs at 2 weeks postpartum, which may be considered an index of prenatal expo­ sure, because it reflects maternal body bur­ dens during pregnancy. The German and CPP cohorts were the most highly exposed cohorts. These results suggest that either the effects on MDI were observable only beyond a certain threshold of exposure or that MDI is not sensitive enough to detect subtle effects of PCBs. The PDI assesses control of the gross and fine muscle groups. The PDI is considered here because mental and motor functioning during infancy are likely to be interdependent (correlations between PDI and MDI range from 0.24 to 0.72 between 3 and 30 months of age) (Bayley 1993). Two studies of six—those from North Carolina and Germany—suggested persistent effects of PCB exposure on PDI score. Child IQ. Results for IQ tests during childhood are presented in Table 3. Out of the nine birth cohort studies, six used IQt­ype tests, and four of those reported a lower IQ score as a function of in utero exposure to PCBs. Thus, most studies suggest that pre­ natal exposure to PCBs affects the develop­ ment of general cognitive competence. In the Netherlands, PCB exposure is related to lower score of general cognitive abilities in children Neuropsychologic analysis of prenatal PCB effects born to younger mothers and parents with lower verbal IQ only, suggesting that parental characteristics and home environment might moderate the effects of the pollutant on cog­ nitive development (Vreugdenhil et al. 2002). Furthermore, as depicted in Table 3, Verbal but not Performance IQ was affected in both the Michigan and the Oswego cohorts when assessed during late childhood. These results suggest long­lasting effects on verbal func ­ tions, whereas visual–spatial functions may be less sensitive to prenatal PCB exposure. This assumption must be explored in more detail using assessments of specific verbal and visual–spatial skills. Verbal versus visual–spatial skills. Table 4 summarizes the results obtained with tests of specific verbal functions. The grouping of results suggests that verbal functions par­ ticularly could be affected by PCB exposure. Vocabulary and verbal comprehension, both highly related to language skills, have been consistently associated with prenatal PCB exposure in all cohorts in which they were part of the study design. However, in the Faroe Islands, the association between expo­ sure and vocabulary was no longer significant when MeHg was included in the statistical model. In the Michigan study, reading com­ prehension was affected by PCB exposure among 11­year­old children. Furthermore, higher­level processing of verbal abilities— verbal abstraction—has also been negatively related to prenatal PCBs at 11 years of age. Unfortunately, this finding was not replicated with the Faroese cohort. Visual–spatial abilities include functions that are relatively independent from lan­ guage and long­term memory. Associations between prenatal PCB exposure and specific visual–spatial functions are reported in four cohorts (Table 4). Measures of visuo­motor integration, mental rotation, and visual dis­ crimination failed to show association with PCB exposure in the Michigan, Netherlands, and Faroese studies. On the other hand, a PCB effect on visuo­motor integration was reported in Oswego. However, this effect was observed at 3 years of age and was not repli­ cated when children were tested again a year and half later. Altogether, these results sup­ port the assumption that verbal functions, notably vocabulary and verbal comprehen­ sion, are more likely to be affected by prenatal PCB exposure than visual–spatial functions. Memory. Associations between PCB expo­ sure and various memory functions are shown in Table 5. In the Oswego and Michigan studies, effects of prenatal PCB exposure on visual recognition memory were observed during the first year of life with the Fagan Test of Infant Intelligence, but no effect was Table 2. Study results for mental and psychomotor development assessed with the Bayley Scales of Infant Development. Cohorta Hokkaido North Carolina Netherlands Michigan Germany CPP Age (months) 6 6, 12, 18, 24 3, 7, 18 5 7, 18, 30 8 MDI – –, –, –, – –, –, – – ↓b, –c ,↓b – Effect PDI – ↓, ↓, –, ↓ ↓, –, – – –, ↓b,↓b – Reference Nakajima et al. 2006 Gladen et al. 1988; Rogan and Gladen 1991 Koopman-Esseboom et al. 1996 Jacobson et al. 1986 Winneke et al. 1998; Walkowiak et al. 2001 Daniels et al. 2003 Abbreviations: ↓, statistically significant decreased performance on the measure; –, absence of significant effect. aCohorts are presented in ascending order of median prenatal PCB exposure. bThese effects are in relation to the sum of PCBs 138, 153, and 180 in breast milk (2 weeks postpartum) rather then in cord plasma. cA negative association between breast milk PCB levels and MDI score approaches statistical significance (p = 0.06) Table 3. Study results for IQ-type tests. Age (years) Test Nonverbal/ Verbal IQ Performance IQ Cohorta Oswego North Carolina Netherlands Michigan Germany CPP 3 4.5 9 3–5 3.5 6.5 4 11 3.5 4 7 MSCA MSCA WISC-III MSCA K-ABC MSCA MSCA WISC-R K-ABC Stanford-Binet WISC IQtotal ↓ – ↓ – ↓b ↓c – ↓ ↓ – – – – ↓ – ↓ ↓ – ↓ – – – – – – Reference Stewart et al. 2003b Stewart et al. 2003b Stewart et al. 2008 Gladen and Rogan 1991 Patandin et al. 1999b Vreugdenhil et al. 2002 Jacobson et al. 1990 Jacobson and Jacobson 1996 Walkowiak et al. 2001 Gray et al. 2005 Gray et al. 2005 Abbreviations: ↓, statistically significant decreased performance on the measure; –, absence of significant effect. Tests: K-ABC, Kaufman Assessment Battery for Children (Overall Cognitive Scale; Melchers and Preus 1994; Neutel et al. 1996); MSCA, McCarthy Scales of Children’s Abilities (McCarthy 1972; Van Der Meulen and Smorkovsky 1985); Stanford-Binet (Broman et al. 1975); WISC, Wechsler Intelligence Scales for Children (Wechsler 1949, 1974, 1991). aCohorts are presented in ascending order of median prenatal PCB exposure. bDecrease is significant among non-breast-fed children only. cDecrease is significant in children born of younger mothers and parents with lower verbal IQ scores only. Environmental Health Perspectives • volume 117 | number 1 | January 2009 9