functions is traditionally made in neuropsy­ chology, notably to evaluate the differential integrity of each cerebral hemisphere (Samson 2005). However, some explanations other than specific effects on verbal hemisphere can be offered. For instance, verbal measures such as vocabulary and reading tests are more likely to be influenced by socioeconomic factors than are visual–spatial measures. If a given socioeconomic factor is related to sources of PCB exposure (e.g., fish products) in a given population, then specific associations between PCB exposure and lower scores on verbal tests could also reflect differences in socioeco­ nomic conditions. Researchers should thus seek the verbal tasks that are less likely to be influenced by such confounding factors, such as measures of phonologic processes. Although the present cognitive analysis highlights the effects of prenatal exposure to PCBs, inconsistencies between studies still persist. Many factors may explain these inconsistencies. First, it is possible that PCB exposure was not high enough to produce statistically significant effects in some cohorts. Strangely, though, differences in exposure severity do not appear to account for observed inconsistencies. Thus, studies reporting the larger number of significant effects on cog­ nitive functions were not the most highly exposed cohorts. This is particularly true in the case of Oswego, which found consistent impairments on cognitive functions although it was among the least PCB­ exposed cohorts. It is also possible that coe­xposure to other contaminants is accountable for inconsistencies Table 8. Study results on auditory and visual functioning. across studies. Strong association between PCBs and another contaminant (e.g., MeHg) makes it less likely to observe independent relation­ ships between PCBs and cognitive outcomes after documenting and statistically controlling for the other contaminant. This might explain the results observed in the Faroese, where statis­ tical control for MeHg altered the associations between PCB exposure and cognitive outcomes (Grandjean et al. 2001). Exposure to another contaminant highly related to PCB levels might also be responsible for the observed cognitive effects if exposure to this contaminant is not documented and accounted for in statistical designs. However, if this were the case here, study results would diverge largely depending on the source of exposure in the population under study. In populations in which expo­ sure arises from fish consumption (Michigan and Oswego), coexisting contaminants (e.g., MeHg) differ from those found in popula­ tions for whom exposure arises from dairy products (Netherlands). The consistent results obtained from those three cohorts suggest that the impairments associated with PCB exposure are more likely to be attributable to PCBs per se. In addition, statistical con­ trol for coexisting contaminants did not alter the relationships between PCB exposure and cognitive outcomes in Oswego (Stewart et al. 2003b, 2005, 2006, 2008). Other explanations could be offered regarding inconsistent findings across cohort studies. Of particular importance is the pres­ ence of essential nutrients that may pro­ mote brain development in the sources of Assessment Hearing BAEPs Visual contrast sensitivity VEPs Cohort CPP Faroe Islands Faroe Islands Faroe Islands Nunavik Faroe Islands Effect – ↓a –b – –c – Reference Longnecker et al. 2004 Grandjean et al. 2001 Grandjean et al. 2001 Grandjean et al. 2001 Saint-Amour et al. 2006 Grandjean et al. 2001 ↓a – – – – – – – – – Measurement MSCA Motor Scale MSCA Motor Scale Huttenlocher gross motor tasks Catsys System hand tremor Catsys System postural sway Catsys System reaction time Rapid pointing movements Rapid alternative arm movements NES-2 Finger Tapping NES-2 Hand-eye Coordination Neuropsychologic analysis of prenatal PCB effects PCB exposure. This may be the case when exposure arises from fish products, which are also potential sources of omega­3 fatty acids (Costa 2007). If we assume that such nutrients have beneficial or protective effects on cognitive development (Koletzko et al. 2008), it is conceivable that the associations between prenatal PCB exposure and cogni­ tive outcomes are stronger than what was actually found in fish­ eating populations. In the Faroe Islands, attempts to control for the beneficial effects of fish consumption dur­ ing pregnancy increased the effects of MeHg on cognition (Budtz­Jorgensen et al. 2007), but no such control was applied for PCBs. The Nunavik study, for which levels of nutri­ ents are documented, will provide a unique opportunity in the near future to learn more about the relationships between nutrients and contaminants. Another explanation that can­ not be ruled out is that different PCB mix­ tures with different congeners across studies result in different outcomes. As emphasized by Schantz et al. (2003), future studies will have to address this issue. Age at assessment can also influence the outcomes of birth cohort studies. Although most cohort studies did not find significant effects of prenatal PCB exposure on the global mental development of infants, effects on global IQ tests were consistently observed dur­ ing childhood in most studies (for example, effects on IQ were found in the Oswego, Netherlands, Michigan, and German studies). An explanation for such findings might be that infant mental development measures have weaker psychometric properties than child­ hood IQ tests in terms of reliability and valid­ ity, especially for predictive validity of later cognitive functioning (Bayley 1993; Wechsler 1991). In addition, because the maturation of certain brain areas, notably the prefrontal cor­ tex, lasts until late adolescence (Segalowitz and Davies 2004), it is possible that some effects appear during development, while others dis­ appear. PCBs were not related to global IQ at 4 years of age in the Michigan cohort but were at age 11 years (Jacobson et al. 1990; Jacobson and Jacobson 1996). Similar results were Reference Vreugdenhil et al. 2002 Jacobson et al. 1990 Després et al. 2005 Després et al. 2005 Després et al. 2005 Després et al. 2005 Després et al. 2005 Després et al. 2005 Grandjean et al. 2001 Grandjean et al. 2001 13 Motor assessment Global motor Gross motor Fine neuromotor Cohort Netherlands Michigan Nunavik Nunavik Faroe Islands Abbreviations: ↓, statistically significant decreased performance on the measure; –, absence of significant effect; BAEPs, brainstem auditory evoked potentials; VEPs, visual evoked potentials. aSignificant increase of auditory threshold in left ear only for sounds of 250 and 12,000 Hz. bDelayed latency of wave V at 20 Hz was significant before controlling for MeHg. cP100 and N150 waves latencies were significantly delayed, and N75 to P100 and P100 to N150 amplitudes were significantly reduced as a function of child’s PCB-153 levels in blood at 5 years of age. Table 9. Effects of prenatal exposure to PCBs on motor function. Age (years) Effect Age (years) 8 7 7 7 5 7 6.5 4 5 5 5 5 5 5 7 7 Abbreviations: ↓, statistically significant decreased performance on the measure; –, absence of significant effect. Tests: Catsys system (Danish Product Development 2000); Huttenlocher gross motor tasks (Huttenlocher et al. 1990); NES-2, Neurobehavioral Evaluation System (Letz and Baker 1988). aSignificant in children with poorer environmental conditions only. Environmental Health Perspectives • volume 117 | number 1 | January 2009