its encoding context (e.g., Rome is the capital of Italy) (Lezak et al. 2004). Although this type of memory was assessed only in the Michigan study, decreased performance as a function of prenatal PCB exposure was observed. Finally, short­term memory has been assessed in four different PCB cohorts with different measures and at different ages. The Memory Scale index of the McCarthy Scales of Children’s Abilities (MSCA; McCarthy 1972) was administered in the North Carolina, the Netherlands, and the Michigan studies. This index integrates scores from four subtests, each assessing different processes related to short­ term and working memory. The Memory Scale index decreased as a function of pre­ natal PCB exposure in the Netherlands and Michigan. The association found in Michigan was attributed to two specific subtests involv­ ing the auditory modality, whereas results on specific subtests were not reported for North Carolina and the Netherlands studies. PCB exposure did not affect learning a list of words in the Netherlands and the Faroese studies. The performance on numeric span measures was altered by PCB exposure at 4 and 11 years of age in Michigan, whereas no such alteration was found for the forward condition in the Faroe Islands at 7 years of age. Regrettably, the Michigan study did not report forward and backward spans separately. Because these two conditions are thought to reflect different processes [the forward condition is thought to assess short­term information retention and attention allocation, whereas the backward condition additionally involves the executive component of working memory (Lezak et al. 2004)], it is not possible to currently identify what caused the decrease in performance on the numeric span measures. The analysis of the effects of prenatal exposure to PCBs on memory components reveals that consistent effects were found on visual recognition memory tasks with young children. Although effects on long­ term verbal memory are not evident, some Table 6. Results for attention. Assessed construct Selective attention Processing speed Sustained attention Cohorta Michigan Michigan Netherlands Faroe Islands Oswego Netherlands Michigan Faroe Islands Neuropsychologic analysis of prenatal PCB effects data suggest that short­term verbal memory is affected by this contaminant. However, the effects could also be the result of a decreased attention allocation or poorer executive func­ tioning, both processes being involved during the completion of tasks. Attention and executive functions. Attention and executive functions (EFs) are unique aspects of human cognition, because they modulate all other cognitive functions. For example, atten­ tion allocation is required to learn new infor­ mation, and sustained attention is necessary to perform optimally on a 2­ hr neuropsychologi­ cal battery testing. EFs are crucial for learning and retrieval strategies and are solicited to adapt to new testing situations. Because most cog­ nitive functions are subordinated to attention and EFs, impairment of these functions might indirectly affect other components of cognitive functioning as well. Table 6 summarizes the effects of pre­ natal PCB exposure on attentional functions. Several aspects of attention can be identi­ fied. First, selective or focused attention is the capacity to highlight the one or two impor­ tant stimuli or ideas being dealt with while suppressing awareness of competing distract­ ers (Lezak et al. 2004). In Michigan, perfor­ mance on the Stroop ColorW­ord test, which requires the capacity to focus attention on the color of a word while ignoring the written name of the color, was not associated with prenatal PCB exposure in 11­year­old chil ­ dren. Nevertheless, in these children, PCB exposure was related to the number of omis­ sion errors on a digit cancellation test, which requires visual selective attention as much as visual scanning and processing speed. Processing speed is a central aspect of attentional functioning. Decreased speed of information processing can have broad­ ranging effects on attentional activities (Ponsford and Kinsella 1992; Spikman et al. 1996). Processing speed was assessed using reaction times during the completion of a cog­ nitive task in the Michigan, Netherlands, and Faroe Islands cohorts. Slower reaction times as a function of prenatal PCB exposure were observed in most assessments. The effects of PCB exposure on processing speed were also tested with neurophysiologic measures. The Netherlands study used an auditory oddball paradigm to assess the P300 wave of the event­ related potentials—a positive deflection of the electroencephalograph voltage occurring about 300 milliseconds after the detection of target stimuli—at the 9y­ear assessment (Vreugdenhil et al. 2004b). Latency of this component is thought to reflect stimulus evaluation duration independently of responsea­ssociated processes (Kutas et al. 1977; McCarthy and Donchin 1981). Higher prenatal PCB exposure was related to delayed P300 latency, which is con­ sistent with behavioral data from other studies. The same result had previously been observed among YuC­heng children severely exposed to PCBs in utero who were assessed with a simi­ lar P300 protocol between 7 and 12 years of age (Chen and Hsu 1994). Those results sug­ gest that speed of information processing is an aspect of attention particularly susceptible to impairment by prenatal PCB exposure. Another aspect of attention is the capac­ ity to maintain an attentional activity over a prolonged period of time, a concept referred to as sustained attention (Lezak et al. 2004). It is measured by observing the evolution in performance over time on a given task. Performance on the Continuous Performance Test, a well­known task of sustained atten ­ tion, did not decline over time as a function of prenatal PCB exposure in the Oswego, Michigan, and Faroese cohorts. However, the fluctuation in reaction times of 9y­earo­ld highly exposed Dutch children was greater than among less­exposed children on the Simple Reaction Time Test, a similar task. The Dutch study is thus the only study sug­ gesting a PCB effect on sustained attention. EFs are recruited for controlled action in new or complex situations, especially when well­learned, routine action schemas Age (years) Effect 11 11 4 4, 11 11 9 9 7 4.5 8, 9.5 9 4, 11 7 ↓ – ↓ – ↓ ↓ ↓ –b – – ↓ – – Measurement Digit Cancellation omission errors Stroop Color-Word completion time KMFF reaction time CPT reaction time Mental Rotation reaction time SRTT reaction time P300 latency CPT reaction time CPT CPT SRTT reaction time variations CPT CPT Reference Jacobson and Jacobson 2003 Jacobson and Jacobson 2003 Jacobson et al. 1992 Jacobson and Jacobson 2003 Jacobson and Jacobson 2003 Vreugdenhil et al. 2004a Vreugdenhil et al. 2004b Grandjean et al. 2001 Stewart et al. 2003a Stewart et al. 2005 Vreugdenhil et al. 2004a Jacobson and Jacobson 2003 Grandjean et al. 2001 Abbreviations: ↓, statistically significant decreased performance on the measure; –, absence of significant effect. Tests: CPT, Continuous Performance Test (Letz and Baker 1988; Rosvold et al. 1956); SRTT, Simple Reaction Time Test (Letz 1994). aCohorts are presented in ascending order of mean prenatal PCB exposure. bDecreased performance as a function of prenatal PCB exposure was significant before documenting rela- tive effect of MeHg. Environmental Health Perspectives • volume 117 | number 1 | January 2009 11