This waveform was first reported in early work by Courchesne and colleagues examining changes in ERP waveforms from childhood to adulthood (Courchesne, 1977, 1978; Courchesne et al., 1981). It is a long-latency, positive slow wave that begins approximately 1000 ms after stimulus onset and continues for up to another 1000 ms (see Figure 3.1 for an example of the morphology of this component). The designation "slow wave" indicates that this waveform does not exhibit a sharp peak but rather encompasses a broad temporal window, slowly increasing in amplitude and then gradually decreasing (see Fig. 3.1). In Courchesne's work, the PSW occurred in 4- to 7-month-old infants in response to infrequently presented stimuli (see Courchesne et al., 1981). The PSW has been primarily examined in infants older than 6 months of age, but it has recently been observed in 4-month-old infants (Webb, Long, & Nelson, 2005). It is generally evoked in response to a single, repeated novel stimulus embedded in a sequence of familiar stimuli (oddball paradigm). This waveform appears somewhat ubiquitous and is nearly always observed in ERP studies of infants and young children, but it does not appear in ERPs elicited by recognition memory tasks in adults (e.g., Nelson et al., 1998). However, there are no specific studies indicating at what age the PSW ceases to be apparent.
Based on an extensive line of ERP research on the development of recognition memory in infants and young children, Nelson and colleagues have concluded that the PSW is elicited by stimuli that have only been partially encoded, and thus require updating in memory (e.g., deHaan & Nelson, 1997; Nelson, 1994; Nelson & Collins, 1991; Nelson & deReg-nier, 1992; Nelson & Monk, 2001). Specifically, Nelson (1994) has suggested that the PSW reflects a process whereby infants have begun to form what he terms a somewhat unstable or tenuous "template" for a relatively novel stimulus that has been seen infrequently (but one that is not completely novel, as no template could exist for such a stimulus). According to this conceptualization, this template must be periodically updated in working memory, with the PSW representing the electrical byproduct of the neural process underlying this updating. Thus the PSW occurs in response to stimuli that have only been partially encoded by the infant, with a stimulus that has been fully encoded into memory generally yielding an ERP pattern that shows a return to baseline following the Nc.
Some investigators have theorized that the PSW represents a process that has generally been interpreted as reflecting the updating of working memory, or context updating, analogous to that described by Donchin regarding the adult P300 wave (e.g., Donchin, 1981; Donchin & Coles, 1988). However, it is critical to remember that the PSW is almost certainly not a P300 per se, due to a variety of anatomical and physiological reasons (Nelson, 1994). In sum, the PSW most certainly indexes some component of memory processes, with much of the evidence pointing toward the idea that it represents updating of memory for those stimuli that have been only partially encoded. Similar to the P300, study of the PSW in children who have experienced maltreatment provides a potential opportunity to examine the developmental course of an ERP waveform strongly associated with memory processes. As will be detailed in a later section of this chapter, Cicchetti and Curtis (2005) have demonstrated hemispheric differences in the PSW in a sample of maltreated and non-maltreated 30-month-old children.
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