A number of investigators have employed ERPs over the past several decades to examine early cognitive and linguistic development (e.g., Alho, Sainio, Sajaniemi, Reinikainen, & Naatanen, 1990; Molfese & Molfese, 1985). To date, however, the large majority of ERP research with infants and young children has investigated recognition memory (see Nelson, 1994, 1996, and Nelson et al., 2006, for a comprehensive review of this area).
Such cognitive research employing ERP methodology with infants examined neurofunctional processes associated with discrimination and recognition memory, primarily utilizing either a standard version of the adult "oddball" paradigm or a combination of infant habituation and the oddball paradigm (Nelson, 1994), with the first such studies published in the early 1980s (Courchesne, Ganz, & Norcia, 1981; Hofmann & Sala-patek, 1981; Hofmann, Salapatek, & Kuskowski, 1981). This experimental procedure involves the sequential presentation of stimuli (usually visual or auditory), some of which occur more frequently than others, with a typical ratio of frequent to infrequent of 80% versus 20%. While an adult subject would be instructed to attend and/or respond to one stimulus or another, for infants this paradigm involves "passive" viewing of (or listening to) the stimuli with no overt response required. In addition to work employing a direct adaptation of the adult oddball paradigm, other investigators have modified the oddball procedure to make it comparable to the infant habituation paradigm, thus including a preliminary familiarization period consisting of the presentation of a single stimulus over repeated trials followed by a sequence of trials in which the familiar stimulus is then presented in conjunction with an unfamiliar, or novel, stimulus (e.g., Bornstein, 1985; Nelson & Collins, 1991; Nelson & deRegnier, 1992; Nelson & Salapatek, 1986).
The rationale for this experimental approach to the study of recognition memory with ERPs dates to early work by Fantz (1961, 1963) that utilized a paired comparison procedure in which it was found that infants looked longer at novel stimuli than at familiar stimuli. This experimental paradigm has been employed in hundreds of behavioral studies examining recognition memory in infants (for reviews, see Aslin, 1987, and Kellman & Arterberry, 2006). The adaptation of this paradigm and modifications of it for use in ERP studies has provided a wealth of data concerning the ontogeny of recognition memory, as well as theoretical models of the neural processes underlying recognition memory (Nelson, 1994). Behavioral measures of the time spent looking in the visual paired comparison task index what decision an infant has made concerning the compared stimuli, but ERPs, on the other hand, have been able to provide a great deal of additional information from data received from electrical activity of the brain, such as when the decision was reached, the neural processes underlying the decision, and what areas of the brain are employed in reaching a decision (Nelson, 1994).
As adapted for the investigation of recognition memory in infants utilizing ERPs, the assumption underlying the oddball paradigm is that, over enough trials, the high-probability events will become relatively more familiar to the infant than the low-probability trials, and thus the two types of events (familiar and unfamiliar) will elicit different cognitive and brain responses. It is presumed that the more frequently seen stimulus will be encoded into memory, while the less frequently seen stimulus will be "less" encoded (i.e., the memory will not be firmly established). Consequently, when the infrequent stimulus is viewed, a neurocognitive operation typically referred to as "updating" must take place in order to more firmly encode this stimulus into memory. Thus, recognition memory is inferred based on differential cortical responses to each of the memory-stimulus types.
In infants, this paradigm typically elicits a middle-latency negative ERP component over central scalp electrodes referred to as Negative Central (Nc; see Fig. 3.1), which is assumed to reflect a general orienting response associated with attention. The Nc has been found to be greater in amplitude following novel stimulus presentations. Late slow waves proposed to reflect recognition memory include the negative slow wave (NSW, associated with novelty detection) and the positive slow wave (PSW, associated with an updating of recognition memory) (see next section of this chapter for more detailed summaries of these waveforms).
The particular modality (visual, auditory) and form (voice, tone, pictures of objects, faces) of the stimulus in this paradigm have many variations, as do the ratio of familiar to unfamiliar and degree of habituation. ERP studies of recognition memory continue to incorporate the oddball paradigm, in part due to its widespread prevalence, reproducibility, simplicity, and applicability across sensory modalities. One adaptation in particular has involved a line of research utilizing faces as visual stimuli in several variations of the oddball paradigm. This approach has particular relevance for ERP studies of infants and children who have experienced maltreatment, in that the use of faces as stimuli—in particular those with expressions of emotion—can potentially tap into not only recognition memory per se but also memory that involves perception and encoding of emotion.
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