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A number of studies have addressed the issue of whether the ERP repetition effect is a modulation of only one or more than one ERP components. Rugg (1990 see also Halgren & Smith, 1987; Polich & Donchin, 1988; Young & Rugg, 1992) recorded ERPs to high and low frequency words on both their first and second presentations. In the first phase of the

experiment, subjects were presented with high and low frequency words which were either repeated after 6 intervening items or were not repeated. In a second phase of the experiment, the non-repeated words from the first phase were re-presented, together with a further set of non-repeating high and low frequency words. The interval between the first and second phases of the experiment was some 15 minutes.

There were two notable aspects of the results of this study. Firstly, the repetition of low

frequency words resulted in a modulation of the ERP between approximately 300 - 500 ms and between approximately 500 - 800 ms post-stimulus onset. In contrast the repetition of high frequency items resulted in a modulation of the waveform in only the first of these intervals and this effect was reliably smaller than was the repetition effect for repeating low frequency words. Secondly, in the second phase of the experiment only the repetition of low frequency words resulted in a modulation of the waveform, and then only in the 500 - 800 ms region of the waveform. Rugg (1990) interpreted the dissociation by frequency of the repetition effects in the earlier and later epochs as evidence for their being two components to the ERP repetition effect. The first, he argued, is sensitive to repetition of both high and low frequency words, and the second sensitive only to the repetition of low frequency words.

Van Petten et al (1991) also investigated the component structure of the ERP repetition effect. EEG was recorded whilst subjects to read passages of text presented one word at a time. Within these passages a number of words repeated one or more times at a variety of lags. The segments of EEG were averaged according to whether a word was presented for the first time or was a

repeat and according to the lag between the presentations of a particular word. Van Petten et al (1991) found that repetition resulted in a positive-going shift in the ERP around 400 ms post stimulus onset which was greater for low than for high frequency words. There was also a negative-going shift in the ERP from approximately 600 ms post stimulus onset which also was greater for low than for high frequency words. Van Petten et al (1991) argued that this biphasic modulation in the ERP could not, unlike the results reported by Rugg (1990), be accounted for in terms of a single positive-going modulation of the waveform. Rather the most parsimonious interpretation of their results was that two underlying components had been differentially modulated by word repetition.

Further evidence for the involvement of two components to the ERP repetition effect was provided by Besson et al (1992). This study involved the presentation of single sentences, one word at a time, which ended with either a congruent or incongruent word. Besson et al (1992) found that around 400 ms, differences between ERPs generated by congruous and incongruous words had the same scalp distribution and magnitude as did the differences between the first and second presentations of incongruous words. This similarity of the effects of congruity and repetition around 400 ms contrasted with their effects in a subsequent region of the ERP. In this region, the repetition of congruous words resulted in a small negative-going shift in the

waveform, whilst the repetition of incongruous words resulted in a larger positive-going shift, similar to that observed by Rugg (1990). Furthermore, whilst the effects of the earlier repetition effect was greatest at centro-parietal sites, and had a tendency to be greater over the right hemisphere than over the left, the later repetition effect was equipotentially distributed over the head. Besson et al (1992) argued that this difference in the scalp distributions of the two modulations of the ERP meant that they could not be accounted for by the action of a single underlying generator.

There have been a number of indications that modulations of other components of the ERP may be sensitive to repetition. Rugg (1987) and Nagy and Rugg (1989) have reported that repetition results in a negative going shift in the ERP around 200 ms and which Rugg (1987) interpreted as reflecting automatic processing. However this effect is not always found in studies of word repetition (e.g. Bentin & McCarthy, 1994; Bentin & Peled, 1990; Rugg, et al., 1988). Nobre and McCarthy (1994) have suggested that semantic priming modulates two different

components in the region around 400 ms post-stimulus, one of which is enhanced by semantic priming whilst the other is attenuated. Further research will show whether a similar distinction can be made between components in this latency range for the repetition effect.

Despite these questions concerning the precise characteristics of the component structure of the repetition effect, Rugg (1990), Van Petten et al (1991) and Besson and Kutas (1993) have all argued that the ERP repetition effect has two main components. They suggest that the earlier component is the N400, and the later one a component generally identified as the Late Positive Component (LPC). However the different authors have assigned different functional roles to

the cognitive processes reflected in these components. I will first briefly discuss the various proposals that have been made to account for modulations of the LPC. Although of

considerable interest, for present purposes the functional significance of the LPC is not of primary importance. In contrast the experiments reported in this paper directly address

interpretations of the significance of modulations of the N400, and I will discuss at some length the different views of the process(es) thought to be reflected by the modulation of this

component.