On the relationship of P3a and the Novelty-P3
Introduction
In 1975, two papers were published that reported the existence of positive ERP components that were either antecedent or temporally coincident with the oddball-elicited P300 (Courchesne et al., 1975, Squires et al., 1975). In both papers, the positivity had a more anterior scalp distribution than the parieto–central distribution that characterized the traditional P300. The component described by Squires et al. was referred to as the P3a and was observed in response to infrequent stimuli during an oddball task or to the same stimuli when unattended; Courchesne et al. described a component they labeled the Novels-P3 that they observed in response to rare, unexpected stimuli inserted unexpectedly as a third stimulus type during an otherwise traditional oddball task. They initially argued that it was the relative uncodability of their novel stimuli that prompted the more anterior P3 response, but it has been subsequently demonstrated by Polich and colleagues (Comerchero et al., 2000) that codability is not a necessary stimulus characteristic.
Research on the Novels-P3 (which we will refer to as the Novelty-P3) has been vigorous and the basic phenomenon has been demostrated in many laboratories (e.g. Knight, 1984, Mecklinger and Ullsperger, 1995, Katayama and Polich, 1998, Spencer et al., 1999, Cycowicz and Friedman, 1999) using visual (Comerchero and Polich, 2000), acoustic (Courchesne et al., 1984), and tactile stimuli (Knight, 1996) with little apparent difference across modalities. Knight and Scabini (1998) describe the Novelty-P3 as an ERP measure of ‘neural activity in a distributed multimodal corticolimbic-orienting system that processes novel events’.
The P3a is a more evanescent component (Squires et al., 1977 but see also Snyder and Hillyard, 1976) and research targeting the P3a, qua P3a, has waned. There are no studies in the literature that directly compare the P3a and the Novelty-P3 and it is increasingly common for the two labels to be used interchangeably. In some recent papers, for example (Knight, 1996, Clark et al., 2000, Comerchero and Polich, 2000), P3a explicitly refers to announced deviants in three-stimulus attention tasks while in other papers (Harmony et al., 2000, Kaipio et al., 2000, Mathalon et al., 2000), P3a is used in its more traditional sense (i.e. as in Squires et al., 1975).
The existence of these more frontally oriented positive components in addition to the more commonly observed parietal (P3b) component has important theoretical implications relevant to attention and orienting and distinctions between the two frontal components may be important as well. To date, however, evidence that these two frontal positivities are distinct and independent has been largely anecdotal or based on preliminary speculations of Courchesne et al. (1975). Since much of this evidence favoring a distinction has not withstood empirical scrutiny (see Section 4) and because of the confusion in terminology, we undertook the present analysis to revisit the two components and to address the nature of their relationship. As part of two independent projects, we elicited the P3a and the Novelty-P3 with prototypical procedures, conducted factor analyses of the data sets containing the two components, then employed a variety of methods recommended by Roemer et al., 1990 for determining whether the principal components and their associated factor scores based on the two sets of ERP waveforms span the same or different factor space. To remain consistent with the initial description and labels of the two components, we refer to P3a as the anterior positivity that is elicited by rare stimuli in traditional two-stimulus oddball tasks and the Novelty-P3 as the anterior positivity elicited by the unexpected novel stimuli delivered to subjects during three-stimulus oddball tasks.
Section snippets
Subjects
Nine University of Delaware undergraduate students (six females) served as subjects in the replication of Squires et al. (1975) and were paid $5.00 per h for their participation. Four additional volunteers were excluded — one had a recent loss of consciousness, one for a hearing disability that made pitch discrimination too difficult, and two for apparatus problems. Twenty-four undergraduate subjects participated in the replication of Courchesne et al. (1975) and these subjects received either
Results
The ERP waveforms from the two data sets are presented in Fig. 1 (left). The ERPs recorded in the Squires et al. paradigm are consistent with the description provided in the original article of ERPs elicited by rare stimuli. The P3a can be seen as a small deflection on the leading edge of the much larger, parieto–central P3b. ERPs recorded in response to the Courchesne et al. stimuli are more centrally focused and the late positive components are less distinct, one from the other. Thus,
Discussion
The present data analysis was undertaken to address the question of independence of two positive ERP components that precede and overlap the ubiquitous P3b component during two and three stimulus oddball tasks. Our results suggest that these components are very similar, potentially the same. To recapitulate the evidence, despite the apparent morphological difference in the ERP waveforms (Fig. 1, left), the factors identified as the P3a and the Novelty-P3 appear identical to the eye (Fig. 1,
Acknowledgements
The research was supported by a grant from the National Institute of Mental Health (MH42465) to Frances K. Graham and Robert F. Simons and contains aspects of both the Ph.D. dissertation of Mark A. Miles and the MA thesis of Xun Chen. Portions of this paper were presented at the 39th annual meeting of the Society for Psychophysiological Research, Granada, Spain, October 1999.
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