Methodological concerns about an event - related potential ( ERP ) study of emotional responsivity : a commentary on Carretié et al . ( 1998 )

In a recent event-related potential (ERP) study published in this journal, Carretié and colleagues concluded that a negative component, labeled N300, " is sensitive to the affective charge of visual stimuli " (Carretié et al., 1998, p. 381). While we concur that a " parietal emotional ERP component " reflects affective significance, we do not feel that this conclusion is supported by the data presented in this study. In particular, there appear to be logical inconsistencies in the data and experimental design as they were reported. Carretié and colleagues presented and analyzed ERP data over a post-stimulus interval of 325 ms. This interval was clearly insufficient to measure time-locked brain activity to complex visual stimuli. The grand average waveforms presented in Figure 2 allow the identification of ERP components P1 (approximate latency 150 ms) and N1 (180 ms). In addition, depending on the particular stimulus category used in this study, P2 and N2 components can be identified as they vary in latency (250-300 ms) and amplitude. Although the rising phase of a prominent P3 component is evident following N2, it does not peak during the 325 ms post-stimulus interval. As pointed out by the authors themselves, ERP components P3 and positive slow wave have been reported to show greater amplitudes to emotional than neutral stimuli The use of such an extremely short recording epoch does not allow the characterization of a late positive complex in this study. The authors indicate that they used a covariance-based principal components analysis (PCA), in which the first four factors accounted for more than 90% of the total ERP variance. However, the Varimax rotated factor loadings presented in Figure 3 are problematical. The most glaring problem is that Factor 1 accounts for more than 75% of the overall ERP variance, but has a maximum loading (> 0.6) between 0 and 170 ms. The ERP averages shown in Figure 2 indicate that this component primarily characterizes the low amplitude (post-stimulus) baseline preceding P1. Since ERP variance represents the mean squared signal amplitude, extraction of this factor using a covariance matrix can only be possible if differences in the variance of the baseline is greater than the variance of the actual ERP. This possibility is unlikely, since the baselines shown in Figure 2 appear to be reasonable. A more likely possibility is that the factors were either extracted in a different manner than what is described in …