On the reliability of augmenting/reducing - Peak amplitudes and principal component analysis of auditory evoked potentials

To investigate the impact of several methodological variations in the assessment of augmenting/reducing, auditory evoked potentials to 1000 Hz tones with varying stimulus intensity (59, 71, 79, 88, 92, 96 dB (SPL)) were recorded at 19 EEG sites in 24 participants during two separate recording sessions. The internal consistency analysis revealed only weak correlations for linear regression slopes based on high intensity levels when compared to slopes based on low intensity levels. For base-to-peak and peak-to-peak ERP component measurements, acceptable internal consistency and temporal stability were confirmed for the N1/P2-based slope, and partly for the P2 slope, whereas pi, NI, and P1/N1 slopes were not reliable. After submitting auditory evoked potentials to a covariance-based principal component analysis (PCA), followed by unsealed varimax rotation, temporal stability of slope measures for the corresponding factor scores substantially increased. The findings suggest that at least five to six intensity-levels are required over a relatively broad range to yield a reliable measure of auditory evoked augmenting/reducing. If measured reliably, P2 slopes may reflect stimulus intensity changes more precisely than N1/P2 slopes, and should therefore be evaluated in future studies of individual differences in augmenting/reducing.