Quantifying Lip-read-induced Suppression and Facilitation of the Auditory N1 and P2 Reveals Peak Enhancements and Delays

Lip-read speech suppresses and speeds up the auditory N1 and P2 peaks, but these effects are not always observed or reported. Here, the robustness of lip-read-induced N1/P2 suppression and facilitation in phonetically congruent audiovisual speech was assessed by analyzing peak values that were taken from published plots and individual data. To determine whether adhering to the additive model of AV integration (i.e., A+V ≠ AV, or AV-V ≠ A) is critical for correct characterization of lip-read-induced effects on the N1 and P2, auditory data was compared to AV and to AV-V. On average, the N1 and P2 were consistently suppressed and sped up by lip-read information, with no indication that AV integration effects were significantly modulated by whether or not V was subtracted from AV. To assess the possibility that variability in observed N1/P2 amplitudes and latencies may explain why N1/P2 suppression and facilitation are not always found, additional correlations between peak values and size of the AV integration effects were computed. These analyses showed that N1/P2 peak values correlated with the size of AV integration effects. However, it also became apparent that a portion of the AV integration effects was characterized by lip-read-induced peak enhancements and delays rather than suppressions and facilitations, which, for the individual data, seemed related to particularly small/early A-only peaks and large/late AV(-V) peaks.