N1 facilitation at short Inter-Stimulus-Interval (ISI) occurs under 400 ms and is dependent on ISI from previous sounds: Evidence using an unpredictable auditory stimulation sequence

The N1 auditory evoked potential amplitude depends heavily on the inter-stimulus interval (ISI). Typically, shorter ISIs result in reduced N1 amplitudes, suggesting a decreased neural response with high stimulus presentation rates. However, an exception known as N1 facilitation occurs with very brief ISIs (∼150-500 ms), where the N1 amplitude increases. This study aimed to further characterize N1 facilitation using an experimental paradigm with a continuous distribution of ISIs (0.25 to 8 s) to identify the specific ISI where N1 facilitation occurs. We also examined the role of ISI history in N1 facilitation and explored correlations between N1 facilitation, overall N1 amplitude and ISI-sensitivity, and results of cognitive tasks. Twenty-nine participants passively listened to a random sequence of auditory clicks at varying intensities (65, 75, or 85 dB) and ISI ranges (0.25-0.5 s, 0.5-1 s, 1-2 s, 2-4 s, 4-8 s) while EEG was recorded. Up to 1800 sweeps were collected in the critical ISI range (0.25 to 0.5 s) where N1 facilitation is expected. Results support N1 facilitation occurring at ISIs under 400 ms (p = 0.03), where N1 amplitudes returned to values seen at longer ISIs (∼1.7 s). Notably, this effect was observed when the ISI two clicks before was shorter than 1.5 s (p = 0.001), but not otherwise (p = 0.37). These findings clarify the temporal dynamics of N1 facilitation and challenge the notion of a rigid, context-independent latent inhibition process explaining this phenomenon.