Objective detection of 40 Hz auditory evoked potentials: phase coherence vs. magnitude-squared coherence.

The relative performance of phase coherence (PC) and magnitude-squared coherence (MSC) for detection of steady-state evoked potentials was studied using 40 Hz auditory evoked potentials (AEPs) in 10 normal human subjects. In addition, simulation experiments were carried out to determine the effects of signal amplitude and phase variability on detection performance. All simulations showed MSC performance to be better than PC performance, with further improvements when MSC was supplemented with weighted averaging. However, human 40 Hz AEP data showed essentially identical detection performance for PC and MSC, with or without weighted averaging. These data support a "phase aggregation" model (at least near threshold) over the more usual model in which an AEP signal is added to a stationary noise. Human data collected under "no-stimulus" conditions agree well with theoretical distributions for both PC and MSC. For equal test time, long analysis periods (with less averaging) yielded equal performance to short analysis periods (with more averaging), for both PC and MSC.