Distinguishing the evoked response from phase reset: a comment to Mäkinen et al.

Mäkinen et al. [Mäkinen, V., Tiitinen, H., May, P., 2005. Auditory event-related responses are generated independently of ongoing brain activity. Neuroimage 24, 961-968] suggest the use of amplitude variance to distinguish the evoked response from phase reset. Because their data do not exhibit a drop in amplitude variance, they conclude that ERPs are generated by 'processes separate from and additive to ongoing brain activity.' We argue that this conclusion is premature because of unrealistic assumptions about the processes underlying an event-related modulation of oscillations. A realistic phase reset model has to consider at least two parameters, degree of phase reset (or 'phase concentration') and amplitude change (event-related increase or decrease in amplitude). With simulated data, we show that a variable increase in amplitude size increases amplitude variance and masks the influence of a phase reset. On the other hand, an event-related decrease in amplitude without a phase reset leads to a sharp drop in variance. Furthermore, simulation of a frequency-specific phase reset shows a drop in variance that may be too small to be detected empirically. Thus, we conclude that amplitude variance is not capable of distinguishing the evoked response from phase reset.