Fig. 2.

Unwrapping the analytic phase of a bandpass-filtered EEG signal yields an almost monotonically increasing ramp function (A). Small discontinuities in the otherwise smooth ramp are phase resets. The time derivative of the unwrapped analytic phase is the instantaneous frequency of the signal (B). Instantaneous frequencies obtained from the analytic phase are instances of angular frequency and, as such, can assume negative values.