This section provides an overview of the most widely used methods for assessing brain connectivity based on phase coupling between neural oscillations. The notion of phase is usually associated to a periodic oscillation, i.e., an oscillation which repeats itself exactly after one period (e.g., a sinusoidal wave). For a periodic oscillation, the phase indicates the fractional portion of the period that has been completed. It is typically expressed as an angle spanning a whole turn (2π) as the oscillation goes through a period. Of note, the value of the phase is meaningless if the origin of the oscillation has no physical meaning (e.g., a trigger or a stimulus). Nonetheless, the phase difference between two oscillations is always well-defined, since the dependence on the origin is implicitly canceled out in the computation of the difference. Indeed, the phase difference can be used to assess phase couplings between oscillations, as discussed below.
