of cortex [71,72]. A bipolar referencing scheme, as commonly used with intracranial data, should therefore eliminate spatially distributed low-frequency effects in the majority of electrodes, and only reveal those effects at the bipolar channels that border the true effect (i.e. at channels that combine activity from one electrode that exhibits a positive theta effect and one that does not; see Figure 5B). Accordingly, negative theta effects have been observed in scalp EEG when using a bipolar reference scheme [32]. A common average referencing scheme, as commonly used with scalp EEG, on the other hand, should preserve spatially distributed positive effects in the majority of electrodes.
