SL estimates that only represent a smaller spatial scale (i.e., less than 5 cm) will not be able to detect these long-range oscillations. However, simulations of broadly-distributed sine and cosine generator sources indicated superior representations of amplitude and phase for CSD measures compared to surface potentials (nose, linked-mastoids, or average reference) when using a less flexible spline constant (m = 5; Tenke and Kayser, 2015). It will be important to further investigate whether SL estimates based on less flexible splines, which can be adjusted to match the spatial scale of surface potentials (Fig. 15), are suited to overcome these limitations for different EEG applications. Importantly, coherence estimates are also subject to adverse effects of volume conduction and the recording reference (e.g., Fein et al., 1988; Guevara et al., 2005), particularly for EEG montages affording insufficient scalp coverage (i.e., 32 or fewer channels; e.g., Marzetti et al., 2007), which are still common in clinical settings as well as basic research.
