*1.4.3. Spatial filtering*

The most commonly endorsed EEG recording methods emanated monopolar signals or Laplacian filtered signals. For example, channel Oz consisted as follows:

$$\text{Monopolar: } \mathbf{y}\_{\alpha}^{\mathsf{M}} = \mathbf{y}\_{\alpha} - \mathbf{y}\_{\alpha 2}$$

$$\text{Laplacian:}\quad \mathbf{y}\_{\text{Ox}}^{\text{L}} = \mathbf{y}\_{\text{Ox}}^{\text{M}} - \frac{\mathbf{y}\_{\text{O1}}^{\text{M}} + \mathbf{y}\_{\text{O2}}^{\text{M}}}{2}$$

Laplacian signals are extracted considering both the sides (like Cz utilizes C3 and C4) [23].
