Introduction to Sleep Electroencephalography

EEG records synaptic potentials from pyramidal cells. It is critical to remember that there is no absolute voltage measurement in clinical EEG. Rather, an EEG trace records potential differences between two electrodes. By convention of the amplifiers used in clinical EEG, an upgoing deflection indicates that input 1 (grid 1) is more negative (or less positive) than input 2 (grid 2). This is an arbitrary but critical rule (Figure 130.1). Thus statements like ‘‘positivity is up’’ or ‘‘negativity is up’’ make no sense unless it is stated whether the positivity is at grid 1 or grid 2. It should be emphasized that the way recordings are displayed can be modified at will with digital acquisition, and this ‘‘post-hoc’’ reformatting is one of the major advantages of modern digital systems.