In EEG terminology, what does undersampling refer to?

Undersampling in EEG terminology primarily refers to aliasing. Aliasing occurs when a signal is sampled at a rate that is insufficient to capture the variations in the signal accurately. According to the Nyquist theorem, to properly reconstruct a signal, it must be sampled at least twice the frequency of its highest component. If the sampling rate is lower than this threshold, higher frequency components of the signal can be misrepresented as lower frequencies, leading to distorted representations of the brain activity being measured. This misinterpretation of frequency is what defines aliasing in the context of EEG.

In contrast, amplitude distortion, frequency drift, and phase lag describe different phenomena that can occur in EEG signal processing but do not relate directly to the effects of undersampling. Amplitude distortion concerns the inaccuracies in the representation of amplitude, frequency drift pertains to slow changes in frequency over time, and phase lag refers to the timing difference between two signals. These concepts highlight problems in signal fidelity but do not specifically address the sampling rate issues inherent in undersampling.