ESTIMATING THE SPATIAL NYQUIST OF THE HUMAN EEG

The discrete sampling of the brain's electrical field at the scalp sur face with individual recording sensors is subject to the same sampling error as the discrete sampling of the time series at any one sensor w ith analog-to-digital conversion. Unlike temporal sampling, spatial sa mpling is intrinsically discrete, so that the post hoc application of analog anti-aliasing filters is not possible. However, the skull acts as a low-pass spatial filter of the brain's electrical field, attenuat ing the high spatial frequency information. Because of the skull's spa tial filtering, a discrete sampling of the spatial field with a reason able number of scalp electrodes is possible. In this paper, we provide theoretical and experimental evidence that adequately sampling the hu man electroencephalograph (EEG) across the full surface of the head re quires a minimum of 128 sensors. Further studies with each of the majo r EEG and event-related potential phenomena are required in order to d etermine the spatial frequency of these phenomena and in order to dete rmine whether additional increases in sensor density beyond 128 channe ls will improve the spatial resolution of the scalp EEG.