How Many EEG Channels Do You Actually Need?

Reducing the number of EEG channels from 19 down to as few as 8 has a surprisingly small effect on a model's ability to distinguish between mental arithmetic and a background state. This was confirmed using a robust evaluation method where models were tested on data from entirely new participants not seen during training. While a ranking method consistently identified the same few channels as being most important, these "best" channels performed no better than randomly selected ones in a controlled test. This finding demonstrates that a feature's statistical importance during training does not guarantee superior performance when generalizing to new subjects.