What is a 'time-variant' signal?
As I've mentioned in a comment, time variant signal is a term which is not commonly used in signal processing, and I also couldn't find a clear definition. After browsing a few texts which use the term (usually written by people not specialized in signal processing), I've come to the conclusion that by time variant signals they actually mean non-stationary signals. The latter being a clearly defined term (check the wikipedia page on stationarity). In short, it means that the signal's properties, such as its mean and variance, change over time. And this is indeed the case with EEG signals, just as it is the case with speech signals.