Everyday non-stationarity

The fact that the statistics of a real-world neural recording change continuously across and within sessions, so that a model fit today is a poorer fit tomorrow and a worse one next week. The sources are layered: on the hardware side, electrode impedance changes as gel dries, skin sweats, or dry pins reseat; on the physiological side, arousal, fatigue, caffeine, medication, circadian phase, and even mood shift the underlying rhythms; and cap re-placement moves the sensors relative to the head.

Non-stationarity is not noise to be averaged away but a systematic drift of the signal distribution, which is why fixed decoders decay and why daily recalibration became the tax that consumer and clinical BCIs alike are trying to eliminate. The wearable, everyday setting is the hardest case because all of these sources act at once and none are controlled — the same person is a moving target from morning to night and from one wearing to the next.