Time constant is expressed in which unit?

Time constant is a measure of how quickly a system responds to changes in its input and is expressed in seconds. In the context of electroencephalography (EEG) and other relevant fields, the time constant reflects the time it takes for a system to reach about 63.2% of its final value after a step input. This is critical for understanding the dynamics of neuronal activity and the response characteristics of EEG signals.

The use of seconds as the unit allows for clear communication of the temporal aspects of neural responses and the filtering characteristics of EEG devices. The other choices represent different physical quantities; for instance, "Mm per second" relates to distance over time, "Cycles per second" is a measure of frequency, and "uV per mm" denotes a potential measurement per distance, but none of these relate to the concept of time as directly as seconds do.