Is there a metric of the overall excitation of the brain or nervous system? I'm thinking of a way to describe what's happening to a single channel EEG/MEG sensor output. I've noticed that there is significant difference in the readings from a quiet observations, versus a person who's engaged in a conversation. I'm looking for some way to quantify the two distinct states, so a computer algorithm may have a chance of distinguishing the two. Potentially, in the future I would like to compare readings from one individual to the next, or compare the arousal of two individuals engaged in a conversation.

It would be helpful to know what are the names of metrics that may be used to measure the overall arousal/excitation of a person, that person's nervous system or brain. I can think of Actigraphy (the study of human motion) as one, its units are arbitrary activity count, or m/s^3. These are scored as "zero point crossings", or "area under the curve" of an accelerometer graph. But are there any other metrics or units of human activity/excitation? Maybe some of them are related to an EEG/EMG readings? Like an obscure algorithm named after some scientist?
MEGs using SQUIDs can detect magnetic flux density measured in teslas. Away from sleep studies and epileptic brain activity, this should be a useful indicator of relative brain arousal. Readings from two different people on this scale should be comparable  based on the orders of magnitude for the unit measurement. As for comparative measurements of brain arousal, I don't think Actigraphy would be useful, nor any other method of measuring somatic motion. These can certainly determine weather a person is sleeping, sedentary, or exercising, but discriminating between different sedentary behaviors would be difficult and determining relative brain activity for those behaviors would be reliant on more traditional methods (EEG,MEG, etc). MRI methods might be useful, as the total change in blood oxygen volume from a base state or inactive state can be distinguished from a more active state.