I don't own a smart watch or fitbit. Having a device tell me that my heart rate is elevated is unnecessary because I can feel that when that happens all by myself.
I have suspected for some time that wearable technology can be used to collect far more data about our brainwave activity than is generally known. Yesterday I came across a description of "far-field potential," which is measurement of nervous system activity from far away in the body as compared to when it is measured up-close like with those electrode webs you sometimes see pictured on people's heads in articles about electroencephalography (i.e., EEGs). The up-close measurements are referred to as "near-field potential."
ABSTRACT. Electrical signals generated in any biological system can travel to the surface by volume conduction. Near-field potentials are generated close to the recording electrode; far- field potentials are generated at a distance from the recording electrode. Biological electrical signals are often dipoles with a negative and a positive end. EEG and evoked potential studies may record one or both ends of the dipole, as either near-field or far-field potentials. The morphology and apparent polarity of recorded waveforms are determined in part by the orientation of the dipole, the direction of spread, and the derivation used for recording.
Ghigo, J., Erwin, A. C., & Erwin, C. W. (1991). Near-field vs. Far-field Evoked Potentials. American Journal of EEG Technology, 31(2), 109–118. https://doi.org/10.1080/00029238.1991.11080362. Online at https://www.tandfonline.com/doi/abs/10.1080/00029238.1991.11080362
Notice that this article is from 1991. That means that this recording of neural data has been possible for over two decades.
[Update: Apparently far-field potential recording between the extremities has been possible since the 1970s, per this paper I came across yesterday:
Somatosensory evoked potential in man: Far field potentials
Potentiel évoqué somato-sensitif chez l'homme: potentiels transmis à distance
Abstract: Three short latency positive potentials evoked by median nerve stimulation were recorded from the scalp, nose and ear of 11 normal adult subjects in leads where the hand or knee contralateral to the side of stimulation was used as the reference site. The short latencies and positive polarity of these components suggest that they are volume conducted far field potentials. Evidence is presented which suggests that the first potential arises in peripheral nerve fibers. Brain stem and dicencephalic structures are suggested as possible sources for the second and third potentials.
The configuration of the response to median nerve stimulation recorded over the cervical spine in ear and hand reference recordings was different. Evidence is presented which suggests that this occurs because the electrode placed on the ear records the far field potentials described in the above paragraph. A far field potential was also recorded in hand--knee leads contralateral to the stimulated median nerve.
Roger Q Cracco, Joan B Cracco, Somatosensory evoked potential in man: Far field potentials, Electroencephalography and Clinical Neurophysiology, Volume 41, Issue 5, 1976, Pages 460-466, ISSN 0013-4694, https://doi.org/10.1016/0013-4694(76)90057-2. Online at https://www.sciencedirect.com/science/article/abs/pii/0013469476900572 and https://pubmed.ncbi.nlm.nih.gov/61849/]
