Most consumer activity trackers are worn on the wrist, but that probably has more to do with where people are used to wearing devices than it has to do with getting good biometric data. EDA can pick up emotional reactions on the wrist, but as can be seen in the van Dooren figure, the responsiveness can vary a lot depending on the exact location and orientation of the sensors. The wrist isn’t a great place to measure heart rate because the wrist is mostly connective tissue and bones, whereas PPG is best measured from highly vascularized “meaty” parts of the body. To get good PPG readings from the wrist it’s important to crank down the tightness of the band to minimize movement artifacts. Once you do that, movement fluctuations on the wrist can be an interesting reflection of motions ranging from walking and running to gestures.
As you move up the arm the nature of the data changes somewhat. EDA becomes less responsive (although with EmotiBit it’s possible to run wired finger clips up to the device) and PPG becomes generally a little better as you land on highly vascularized musculature. Having a super consistent PPG signal is super important to calculate heart rate variability (HRV, a measure of sympathetic/parasympathetic activation) because it takes about 5-10 minutes of reliable heart rate data to calculate a single data point for HRV.
The forehead can be an interesting location for EmotiBit to measure biometrics, with strong EDA and PPG signals. Temperature data on the forehead is also interesting since it’s reflective of core temperature (that’s where your school nurse will likely take your temperature these days) and shows fluctuations with emotional changes. Head movements can reflect both changes in gaze as well as nods and other social interactions.