Re: Cognitive Acuity ?
From the video where Meta show off their ideas for goggles, one of the goals is to accurately track the pupil.
There are good technical reasons for this, mainly rendering in full detail only the part of the image that will hit the fovea, using lower res for the rest of eye (both in pixels rendered and in colour detail) - and maybe even not rendering at all the bit of the image that lies in your blind spot. Doing this obviously reduces the amount of computation and bandwidth needed for the video (and isn't a new idea, VR creators were trying to do the same in the 90s).
Meta are even playing with changing the apparent depth of field of the image presented, as determined by measuring the pupils, including how dilated they are. Again, using the data to improve the results.
Now, if they just happen to "accidentally" happen to also deduce whether you are enjoying what you're looking at by the pupil response and that "leaks" out to the advertisers...
(PS if you've got various light sensors looking closely at the User's face and eyes, you can use them to, say, determine the heartrate as well. No need to try to tap into the User's heart monitor watch, which is just doing exactly the same trick - and having to do it on a tougher target, the wrinkled, tanned, hairy and dirty back of the wrist).