The idea is that raytracing can degrade gracefully, so there's a sliding scale between rendering time and rendering quality for every pixel. And yes, this means that, theoretically, the computer could naturally degrade pixels that you're not currently looking at.
I suspect this would look highly unnatural in practice, though, as even static scenes would flicker and change, especially with reflections and any surface or technique that uses random sampling methods (which is virtually every algorithm that is both fast and looks good).
> I suspect this would look highly unnatural in practice
In pair tests test subjects were unable to see a difference between the foveated image and the normally rendered one. The flicker can be removed by blurring the image to the point where flicker no longer exists. Because your brain is used to a very low sampling rate outside the fovea, it actually helps in hiding the artifacts, because they occur naturally in your vision.
I suspect this would look highly unnatural in practice, though, as even static scenes would flicker and change, especially with reflections and any surface or technique that uses random sampling methods (which is virtually every algorithm that is both fast and looks good).