> A hypothetical experiment for determining this would be...

Or at least what I described here is what I interpret the concept to be.

>
>
>   4. Stop when they say that the objects are clearly only in their peripheral
>      vision field.  That is, you stop when they report that it's iffy as to
>      whether they even see it.

More likely it would be at a point where the object becomes hard to recognize,
since we can see in our peripheral vision objects that we can't clearly recognize.