In the valley of the blind, the one-eyed man is king.
The following is only opinion.
If you can help document my supposition, or refute it, I would greatly appreciate your comments.
Presbyopia is associated with the inability to have the muscles of the eye properly adjust to close distances and is typical in people over 45 years of age. Because of presbyopia, people who do a lot of close visual work (such as reading or using a computer) frequently need either "reading glasses" or "computer glasses" or alternative glasses such as bifocals or progressive glasses.
Vision, however, is subjective. Only the patient can tell the doctor when their vision is clear, and sometimes even the patient cannot tell. ("Which is clearer, ONE or TWO.")
The most frequently used device for measuring corrective vision is the phoroptor. While the phoroptor may be faster and easier to use when quantifying vision adjustment than using a manual lens frame, the restricted vision field of the phoroptor may give an INHERENTLY INACCURATE prescription with older eyes than the less-restricted visual field of a lens frame. (This is similar to "pin hole vision" in which an image becomes clearer when viewed through a screen or a viewer with multiple pin holes. While the disclaimer on "pin hole glasses" may question their validity, the concept as applied to the phoroptor may be valid even if the glasses are not.)
The "standard" Snellen vision test requires the interpretation of barely legible letters at a simulated 20 foot distance. The Snellen test also inadvertently encourages over-refraction since the benchmark for correct refraction is 60% letter recognition (i.e. clearly identify 3 of 5 letters). In a highly assertive literate society, that 60% score may subconsciously be viewed as "failing the test" whereas a 100% "passing" score inadvertently causes excess-refraction.
Such excess-refraction from using a phoroptor may be as much as 1 diopter in each eye. Unless the patient is aware that their progressive glasses are too strong, that effect on their ability to read will lead to a loss of reading comprehension or an assumption of increased stupidity.
Our "high tech" culture, and the unintentional misdiagnosis by Optometrists and Ophthalmologists, creates a "technical disability" for older people with presbyopia similar to starvation from an inability to chew that killed many Native Americans by age 45.
While a loss of comprehension and mental function likely is associated with old age, the question that needs to be re-evaluated is "How much of dementia is caused by "old age" versus how much of it is caused by habituation to misprescribed and misdiagnosed glasses?
Your feedback would greatly be appreciated.