Presbyopia: The standard explanation of this phenomenon in the commonly accepted medical model is that with increasing age, degeneration of the structure of the lens of the eye results in increasing stiffness and a reduction in plasticity of the lens of the eye. The internal eye muscles become less able to deform the lens of the eye. The ability to alter the shape of the lens is affected resulting in an inability to focus, usually at near distances, resulting in a need for reading glasses to enable focusing at close distances such as are typically encountered in "reading". it is unlikely that there is much evidence base for this opinion as research would demand the enucleation of apparently normal eyes to study" lens viscosity" and "pressure deformability".

Cataract surgery (a method whereby researchers may have access to fresh human lens material under normal circumstances), does not usually remove the lens and capsule and in removing the lens alters the lens structure, making any conclusions about lens deformability impossible.

In the Paill Spectrum model, this same effect of Presbyopia would be seen simply due to progressive damage to the internal eye nerves and muscles. the internal nerves and muscles of the eye are unable to deform the lens of the eye because they are damaged by a disease process directly reducing the function of the eye nerves and muscles.

Same effect. Different explanations. Very different consequences in planning intervention or treatment, depending on the explanatory model being used. The stiff lens theory is unlikely to go away because it seems so intuitively correct. however the evidence base for this conclusion may well be very deficient .

The Paill Spectrum model is perhaps a better explanation for the development of symmetric and assymetrical presbyopic visual acquity deterioration, in the presence of nasal medical pathology.