Otis makes statements about what he calls the "behaviour of the natural eye"
in response to "the average visual environment".  He bases these statements
on animal research using neo natal chickens and primates with simulated
refractive error. They were designed to study how the eye corrects itself
when presented with large congential (present at birth) refractive error.
He concludes that using a plus lens for near work will reverse myopia in
adult humans.

Lets look at what happens to the eyes in the research.   A large amount of
congenital hyperopia is rapidly corrected by modification of the growth of
the eye and the adult animal ends up emmetropic or nearly so.  A large
amount of congenital myopia is more slowly and less thoroughly corrected,
and (this is important) if the myopia simulating device is removed for a
minimun of 2 hours a day, the myopia is not corrected.  A congenital myope
who sees clearly without accomodation for a few hours a day may well end up
still myopic as an adult.

Now lets look at the "visual environments" of the experimental animals.  For
the hyperopes, retinal blur is present at all times and at all viewing
distances.  Blur can be only be relieved through accomodation by flexing the
muscle controlling the lens inside the eye and the use of accomodation at
any time or viewing distance will always reduce retinal blur.

For the myopes retinal blur is present only some of the time and at some
viewing distances.  Blur cannot be relieved ever for most distances (beyond
far point).   Accomodation will increase blur for most viewing distances
(beyond far point), and will reduce blur for some viewing distances (closer
than far point).  A viewing distance exists for which there is no retinal
blur when accomodation is not in use (far point).

For emmetropes, retinal blur is present only some of the time and at some
viewing distances.    Blur cannot be relieved ever for a few distances
(closer than nearest point of  most accomodation).   Accomodation will
increase blur for most viewing distances (anything further than about 6ft),
and will reduce blur for some viewing distances (closer than about 6ft).
Viewing distances exists for which there is no retinal blur when
accomodation is not in use (farther than about 6 ft).

The tricky bit for the animal is to decide how to respond to the retinal
blur.  Increasing the rate of growth of the eyeball will fix the blur found
in hyperopia, but will make the blur found in myopia and emmetropia worse.
Decreasing the rate of growth of the eyeball will fix the blur found in
myopia, but will make the blur found in hyperopia and emmetropia worse.  Not
changing the rate of growth will maintain the blur of emmetropia and will
not make myopic or hyperopic blur worse; neither will it improve them.

As seen above, there is a major difference between the blur found in
hyperopia and the other two conditions:  it is present at all times and at
all viewing distances and can only be relieved by accomodation.  So, if the
eye determines that blur is present all the time, it would be very safe to
increase the rate of growth of the eye.  This also explains why, when
experimental conditions are designed to subject the eye to constant blur, as
with fogging lenses or by the use of a confined environment, the eye
responds by speeding up growth.  If the eye was, in fact, not hyperopic,
then the increased growth will result in myopia.   The eye must be very
careful to not increase growth in response to conditions of confined
environment and the research bears this out: as long as there is at least 2
hours a day without retinal blur, the eye will not respond with increased
growth.

There is a minor difference between the blur found in myopia and that of
emmetropia; the emmetrope will have a larger range of conditions in which
retinal blur is absent when accommodation is not in use.  However, for some
low amounts of myopia, say between -1.00 and -3.00, there will be little
difference: the emmetrope has little retinal blur for viewing further than 6
ft away and the low myope will have little retinal blur for distances
closer.  It is much more difficult for the eye to determine whether the
sometimes retinal blur condition is emmetropia or myopia and thus we are not
surprised that the eye is more cautious in decreasing growth.  This also
explains why the eye does not self correct  low amounts of myopia very well.

Now, how would using a plus lens at near affect things?  For the hyperope,
it would introduce a period of no retinal blur when accomodation is not in
use and may stop the eye from correcting itself.  For the emmetrope, it will
provide more time with no retinal blur and may help the eye not change.  For
the myope, it will provide more time with no retinal blur, making myopia
even harder to differentiate from emmetropia and thus may stop the eye from
slowing growth and self correcting.

So what do you think?  Leaving aside the question of neonates vs adults,
does the animal research suggest that using a plus lens at near is likely to
prevent or reverse myopia?

Dr Judy