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Vernier- acuity of the eye is remarkable.

It is well below the diameter of the cones in the retina, (which subtend about 
24 seconds of arc);  vernier acuity being around five to ten seconds of arc 
subtense at the eye.  Hence acuity is not based on just the distinguishing 
between the separate retinal elements of cones.

It indicates there is considerable 'computer enhancement' at retinal level 
with integration and summation/cancellation effects occuring across a large 
number of retinal receptors horizontally to be able to discriminate the 
discontinuity in a line.  Not forgetting the 'line' is not a line at all but 
a diffraction pattern between a light and dark edge presented to the retina, 
blurred, with many abberations.

Also, if you were to cut a small section in the retina and look at the actual 
image there in a living eye, not only is it a horrible, blurred, abberated 
diffraction mess, it is moving around over an area of a huge number of 
receptors at any time in almost a random pattern as the saccadic movements of 
the eye muscles do not hold the eye still at all but are 'jiggling' about 
massively at retinal level. The 'jiggle' is actually a necessary part of 
vision for if it was not there, the receptor photochemicals become fatigued 
almost immediately on 'firing' the receptor, and there is a distinct time lag 
before they can fire again.
Experiments to give a true stabilised retinal image just give an image which fades into blindness!

The brain somehow stabilises this perceived image, not only considering the 
retinal receptors themselves and their perceived spacial sense relative to 
one another (and their postional sense in the brain);  but also like a 
gyro-stabilised platform,  relative to the positional spacial sense of the 
body as it 'sees' itself in the world.

A truly astonishing achievement.  The psycho/physiology of vision perception 
is amazing and yet poorly understood.

Douglas Denny.
Chichester. England.




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