The Model

The eye is a unique and very powerful image gathering device.  As is usual, we often make analogies to try to understand something that we do not know as well (Hofstadter, 1985a).  It helps us understand the new idea by allowing us to map it onto aspects of a familiar idea.  However, there are traps.  Such a mapping may blind us to the differences between the two objects or concepts under consideration.  Often such blindness goes unnoticed as we slip along with our analogy because the mapping seems so automatic in our brain (Hofstadter, 1985b).  What I want to do in this section is discuss a couple of possible analogies with the eye, one common and one much less so, look at them to highlight how the eye is very different.  These two analogies are the eye as a camera, and the retina as a collection of pixels.

The Eye as a Camera

This analogy is very common and has been used in many textbooks.  The strength of this analogy is that it clearly gets across how the optical aspects of the eye work [I will try to get a figure with the parts of an eye in here to help this discussion].  Basically the cornea and lens of the eye invert the image of the outside world as it come through and focuses that image on a thin surface on the back of the eye, the retina.  The analogy is laid out in the table below.

Feature Camera Eye
Focusing Elements Lens Cornea and Lens
Effect on Image Upside Down and Backwards Upside Down and Backwards
Where Focused On Thin Layer at Back (film) On Thin Layer at Back (Retina)

While both do invert the image, the focusing is still rather different between the two systems, but its details is now what I want to discuss now.  I want to look at that thin layer at the back of the imaging device.  Film is not like the retina.  First, film is passive and the retina is an active system.  Second, each particle on the surface of the film act independently of the particles around it.  In other words, each particle absorbs the light that falls on it and response in its chemically useful way.  It does not interfere with or nor is it interfered with by its neighbors.  This very useful feature of film is not only not true of the retina, it would not be useful on the retina.

Examine these illustrations.  In each one the perception of stimulus does not match the physical parameters of the stimulus.  Such observations lead to the belief of what has been called lateral inhibition.  This is the belief that light falling on one receptor in the eye can reduce the ability of a nearby receptor to respond to light.  This would be like having light falling on one particle of a piece of film preventing surrounding particles from being able to respond to light.  Ratliff and Hartline (1959) got some of the earliest and clearest physiological evidence showing lateral inhibition.  They used the horseshoe crab (Limulus) which as a compound eye.  In this case each little eyelet or ommatidium inhibited its neighbor when it received light.  Now lateral inhibition is mutual across the retina which is why the effects illustrated are seen.

To see how this feature of vision is worked out in our eyes look here.