Rethinking the Process of Vision

I had been aware of the low density of cone receptors extending to the peripheral retina but did not give them much thought other than thinking that their function might be to slightly alter (due to their low density) the blue response that I had seen for this region. In retrospect, however, I see that this cannot be their function and that they are probably involved in detecting the high spatial frequency part of the 2-D Fourier transform that the retina encodes. As we develop the details of how the eye performs this transform the role of these receptors will be clarified.

Details about these peripheral cones abstracted from a web site:

“Although peripheral cone density falls as low as 4000/sq mm, versus
200 K/sq mm for the maximum rod density, it is important to realize
that the cone diameter at these eccentricities is as large as 5 - 9 um.
The cone inner segments therefore occupy as much as 1/3 of the area
of peripheral retina, and catch as much as 1/3 of the light captured by
the two types of peripheral photoreceptors.” (underline is mine)

It seems obvious that these cones probably play a more significant role than I had thought by virtue of their capturing a great deal of light in the peripheral retina. Note that the large 5-9 micron dimension refers to the inner segments and not the light-detecting outer segments of these cones. I do have (but will find) the size of the important outer segment then being able to assign a wavelength response to the centers. This response, however, must lie at short wavelengths as these are the only wavelengths that are refracted to these high retinal angles.

Recall that I had proposed that the peripheral retina acts as a “wide angle light meter” controlling the (mechanical) constriction of the pupil and thus light entrance into the eye. There seems to experimental evidence supporting this proposal noting that receptors in the peripheral retina are “ganged together” in parallel. And too, that I propose that it is the physical diameter of the rods, and thus their characteristic rod-to-rod appositional distance, that determines the absolute short wavelength of visual response. (If, for example, one has genetically-determined , slightly smaller rod receptors one ventures into the biologically-damaging ultraviolet region – not good!

Knowledge of sub-retinal “circuitry” of the peripheral retina (that undoubtedly exists) will probably shed more light on this subject.

GCH

11/07/07

Tucson, AZ

This entry was posted on Wednesday, November 7th, 2007 at 8:20 am and is filed under Running Commentary . You can follow any responses to this entry through the RSS 2.0 feed. You can leave a comment, or trackback from your own site.