The Directional Stiles-Crawford Effect - an Explanation
August 1st, 2003 | No Comments »A puzzling directional effect regarding light entering the eye was first dicovered in 1933 by W. Stiles and B. Crawford and has since carried their names. In this effect “a light ray entering the eye through the center of the pupil is several times more effective than one incoming through the pupillary periphery.” I am quoting here from a summary paper on polarization effects by Zhevandrov (1.) that describes the effect very succinctly. A schematic representation is shown as Figure 1 - again from Z’s. paper:
Another quote from Z. “The effect is more pronounced in light-adapted cone cells at the foveal portion of the fundus, being virtually absent in the rod-lined surfaces of the retina.”
The geometric model for interreceptor retinal light detection devices would seemingly provide an explanation for the SC effect. As we discuss elsewhere on the webpage, the cone/cone devices characteristic of the fovea display a viewing angle of less than five degrees. We also show that the light detection sensitivity of this device structure is very uniform within this viewing angle. I might suppose that this should not be surprising as the overall light level within the eye we believe to be controlled by the “wide angle light meter” formed by the array of rod/rod devices that constitute the peripheral retina which extert control of the light entrance aperture to the eye - the iris. Retinal light detection devices then would not have to deal with large fluctuations in intensity and can concentrate on the function of imaging.
We have further proposed that signal generation within the interreceptor devices is via an electrical dipole mechanism formed within the thylakoid disk stack. Information defining the direction of the incoming light ray is derived from a comparison of the signal generated in adjacent receptors from light absorbed within each receptor. This is shown in Figure 2 (with the fundamental operation of the device discussed in greater detail elsewhere on this webpage.) The single ray shown within the viewing angle represents a ray of normal incidence which, in effect, provides a “null” signal, i.e., depositing equal amounts of energy directionally along the length of each receptor. Now, if a non-axial ray entering the device outside of the viewing angle is incident as shown ( at 45 degrees) the polarization signal generated will obviously be smaller. It is interesting that this may be so even if the non-axial ray deposits an amount of energy approximately equal to a ray entering within the viewing angle. Does all depend on the magnitude of the polarization signal generated?
This should be readily calculable.
