UNDERSTANDING THE QUANTUM LIMIT OF VISION IS THE KEY
September 13th, 2006 | No Comments »A quote from Albert Rose’s book “Vision Human and Electronic” (Plenum Press) p.29:
“In the scheme of evolution, vision has an almost unique role. One can conceive, for example, that further evolutionary developments might lead to a larger brain capacity, a more evolved nervous system, or a variety of enhancements of current functions. It is not conceivable that the sensitivity of the visual process can be significantly enhanced. The visual process is at an absolute terminal point in the evolutionary chain. To the extent that the visual process now succeeds in counting each absorbed photon….”
I believe that an understanding of how the eye detects at the quantum limit, i.e., is able to “count” individual photons or quanta of light will lead (finally!) to a fundamental understanding of the vision process. The single credible explanation for the light sensitivity of the eye extending to the ultimate level of sensitivity - the quantum limit - must invoke very fast time of the order of 10>-12 sec in the detection process . This time is, in essence, involves the “stopping time” of light (or the photon) in the absorbing mass constituted by the retinal outer segments and the time-associated reduction of noise that competes with signal.
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A correspondent provides references to femtosecond chemical measurements that support the above:
“As established experimentally, the quantum of light impinging on a rod and being absorbed by the molecule of a rhodopsin, “launches” the photochemical reaction (occurrence of electric impulse) for 0.2 picoseconds !,( El-Sayed M.A, Tanaka I, Molin Y. (1995). Ultra Fast Processes in Chemistry and Photobiology, Blackwell Science., Kandori1, H, Shichida, Y., Yoshizawa, T., (2001). Photoisomerization in Rhodopsin, Biochemistry 66) Curiously, that is exactly the time required for a quantum of light to reach a photosensitive cell, as the length of a rod is about 60 μm”
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In explaining the quantum limit capability, it must be realized at the outset that the retina of the eye as taught in this work should be thought of as a circular array of generic “quantum wires” spaced according to physics principles to detect three discrete electromagnetic wavelengths. At the point where light interacts (the length of the outer receptor segments) this array forms a huge number (200,00+ in the fovea alone) of high speed, probably picosecond (or 10>-12 sec.) responsive, optical detection devices. These devices are formed between quantum wires as I have proposed (see “Diagram of the Fundamental Light Detection Device…” on the web page). Each of these devices independently is sufficiently sensitive and free of noise to detect single quanta of light. One envisions then a dense, circularly symmetric, light detecting image-forming array with each element (“pixel”) able to detect (“count”) single light quanta and form an electrically useful signal that is transferred forward in the vision process. In electronic light detection technology this would be termed the “first stage” of the detector.
The total image in the above is therefore present in very fast time - probably ranging from the 10>-12 sec to 10>-9 sec (the latter to allow for retinal processing) time domain. This image thus is ‘there” and being continuously “updated” - and “viewed” by the visual centers of the brain - in this time frame. More about this next.
(I have completed a preliminary analysis that I will present shortly showing that, in the visual scene, sufficient photons are incident on the retina in this time frame to produce a statistically proper image. Remember that the retina is at the Fourier or focal point of the optical system of the eye. The pupillary constrictive system of the eye evolved to continually reduce photon intensity on the retina to biologically tolerable values. There is no dearth of photons incident on the retina!)
Then …the “instantaneously updated” visual image is transmitted via the 1.9M fibers of the optic nerve to the visual centers of the brain. This conduction path is centimeters (how many?) long and the conduction is known to be ionic. Ionic mechanisms are slow and might generally be said to lie in the millisecond (10>-3 sec.) range. I would guess that evolution employed the combination of the finite length of the optic nerve (the distance from the eye to the brain centers involved) and the slow ionic conduction mechanism to arrive at a proper “reaction time” consistent with muscle movement etc. of the human system. In a sense the visual image had to “slowed down” to achieve this match. This is the oft-quoted “reaction time” of the eye approximating 0.2 sec. I propose that this time actually contains two separate times as discussed herein and that the measured “reaction time” of the eye has nothing to do with the process of forming the visual image.
We thus now have the picture of an “instantaneously updated image” that is both spatially and temporally coherent being transmitted through the coherently arranged fibers of the optic nerve to the brain - with the image itself appearing at the visual cortex of the brain!
With this realization one is struck with the question: just what is viewing what here? The traditional view of a passive eye/retina transmitting images to the visual cortex of the brain, OR the brain “looking out” through the coherent optic nerve bundle to the retina acting as a “window on the external world” with it’s imagery changing and being continuously updated in very fast time. Questions?
GCH
9/13/06
