Beyond an Explanation to Speculation About the Process of Vision
October 3rd, 2006 | No Comments »I now step beyond simply proposing an “explanation” for the nature of the vision process to speculation based on where that new explanation leads. I had wished that others would begin this process……and a few are actually beginning to do that!
In recent comments I have made the point that the well documented result that vision has evolved to the “quantum limit”, i.e., that the retina is capable of “counting” single quanta (or photons), is the key to understanding the process of vision. One must realize that the historic “wave particle duality” dilemma of physics asserts itself here! This single, well documented result that quantum effects are involved at the ultimate limit of vision must be confronted. This association forms the basis for, and provides great insight into understanding the true nature of the process.
NOTE:
To avoid confusion I again remind that the explanation for light interaction that I propose shows (proves) that the eye evolved to detect light as the wave of classical physics. It is the absorbing mass (the electron) of retinal receptors that is quantized being necessarily reduced to “quantum confinement” dimensions for the interaction to occur. The process can then be described either way, i.e., in classical or quantum terms. I use the term “photon” but would rather term it in light of the forgoing a “quantized interaction”.
Based on my initial proposal that the light detection centers of the retina were actually receptor appositions (cone-to-cone, cone-to-rod, etc) and not the receptors themselves (and all that follows from that explanation) single photon sensitivity can only mean that each receptor site acts individually to detect single quanta. This is a daunting thought! The small central fovea contains 200,000+ detection sites – with each site acting individually! Moreover, it follows that the “stopping time” of light within retinal outer segments is in the ephemerally short time of picoseconds (10-12 sec). It is within this time frame that what might be termed a “quantal image” is “there”. Thus an image composed of millions of individual quantal interactions is resident on the retina in this brief period of time!
I will discuss below whether we should even think of this as a “time” at all or are we now approaching the “instant” of perhaps a new “quantum reality”! We are certainly venturing into the unknown area of physics where wave and particle interact.
But first, I worked for many years in photonic technology developing, among other applications, optical systems capable of single photon detection. In general there are two paths to gaining this level of sensitivity. The first involves use of cryogenic cooling to cool the light detecting element in order to reduce electronic noise and reveal the photon level signal. I remember a system built by an aerospace group in Los Angeles that used a silicon photo detector and that did an admirable job of single photon counting but which required cooling with liquid helium to a few degrees above absolute zero. Another path to attaining this level of sensitivity involves accepting the level of noise and providing a post-detector electronic amplification of a million (106) or so to raise the small single photon signal to a measurable electronic level. This is the route taken by the “vacuum photomultiplier” light detector that can actually detect single (or at most a very few) photons at room temperature. Rose in his analysis (previously referenced) of the vision process could only propose that the eye somewhere contained such an amplifier but he could not explain where or how this might be accomplished in a biological system. The eye, however, is superior to both of these approaches with its ability to detect single photons above room temperature, i.e., at body temperature. How does it accomplish this?
I have proposed that the answer lies in the time scale. In designing radiation detection systems one employs the concept of “matched bandwidths” matching the time in which the signal is absorbed (“is there”) with the “time constant” of the following electronic amplification system. Noise (for example, random thermally induced electrons) is a time-integratable function with the random events appearing in, and integrated over, time. Shortening the “time constant” of the detection process reduces the number of such events that can occur within the detection window thus reducing the overall noise level. A group of us published a paper on this subject a number of years ago (1.) noting how the effect could be used to increase the sensitivity of radiation detection systems.
(1.) Tove, P.A., Cho, Z.H., Huth, G.C., “The Importance of the Time Scale in Radiation Detection Exemplified by Comparing Conventional and Avalanche Semiconductor Detectors”, Physica Scripta, 13, 83-92, 1976
It would seem that the factors of the vision process as I have outlined above seem to conform to such a detection scenario with the quantum level signals being absorbed in very short, picosecond time. In the initial paper of this web page I discussed the mechanism operative in the light detection device formed between adjacent retinal receptors that seem to conform to these requirements.. See http://www.ghuth.com/?p=58 for a diagram of the structure. This device has the small, sub-micron dimensioned scale required for operation in the very fast time domain. I had proposed that the necessary electrical signal follows from photoisomerization of the retinal molecules contained within each receptor. A correspondent has noted that this reaction has been experimentally shown to occur in this time scale.
“As established experimentally, the quantum of light impinging on a rod and being absorbed by a molecule of rhodopsin ”launches” the photochemical reaction (occurrence of electric impulse) for 0.2 picoseconds”, Ref. El-Sayed, M.A., Tanaka, I., Molin, Y., 1995, Ultrafast Processes in Chemistry and Photobiology, Blackwell Science; Kandoril, H., Shichida, Y., Yoshizawa, T., Photoisomerization in Rhodopsin, Biochemistry, 66, 2001.
From all of this one envisions a “quantal image” comprising millions (corresponding to the number of retinal receptor appositions) of individually quantum receptive “picture elements” at the plane of light interaction with the retinal outer segments. This is an awesome array to contemplate! It must be faced that whatever is occurring here lies in the domain of quantum physics with all of the uncertainty (sic!) that this statement implies.
The term “coherent” (either spatially, temporally or both) seems to apply here and I will revert to what is a classical concept of “an image” being formed and transmitted onward realizing that this probably is not the true state of affairs.
Thus leaving the quantum realm, we assume that the “quantal image” proceeds through retinal neuronal circuitry and thence through the, assumed to be, imaging bundle of the optic nerve to the visual centers of the brain.
It is not clear whether the three (RGB intensity sensitive) Fourier transforms from the retina are processed (synthesized?) in neuronal circuitry on the retina of if this occurs in the brain. The latter would require that three “coherent” quantal images be transferred – in time. My sense is that this does not occur and that only one image is transferred.
One gets the sense of a vision process where an “instant” (in picosecond or less time) quantum defined image (whatever that means!) continually updates an ongoing “vision stream” to the brain. The element of time itself (processing time?, signal transfer time? etc.) is perforce involved in the post-quantal image formation time.
Thus the vision process within itself, following the initial quantum image sensation stage, looks back in time! WE CAN NEVER SEE THE “QUANTUM INSTANT” OF IMAGE FORMATION ON THE RETINA! In another sense we are viewing the abyss of quantum physics. We can never reach the instant that is before our eyes!
I am aware (painfully aware) that none of this breaks any fundamental new ground in understanding the reality of the quantum nature of reality. All that one can now say is that “the classical wave and quantized particle aspects of that reality come into coincidence at the plane of the outer segments of retinal receptors”. The quantal array of receptors that I propose form the retina and that results in the vanishing “instant” of time that is perceived can said also to exist in the instant that is “freeze framed” in a photograph! The vision process, however, produces a “moving or continuous instant” and quantum effects are undoubtedly involved. Questions?
I also believe new is the proposal that the well known “arrow of time” of physics exists within the vision process itself. As stated above, we can never know this instant (the “quantized” instant). The instant (timeless?) quantum realm exists on the retina itself but all following is in classical “time”. The ideas of Julian Barbour (“The End of Time, the Next Revolution in Physics” as previously referenced) and Ernst Mach (that reality consists of timeless “sensations”) that time itself is an illusion become perhaps more real. Using classical; “time terminology we are always even within ourselves looking into the past!
And too, reaching a fundamental understanding of the Fourier transforming process is involved. I have shown that the vision process is based on this process. Brain Hagan in his brilliant paper (again, previously referenced) presents a geometrical/mathematical explanation that “the wave is the particle” but this does little (at least to me) to provide understanding in this situation. Questions?
I am going to leave this here although I have many other thoughts and will add them shortly.
ADDED:
I learn from my son that sub-retinal neurons (terminology?) are surprisingly (probably only to me!) analogue in nature . as opposed to neurons in the brain that react digitally. This would seem to support some of the above assertions
GCH
10/04/06
Updated 10/06/06
