I have not had much time to think about vision lately, but the following text written some weeks ago seems sufficiently important to introduce even in it’s incomplete form. I will add Comments shortly on the subject of the quantum aspects of the vision process.
“Experimental data shows that the eye can detect (‘count’) single photons. This capability defines the ‘quantum limit’ of the vision process. As Albert Rose stated eloquently (paraphrased) ‘this is the end of the line in the evolutionary process – the sensitivity of vision cannot be further increased’! There is really no debate about this, but, the mechanism underlying this sensitivity has never been explained.
It is obvious that invoking the terms ‘photon’ and ‘quantum’ that vision actually interacts with the reality of quantum physics and that these physics must be involved in any ultimate explanation of vision! There are many in the physics community attempting to address the subject of a quantum connection with neuroscience……and the retina as noted by Roger Penrose is a direct extension of the brain! This subject has not, and could not be, addressed in the literature of vision as long as the eye was perceived to be the analogue of a “camera” with the usual line of thought in this regard involving long image integration times of the order of photographic film, i.e., on a macro time scale of the order of fractions of a second. This is the overarching view tacitly assumed by vision science to this day.
From the view of photonics technology there is a glaring disconnect here. The only way that this field knows of to detect light at this low level in contemporary imaging systems (“photomultipler” tubes, etc.) is to either apply large electronic amplification (not at all evident or possible in biological systems) or, to reduce the temperature of the system to close to absolute zero to reduce signal-obscuring noise. Neither of these considerations is evident in the eye and, moreover, vision accomplishes this feat even above room ambient at body temperature.
I propose an explanation positing that the only way to explain this (and it must be explained!) is that the logically spaced array (of a 100 million or so receptors) of discrete light detection elements must be considered independently with each possessing electronic characteristics consistent with the requirements for detecting single quanta. Considering individual light detection elements (or devices) is the only way to explain detection at this ultimate level of sensitivity.
The characteristics of these individual devices includes both: a.) the sub-optical wavelength dimensionality of each element (characterized by the appositional “electromagnetic antenna” distances between two retinal receptors) and, b.) a new approach that considers that a vastly reduced time scale is involved reaching at least into the sub-picosecond (10-12 sec) time domain. Electronic (thermally generated) noise is a time integrated function with shorter times reducing the number of discrete, unwanted electronic noise events that are included in any event detection “time window”. The integration time of the eye may extend even to femtosecond (10-15 sec) time or less. The key to this sensitivity lies, I believe, in a paper that a number of us published some years ago (referenced elsewhere) to the point that the only way that signal obscuring thermal noise can be reduced is by considering that the detection process occurs on a very fast time scale. Noise is a time-integrated function!
The idea that the retinal image is acquired in very fast time is supported by data indicating that this is the “switching time” of the fundamental, signal-producing’ retinal molecule with in the rhodopsin complexes contained in receptors of the retina.
One most note that this combination of sub-micron dimensionality and brief time scale brings the process of vision into the realm of quantum physics – where we must go!.
If the situation is as I propose, the visually acquired image can be said to be “quantum noise limited envisioning a two dimensional array (the retinal surface) with each receptor capable of detecting a single photon quantized events – a ‘quantal image’. This is the scenario envisioned by Albert Rose in his splendid book. It represents, however, a classical view of what must be viewed as a quantum construction! How must one treat an ‘array of quantum events’? I will leave this line of thought for a moment and return to what visual image processing from the retina to the visual cortex of the brain must consist of.
One will then ask: how is such an ephemeral (i.e., in time) signal perceived by the much slower human nervous system? I have proposed that transmission of the visual image information through the two million or so individual fibers that comprise the optic nerve accomplishes this function ‘slowing down the image’ to ‘human nervous system time’, or, ~10-2 seconds. Transmission through this nerve bundle has been historically assumed to be ionic that would have the proper time constant. A recent hypothesis proposes that this transmission may actually use solitons (coherent phonons) in a lossless mode.
I might suppose that some sort of time integration of acquired “quantal image” information must occur somewhere in this process. .a new window for the vision/mind process into what I term “quantum reality” is provided by this logic.
From the above it should be obvious that in both space and time this explanation approaches the realm of quantum physics.