On the Visual Response of Nocturnal Species
January 24th, 2007 | No Comments »No visual system ’sees color in the dark’! The basis of my explanation for the vision process is that the diameter (or lateral dimension) of retinal receptors controls the response of the retina to wavelength and not pigment molecules within each receptor (rhodopsin acts as a structural component to position the retinal electronic signal-producing molecule within the differing diameters of cones and rods). It is receptor diameter (of cones and rods) that determines the center-to-center appositional distance between adjacent receptors. A new form of ‘optical antenna’ that I describe is formed where the dimension (length) of these antennas determines wavelength response. Importantly, there are only three geometrically determined lengths. The sensation of the ‘hues of color’ requires that two receptor sizes be admixed on the retinal surface. This sensation is not the result of pigment molecules or complexes contained within individual retinal receptors! What we term the ‘hues of color’ follow from the eye receiving sufficient light intensity on either side of a geometrically-defined mid-band wavelength to determine a valid ratio of the two intensities. The hues of color can only be determined if two different sizes of receptors are present. Vision otherwise is monochromatic. The geometrically defined mid-band reference is exactly the ‘fulcrum’ point the presence of which was brilliantly deduced by Edwin Land. Thus, the biological retina needs no laboratory instrumentation to define wavelength - it is determined by light refraction within the eye and geometry!
On the subject of nocturnal vision it is crucial to note the unassailable measurements (see my previous Comments on this subject) demonstrating that evolution has brought vision to the ultimate sensitivity, i.e., that the vision process is able to ‘count’ single (or at least a few) photons. This statement has fundamental meaning (at least in physics) but seems to have been disregarded by vision science. The sensitivity of vision cannot be further increased! Any idea therefore that such statements as ‘the hues of color can be sensed in the dark’ are meaningless. Vision cannot discern the hues of color in the dark for the simple reason that too few high energy (blue, for example) photons interact on either side of geometric mid-band to determine a useful ratio. The only photons available are of lower energy in what is termed the near infrared. An analogy - water cannot run uphill!
Also, low light level night vision systems developed extensively by the military knowlegably employ technology that extends the response of such systems beyond the visual bandwidth into this near infrared (or NIR) region that extends roughly from 700-1000 nanometers (or from 0.7-1.0 microns).- using the only photons that are available. (one will remember that the human visual band extends from 400-700 nanometers or 0.4-0.7 microns). Such night vision systems generally use what is termed an ‘S-1′ photosurface that is sensitive to wavelengths in the NIR region but is to a large degree insensitive to visible light.The NIR signals are amplified and converted to a visible phosphor display for human viewing.
It follows therefore from my explanation of the vision process that the retina of any specie that evolved to possess the capability for nocturnal vision would be found to be composed of receptors having a larger diameter than the human variety (the diameter of cones found on the human retina is about 2 microns). I recently came upon the following paper describing the retinal morphology of the Gecko family of lizards - and it goes further in validating my concept than I ever could have expected! The dimensions of retinal receptors of both nocturnal and diurnal species are measured with, in summary, nocturnal retinal receptors being large (very large) with those of the diurnal species being close to the dimensions of human ‘daylight seeing’ vision. What could be better!
The abstract of this paper follows with the emphasis being mine:
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Biomedical and Life Sciences and Medicine
Issue Volume 29, Number 7 / July, 2000
(1) Lehrstuhl für Tierphysiologie, Fakultät für Biologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany
Abstract: Geckos comprise both nocturnal and diurnal genera, and between these categories there are several transitions. As all geckos depend on their visual sense for prey capture, they are promising subjects for comparison of morphological modifications of visual cells adapted to very different photic environments. Retinae of 22 species belonging to 15 genera with different activity periods are examined electron microscopically. Scotopic and photopic vision in geckos is not divided between ldquoclassicalrdquo rods and cones, respectively; both are performed by one basic visual cell type. Independent of the activity periods of the individual species, the visual cells of geckos exhibit characteristics of cones at all levels of their ultrastructure. Thus, gecko retinae have to be classified as cone retinae. Only the large size and the shape of the photoreceptor outer segments in nocturnal geckos are reminiscent of rods; the outer segments are up to 60 micrometers in length and up to 10 micrometers in diameter. The visual cells of diurnal geckos have considerably smaller outer segments with lengths ranging from 6 to 12 micrometers and diameters ranging from 1.3 to 2.1 micrometers. Nocturnal and diurnal species differ in the structure of their ellipsoids. One type of visual cell in nocturnal geckos has modified mitochondria with either rudimentary cristae or no cristae at all, and one type of visual cell in diurnal geckos possesses an oil droplet. The visual cells of Phelsuma guentheri and Rhoptropus barnardi are intermediate between those of nocturnal and diurnal species.
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One sees that the receptors of the nocturnal species are measured as being 10 micrometers in diameter while receptor diameter of the diurnal varieties are smaller being from 1.3-2.1 micrometers. Retinal receptors of the nocturnal gecko are thus some 5X larger than the human variety. I wold project that this dimension extends the visual range of the nocturnal species into the near infrared or NIR region of the spectrum.
The smaller receptors of diurnal gecko species are found to be similar in size to the long wavelength-defining cones on the human retina. This leads one to believe that the vision of these species possess a ‘daytime seeing’ visual band much as we humans. That the receptors are slightly larger than human variety by perhaps 2x which would indicate a shift toward sensitivity at longer wavelengths - but not nearly to the 5x increase of the nocturnal specie.
To go further in defining the visual characteristics of these (or any) specie I would require a more detailed morphology (’retinal map’) of a distribution of receptors - assuming that more than one size were present. This would be very similar to the retinal map of the human retina published by Osterberg (and referenced in my paper). If only one size of receptor is present vision as the above reference seems to indicate, vision will be to a large degree monochromatic, i.e., centered around the wavelength determined by the single receptor apposition dimension. In antenna terminology this is a “high Q” antenna geometry. As noted above, any possibility of sensing the ‘hues of color’ would require that two different sizes of receptors be present and admixed across the retinal surface.
Now, what exactly would all of this conclude that the nocturnal Gecko sees? Given only the above information I would conclude that vision of the nocturnal Gecko lies in a narrow band somewhere in the infrared region of the electromagnetic spectrum beyond the visible. The retina would not be sensitive to visible light! Thus under no circumstances could it discern the, what we term, ‘hues of color’.
Let’s be more precise in defining the wavelength that it does see. Assuming that the long wavelength (red) limit of the human visible band is 700 nm (or 0.7 microns), and that this corresponds to a wavelength-defining receptor dimension of ~ 2 microns, the Gecko receptor being 5X larger.( assuming a linearity of response that may be reasonable) would be responsive at ~ 3.5 microns. This is somewhere between what is termed the Short Wave Infrared SWIR band (1-3 microns) and the Mid Wave Infrared MWIR band (3-5 microns).
Since I have no means of imaging at these wavelengths I will illustrate what I believe the Nocturnal Gecko sees using the Near Infrared NIR band ( from 0.7-1.0 microns).These images can be obtained using an NIR filter (that passes NIR and blocks visible wavelengths) and a state-of-art digital camera. It is not quite this easy …but almost. . A website that contains teaching and instructve NIR images is:: Infrared (IR) Basics for Digital Photographers-Capturing the Unseen.
I believe that these NIR images make the point that I am trying to make, namely, that what the Gecko sees are contrasts in black-and-white that correspond to the blue-to-red visual band that we sense in the human visible band as the ‘hues of color’. Again, I would contend that the species does not at all, and cannot, see ‘color’ as we define it.
At NIR wavelengths the following light reflective characteristics are observed: a.) light reflection from green foliage, a probable background comprising a Gecko’s vision, is greatly enhanced appearing as bright white, b.) surprisingly, and perhaps counter intuitively, red colors at the long wavelength end of the visible band appear even whiter, and c.) blue colors at the short wavelength end of the visible spectrum appear dark even to black.
From this one might imagine the characteristics of nocturnal Gecko - a (even enhanced) black and white gray scale corresponding to light reflected from objects in the visible wavelength (RGB) band. This would be set against the enhanced white reflection from green foliage.
The following series of images illustrate this effect. The first is a normal visible range, unfiltered image of spots of blue and red paint together with a bit of green ‘foliage’ (one knowledgeable of leafy things might recognize parsley). The image shows exactly what one would expect in the visible range. In the next image the same scene is photographed in the NIR region using a Hoya 72 filter that blocks essentially all of the visible band. One sees that the red is almost invisible against the white background (Indeed, the above referenced NIR website indicates that the color red can be viewed in the NIR as whiter than a white background). The blue spot in contrast appears a very dark gray even to black. Thus, in the NIR region there is a grayscale across the visible band extending from red to blue. The green ‘foliage’ is as expected highly reflective in the NIR showing a high contrast white.
Please note that these images are quite un-optimized with, for example, the color spots having some shine or reflectivity. Ideally a matte surface would yield even more black and white contrast.
Next, we attempted to simulate ‘what a Gecko might sees in a usual (to him or her) outdoor setting using my desert backyard at night after sunset. The same two bottles of blue and red paint were set into some local foliage (?) together with a blue colored tweezer that was available. Note again that these two photos are not optimized with the blue and red colors changed by viewing through the reflective glass of the bottles and the objects themselves were placed deeply into the vegetation. If they had been on the same plane as the vegetation their reflectance would have been more pronounced. Note the intense white representation of the green foliage.
But here they are:
Outside NIR image of the above scene …note blue tweezer and red and blue bottles of lacquer:
I expect a rejoinder to my proposal to be that “microspectrophotometric measurements of Gecko (or other nocturnal species) retinae show a pigment response that belies your NIR prediction!”. I happened to come across a site today that used this rationale in measuring the response of another nocturnal specie - the owl. In that work the retinal pigment response was in the same (always the same!) wavelength region as measured human pigments, i.e., at ~ 500 nanometers or the approximate middle of the human visual band. Please see my entry on this site of January 18,2006 ” More on Retinal Pigments” for my thoughts on this type of measurement. In summary,it is my belief that retinal correct response can only be measured with the organ in the living state, i.e., where wavelength-defining spacing between receptors is maintained. Any single receptor - and pigment or pigments thought to be contained therein - represents the ubiquitous (i.e., to all receptors) spatially quantum-confining electron central space. The energy gathering ’surround’, provided in the living state by specific retinal apposition dimension, defines wavelength. When a receptor is ‘teased’ from a section of dead retinal tissue and flattened for microspectrophotometric measurement on a microscope slide. The milieu in which it finds itself (some kind of preservative solution) will effect some dimension of lateral index of refraction gradient….but this will NOT be the state that the receptor was in on the living retina. This refractive index (that will probably vary a bit) gradient always seems to produce very nearly the same ‘pigment’ wavelength response - this type of measurement always yields almost the same curves! Why hasn’t anyone noticed this? Please see my Comment noted above.
The rhodopsin protein component of the retinal/rhodopsin complex that resides within receptors serves in this explanation the function of a variable geometric spacing element assuring energy transfer within the different sizes of receptors - that we have termed cones and rods. I believe that it is a misconception to believe that they are independent ‘pigments’ within themselves.
Just to remind again, the three ‘lengths’ that derive from an admixture of two diameters correspond to the exact ends and geometrically determined center of the band of vision. The size of each component determines the placement in the electromagnetic spectrum of the band.
GCH
1/21/07
