ON THE VISION OF CATS AS PREDICTED BY THIS WORK

I have said repeatedly in this work that, given a measurement of the distribution of receptors on the retinal surface of any specie, the fundamental principles uncovered for the human eye would allow precise definition of the visual characteristics of that specie. I have searched for such data and a paper  The Distribution of Rods and Cones in the Retina of the Cat (Felis domesticus) Roy H. Steinberg, Miriam Reid, Paula L. Lacey. Comp. Neur., 148, 229-246, provides a test for my proposal.

A quote from the introduction to that paper :

“Although a large amount of physiological research is currently being carried out on the cat retina, little information concerning the distribution of receptor densities is available…”.

The reader might first familiarize himself/herself with these principles applied to the human retina in the Comment that I wrote on 7.31.09  “A PRIMER ON GEOMETRY AND VISION” and, if necessary, repeat over and over the phrase that “cones do not detect color…cones do not detect color…and on”. It would be useful in the context of this work to cease using the terms “cones” and “rods” with all of the  baggage that they carry and substitute “large or small diameter receptors”. For clarity in the following, however, I will continue to use the old terms.

First to be noted is that the receptor distribution of the retina of the cat as measured in the Steinberg paper can be seen to reflect , in an overall sense, the general plan of the human retina. The cat retina has with a central region (their “area centralis”) that contains, as does the human retina, the highest density of cones. The distribution of rods also mirrors that of the human  but, as will be noted, of much greater density. In essence, the result is that there is no hexagonally arrayed,  tightly packed all-cone region on the cat retina  – the fovea in the human.

The central region of cone density (or, in the cat, cone/rod and rod/rod appositions) is the area where the image is acquired.  In the cat it would seem that the  image acquired will be more diffuse than human reflecting the spread (or diffuseness) of cones in the central region (I suppose that this might be termed visual acuity). Again, in cursory reading, I find that this seems to have been noted as characteristic of cat vision.

I would note again that, as I propose,  the visual image is acquired both in  human and cat in the Fourier domain, i.e, implying that the retina lies at the focal (or Fourier) plane of the optics of the eye. To satisfy the Fourier equation receptor appositional light detection centers possess  the ability to detect both the  intensity and phase of incoming light for the image formation process.

A place to begin is to reproduce their Figure 13a comparing the densities of cone receptors in the human and cat retina. These densities both peak at zero degrees but the density of cones in the human retina reaches 150,000 / mm2 while in the cat this value is only 25,000 / mm2.


Figure 13b from their paper shows the distribution of rod receptors in both the human and cat retina. It is readily apparent that the overall shapes of the two curves are similar in shape. The cat retina, however, contains a maximum rod density of almost 500,000 / mm2 while the value for the human variety is only 150,000 / mm2.. It becomes immediately obvious why the cat retina does not contain a central tightly packed all-cone region.


This is shown clearly in their Figure 2 with photomicrograph 1 depicting the array of receptors at the point of maximum cone density (i.e., near zero degrees eccentricity).

For comparison, the following is Osterberg’s classic measurements  published in 1935 of  the distribution of cones and rods on the human retina. I would note that these are the measurements upon which this entire explanation of light interaction on the retina is based. A simple calculation of the number of receptor appositions as a function of eccentricity reveals the three wavelength peaks that underlie the trichromicity of vision and all that follows. Refer here to my geometric “Rosetta Stone” diagram in the original paper.
I have proposed that an admixture of retinal receptors of two sizes on the retina is required to elicit the sensation that we term “color”  in vision. In such as admixture (as on the human retina) I have noted that there is geometrically-defined center (a rod-to-cone apposition) that defines an exact mid-band wavelength from which the sensation of color is obtained. This center (at 8-9 degrees on the human retina) provides the wavelength reference from which other wavelengths can be quantified.  On the human retina this corresponds to, if the entire band lies from 400-700 nanometers, 550 nanometers.  I wonder if anyone has yet appreciated the profound nature of this finding – the wavelength of light is geometrically defined.

This wavelength reference must undoubtedly be the “fulcrum” that Land deduced from external measurements must be present. The sensation of “color” is then synthesized by the eye or brain somehow from what Land termed “lightness records” deriving rom light interaction on either side of the fulcrum reference point.

Humans see color therefore because their retina possesses three discernible  light interaction regions and a resultant “fulcrum” central wavelength reference point. As taught by Land, any specie that senses what we term “color” must have three differential light interaction regions on it’s retina.

On the human retina these regions are: a.) a central all-cone fovea composed of hexagonal tightly packed cones providing long wavelength interaction and, that in  fact defines the long wavelength limit of the visual band, b.) the mid-band fulcrum reference defined by an admixture of two sizes of receptors, and finally to, c.) the region sensitive to short wavelengths primarily rod-containing  region beyond that defines the short wavelength limit of visual response.

From the Steinberg paper, the sizes of cone and rod receptors of the cat are generally similar to the human variety with rods of about 1 micron diameter and cones slightly large than human as measured at 2 microns. (they find that as one approaches the peripheral retina the size of rods slightly increases for which  I will propose a tentative explanation in the following). The ratio of the sizes of cones to rods in the human retina  is ~1.8:1. In the cat, therefore, according to this work, this ratio is ~2:1 reflecting the slight increase in cone diameter. Thus human and cat rod receptors are of approximately the same diameter with the cone receptors of cats being slightly larger than the human variety.

In support of this the rod-around-cone motif observed for the cat retina (see Figure 2 above) contains, perhaps primarily, the eight-around-one motif of the human retina (and most other species – ref) but also a sizeable number of nine-around-one centers that would follow from the larger size of the cat’s cones. I would note again that these motifs are the only ones that are geometrically possible using the above ratio values.

For those who have not read this work – this ratio of the sizes of receptors exactly determines the visual band of the specie.

Although the cat retina contains an admixture of two sizes of receptors (as does the human retina) an entirely different situation obtains. It is seen from these measurements that the density of rods greatly overwhelms the density of cones in the area centralis. This precludes the formation of a tightly packed all-cone “fovea” as in the human specie.

Cat vision therefore sees only two wavelengths  instead of  the three that we see! The necessary third, long wavelength sensitive region provided by the all-cone human fovea is missing in the cat retina.  The cat retina does not detect the three “primary” wavelengths necessary to synthesize the hues of color, nor does it contain a geometric “fulcrum” point necessary to accomplish this.

The cat can therefore not see the hues of “color” that we see! Here I ask the reader to repeat again that “cones do not detect color”………). What the cat must see from this explanation is an image, that must in some sense appear as black-and-white, viewed within the bandwidth from 400-600 nanometers.
In “human color terms” cat vision would be sensitive from blue to perhaps green-yellow. Even a cursory exploration of the literature on cat vision seems to bear this out. From a  Wikipedia entry  “The Senses of Cats”

“Cats can see some colors, and can tell the difference between red, blue and yellow lights, as well as between red and green lights.[3] Cats are able to distinguish between blues and violets better than between colours near the red end of the spectrum.[4][5]

The emphasis is mine. Also, I would disagree with the sentence that “cats can see some color,…”. More properly stated this sentence should read: “cats are able to discern some wavelengths in the visible band”.

At the start of my thinking on this paper I imagined, since I vaguely believed that cats possess excellent night vision, that their retina would necessarily possess larger cone receptors for night viewing (larger center-to-center distances between receptors extend wavelength sensitivity into the near infrared). In first glance at the paper this seemed to be so. But, the analysis does not at all bear this out as described above. I would believe that the excellent night vision of  cats follows from the seemingly much larger physical size of the structure of their eye. I have not had time to look into this, but the huge density of rod receptors  on the cat retina (3-4 times greater than the human retina), and the seeming fact that the rods on each retina are of the same size, foretells a much larger eye structure. I would appreciate comments on this subject.

In analogy to my view of the functioning of the human retina, the combination of  a higher density of rods distributed over a larger area  (almost the total area) of the retina in the cat functions as a (very!) wide angle light meter controlling pupillary constriction and light entrance into the eye. This combined with the seemingly larger  (ref) eye of the  cat functions to acquire a much greater amount of light than the human eye.

The finding of the paper that the size (diameter) of rod receptors increases near the  peripheral retina and the ora serrata. I vaguely recollect that this may also be true on the human retina. Remember that it is the diameter pf rods that determines the exact short wavelength end of the visible band of the eye. This is an important diameter as this region abuts the potentially biologically-damaging ultraviolet region. It might be therefore that nature employs larger rod size as a protective measure to “bend the band upward” toward slightly longer wavelength interaction in this sensitive region.

I conclude that this analysis concludes that cats cannot detect “color,” as humans do,  and correspondingly, that we must begin to realize the distinction between detecting optical wavlengths and detecting color.

Respectfully submitted,
GCH
8.17.09