A Henrik Ibsen Quote and from Today’s NYT: “Growing Older, and Adjusting to the Dark”
March 14th, 2007 | No Comments »From Garrison Keillor’s “Writer’s Almanac” of March 13th (I heartily recommend the RSS feed for this site!). From Act 2 of Henrik Ibsen’s play Ghosts that opened in 1891:
“I almost think we’re all of us Ghosts… It’s not only what we have inherited from our father and mother that walks in us. It’s all sorts of dead ideas, and lifeless old beliefs, and so forth. They have no vitality, but they cling to us all the same, and we can’t get rid of them. Whenever I take up a newspaper, I seem to see Ghosts gliding between the lines. There must be Ghosts all the country over, as thick as the sand of the sea. And then we are, one and all, so pitifully afraid of the light.”
I think perhaps appropriate to the realm of science!
Segueing to……….. from the New York Times of the same date: “Growing Older, and Adjusting to the Dark” (Science Times, p.D7)…..again the same old stuff over and over again about “cones giving us color vision” and that “….rods provide only black and white images..” Tsk, tsk!!!
I think that another summary of my explanation for light interaction with the retina and the vision process is in order:
1.) The visual image is formed on the retina from light falling on the matrix of cones and rods extending from the all-cone fovea to approximately 20 degrees of retinal angle.
2.) The hues of color are determined by determination of a ratio of light falling on either side of a fundamental, geometrically-determined reference point (’primary green’) at 7-8 degrees of retinal angle..exactly as Edwin Land deduced. This point corresponds on the retina to where there are - a sufficient density of rods to completely surround each cone. Land showed that a slight shift of this reference point (and assuming that the diffractive properties of the eye do not change) can drastically change perception of the hues of color. A slight shift of this point (by, for example, genetic alteration of the size ratio of retinal receptors) is in my mind undoubtedly the explanation for ‘color blindness’ - and NOT the ‘absence of one of the classes of cones’ that, in fact, do not exist at all.
3.) The all-cone fovea itself extending to one degree of retinal angle is responsive solely to long wavelength (’primary red’) and, in fact, specifically defines the exact long wavelength end of the the visual band.
4.) The transition to predominantly rod density from the 7-8 degree ce mid-point reference to approximately twenty degrees detects the short wavelength (’primary blue’) input to the image.
5.) The three ‘primary wavelength’ regions described above detect the varying intensity of these wavelengths and are not a spectral response as universally assumed.
6.) The predominantly rod-containing peripheral retina beyond 20 degrees serves the sole function of a ‘wide angle light meter’ controlling the mechanical action of pupillary constriction that limits light entrance into the eye. This region is solely short wavelength (blue) sensitive and exactly defines the short wavelength limit of the visual band…a dangerous point being adjacent to the biologically-damaging UV region. In addition to being sensitive to the intensity of light , pupillary constriction is solely short wavelength sensitive being controlled by the response of the peripheral retina.
7.) The ratio of the sizes of the cones to rods on the human retina is ~1.8:1 corresponding to the visible band from 700-400 nm.
8.) A mix of two receptor diameters must be present to produce the sensation of color. The response of a retina with receptors of only one diamter would have a monochromatic response.
9.) The absolute diameter of retinal receptors determines (by setting center-to-center appositional length) the wavelength sensed. Smaller receptors connote shorter wavelength response (insects and birds / UV) and larger receptors sensing intothe near infrared region of the spectrum (fish).
I would guess that the comment in the NYT piece about “the less responsive muscles in the iris…” is the primary effector of reduced night vision with aging. Constriction of the pupil of the eye (as I have noted above) is mechanical and I would assume subject to decline ..as all of our other muscles are! Perhaps eye exercises would forestall this decline…?
The comment “There is also evidence that as we age we lose more rods than cones” and “But an autopsy study of older adults found that while the cones remain intact, almost third of the rods in the macula (generally the imaging region as defined above) had been lost”. Maybe so….but if true ..perception of color would change with age!! This seems amenable to experiment. Has this ever been investigated?
I have also noted the possible relationship between the short wavelength limit-controlling function of the all-rod peripheral retina and the onset of the condition of macular degeneration. Once one realizes that the the absolute wavelength response determined by this region is perilously close to biologically damaging ultraviolet radiation one can speculate about this connection. Moreover, following from this explanation it is the diameter of rod receptors in an individual that sets this limit..which must be genetically inheritable.
GCH
3/1/07
