Eye and Vision

"everything there [Emerald City] is of a green color -- just as everything in this Country of the Gillikins is of a purple color."
"ls everything here purple?" asked Jack.
"Of course it is. Can't you see?" returned the boy.
"I believe I must be color-blind" said the Pumpkinhead, after staring about him.
--L Frank Baum, The Land of Oz, 1904

Purves et al Chapter 11 covers optics, the eye and the retina, also some pictures from chapter 12
Note that the Washington University Medical School's Neuroscience Tutorial has good coverage on this topic:
Eye and Retina
More on color vision and visual transduction can be found in my signalling course where the text figures refer to Alberts et al., Molecular Biology of the Cell, 3rd edition, New York, Garland:
Vertebrate Vision

Pep talk

Transduction (how do we get from energy to nerve response?) first and best understood for light.
Centures old literature on light (e.g. Newton) and color vision
"Medically," vision is very important to the quality of life
"the eye is the window to the brain" -- (NEI phrase) physician can actually look at CNS.
For instance, increased crainial pressure (like from tumor) shows up as papilledema
Funding for research and private foundations.
Vision is fundamental to the nature of human experience.

Fig. 11.9A
Rhodopsin is the visual pigment
Protein plus chromophore (11-cis retinal) makes visual pigment
George Wald 1967 Nobel Prize

Fig. 11.9B
isomerization of retinal

Physics
(A rod can see one quantum, see below)
Energy of one photon = Planck's constant (h) x the frequency (nu)
Frequency = speed (c) / wavelength (lambda)
frequency = for 500 nm (blue-green), 3 x 10 to the 8 m/s divided by 0.5 x 10 to the -6 m = 6 x 10 to the 14 sec to the -1
E = 6 x 10 to the 14 per sec x 6.63 x 10 to the -27 erg-sec = 3.96 x 10 to the -12 erg

Light

Demonstration: A Mercury arc lamp feeds into a monochromator to make monochromatic lights of the spectrum. In your text, the spectrum is described in Fig. 10-12. We have a monochromator which generates a spectrum using a grating (the obvious alternative being a prism). On the front of the monochromator is a slit that picks off a small section of the spectrum. Using another monochromator, where I could look inside, I obtained this picture to show how the slit selects a portion of the spectrum. The slit picks off 6.4 nm/mm, i.e. if it is 1 mm open, it lets through 6.4 nm of the spectrum. With the slit set at 1 mm, we project the beam onto a screen. Cranking the monochromator, we go through ROYGBIV (red orange yellow green blue indigo violet). We note that it is brighter at some wavelengths than others.I scanned this graph of the spectral output of various light sources to demonstrate that there are "lines" in the mercury arc (HBO) spectrum at 580, 550, 438, 405 and 365 nm. At a setting of about 579 nm, light looks uniquely yellow (Just 5 nm higher looks orangish, while just 5 nm lower looks greenish). At 365 nm (we need an additional filter), the screen looks dark but an index card looks blue. That is fluorescence. A short wavelength excites electron orbitals, then there is some radiationless deexcitation, so, when the electron falls to its ground state, there is less energy and a longer wavelength. Neutral density filters are used to attenuate light. The values are in log to the base 10. A 0.3 log unit filter would cut the light in half (you do the arithmetic) [it doesn't seem that much, does it?] and 0.6 would cut it to 1/4. A 0.3 plus a 0.6 is the same as the 0.9.

Fig 11.14
"Light" is a portion of the electromagnetic spectrum - 400-700 nm
400 is violet, 700 is red
For over a century, it has been known that insects see "near" ultraviolet (UV) (300-400 nm)
My research contributions in the 1970's concerned UV sensitivity in Drosophila:
site1, site2, site3
Starting in the early 1980's, researchers showed that various vertebrates, fish, birds, eventually mice, see UV.
Snakes "see" infrared (IR) (heat of warm blooded prey) with pit organs - pinhole eye, (reference: RIGamow and JFHarris, The infrared receptors of snakes, Scientific American, May 1973, 94-100)

Eye dissection

The eye with orbital fat
The eye with orbital fat removed clearly showing optic nerve
Here is a sheep eye showing the exit of the optic nerve
Retina is white film, black is pigment epithelium
Tapetum gives eye glow, reflecting light back through retina increases sensitivity for nocturnal vision
An albino eye cut around the orbit is better for showing ciliary muscle
Here is the lens of the eye

Eye structure

Fig. 11.1
the eye picture of an ophthalmologist's office
cornea, iris, pupil, conjunctiva, sclera, extraocular muscles
lens, aqueous, vitreous, retina, fovea, optic n.
there is a blind spot where the optic nerve exits

Disorders

Fig. Box A
refractive errors
diopters - reciprocal of focal distance in m
cornea is 0.024 m, 42 diopters
Emmetropia-normal,
Hyperopia-far-sighted, need convex lens,
Myopia-near-sighted, need concave lens, involves abnormal elongation of the eye
visual angle, acuity - Snellen eye chart - 20/20 is seeing letter 5 min (1/60 degree)
1980's radial keratotomy (RK)
LASIK
Both change shape of cornea

Fig. 11.2
Presbyopia
Accomodation
TRANSPARENCY Fig from another book,
(1) ciliary muscle relaxed, suspensory ligaments taut, lens thin for distance vision
(2) ciliary muscle contracted, suspensory ligaments relaxed, lens thick for near vision
loss of accomodation with age explains Presbyopia
Benjamin Franklin developed bifocals

Other disorders:

Glaucoma
pressure is too high because aqueous humor does not drain well, ganglion cells die, treated with drops or surgery, canal of Schlemm

Floaters
in vitreous especially in people with myopia

Diabetic retinopathy
blood vessels overgrow, leak, blast holes in retina with laser decreases angiogenesis

Cataract
lens becomes opaque, remove and often replace with intraocular lens, made of polymethyl methacrylate, known to be tolerated since pieces from airplane visors would lodge in pilots under fire (and since about 1988, these have been doped with UV blockers)
Humans see UV, but only if the lens is removed (aphakia), one of my interests.

Retinopathy of prematurity
High O2 in incubator causes overgrowth of blood vessels
"Little" Stevie Wonder

Fig. 11.3
ophthalmologists view of eye
optic disk = papilla (where optic nerve exits and site of blood supply)
fovea, site of high acuity (cone vision) - point of fixation
Macula lutea pigment
This picture better shows a yellow pigment that absorbs blue light

Box B (Chapter 11)
demonstrates the blind spot

Fig Box D Chapter 11
Retinitis pigmentosa is tragic, people can see when young, lose rod vision (tunnel vision [ring scotoma] because rods are in mid-periphery).
Rods go first and eventually cones which is strange if rod molecules are mutant.
There are autosomal and X-linked types, dominant and recessive.
There are other genetic degenerations and stationary (not progressive) blindnesses are in molecules of transduction cascade as well as in other rod and cone molecules.
There is a web site where information relevant to the retina, especially genetic causes of blindness, accumulates (site)

Box C Chapter 11
Age-related (it used to be called "senile") macular degeneration (AMD)
People about 80 yrs old lose high acuity vision (cannot read)
"Wet" (10%) is a sudden and treatable (laser surgery) medical emergency from blood vessel leaking
"Dry" complicated but may have an genetic basis too, worse in smokers
More on genetic blindnesses can be found in my signalling course:
Retinitis pigmentosa and age related macular degeneration

Rods and Cones

Fig. 11.5C
Photoreceptors- 125 million receptors 20/1 rods to cones
(converge on 1 million ganglion cells)

Fig. 11.13A
Rod and cone number as a function of visual angle [angle is the way to express it] (note blind spot)
Rod, peripheral vision, dim black and white, sensitive - "scotopic"
[Explains ring scotoma (loss of vision in mid periphery) in RP]
Very sensitive - 1 photon

Landmark paper
People can see light of 6-14 quanta over a 500 rod area (SHecht, SSchlaer and MHPirenne, Energy, quanta and vision, J. Gen. Physiol., 25, 819-840, 1942)

Cone, fovea, color, acuity - "photopic"
Shown in rats, rods are supported by retinal pigment epithelium

Fig. 11.6
disk shedding

RPE: (1) melanin that blocks light reflection
(2) metabolism to provide 11-cis retinal (chromophore ofvisual pigment, rhodopsin)
(3) phagocytosis and recycling of shed rod tips
Cells are postmitotic and the indigestible residue of the phagolysosomal system is lipofuscin, a fluorescent aging pigment, a topic on which I've done research.

Fig. 11.13B
fovea is pit without rods and with cells and blood vessels out of the way

Color vision

PKKaiser The Joy of perception, a web book
Follow leads, table of contents, 3-fun things in vision, afterimages

Fig. 11.14A
spectral sensitivity of rods and 3 cone types
confirms Young -Helmholtz trichromatic theory
3 kinds of cone 420 530 560
3 kinds of cone opsins which are evolutionarily related in humans and OW monkeys

Fig. 11.16
green and yellow (middle and long wavelength) cone opsins are near each other on X
(blue cone opsin is on human chromosome 7, rod on chromosome 3)

Fig. 11-14B
technique to see cones

evolution
bottleneck hypothesis color vision re-evolves after nocturnal life (where adaptive pressure for cone vision is relaxed) early in mammalian evolution
Red or green color blindness - on X, thus preferentially in males.
Blindnesses were thought to be from altered genes, but numbr of copies in human population is variable, and cross-over accidents can even make chimeric genes.
Female "carriers" should actually be mosaics of color blind vs normal retina because of Mary Lyon X-inactivation hypothesis
See recent evolution in superfamily of G-protein-coupled receptors (7 transmembrane domain receptors)

Phototransduction

I was in graduate school when a seminar speaker (also in a paper) argued that the separation of disks from the plasmalemma meant that there must be an intracellular signal that diffuses across the cytoplasm; at that time, they thought it was Ca2+

Fig 11.7
response is hyperpolarization
response is slow (this is cone, rod is even slower)
"You can walk through the forest with nothing but starlight, but you cannot run."

Fig. 11.9
details of transduction cascade
Stacks of disks. Lumen is like outside cell
vitamin A is the chromophore
Transducin activates cGMP PDE, less cGMP (ligand) and channel closes so...

Fig. 11.8A
...cell hyperpolarizes...

Fig. 11.8B
...since sodium channel closes.

Retinal processing

Fig. 11.21A
center surround (excitation vs inhibition or vice versa) receptive fields
This kind of processing emphasizes contrast detection.

Feature detection - introduction and summary
Lateral inhibition H. K. Hartline - 1967 - Nobel
"primary physiological and chemical visual processes"
Limulus horseshoe crab
Mach (Ernst) bands - see edges (bright-dark contrast) especially well

TRANSPARENCY
(from Classic paper W.H.Miller, F. Ratliff & HK Hartline, How cells receive stimuli, Scientific American, September 1961, 222-238)
at a light-dark boundary the response "boundary" from the array of corresponding nerves is exxagerated

TRANSPARENCY
(from Classic paper F. Ratliff, Contour and contrast, Scientific American, June 1972, 90-101) The moon seems bright because it is next to a dark edge, relative to the nearby sky which is next to a shallow gradient of dark.

Personal reflection. When I applied to graduate school, Rockefeller invited me down for an all day interview (since I was already in Manhatten at Columbia College "the gem of the ocean"). Lunch (and a lab tour) was with Floyd Ratliff. Unfortunately for me (I was rejected by Rockefeller), I thought he was mispronouncing "stimulus" when he kept saying "Limulus." I learned about Limulus the very next week in physiological psychology. (You never know when the knowledge you are armed with will come in handy.) Oh well. I went to Wisconsin (go Badgers!) and loved it so much that I gave them my first born.

Purves home page interactive Demos, find Craik-O'Brien-Cornsweet

Blobs in Hermann grid (Michael Bach, follow leads Optical illusions and visual phenomena, 6 down 2 over - Hermann grid) explained by more inhibition at corners

Sensory processing (in lots of sensory systems) uses lateral inhibition to give feature detection, which for vision is color, contour and contrast and movement. In other words, the brain does not keep track of a point by point stimulation of each rod and cone, but rather reduces that information into evolutionarily (depending on the species) relevant fetures. The retina does this by lateral connections, and the input to the nervous system, like at the thalamus and cortex, processes further.

Lower animals, like frogs (with less brain power) have more retinal processing (since there is less that can be done in the brain). The process of lateral inhibition gives feature detection, for features such as contrast and movement. Some ganglion cells in frog retina fire preferentially to small dark spots moving rapidly through receptive field ("bug detectors"). The idea is that there are ganglion cells which fire only with quickly moving small dots, stimuli resembling flies which frogs must detect expeditiously in order to catch them on their sticky tongues.
Classic Paper J.Y.Lettvin, H.R.Maturana, W.S.McCullock & W.H.Pitts, What the frog's eye tells the frog's brain, Proceedings of the institute of radio engineers, 1959, 47, 1940-1951.

Primate ganglion cells -
Magnocellular (large) 10% - movement
Parvocellular (small) 90% - form and detail
cat ganglion cells -
morphology and function WXY
color opponent cells

Fig. 11.18AA
glutamate is "excitatory" transmitter (usually would give an EPSP)
released in dark - but less glutamate in light on
off bipolars respond (depolarize) to increased glutamate (dark) with EPSP
off thus hyperpolarize to light and decrease firing of off ganglion cell
on bipolar hyperpolarize to glutamate (dark) probably through metabotropic
thus on depolarize in light and increase firing of on ganglion cell

Fig. 11.17AB
all this comes in center surround organization mediated by horizontal connections

Color contrast

(not well covered in book)
If red in center of visual field excites ganglion cell filing, then red in periphery inhibits.
In this same cell, green in periphery excites and green in center inhibits.
Also there are cells that are the opposite.

In the LGN, there are center - surround cells and color processing

End

My interests center around vision, so a visit to the research interests of my home page will offer various topics about vitamin A, ultraviolet light, and Drosophila mutants. Dr. Fliesler in SLU's Ophthalmology Department and Dr. Ariel in SLU's Anatomy and Neurobiology Department are some of my fellow wizards in visual science.

Exam questions from 2005 - 2007 relating to this outline

Hartline won a Nobel Prize for showing that lateral inhibition contributes to detection of what feature in Limulus?

contour and contrast

What is the specific defect in AMD (age-related macular degeneration)?

cone vision in fovea

If stimulation in the center of the receptive field of an on-center ganglion cell increases firing a lot, how would stimulation of the whole receptive field with the same intensity affect
firing?

way less

Out of the "corner of your eye," you see a dim star. You do not see it when you look right at it. Why can you see it when you look away from it?

rods are more sensitive and they are off fovea

When would you use Planck's constant in calculations concerning sensory transduction?

to calculate the energy of a photon of light

How does PDE (phosphodiesterase) affect the membrane potential in the rod?

decreases cGMP, channel closes, cell hyperpolarizes

Why is there a "ring scotoma" (dougnnut-shaped blind area) in people afflicted with retinitis pigmentosa?

rods go first, that is where they are prevalent

"There are bug detectors in the frog retina." State this in a less casual way.

Movement of a small dark spot in the receptive field preferrentially increases firing in some ganglion cells.

"Three nuclear layers and two plexiform layers" describe the histology of what sensory structure?

retina

What is the status of cation channels of rods in the dark?

open

Unless something is wrong with them, the proteins in the cornea and lens transmit light quite well. How come, by contrast, the G protein-coupled receptor (rhodopsin) absorbs light?

is has a chromophore, a pigmented portion, 11-cis retinal

Capillaries are in the choroid, a pigmented layer behind the retinal pigment epithelium. Why is it that an ophthalmologist can see the other blood supply?

those pass on the vitreal side of the retina

Scotopic sensitivity would be highest at about 15o off fovea. Why?

that is where rod concentration is highest

The age pigment, lipofuscin, is the indigestible residue of the phagolysosomal system. How does this apply to the rods and retinal pigment epithelium?

rods shed their tips and the RPE phagocytoses them

What is the cause of blindness in diabetes, and how does laser photocoagulation of the retina slow the progression of diabetic retinopathy?

angiogenesis, formation of new blood vessels that leak, burning out part of retina, though it causes some vision loss, inhibits angiogenesis

How come surgical manipulations of the cornea such as LASIK and RK can be so effective in correcting myopia?

because the curved cornea-air interface, with its huge difference in index of refraction, is a powerful lens and these surgeries change the shape of that lens

What is the name and cause of the disorder that makes people over 40 need reading glasses or bifocals?

presbyopia is from stiffness of the lens

"You can walk through the forest with nothing but starlight, but you cannot run." Why not?

phototransduction is slow especially in rods

Pick a person, any person. How many copies of middle- and long-wavelength (green- and yellow-) coding genes would you find on one X chromosome?

cannot say. could be one, 2 or even more

You are recording intracellularly from a rod. What would happen if the cGMP decreased?

it would hyperpolarize

What is the term for the enhancement of contour-contrast information in which the gradient of neural responses across the retina is more exaggerated than the intensity gradient?

hyperpolarization, feature detection, Mach bands

"Magnocellular" and "parvocellular" are terms applied to retinal ganglion cells based on size and processing (movement vs contrast respectively). What is the difference in projection to the brain?

they project to different layers in the LGN. M-> 1 & 2, P -> 3-6

Long before they could record intracellularly from single rods or cones, they knew that a rod could see a single photon of light. How?

testing whether subjects could see carefully calibrated lights (psychophysics)

Under what circumstances would a person be able to see ultraviolet light?

if the lens is missing

"The blue cone opsin is on human chromosome 7 while rod rhodopsin is on 3." Where are the genes for yellow- and green-absorbing opsins?

the X

There is a huge amount of phagocytosis that retinal pigment epithelium (RPE) cells must do. Phagocytosis of what?

the tips of rod outer segments are shed daily

"The blind spot is at about 15o on the nasal retina at the horizontal plane." Why is there a blind spot?

there can be no receptors where the optic nerve exits

Abnormal elongation of the eyeball is associated with what refractive error?

myopia

Cones would be preferentially damaged in what debilitating disorder affecting the elderly?

age related macular degeneration (AMD)

A woman has lots of sons, half red-blind (protanopic) and half with normal color vision. How would her retina be with respect to red color vision?

mosaic

Specifically, what gates the channels in the rod plasmalemma?

cGMP

Hartline found that there is increased firing from brightly lit facets (of a crab's eye) near a dark area than a brightly lit area near only brightly lit areas. Why?

less inhibition

Rhodopsins (such as the 4 of rods and cones) are sometimes called visual pigments, yet proteins usually do not absorb visible wavelengths of light. What makes rhodopsin a pigment?

11-cis retinal

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