ANATOMY OF VISION II1

An audiotutorial introduction to the form and function of the human primary visual pathways

form:
for the tape click here

Paul Pietsch, PhD,
Professor Emeritus,
School of Optometry
Indiana University

Web Contact:
pietsch@indiana.edu

function:

SOUND CONTROL

65-min tape 		Hi Doc!
Say, before you tackle this tape, do the both of us a big favor, hah! Go and do the first lesson in this vision anatomy series -- the Overview of the visual system (click).
When you can handle the check list over there, come on back.
And after you do get back, and before you mash the tape button, take another squint at those pictures at the top of this page. They'll show you at a glance what every pushcart vendor, housewife and radio announcer/producer ought to know about the form and function of the main visual pathways in us human beings.
Now, in case you hit the books, Doc, let me tell you what some will tell you, and why -- and why it's useful.
First off, at the LGB (lateral geniculate body), we can divide the visual pathways into an Anterior Division and Posterior Division. Some neuro-anatomists call that anterior division the retino-geniculate tracts (between the retinas and the LGBs); and the posterior division, the geniculo-calcarine tracts (between the geniculate body and the calcarine fissure). The man on the tape calls those geniculocalcarine tracts the optic radiations. That's what you'll find in most of our captions and legends.
But tracts or schmacks, remember Doc, they're still axons -- output nerve fibers!

Now keep this in mind too, Doc. That anterior division is made up of axons from cells in the eyeball called retinal ganglion cells. Axons of the retinal ganglion cells are the business end of the optic nerves, the optic chiasm and the optic tracts; and they synapse (contact) on neurons in the LGB.
The LGB (some people like to call it the LGN4) is a piece of the thalamus; LGB cells process sensory input (like other nuclei of the thalamus do for input destined for the cerebral cortex). Anyhow, the LGB signals the primary visual cortex [and other places in the brain, too, but for now let's stay with the visual cortex (aka lips of the calcarine fissure or area 17 or striate cortex)].
The LGB's output axons are the optic radiations.
Since you probably don't have what the tape tells you to take out of your school bag, your old pal Cranky will give you some substitute stuff:

Diagrams: Front Versus Top Views
See if you can draw your own diagrams from memory after you're done, Doc!
front view (looking at a patient):
 top view:

THE CHIASM RULE
It's convenient to divide each retina in hemiretinas -- NASAL(N) and TEMPORAL (T) [lateral and medial] and name the fibers accordingly.
In terms of mapping field information, N fibers of one eye pair with T fibers of the other; or N+T' and N'+T.
And at the optic chiasm, by virtue of the CHIASM RULE, the corresponding members of the pair get together. This paring-up at the chiasm is the anatomical basis for:

fields

ANTERIOR DIVISION{Back to Cranky's substitute stuff}

That white arrow over there on the model points to the visual fields -- or to what we get when we project the overlapping fields of view of the two eyes onto a flat screen.
Notice, first off, how the quads (quadrants) in the fields flash to quads of the retinas. Note how that red zone in lower left field flashed onto the upper right quads of the two retinas. (Remember the left eye has a right side, too, Doc! Both eyes see some of both fields; your beak [nose] blocks what's far out in either your left or right field) Check out the silver quads. And on and on...

Why? Optics! That's the answer, optics -- lenses! Images project upside down onto the retinas. It's as though the entire field does a 180-degree cartwheel as it flashes onto the two retinas. The axis of rotation runs through central field and central retina. Check out that front-view diagram (click).


Now that Little Man in the eyeball2 does a flip not because he's an acrobat but -- again -- because of what the eye's optical devices (cornea and lens) do to an image.
This "optical flip" plus neuroanatomy (where the various nerve fibers go) make for what the tape calls the rules of opposites -- fields versus where the vibes go to the eye and, eventually, to the brain:

  • what's up goes down (and vice versa)
  • what's left goes right (and, of course, vice versa)
  • what's temporal (lateral) goes nasal (medial):

    The posterior division will let us add more to the rules of opposites, but before moving on to that, let's take a closer look at the at the beginning of the primary visual pathways; i.e., at the eyeball -- (click)..


(Dr. Randy Harris made this Triagram or 3-D diagram.)
close view of eyeball:
This is a right eye, Doc! How do we know that?
  • First off, notice the angle of the optic nerve. The optic nerve in our picture is angled back to plug into the optic chiasm. Also check out a real optic nerve in an MRI [click].)
  • Notice the Optic Disc and that it lies on the inner side of the macula,the zone of the retina that sees the central area of the visual field.
    The optic disc is also called the Optic Nerve Head; it's where the optic nerve fibers form up into the optic nerve and leave the eyeball. There aren't any rods and cones (photoreceptors) at optic disc. Functionally, the absence of those cells creates a hole in the retina at the optic disc -- the famous BLIND SPOT. (The mind usually ignores that blind spot and it takes special techniques to demonstrate its existence.)
  • Anyhow, an anatomist would say the optic disc lies nasal to (on the snozola or inner side of) the macula.
    The central area of the macula is called the fovea (where our vision -- or a monkey's -- is louse-picking sharp); the center of the fovea, called the foveoli, is where our vision is at its most acute. We use that foveola when, for instance, we thread needles, or, if we're chimps, when we supplement our diet by hunting for juicy nits while grooming a family member.
  • But, if you ever map somebody's visual fields, you'll see that the blind spot and macular zones are reversed -- on the map, that is! Why? The reason is the same as for why Flip's image stands on its head. So don't get uptight already when you see the blind spot mapped temporal to the macular map. (Just remember our pal Flip -- and the rules of opposites.)

{Back to Cranky's substitute stuff}

POSTERIOR DIVISION Optic Radiations (geniculo-calcarine pathways):
Lateral Dissection of the White Matter of Left Cerebral Hemsiphere:

Note: Meyer's temporal loop is part of the optic radiations. These fibers are from neurons in the lower, lateral sector of the LGB. The loop in question is a result of the downward, foward growth (in the fetus) of the temporal lobe in the human brain. The fibers of Meyer's loop swing foward and around the INFERIOR or temporal horn of the lateral ventricle before turning back toward the occipital lobe, there to plug into the anterior one-third of the calcarine fissure.

{For a diagram and more detailed discussion click here.}

THE OCCIPITAL ("OPTIC") LOBE

Central field (macular vision) maps onto the occipital pole (nearest the surface of the skull). The monocular zone of a visual fields -- from way out temporally -- map onto the anterior (and deepest) one-third of the visual cortex. The middle third, between the anterior and posterior thirds, receives binocular input from the paramacular (aka binocular peripheral) zones of the two retinas.

BRODMAN MAP AREAS ON THE MEDIAL SURFACE OF THE OCCIPITAL LOBE

HEMIANOPSIA
What kind of blindness will these lesions produce?
1. Total blindness in the affected eye -- anopsia
2. LEFT HOMONYMOUS HEMIANOPSIA with macular splitting
2X. LEFT HOMONYMOUS HEMIANOPSIA sometimes with macular sparing

3. BITEMPORAL (HETERONYMOUS) HEMIANOPSIA


Take note, Doc! The partial blindness (hemianopsia) is named for the affected FIELD, not the sick side of the brain!

Also, some books use the term hemianopia (no s). That's okay with Cranky, understand, but as the man on the tape says, OP means eye and OPSIS means see. Ergo, an-OPIA, means "no eye"; an-OPSIA means "no see." But go ahead Doc, take your pick.

Slide 20

Legend for Slide 20:
LGB, lateral geniculate body (nucleus); P, pulvinar (of thalamus) on reader's left; Nuc. III, oculomotor (or 3rd cranial nerve) nuclear complex; centr. gray is a cylinder of central gray matter surrounding the cerebral aqueduct or iter; post com., the posterior commissure is a bridge over the central gray matter used in pupillary reflexes; pre-text., the pretectal area(or nuclei), for which this plane of sectioning is named; {For a wider view, click here!}

{back to the diagram of the optic radiations}

 {To get back to discussion of Meyer's loop click here.}

(legend, ctd.)

br. sup. col., brachium of the superior colliculus, a branch of the optic tract with input for internal eye reflexes; (on the left side the latter appears as a dark triangle, apex skewed left, while on the reader's right the fibers in question appear as a dark tuft squeezing between the pulvinar and the medial geniculate body (nucleus),m; the apparent difference in the two sides is because the section is slightly (but fortuitously) cockeyed;sub. nigra, substantia nigra; the big F, the interpeduncular fossa -- so-called because it lies between the cerebral peduncles (pes pedunculi, crus cerebri); U, uncus (contains the amygdala, a basal nuclues), a primitive piece of the temporal lobe. The splenium of the corpus callosum lies at 12 o'clock. Notice the pineal body between the corpus callosum and the posterior commissure. This section is just anterior to (in front of ) the tectum (superior colliculi); thus the term "pre-tectal" area.

EYES IN MRI3

{back to close view of eyeball}

Ventricles:

Coronal Section Level of the LGB's
{To return to slide 20, click here!}

optic chiasm and intracranial segment of NII
{back to close view of eyeball}

DIAGRAM OF THE OPTIC RADIATIONS

Legend:
This diagram depicts a left side of the brain -- left LGB, left optic radiations and left calcarine fissure. [BLUE = LATERAL, RED = MEDIAL in the LGB]
Note: the uncus (U in Slide 20) is on the inner surface of the temporal lobe, a short distance behind (posterior to) the temporal pole; we use it as a surface marker.
The LGB looks like a cap for Dopey the 7th Dwarf, with a blunted apex and a broad base.
We can divide the LGB (precisely) from apex to base into LATERAL (here blue) and MEDIAL (red) halves. The lateral half-LGB (blue) receives signals on nerve fibers from below the equator of the retinas. The medial half-LGB (red) receives signals from above the retinal equator.
Note what this means in terms of the optical flip. Upper visual field flashes to lower retinas -- and thus to the lateral (blue) half-LGB. And, of course, vice versa for the lower visual field: lower-field vibes end up in the medial (red) half-LGB.
BUT THAT'S NOT ALL, DOC! When it comes to output, the lateral half-LGB (blue) signals the lingual (lower) lip of the calcarine fissure (blue). Of course, the medial half-LGB (red) sends its fibers to the cuneate (upper) lip of the calcarine fissure (red).
OR, In terms of visual fields: what appears in the upper half of the visual field flashes to the lower (lingual) lip of the calcarine fissure and and vice versa for the lower visual field. It's the rule of opposites all over again -- fields vis-à-vis occipital cortex
BUT WHAT ABOUT THE LGB and the rule of opposites? Does the LGB violate the rule? The answer is NO!
The rule holds for the LGB, too. How's come? The reason has to do with the embryo. In the embryonic LGB, what's blue here (lateral) is down and what's red (medial) is up. During development, the LGBs rotate so that what's down goes laterally and what's up takes a medial position. 		Legend, ctd.

There's also this about rotation of the embryonic LGB. What becomes the apex points posteriorly -- like the occipital pole. And the base of the embyo's LGB faces forward.
Notice how fibers from the apex go to the back of the calcarine fissure. Those from the base go to the anterior (front) of the calcarine fissure.
Now the apex of the LGB receives signals from the maculas of the two retinas. The middle zone receives signals from the two para-macular zones of the two retinas. But the base of the LGB receives signal from the nasal extreme of the opposite retina only.
What that means in terms of mapping onto area 17 (primary visual cortex) is this:

  • macular vision registers at the posterior third of 17 (i.e., occipital pole)
  • para-macular (bionocular peripheral) register on the middle third of 17
  • far peripheral vision -- monocular -- registers on the anterior third of 17
Can you figure out why the Loop of Meyer (from the lateral base of the LGB) carries signals about only the upper, outer (temporal-superior) quadant of the opposite visual field?
Can you dope out from the diagram why the words you're now reading at this very moment are flashing to the posterior tip of your own occipital lobe? {return to the dissection of the posterior division}

Footnotes:

  1. An item in the Electronic Reserve collection of the Indiana University School of Optometry Library
  2. This drawing of the eye is part of an audiotutorial created by Anthony J. Adams, OD, PhD, University of California at Berkeley, School of Optometry and produced by the School's Multimedia Center under an NIH grant to Dr. Adams. We are grateful to him for permission to use it.
  3. The MRIs were a gift of the late Dr. Hiroharu Noda.
  4. LGN = lateral geniculate nucleus! So which is it, LGB or LGN? A human LGB is actually a composite of two separate nuclei -- the dorsal and ventral lateral geniculate nuclei -- that grow together into a single mass in both primate evolution and human embryonic development. The ventral, phylogenically more primative group, are made up of relatively large neurons; in humans those cells form into two so-called magnocellular layers. The counterpart of the dorsal LGN of 'lower' mammals is made up of four layers of much smaller cells -- parvicellular layers (parvi is Latin for small). The six layers are crescent-shaped and stack in our heads, with the two magnocellular layers toward the base. The magnocellular and parvicellular layers have different function. We'll get into the details in a later lesson. For now, appreciate that the term LGB encompasses both nuclei. So is it LGB or LGN? As long as you're dealing with a human or an ape, and not a possum or a wombat, LGN will convey what you're trying to say (even though LGB is anatomically more precise).

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