ANATOMY OF VISION III,¹

-- Some fine points

SOUND CONTROL

89-min tape

Hi Doc. Back for more neuro, eh! Before you mash that play button, let old Cranky give you a few tips. And before it slips my mind, I better warn you of a blank space at about 46 minutes. That was where the tape guy was flipping -- the casette, not his lid. Anyhow, the blank lasts a couple of minutes. So when silence starts, if you aren't at about 80 plus minutes, don't quit or write nasty letters to the president of Indiana University, okay! The lesson puts polish on much of what we've already covered in the two earlier lessons in the Anatomy of Vision series. Ergo, if you haven't done those lessons, do yourself a favor, Doc. Go do 'em: Anatomy of Vision I Anatomy of Vision II So okay already, polish --bolish! (as the natives of a tiny island off the cost of North America might tell us)**: Don't forget the main points about the form and function of the visual pathways ( and if you don't remember, click here!) Now the students who used to take this course knew a lot about the eyeball before they ever listened to the tapes. If you know all about the eye, swell! But if not, and you need a fast overview of what you'll need to know for this tape , click here. The tape opens with quizzes on brains and slides and stuff most of us usually don't carrying around in our back packs (Frankenstein's assistant excepted). The easiest way to compensate is for your pal Cranky to slip you some pictures. Here's Cranky's control list of the important stuff (ignore the other things): [Remember to come back to the list if you get lost or tangled up or whatever.] Handouts (also what the tape man calls slide 74) Gross brain substitutes. Occipital Lobe (areas 17, 18 and 19) : when the tape man starts talking about the "uncut hemisphere" he's talking about what some people call the 'Optic Lobe' (click to get there). Coronal section through LGB Slide 20 Cadaver head [replicas models] -- (subs for 'Netter') For the UNCUS see U in Slide 20 Retina and optic nerve optic chiasm diagram LGB fiber projections -- to and from it Internal Capsule (MRI) Optic radiations Special dissection of the optic radiations. The Line of Gennari -- why area 17 is also called the 'striate' cortex -- a sketch of you'd see in a microscopic section of the visual cortex.

Slide 74 (see footnote 3)

Legend and notes for Slide 74 t= temporal, n= nasal, s = superior, i = inferior. The macular bundle (m) maintains its identity throughout the retinogeniculate pathways and is represented in black; but the equator and longitudinal meridan pass through m as well; i.e., m is binocular, contains corresponding t and n' fibers; m's lateral side contains RG fibers from below and the medial side from above the retina's equator. Item 1, the quadrants of retinas, represent the visual receptors. In the retina itself, the fibers require special attention. In item 2, the map does show how the RG fibers are organized just behind the eyeball (retrobulbar portion of the optic nerve); why they're organized like this is also taken up separately -- with the organization of fibers on the retina. Item 3 shows how fibers from various quarants of the retina map through most of the course of the optic nerve. Remember that fibers -- represented here -- are the 180-degree opposite of the zones of the visual fields. Item 4 is blank (and ignored). Why? The relative positions of the fibers become too complicated for a simple generzalization as the optic nerve approaches the optic chiasm. On item 5, we need a separate diagram to treat the optic chiasm (click here). In item 6, only the inferior (i) and superior (s) fiber zones are mapped, partially for simplification. But corresponding n and t fibers converge throughout much of the course of the optic tract. The corresponding fibers from the two eye twist and turn through much of the course of the optic tract -- to the extent that generalizations about distribution are not possible beyond what is depicted in the diagram; namely, RG fibers from below the retinal equator (TI, NI') tend to lie on the lateral side of the optic tract; those from above retinal equator (TS, NS') tend toward the medial portion of the optic tract. As the optic tract approaches its termination in the LGB, the macular bundle emerges on the tract's super surface; the inferior fibers lie on the lateral side and the superior fibers on tract's medial side, as represented in the diagram. Item 7. Towards the end of the optic tract macular fibers push to the tract's superior surface; the inferior fibers move laterally and superior fibers medially. The mapping of RG fibers approximates the receptive zones on the LGB. The LGB will be considered separately. But some generalizations can be made here: the LGB's lateral half receives inferior RG fibers and then projects to the lingual (lower) gyrus; the medial half LGB receives superior fibers and projects to the cuneate (upper) gyrus. Area 17 is synonymous with: primary visual cortex; striate cortex; calcarine cortex; visual area I. The apex of the LGB (receiving station for m) projects to the posterior one-third of 17 and the occipital pole; the middle zone recieves binocular peripheral fibers and projects to the middle one-third of 17; the base receives nasal fibers (N')from the contralateral retina (but no T fibers) and projects to the anterior one-third of 17; i. e., these are monocular areas of the LGB and 17; they correspond to far peripheral visual fields.

legend: 1. Optic Nerve (stump) 2. Optic Chiasm 3. Optic Tract (shaft of arrow crosses the uncus) 4. Tuber Cinerium (of the hypothalamus) m, Mammillary Bodies (rear of the hypothalamus) {for dissection scroll right}--> To return to Cranky's list, click!

scroll right, for medial view of Brodmann areas -->	Some investigators use the terms visual areas I, II and III for 17, 18 and 19, respectively. Scroll right for a lateral view --> Scroll down for more information!
	An occipital lobe receives signals about the opposite visual field. Area 17 is the primary target for the geniculocalcarine pathway. But experimental evidence indicates that area 18, via branches of the fibers to 17, also receives a smaller, mirror image projections of what maps onto 17. In turn, area 19 receives a mirror image of what maps onto 18. (The projection onto 19 appears seems to be from 18.) Now here's a freebe for you, Doc! Some the neuro big shots tell us that 18 has been found in all mammalian species but that only some of us have 19 (visual area III) . And let me remind you that in animals without any visual cortex at all (e.g., salamanders), the main target of the optic tract is the brainstem -- mainly, but not exclusively -- the optic tectum (aka, superior colliculus).

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Coronal Section Through LGB Check out Slide 20 for a stained section of the LGB (click)

A QUICK LOOK AT THE EYEBALL:

	The back of the eye 2 is made of three distinct coats: the outer sclera, the choroid and the business end of the eye, the inner retina, the beginning of the anterior division (retinogeniculate pathway) of the visual pathways. The retina is really a piece of brain displaced onto the face during embryonic life. Like brains in general, it processes information; i. e.,, in the philosophical sense of the word, the retina thinks! It's made up of several distinct layers, the retina (10 in the conventional wisdom) -- layers of nerve cells (neurons), their support cells, plus their fibers (axons and dendrites) and elaborate connections (synapses). We need a microscope to see those layers: (scroll down). Or else click back to Cranky's suggestions)
Light is the stimulus for vision. It's absorbed by the famous rods and cones, collectively the receptor cells. Energy (light) absorbed by a special part of receptor is used to make a signal -- a neural signal that's relayed (via synapses) to other parts of the retina and (eventually) to what are called retinal ganglion (RG) cells. Fibers of the RG cells literally are the business end of the anterior division of the visual pathways. Notice (in the picture on the right) how fibers of RG cell swoop over the inner [sic] surface of the retina literally to form the optic disc. Notice also how the outer coats of the eyeball (sclera and choroid) bend out of the way to make room for the RG fibers. (The broken arrow identifies slips of connective tissue, attached to the sclera, that create a sieve-like membrane at the optic disc. Bundles of RG fibers (about a thousand per bundle) pass through the holes in the sieve, en route becoming constituents of the optic nerve, per se. A map of the visual field projections onto the retina coincides with the receptors, not the distribution of fibers. What's the big deal about that? Lesions in the visual pathways obviously show up in the mapping of the visual fields. But, because of the arrangement of RG fibers in the retina, the patterns of deficits vary greatly depending on whether the lesion is in the eye or further along in the visual pathways. But we need a diagram to appreciate the general plan (click).

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Retina diagram RG fibers from the macula form a separate bundle (the papillo-macular bundle) in the retina and throughout the anterior division of the visual pathways. Macular fiber make a sharp, right-angle bend into the optic disc and actually make the lateral side of the optic nerve. For a short distance behind the eyeball, a "macular bundle" (m) lies on the outer side of the optic nerve. Injury there can affect macular vision (which can be bad news). The diagram shows how peripheral -- non-macular (TS, NS... etc) -- fibers from the various quardrants of the retina enter the optic nerve head. Note their relationships in the so-called retro-bulbar portion of the eyeball (retro means in back of )! A few millimeters beyond what the diagram depicts, the macular bundle moves into the core of the optic nerve. Through much of the optic nerve's intracranial course, the various fibers take relative positions that look much like the quadrants of the retina. (Note item 3 in the drawing of the pathways.)

Chiasm Details

chiasm diagram: {back to slide 74 -- composite diagram}

Chiasm Rule: {back to slide 74 -- composite diagram}

To follow the various fibers through the optic chiasm, it's useful to divide the chiasm into three planes lower (inferior) middle and upper (superior). Temporal inferior (TI) fibers move through the lateral edge of the lower plane and pass into the lateral, inferior part of the ipsilateral (same side) optic tract. En route to optic tract the TI fibers in question lie close to the lateral edge of the optic chiasm. The nasal inferior fibers (NI) have a goofy course; they hug the inner side of the optic nerve and then ride the anterior edge of the optic chiasm over to the contralteral (opposite) side; there, believe it or not, those NI fibers bend into the terminal stump of the contralateral optic nerve, as the famousanterior genu(knee); the NI fibers then proceed posteriorly, eventually to join with uncrossed TI fibers (not shown in the diagram) on the outer side of the optic tract. The anterior genu (knee) must be taken into account in evaluating visual field maps for lesions in the visual pathways. (Note: some older textbooks show diagrams of a posterior genu. The alleged posterior genu has not actually been observed by investigators and, therefore, is not represented here.) Macular fibers are much more numerous than the diagram may suggest; they're found in all three strata of the optic chiasm but are most concentrated in occupy the middle stratum. Macular fibers representing the nasal hemi-retina take a fairly straight course through the chiasm and into the contralateral optic tract. In the upper third of the chiasm, the uncrossed temporal superior fibers(TS) move into the medial side of the ipsilateral optic tract. The crossed nasal superior fibers, NS, also enter the superior plane of the chiasm and move posteriorly as they cross but take a generally straight course, eventually to join the TS fibers on the medial side of the optic tract.

The LGB: Projections of Fibers from the Retina

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Slide 20 (For plane of sectioning, click here.) Take a close look at that LGB on the left and notice thick pink and thin navy blue concentric layers. The pink layers are cells (of the lateral geniculate nuclei); the dark blue are layers of fibers (their myelin wraps take the blue stain). We can't see it in a single section (because the LGB is a 3-dimensional object). But apes and humans have six distinct layers of cells in an LGB. The layers curve but are conventually numbered from "base" to "apex" as 1 thru 6. The layers receive fibers from different territories of the retinas. For a diagram, scroll right-->.
RETINOGENICULATE MAPPING ---> Harrington tells us that a mathematically precise relationship exists between points in visual space and specific receptors in the retina5. This precision is maintained in the mapping of the retina onto the LGB and, in turn, of the LGB to the calcarine fissure (and to other areas of the brain, such as the superior colliculi, of which we'll say more in a later lesson). But the mapping in question, while highly specific, is far from simple. (The LGB, like the rest of the thalamus, is no simple relay station; its cells sort and process visual sensory information before transmitting elsewhere.) Branches from crossed (nasal) fibers synapse with specific groups of neurons in layers 1, 4 and 6. The other layers -- 2, 3 and 5 -- receive uncrossed (temporal) fibers. Electrophysiological data indicate that groups of neurons receiving signals from corresponding (homonymous) foci on the two retinas are organized into specific columns within the LGB. A given column, in turn, distributes the output to highly specifric places in area 17, as well as 18 (via collateral branches) and to other parts of the brain (e. g., midbrain) (For a diagram scroll right) ---> Some layers serve different functions from others (scroll down).	Recall that the nasal-most zones of the retinas correspond only to the far peripheral zones of the visual field and, therefore, are monocular (meaning that they're represented in only one eye.) Obviously, the monocular fibers are crossed. Since there are no corresponding temporal fibers for the latter, monocular columns in the LGB are made up of cells in only layers 1, 4 and 6. (Inspired by LeGros Clark, 1932.)
Layers 1 and 2 are made up of relatively large cells and, accordingly, are called magnocellular layers. These two layers are homologous to the ventral lateral geniculate nuclei of "lower" mammals. In primates the main output of layers 1 and 2 is to the brainstem (pretectum and midbrain) and seems to be used primarily in ocular reflexes and eye movements; but layers 1 and 2 also signal area 17 (possibly to integrate higher visual finctions with various reflexes); 1 and 2 may also send to the hypothalamus. Layers 3-6 are mode up of much smaller cells, hence the name parvicellular layers (parvi means small). The principal output from the columns in 3-6 is to the primary visual cortex (area 17). Areas 18 and 19 also receive projections from the LGB. Some evidence suggests that the LGB receives feedback signals form the visual cortex. (Scroll right for diagram.)--> To return to Cranky's list, click!

INTERNAL CAPSULE, as seen in the living brain (MRI):4 Emerging from the LGB, the fibers of the geniculocalcarine pathways move up into the retrolenticular portion of the internal capsule before spreading out as the optic radiations. (Recall the etymology of retro-lenticular -- behind the lenticular nucleus; i.e., the globus pallidus and putamen, collectively.) This particular MRI fortuitiously shows a portion of the corpus callosum known as the tapetum, which permits directly intercommunication between the right and left occpital lobes . The evidence suggests that 17 of one side communicates with 18 of the other; i. e., not 17 to 17. {Back to Cranky's list}

OPTIC RADIATIONS (Geniculocalcarine tracts)

(left side of dissected human brain, lateral perspective) Scroll right for a diagram ---> For another perspective on the optic radiations click here.

(scroll right for more--->

scroll right for more--->

Microscopic view of Area 17 (striate cortex, calcarine cortex, primary visual cortex) -- a sketch:

The line (stria, band) of Gennari comes to an abrupt end where area 17 terminates and area 18 begins. Under the light microscope, at low power, in a stained (luxol blue-cresyl violet) section including both areas, Gennari's line appears as closely packed, purple, hachure marks roughly half way between the surface of the cortex and the white matter (medullary substance).
The cerebral cortex is layered -- six layers in the conventional wisdom, numbered I-VI from the surface to the white matter. The line of Gennari is found in layer IV (and is analogous to much thickened outer band of Baillarger in other parts of the cortex). Layer IV is prominent in sensory-type cortex.

Plane of sectioning for slide 20:
{Back to slide 20}

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SPECIAL DISSECTION OF THE OPTIC RADIATIONS (five viewing angles):

View 1 Scroll right for 2----> Scroll down for 3

2 (Top-right view of same specimen as in 1

3 (Left, lateral view of 1): That big pit is where the lenticular nucleus was scouped out to expose the lateral side of the internal capsule. Most of the outside of the occipital lobe was shaved away to expose the optic radiations. (The frontal pole points left and the occipital pole, still there, points right.) What can we take away from a picture like this? For one thing, Doc, the optic radiations -- like the internal capsule, the corpus callosum, certain bundles and so-called 'fasciculi' -- are part of the cerebral hemisphere's massive white matter, (aka, medullary substance); the cortex, recall, is gray matter (location of the neuronal cell bodies). What makes for 'whiteness,' incidentally, is the suet-like myelin sheath around each of the millions and millions of fibers. The cortex, on the surface of the cerebrum, has many, many fibers too, but no myelin. Functions? White matter communicates, gray matter processes ('thinks'). For a closer look at view 3 click here.

4 (Closer view of the perspective in 3): {back to View 1} Scroll down for 5.

5 (A view of 4, but from the other side.) Note the lips of the calcarine fissure, Doc! (Anatomists actually call them 'lips,' believe it or not (some do, anyway). In this picture, we're looking at the primary visual cortex (aka area 17, or striate cortex) -- cuneate gyrus above, lingual gyrus below. You've got one of those things inside your own head, Doc! {back to View 1 of dissection} (back to Cranky's list)

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1. An item in the Electronic Reserve collection of the Indiana University School of Optometry Library
2. These pictures of the eye are modified from graphics in an audiotutorial created by Anthony J. Adams, OD, PhD, University of California at Berkeley, School of Optometry and produced in that School's Multimedia Center under an NIH grant to Dr. Adams. We are grateful to him for permission to use his materials.
3. Acknowledgement. Jacque E. Kubley drafted this diagram in the academic year 1972-73 from an original drawing of the author (Pietsch) while Kubley was in the employ of the Indiana University School of Optometry as a graphic artist and photographer.
4. This MRI was a gift of the late Dr. Hiroharu Noda.
5. Harrington, D. O. The Visual Fields, Mosby, St. Louis, 1976.

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