Lecture 31: Visual System II, Central Visual Pathways

Question 1

How can seeing be defined?

Answer 1

Using optical input to the two eyes to know what things are, where they are and to guide action based on this knowledge

Question 2

What are the central projections of the optic nerve?

Answer 2

1. Superior colliculus
2. pretectal complex (midbrain)
3. Accessory optic nuclei (midbrain)
4. suprachiasmatic nucleus (hypothalamus)
5. Lateral geniculate nucleus (Palmer does researchhere)

Question 3

What is the function of Suprachiasmatic nucleus (SCN)?

Answer 3

Optic nerve input synchronizes circadian rhythms to the light dark cycle
1% RGCs project here

Question 4

What is function of superior colliculus?

Answer 4

Generates orienting head and eye movements
Also gets extensive inputs from visual and other cortices
Mostly “M-cell” (movement) input from retina

Question 5

What is the function of pretectal nuclei?

Answer 5

Efferents to Edinger-Westphal for parasympathetic fibers

-pretectum is part of tectum in midbrain

Question 6

What is function of accessory optic nucleus?

Answer 6

Reflex following movement.

Inputs from other category

Question 7

What is the Retino-geniculo-cortical (striate) pathway?

Answer 7

Photoreceptors  LGN  primary visual cortex (striate) in the occipital lobe

Question 8

What is the Optic nerve

Answer 8

distal to optic chasm (most anterior)

Question 9

What is the Optic tract

Answer 9

proximal to optic chasm (more posterior)

Question 10

What are the hemifields that each eye have?

Answer 10

1. Nasal hemifield

2. Temporal hemifield

Question 11

What will pathology in optic tract involve?

Answer 11

Both eyes because optic tracts contains optic fibers from ipsilateral temporal hemifield and contralateral nasal hemifield

Question 12

What will pathology in optic nerve involve?

Answer 12

Complete blindness in only one eye

Question 13

Where is the right visual field represented in the cortex?

Answer 13

In the left (contralateral) cerebral hemisphere

Question 14

What axons cross in the optic chiasm and what axons remain ipsilateral?

Answer 14

Nasal hemifield cells cross

Temporal hemifield cells remain ipsilateral

Question 15

If you view something from the right side, where does the light hit the eye?

Answer 15

On the temporal hemifield of the left eye
And
On the nasal hemifield of the right eye

Question 16

What is a homomynous defect?

Answer 16

When you have a defect on the same side of both eyes (nasal on one eye and temporal hemifield in the other)

Question 17

What is a hemiopsia?

Answer 17

When you lose sight on one hemifield

Question 18

Where do the two visual fields (R and L) overlap?

Answer 18

From -60 to 60 degrees

Question 19

What is function of LGN?

Answer 19

Specific thalamic relay nucleus for the primary visual cortex
Main pathway for seeing
Inputs for M and P cells

Question 20

How many layers are in the LGN? How are they organized?

Answer 20

6 layers total (lamina)

Organized according to the type of RGC afferents making synapse at each one

Question 21

Where do M cells terminate in LGN? P cells? Contralatearl vs. ipsilateral eye?

Answer 21

1 and 2 (magnocellular layer)
3-6 (parvocellular layer)
Contralateral and ipsilateral layers are SEPARATE (1,4,6 and 2,3,5 respectively)

Question 22

Where do the on/off center cells segregate?

Answer 22

In the parvocellular layers

Question 23

If you see layers 1, 4 and 6 lesioned on the right side of LGN, what eye is lesioned?

Answer 23

The contralateral or the left one

Question 24

How is the LGN organized?

Answer 24

Lateral geniculate nucleus (LGN) is organized in a retinotopic map
Translation: two eyes project into LGN in such a way that adjacent points on the retina are represented by adjacent points in LGN.
Much more space is given to the fovea and central visual field (presumably to focus more on acuity)

Question 25

What is the role of the brainstem in waking vs. sleeping state?

Answer 25

When awake, brainstem sends signals to keep resting membrane potential When sleeping, brainstem no longer sends signals and allow thalamicocortical cells (TC) to hyperpolarize

Question 26

What is the striate cortex? Where is it located?

Answer 26

V1, primary visual cortex It is the largest functional area of the cerebral cortex -controls visuotopy It is located on the upper and lower banks of the CALCARINE FISSURE

Question 27

In the striate cortex, how are the upper and lower visual fields represented?

Answer 27

The upper visual fields are represented on the lower bank of the calcarine fissure Lower visual fields are represented on the upper bank of the calcarine fissure

Question 28

What does it mean to call thalamus a gate?

Answer 28

It means thalamus controls waking and sleeping state…so it dictates whether to allow external stimuli to activate your cortex or not

Question 29

Where do M and P cells terminate in the striate cortex?

Answer 29

M and P cells terminate in layer IV (out of VI in cortex) But they terminate in NON overlapping layers WITHIN layer IV M cells send shit up to areas II and III -then those neurons send shit to Areas 18 and MT -also projects to V and VI P cells terminate in IV C (beta) -go up to layers 2 and 3 -go to layers V and VI (which then go to pons, LGN respectively) -also go to areas 18 and V4

Question 30

What is the significance of layer IVB of the striate?

Answer 30

- orientation selective - direction selective - stereoscopically selective

Question 31

How are receptive fields represented in visual cortical cells (visual cortical cells)?

Answer 31

They are organized into columns and segregated from bright and dark On-center and Off-center receptive fields in IVCalpha are segregated (can be tilted 45 degrees or can be alternating columnar…off-center – on-center – off –center) -different IVB cells are stimulated better by different orientations of stimuli

Question 32

What types of modular organization is present in visual cortex?

Answer 32

i. orientation selectivity ii. direction selectivity iii. ocular dominance iv. color

Question 33

What is the ocular dominance (OD) modular organization

Answer 33

-OD bands in cortex project in alternating zones -some lamina are represented by ipsilateral eye -some laminae are represented by contralateral eye -depending on where axons come from in the eye, there are also alternate inputs into the striate cortex Significance: it means cells of thalamus or cortex were driven ONLY by cells coming from one eye -each is ISOLATED in its representation…no overlap between the two

Question 34

What is the color modular organization?

Answer 34

Different blobs in striate control for color (separated in spaced blobs) Located in layers 2 and 3 of striate The ones that stain positive cytochromoxidase as a blob, means that’s where the color is being processed

Question 35

What is the relationship between blobs (color) and ocular dominance modular organization?

Answer 35

If you lift up layers 2 and 3, you will see the OD organization in layer 4

Question 36

What is organization selectivity modular organization?

Answer 36

It is characterized by “pinwheels” (the colored pinwheels, remember?) - orientation is around the clock (around the pinwheel) - it means these are the organization of how all possible representation of visual field are reflected in the striate

Question 37

When you take the OD bands, CO blobs and pinwheels are put together? Composite modular organization?

Answer 37

The color blobs are always sin the middle of the OD bands - centers of the pinwheels are always on the CO blobs…so always in the center of the OD bands) - this organization (color blobs in middle of OD bands in middle of pinwheels) is repeated again and again

Question 38

What is the function of lateral connection in striate?

Answer 38

Allows the different modalities of the cortices (orientation, color, edges) to interact with one another to form tangible precepts of reality

Question 39

What are the 3 functions of lateral connection in striate?

Answer 39

i. excitatory Significance: interneurons can excite neurons whose receptive fields do NOT overlap at all with the receptive field of the cortical area that was excited ii. like orientations iii. between columns whose receptive fields are non-overlapping space Example: allows brain to see a triangle in “kaniza triangle” because the cells that do see a contrast on the edges of triangle are able to relay that signal to make an illusory edge of a triangle Production of a “precept” of an edge

Question 40

Things to remember about Retino-GENICULO-strate pathway

Answer 40

-retinotopically organized -simple map with central magnification (fovea) M&P cells in separate layers Ipsi and contralateral eyes in separate layers Gating by extraretinal inputs

Question 41

Thing sto remember about Retino-geniculo-STRIATE Pathway

Answer 41

-retinotopically organized LGN axons terminate in layer IVC with M and P inputs and LE/RE inputs separate Molecular: orientation, ocular dominance, color Binocular cells (stereopsis) – Layer IVb Diverse outputs: superior colliculous, pontine nuclei, LGN and other cortical areas M and P largely separate all the way Long range connections modify responses elicited from the receptive field