Lecture 14 - Visual Pathway & Visual Field Defects

What are the components of the visual pathway?

Retina, optic nerve, optic chiasm, optic tract, lateral geniculate nucleus (LGN), optic radiations, and visual/striate cortex.

What is the optic chiasm the location of?

Crossing of nasal retinal fibers.

Where is the optic chiasm located?

In the subarachnoid space.

What surrounds the optic chiasm?

Meningeal sheaths and cerebrospinal fluid (CSF).

What structure is above the optic chiasm?

Floor of the third ventricle.

What structure is above the optic chiasm?

Hypothalamus.

How far below the optic chiasm is the pituitary gland?

Approximately 1 cm below the optic chiasm.

Where is the lateral geniculate nucleus (LGN) located?

On the dorsolateral thalamus.

What is the lateral geniculate nucleus the site of?

Termination of retinal ganglion cells from the optic tract and beginning of optic radiations.

What is the structure of the lateral geniculate nucleus?

Cone-shaped, layered structure.

Which LGN layers are magnocellular?

Inferior layers 1–2.

What are the magnocellular layers called?

M1 and M2.

Which LGN layers are parvocellular?

Superior layers 3–6.

What are the parvocellular layers called?

P1, P2, P3, and P4.

Which LGN layers are koniocellular?

In-between layers.

What are the koniocellular layers called?

K1, K2, K3, K4, K5, and K6.

Does the LGN receive retinal projections from both eyes?

Yes.

Which LGN layers receive crossed/contralateral fibers?

Layers 1, 4, and 6.

Which LGN layers receive uncrossed/ipsilateral fibers?

Layers 2, 3, and 5.

What are optic radiations?

Fibers beginning at the lateral geniculate nucleus and traveling to the visual cortex.

What is another name for the visual cortex?

Striate cortex, Brodmann area 17, and V1.

Where is the visual cortex located?

Medially at the most posterior aspect of the occipital lobe.

Which part of the visual cortex is most buried internally?

The most posterior portion.

What is the approximate thickness of the visual cortex?

Approximately 2 mm.

Where do macular fibers terminate in the visual cortex?

In the most posterior portion of the cortex.

Where does the superior macula terminate in the visual cortex?

Cuneus gyrus.

Where does the inferior macula terminate in the visual cortex?

Lingual gyrus.

Which part of the cortex contains the most peripheral fibers?

The most anterior portion of the cortex.

How is macular representation within the striate cortex compared to retinal area?

Disproportionate to its overall retinal area (cortical magnification)

Why is macular representation more extensive?

For fine, detailed vision.

Why is macular representation disproportionate?

Higher cell density in the fovea than the periphery.

What is the retinal ganglion cell density in the fovea?

30,000 RGCs/mm².

What is the retinal ganglion cell density 10 degrees away from the fovea?

5,000 RGCs/mm².

How is the striate cortex organized?

Horizontal layers and vertical columns.

How many main horizontal layers are in the striate cortex?

Six main layers, some with sublayers.

What do the horizontal layers correspond to?

Input from both eyes.

What type of information is associated with magnocellular input in the cortex?

Motion and low spatial frequency.

What type of information is associated with parvocellular input in the cortex?

Color and high spatial frequency.

What are ocular dominance columns?

Respond preferentially to visual information from one eye.

What are orientation columns?

Respond preferentially to different stimulus orientations.

What does the LGN modulate?

Transfer of visual information to the cortex.

What is spatial resolution?

Smallest detectable difference between separate objects that can be distinguished.

What is temporal resolution?

Shortest time period between two separate stimuli that can be distinguished.

What is contrast sensitivity?

Ability to distinguish an object from its background.

What is another name for retinal input to the LGN?

Retinogeniculate pathway.

Which retinal ganglion cells project to magnocellular layers?

M (parasol) cells.

Which retinal ganglion cells project to parvocellular layers?

P (midget) cells.

Which retinal ganglion cells project to koniocellular layers?

All other cells.

Do retinal axons contribute to the majority of synapses within the LGN?

No.

What do retinal axons drive within the LGN?

The majority of LGN function.

What is the largest source of synaptic input to the LGN?

Striate cortex.

What type of pathway is cortical input from striate cortex to the LGN?

Modulatory pathway.

What structure provides inhibitory control to the LGN?

Thalamic reticular nucleus.

What are the functions of the thalamic reticular nucleus?

Suppress irrelevant visual input and modulate response rate.

Which brainstem structure sends input to the LGN related to motion processing?

Superior colliculus.

What superior colliculus functions are related to LGN input?

Saccades and head movements.

What cortical area are superior colliculus projections related to?

Middle-temporal (MT) area of cortex.

Where does LGN information mainly project in the striate cortex?

Layer IV.

Which striate cortex sublayer receives magnocellular input?

4Cα.

Which striate cortex sublayer receives parvocellular input?

4Cβ.

What is reciprocal innervation between the LGN and striate cortex?

LGN projects to striate cortex and striate cortex projects back to LGN.

What is the function of LGN interneurons?

Inhibitory control within the LGN itself.

What type of receptive fields do LGN neurons exhibit?

Center-surround receptive fields.

What exists between the center and surround of LGN receptive fields?

Spatial antagonism.

What do LGN receptive fields mirror?

Retinal ganglion cell receptive field structures.

What can LGN receptive fields correspond to?

Illumination or color.

What is color opponency?

Optimal colors stimulating receptive field center and surround differ.

What is an example of color opponency?

Red ON-center.

What happens when long-wavelength light stimulates the receptive field center of a red ON-center cell?

Fires.

What happens when low-wavelength light stimulates the receptive field surround of a red ON-center cell?

Stops firing.

What are the characteristics of the inferior layers (M1–M2) of the LGN?

Magnocellular.

What type of spatial resolution do magnocellular layers have?

Lower spatial resolution.

What type of temporal frequency do magnocellular layers have?

Higher temporal frequency.

What type of contrast sensitivity do magnocellular layers have?

Higher contrast sensitivity.

What type of responses do magnocellular layers generate?

Transient, rapid responses.

What type of illumination do magnocellular layers respond to?

Monochromatic illumination.

What stimulates ON-center magnocellular cells?

Small light stimuli on dark background.

What stimulates OFF-center magnocellular cells?

Small dark stimuli on bright background.

What are the characteristics of the superior layers (P3–P6) of the LGN?

Parvocellular.

What type of spatial resolution do parvocellular layers have?

Higher spatial resolution.

What type of temporal frequency do parvocellular layers have?

Lower temporal frequency.

What type of responses can parvocellular layers generate?

Sustained responses.

What type of color opponency do parvocellular layers exhibit?

Red / green color opponency.

What are the four red/green opponency types of parvocellular cells?

Red ON-center, Green ON-center, Red OFF-center, and Green OFF-center.

What are the characteristics of the in-between layers (K1–K6) of the LGN?

Koniocellular.

What type of contrast sensitivity do koniocellular layers have?

Higher contrast sensitivity.

What type of responses do koniocellular layers generate?

Transient, rapid responses.

What type of color opponency do koniocellular layers exhibit?

Blue / yellow color opponency.

What type of stimulus do koniocellular layers respond to?

Illumination.

How can koniocellular cells respond to brightness changes?

Can respond to any stimulus brighter or darker than surround.

What inhibits koniocellular cells?

Any stimulus covering receptive field surround.

What is the afferent input to Layer IV of the striate cortex?

LGN.

Where does information move after entering Layer IV?

To more superficial layers.

What is introduced when information moves from Layer IV to more superficial layers?

Binocular input.

Which layers of the visual cortex project to other cortical areas?

Layers III and IV.

What is the function of Layers III and IV projections?

Higher order processing.

Which layer of the visual cortex projects to the brainstem?

Layer V.

Which brainstem structures receive projections from Layer V?

Superior colliculus and pons.

What are the functions of Layer V projections?

Motion and eye movements.

Which layer of the visual cortex projects back to the LGN?

Layer VI.