Lateral Geniculate Nucleus & Functional Testing - Visual Neurophysiology and Perception Spring 2026

along their axons and out of the optic nerve

The action potentials generated in RPCs are carried where?

to the dorsal lateral geniculus (LGN) of the thalamus

In primates, 90% of the RGCs project to where?

glutamate

RGCs release ______ onto target neurons

LGN in Brain (Pic)

LGN in Brain (Pic)

true

True or False:

In general, the response of the LGN neuron is similar to that of the RGC that provides input to it

Yes

Do LGN neurons have center-surround spatial antagonism?

LGN neurons have stronger center-surround antagonism

Is center-surround antagonism stronger in RGCs or LGN nuerons?

minimal response

Diffuse light to LGN neurons results in what d/t strong center-surround antagonism?

When shadows, increased light, or highlights are present

When will the LGN neurons send stronger responses to the visual cortex?

No -- there is not dramatic processing of the visual signal here but it is not only a relay station either

Is the LGN a "relay station"?

Yes -- increased lateral inhibition (center-surround antagonism) at this structure

Is there modification to the visual signal at the LGN?

gate

The LGN can be thought of as a "____-"

Controlling the flow of info to higher brain centers so that only the most relevant or interesting info passes through

Why can the LGN be thought of as a gate?

inhibitory

In addition to excitatory input from the retina, LGN also receives _____ input

-From the visual cortex

-Mesencephalic reticular formation for saccadic suppression

-Inter-neurons within the LGN -- Magno neurons can inhibit Parvo

Where does the LGN receive feedback inhibition from?

true

True or False:

Are different qualitative attributes of the visual image separated out within the retina into and carried onto the brain (LGN) via different pathways?

Separate RGC classes transmit different sensory messages to the brain

What is the basic organizational principle that is often applied to the visual pathway?

True

True or False:

The cells of the LGN have a specific anatomical orientation

6

How many layers are in the LGN in cross-section?

each layer received input from either the ipsilateral or contralateral eye

What is the importance of each of the 6 LGN layers?

true

True or False:

The cells in each of the different layers of the LGN have a different physiological property

Magno Layers

What is the name of the 2 bottom-most layers of the LGN?

larger cells

What are the characteristics of the Magno layers compared to Parvo?

3, 4, 5, 6

What are the "parvo" layers of the LGN?

1, 4, 6

What layers of the LGN receive contralateral eye input?

2, 3, 5

What layers of the LGN receive ipsilateral eye input?

Parvo (90%); Magno (10%)

Magno vs Parvo Physiology of LGN Neurons

Which are more predominant?

None

Magno vs Parvo Physiology of LGN Neurons

Color Opponency of Magno Neurons present?

More transient

Magno vs Parvo Physiology of LGN Neurons

Response duration of Magno Neurons?

fast (high speed of transmission)

Magno vs Parvo Physiology of LGN Neurons

Response Timing of Magno Neurons?

Large receptive field w/ center-surround antagonism

Magno vs Parvo Physiology of LGN Neurons

Receptive field of Magno Neurons?

LOW

Magno vs Parvo Physiology of LGN Neurons

Magno: Better responses to what spatial frequencies?

Yes

Magno vs Parvo Physiology of LGN Neurons

Color Opponency of Parvo Neurons?

more sustained

Magno vs Parvo Physiology of LGN Neurons

Response Duration of Parvo Neurons?

slower -- more delayed

Magno vs Parvo Physiology of LGN Neurons

Response timing of Parvo Neurons?

Smaller receptive field w/ center-surround antagonism

Magno vs Parvo Physiology of LGN Neurons

Receptive Field of Parvo Neurons?

HIGH

Magno vs Parvo Physiology of LGN Neurons

Parvo: Better responses to what spatial frequencies?

true

True or False:

The differences in the responses of the two classes of LGN neurons should remind you of the differences between parasol and midget RGCs

Parvo (P pathway)

Midget RGCs project to what LGN neuron?

Magno (M pathway)

Parasol RGCs project to what LGN neuron?

where; what

The idea emerged in the 1980s that there where two major visual pathways -- a ____ system and a _____ system

-flicker

-movement

-alters us to the presence of a visual stimli

What is the Magno system good at detecting?

providing info regarding the spatial details of a stimulus

What is the Parvo system good at?

small bistratified RGCs

What is the retinal component that connects to the konicellular pathway of the LGN?

limited (Blue ON, Yellow OFF)

Does the konicellular pathway of the LGN show color opponency?

large receptive field

What is the size of the receptive field of the konicellular pathway of the LGN?

No

Does the konicellular pathway of the LGN have center-surround antagonism?

lower

What spatial frequency does the konicellular pathway of the LGN best respond to?

slow responses -- very slow speed of transmission

What is the speed of responses of the konicellular pathway of the LGN?

IN BETWEEN the 6 principle LGN layers

Where are the konicellular pathway neurons of the LGN located?

The idea is that different channels in the visual pathway are tuned into specific spatial frequencies that exist within a given image

Why is the visual system classified as a Fourier analyzer?

Yes

Does each cell of LGN have a peak sensitivity in terms of spatial frequency?

Glaucoma is a group of progressive optic neuropathies that have in common a slow progressive degeneration of RGCs and their axons; resulting in a distinct appearance of their optic disc and a concomitant pattern of visual loss

What is the definition given for glaucoma?

Intraocular pressure

What is the most important known risk factor for primary open angle glaucoma?

medical or surgical procedures designed to lower IOP

All current therapies for this disease involve what?

Yes

Do the results of wide-scale clinical trials support that IOP reduction is beneficial in decreasing POAG incidence/progression?

RGCs

_____ are the only cell that is affected in human glaucoma

The initial damage to the RGC axon occurs at the lamina cribosa. This is AXONAL injury -- not CELL BODY injury.

Where are RGCs injured in glaucoma?

sheets of the lamina cribosa get compressed and pushed outward

Earliest changes in the optic nerve in POAG show what happening to the lamina cribosa?

superior and inferior; arcuate defect superior and inferiorly and early cupping changes superior and inferior

Where are the thinner lamina cribosa beams & bigger pores? What is the clinical implication of this?

superior or inferiorly

In glaucoma, the earliest changes appear ____ or ____

vertical

The (vertical/horizontal) C/D ratio correlates to glaucomatous progression

RGC axons are pinched at the lamina cribosa, disrupting axoplasmic flow. This flow constantly carries vital messengers (neurotrophins like BDNF) from the brain target back tot he RGC cell bodies of the retina. If the axonal transport is affected and BDNF does not reach the RGC cell bodies, the cell will initiate its own death.

What is the neurotrophic deprival hypothesis that explains why RGCs die in glaucoma?

The RGC will initiate its own death

What happens if the BDNF neurotrophin is affected and does not reach RGC cell bodies?

Yes -- RGCs that do not reach the correct target eliminate themselves

Is BDNF important embryonically?

apoptosis

Neurotrophin deprivation will genetically program cell death called ______

-Each affected cell expresses enzymes to digest its own DNA and other cellular components

-The cell then fragments into small vesicles (no membrane rupture) and removed without inducing inflammation

How does a cell undergo apoptosis?

No

After apoptosis of a RGC, will inflammation result?

No Neurotrophins = Apoptosis

Neurotrophins Present = RGC Growth (Pic)

No Neurotrophins = Apoptosis

Neurotrophins Present = RGC Growth (Pic)

Yes -- meshes well with classical mechanical theory of damage

is neurotrophin deprivation an attractive theory for glaucoma?

by cutting the optic nerve (in vivo) or depriving cultured RGCs (in vitro)

How can you indice neurotrophin deprivation experimentally?

Apoptotic

________ RGCs have been detected in human glaucoma

No

Is vascular compromise mutually exclusive to neurotrophin deprival hypothesis?

Yes

Does decreasing the blood supply to the optic nerve also result in RGC death?

acute ischemia of the optic nerve

What is ischemic anterior optic neuropathy d/t?

optic nerve pallor

What does ischemic anterior optic neuropathy result in?

No

Does ischemic anterior optic neuropathy result in optic nerve cupping?

Yes -- elevated IOP is a causative factor for glaucomatous damage

Does the experimental elevation of IOP in an animal induce RGC death?

Yes

Does chronic and moderate elevation in IOP induce RGC death & optic nerve cupping?

Rat Glaucoma Model -- Disrupting TM outflow w/ Gradual Elevation of IOP (Pic)

Rat Glaucoma Model -- Disrupting TM outflow w/ Gradual Elevation of IOP (Pic)

50

Research on human cadaver eyes from glaucoma patients suggested that VF defects were not detectable until about _____% of RGCs had died

Yes -- loss seen when 50% of RGCs already gone

TAKE HOME MESSAGE: When a patient presents with a VF defect using standard perimetry, has there been a least moderate damage already?

Glaucoma Detection Model (Pic)

Glaucoma Detection Model (Pic)

does not test very many spots in a patient's central field of vision

What is a limitation of standard 24-2 or 30-2 automated VFs?

peripheral

What is the long standard belief? Glaucoma affects _____ vision first

perimacular RGCs

Recent work by Don Hood shows that _____ are affected early in glaucoma

structural OCT and 10-2 functional assessment of the macular region

On what tests can you pick up the early changes in vision and perimacular RGC loss in glaucoma?

Yes -- do not necessarily show up on 24-2 VF

Are macular defects in patients with POAG easy to miss?

designing clinical tests that better detect early RGC loss in glaucoma patients

What is the emphasis on in early detection for glaucoma?

-OCT

-Scanning Laser Polarimetry

-HRT

-FAF

What are the structural tests that are involved in the early detection of glaucoma?

-VF (speed, reliability)

-SWAP

-Frequency doubling

What are the functional tests that are involved in the early detection of glaucoma?

-clinician grading of the ONH

-fundus photo

20-40 years ago, _____ were the main structural assessments of glaucoma

Yes

Can certain diseases impact one pathway more than another?

You might be able to detect the disease earlier with special clinical tests

Why is it important to know which pathway of vision a certain disease will impact?

S cone driven konio

_______ pathway shows reduction in diabetic retinopathy and glaucoma

stimulus/background

You can adjust the ______ parameters to selectively identify a specific cone class

blue; yellow

To isolate S cones, use a ____ stimulus light on a ____ background

increases

Isolating S Cones

Based on Weber's Law, as intensity of a background increases, the absolute intensity of the target light at detection threshold (increases/decreases) -- so that the ratio of the target light to the background intensity would change

Yes

Do cones adapt to light?

M and L cones will adapt to the yellow light background, the threshold is raised (sensitivity decreased), and increment threshold will then be mediated by S cones

What is the wrinkle to Weber's Law when using a yellow light background compared to a white light?