Visual Input (Dr. Bianco)

Question 1

What is stereopsis ?

Answer 1

The perception of depth produced by the reception in the brain of visual stimuli from both eyes in combination; binocular vision.

Question 2

Are the blindspot and fovea located more on the nasal side or temporal side of the retina ?

Answer 2

The nasal side.

Question 3

What are the two most common eyes mvnts ?

What is their purpose ?

Answer 3

Smooht pursuit eye mvnts (left-right) and convergence eyes mvnts (forward-backward).
Both are voluntary eye mvnts that maintain the image of a moving target on the fovea of both eyes.

Question 4

What are saccades ?

Why are they useful ?

Answer 4

Saccades are v fast eye mvnts that redirect gaze. These mvnts of different amplitudes occur several times per second. They enable fast foveal sampling of interesting/important object features and prevent image stabilization of the retina (stabilized images fade rapidly).

Question 5

Does the retina detect:

• the absolute value of of light intensity ?
• the differences in light intensity ?

Answer 5

DIFFERENCES !

Like most processes, the nervous system is good at detecting change, not absolute things.

Question 6

What imp features can the retina detect ?

Answer 6

Wavelength = color
Contrast = edges
Spatial frequency = size
Motion (more complicated)

Question 7

What is spatial frequency ?

Answer 7

Spatial frequency refers to the level of detail present in a stimulus per degree of visual angle. A scene with small details and sharp edges contains more high spatial frequency information than one composed of large coarse stimuli.

Question 8

What is contrast ?

Answer 8

The state of being (strikingly) different from something else in juxtaposition or close association.

Question 9

What is parallel processing ?

Answer 9

For each location in the visual space, a wide range of features are detected and extracted by subset of retinal neurons w/ different response properties.
Parallel processing is the simultaneous detection and transmission of multiple types of info from the same image.

Question 10

How is the retina structured ?

Answer 10

It is a layered structure: pigment epithelium, photoreceptor (PR) outer segment (rods and cones outer segments), outer nuclear layer (inner segments of rods and cones), outer plexiform layer (PR-BP cell synapse or PR-HC synapse), inner nuclear layer (BP cells), inner plexiform layer (BP cell-GC synapse or AC-BP synapse), GC layer and nerve fiber layer

Question 11

What are the dimensions and properties of the human retina ?

Answer 11

It is 0.5mm thick and transparent to light.

Question 12

What is the fovea ?

Answer 12

The fovea is the center most part of the macula. This tiny area is concentrated w/ cones and covers about 2 degrees of the visual field. This is the only sport containing no blood vessels.

Question 13

Where is the blind spot ?

Answer 13

In the area where the optic nerve connects to the retina in the back of each eye, which known as the optic disk (optic nerve head). There is a total absence of cones and rods in this area, and, consequently, each eye is completely blind in this spot, hence the name blind spot.

Question 14

What is retinas pigmentosa ?

Answer 14

The loss of rods followed by progressive loss of cones, which leads to night blindness and loss of peripheral vision

Question 15

What is age-related macular degeneration ?

Answer 15

Cone degeneration in fovea which leads to loss of central vision.

Question 16

What is the limit to the maximal spatial acuity of the retina ?

Answer 16

The number of PRs (like the pixels of a camera).

Question 17

How are PRs connected to BP in the fovea ?

In the peripheral retina ?

Answer 17

In the fovea, densely-packed cone photoreceptors transmit signals from very small areas and can have a one-to-one relationship with ganglion cells.
In the periphery, signals from multiple photoreceptors are ‘pooled’ onto single ganglion cells so although sensitivity is high, the spatial acuity (or resolution) is lower in comparison to the fovea and responses are slower.

Question 18

How are the PRs structured ?

Answer 18

PR outer segments are tightly packed with membranous discs (rods) or lamellae (in cones) containing the visual pigments (either rhodopsin or one cone opsin type) at about 25,000 molecules/μm2.

Question 19

Of what large family of receptors are the PRs part of ?

Answer 19

GPCRs.

Question 20

What process kicks off phototransduction ?

How does this process work ?

Answer 20

Isomerisation of retinal: when the 11-cis-retinal molecule absorbs a photon, it flips to the all-trans-retinal form.
This causes a conformational change in the opsin molecule, unfolding the C-terminal region to reveal G protein (transducin) binding sites.

Question 21

What are the 3 types of cones and what do the types distinguish ?

Answer 21

Short = blue (419)
Medium = green (531)
Long = red (559)

Question 22

How is wavelength sensitivity determined ?

Answer 22

By differences in opsin amino acid sequences.

Question 23

At what wavelengths are rods more sensitive ?

Answer 23

490nm

This explains why blue objects appear brighter at night.

Question 24

What are the principal steps of phototransduction ?

Answer 24

a) one rhodopsin molecule absorbs one photon
b) 500 transducin molecules are activated
c) 10E5 cGMP molecules are hydrolysed by PDE (posphodiesterase)
d) 250 ion channels close
e) rod cell membrane is hyperpolarised by 1 mV
f) less Glu release w/ light

Question 25

Are rods or cones more sensitive to single photons ?

Is this always accurately measurable ?

Answer 25

Rods.
Occasionally, when excited, rods do not responds and sometimes there is a response even when there is no light pulse (a false signal). This is due to spontaneous thermal isomerization of retinal and makes dark vision a bit noisy.

Question 26

How do PR responses vary w/ light intensity ?

Answer 26

Photoreceptor responses to light are GRADED and proportional to light intensity (which is inversely prop to Glu release).

Question 27

What is the problem w/ rods ?

Answer 27

Although they give faster responses to bright light, rods saturate at high intensities so they don’t work in bright light.

Question 28

How are cones different from rods in terms of sensitivity ?

Answer 28

Cones start to work at higher light intensities and respond more quickly than rods. They also recover more quickly after the light is turned off. Cones can adapt to bright light and do not saturate unless exposed to very high light levels.

Question 29

What is the dark current ?

Answer 29

In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current. This is called the dark current and keeps the cell depolarized at about -40 mV.

Question 30

What is mesopic vision ?

Answer 30

Mesopic vision is a combination of photopic vision (cones, bright light) and scotopic vision (rods, dim light) in dim light.

Question 31

What are the 3 types of BP cells ?

Answer 31

ON, OFF and Rod.

Question 32

How does the ON pathways in cones work ?

Answer 32

The PRs in the cone sense light, so the cone cell releases less Glu to the ON BPC which expresses an inhibitory metabotropic mGluR6 receptor. The BPC is thus less inhibited and can release more Glu to a RGC, causing it to depolarize and fire APs.

Question 33

How does the OFF pathways in cones work ?

Answer 33

The PRs in the cone sense light, so the cone cell releases less Glu to the OFF BPC which expresses an excitatory AMPA receptor. The BPC is thus less stimulated and releases less Glu to a RGC, causing it to hyperpolarize (and thus not fire APs).

Question 34

What is the major difference between the ON and OFF pathway in cones ?

Answer 34

The difference come from 2 distinct glutamate receptors on the dendrites of the BPCs.

Question 35

How does do the ON and OFF pathways work in rod cells ?

Answer 35

Both ROD BPCs express a inhibitory metabotropic mGluR6 receptor, so both ROD BPCs depolarise in light
ON: ROD BPC synapses an amacrine cell that further depolarizes an ON BPC vias gap junctions, thus leading to more Glu release and further depolarization of the RGC.
OFF: ROD BPC synapses an amacrine cell that release the inhibitory neurotransmitter Gly onto an OFF BPC, which thus released less Glu and causes the RGC to hyperpolarize.