Visual Processing

Question 1

Photoreceptors

Answer 1

The photoreceptors are specialized neurons that are located in the retina

They are light-sensitive neurons that convert light energy into electrical energy in cells.

Question 2

Where are photoreceptors located?

Answer 2

In the retina — they are specialized neurons that convert light energy into electrical signals (phototransduction).

Question 3

What are the two types of photoreceptors?

Answer 3

Rods and cones.

Question 4

How many rods and cones are in each retina?

Answer 4

~120 million rods and ~6 million cones.

Question 5

Do photoreceptors fire action potentials?

Answer 5

No — they respond with graded membrane potentials proportional to light intensity.

Instead of generating a binary on or off signal, photo receptors now adjust their membrane potential in a continuous proportional manner based on the intensity of the light they detect

Large or bright light causes a large graded membrane potential while with a dimmer light the change

Question 6

What are the three main parts of a photoreceptor (Rods and Cones)?

Answer 6

Outer segment: Membrane discs with visual pigments.

Inner segment: Nucleus and organelles for protein synthesis.

Basal end: Synapse that releases glutamate.

Question 7

What pigment do rods contain?

Answer 7

Rhodopsin (opsin + 11-cis retinal).

Question 8

What is the function of the retinal pigment epithelium (RPE)?

Answer 8

Nourishes photoreceptors, absorbs stray light, and regenerates visual pigments.

Question 9

Why are photoreceptors located near the RPE and choroid?

Answer 9

For access to nutrients, energy, and waste removal via the vascular choroid.

Question 10

Outer Segments (OS) of photoreceptors

Answer 10

Contains stacks of membrane discs with visual pigments — the site of phototransduction.

Question 11

Outer Nuclear Layer (ONL)

Answer 11

Houses the cell bodies of rods and cones.

Question 12

Outer Plexiform Layer (OPL)

Answer 12

Synapses between photoreceptors and bipolar cells.

Question 13

Inner Nuclear Layer (INL)

Answer 13

Contains the cell bodies of bipolar, horizontal, amacrine, and Müller glial cells.

Question 14

Inner Plexiform Layer (IPL)

Answer 14

Site of synapses between bipolar cells, amacrine cells, and ganglion cells.

Question 15

Nerve Fiber Layer (NFL)

Answer 15

• it is the front most layer of the eye; Contains axons of the ganglion cells, heading toward the optic nerve.

Question 16

Ganglion Cell Layer (GCL)

Answer 16

Contains the output neurons of the retina — the ganglion cells.

Question 17

: What do Müller glial cells do?

Answer 17

Act as optical fibers guiding light through the retina and reducing light scatter.

Question 18

Why do we have a blind spot?

Answer 18

It’s where the optic nerve exits the eye — no photoreceptors are present.

Question 19

Why don’t we notice our blind spot?

Answer 19

The brain fills in missing information, and each eye’s blind spot is in a different location.

Question 20

Why would the structure make physiological sense

Answer 20

The photoreceptors are placed directly by the RPE

And behind the PE we have the choroid layer which is highly vascularized providing the photoreceptors with sufficient energy, nutrients and waste removal to function

Question 21

What is phototransduction?

Answer 21

Conversion of light energy into an electrical signal in photoreceptors.

Question 22

Are photoreceptors more active in light or darkness?

Answer 22

: Darkness — they’re depolarized and release glutamate continuously.

Question 23

What happens when light hits photoreceptors?

Answer 23

They hyperpolarize → reduce glutamate release → signal light to bipolar cells.

Question 24

Cons vs Rods

Answer 24

rods are perfect for low light conditions, whereas cones (3 types) are specialized for detecting wavelengths

Question 25

Rods in the DARK

Answer 25

For rods, their key light sensitive pigment is called Rhodopsin, which is made up of 2 parts; opsin and 11-cis retinal The actual light sensing part is the retinal In the absence of light, 11-Cis retinal remains bound inside the opsin and rhodopsin remains inactive Inside the photoreceptor's outer segment the enzyme guanylyl cyclase is constantly producing cGMP CGMP binds to CNGs which are found on the membrane and they keep them open allowing the sodium and calcium to flow into the cell (positive) If you keep bringing in a lot of positively charged ions you get closer and closer to depolarizing the membrane This is called the dark current and it keeps the cell depolarized at like –40mV Because it is constantly depolarized, the photoreceptor is constantly releasing glutamate onto the downstream bipolar cells Note that to maintain homeostasis, the cell uses the NCKX (Sodium-Calcium- Potassium exchanger) - it kicks out calcium

Question 26

Rods in LIGHT

Answer 26

A photon of light is absorbed by the 11-cis retinal and it changes its shape and becomes an all-trans retinal, activates opsin converting Rhodopsin to its active form called Metarhodopsin-2 Metarhodosin-2 then activates a G-Protein called transducin which is made up of subunits alpha, beta and gamma Upon activation the alpha subunit exchanges GDP for GTP and this becomes active and activates the phosphodiesterase-6 (PDE6) which breaks down cGMP to GMP and this leads to a drop in intracellular cGMP With less cGMP the CNGs will close and the cell membrane becomes hyperpolarized moving towards –70 mV and glutamate release is reduced Reduction in glutamate signals to the bipolar cells that light has entered Recovery Low Ca levels are recognized by protein GCAP (in the dark it is inhibited by high calcium levels) GCAP will be activated and will stimulate the guanylyl cyclase to start producing the cGMP again to help with the recovery process This opens the CNG channels and allows for another round

Question 27

Photobleaching

Answer 27

when opsins can no longer detect light until it is reconfigured back into the 11-cis retinal

Question 28

Cones are for bright lights

Answer 28

Cones are less sensitive than rods; they are responsible for vision in bright light and for distinguishing colors, but they don’t operate in dim conditions. Rods can detect single photons. But they operate only in low light: in daylight they are “bleached out”, i.e. their rhodopsin is broken down so they can’t sense light. When the lights go dim the rods dark adapt, i.e. they rebuild their stores of rhodopsin over ~30 minutes.

Question 29

Photoreceptors are not distributed uniformly

Answer 29

cons, are most densely packed in the macula, a central disk 5.5 mm across, and especially in its central pit called the fovea, 1.5 mm across (2° of visual angle — about as wide as 2 fingers viewed at arm’s length). We use the fovea for detailed vision. 5° away from its center, acuity is quartered; at 20° it falls below the standard for legal blindness. So you're sharpest vision is limited to this little area

Question 30

saccades

Answer 30

the rapid, jerky eye movements your eyes make when you shift your gaze from one point to another. Saccades create the illusion of a wide, stable, detailed visual field by rapidly sampling small, detailed parts of a scene and integrating them in the brain’s visual cortex.

Question 31

What keeps rods depolarized in the dark

Answer 31

cGMP binds to and opens CNG (cyclic nucleotide–gated) Na⁺/Ca²⁺ channels → “dark current

Question 32

What is the dark current?

Answer 32

The steady influx of Na⁺ and Ca²⁺ that keeps the photoreceptor at –40 mV

Question 33

What enzyme maintains cGMP levels in the dark?

Answer 33

Guanylyl cyclase.

Question 34

What maintains ion balance during the dark current?

Answer 34

NCKX (Na⁺/Ca²⁺/K⁺ exchanger) removes Ca²⁺.

Question 35

What happens when light activates rhodopsin?

Answer 35

11-cis retinal → all-trans retinal → activates opsin → forms metarhodopsin II.

Question 36

What G-protein does metarhodopsin II activate?

Answer 36

Transducin (α, β, γ subunits).

Question 37

What enzyme does transducin activate?

Answer 37

Phosphodiesterase-6 (PDE6), which breaks down cGMP → GMP.

Question 38

What is the result of cGMP breakdown?

Answer 38

CNG channels close → hyperpolarization → reduced glutamate release.

Question 39

How does recovery occur?

Answer 39

Low Ca²⁺ activates GCAP → stimulates guanylyl cyclase → regenerates cGMP.

Question 40

What is photobleaching?

Answer 40

When opsins can’t detect light until 11-cis retinal is regenerated.

Question 41

What vitamin is required for retinal regeneration?

Answer 41

Vitamin A

Question 42

Where are cones concentrated?

Answer 42

Fovea (central macula).

Question 43

Where are rods concentrated?

Answer 43

Peripheral retina.

Question 44

Why is peripheral vision better in the dark?

Answer 44

Rods dominate the periphery and are more sensitive to dim light.

Question 45

Retinal Layers (outer → inner)

Answer 45

RPE – pigment & recycling Photoreceptor layer Outer nuclear layer (ONL) – photoreceptor cell bodies Outer plexiform layer (OPL) – synapses with bipolar & horizontal cells Inner nuclear layer (INL) – bipolar, horizontal, amacrine, Müller cells Inner plexiform layer (IPL) – bipolar → ganglion synapses Ganglion cell layer (GCL) – output neurons Nerve fiber layer (NFL) – axons to optic nerve

Question 46

What is the signal flow in the retina?

Answer 46

Photoreceptors → Bipolar cells → Ganglion cells → Optic nerve → Brain.

Question 47

What do horizontal cells do?

Answer 47

Lateral inhibition → enhance contrast, form center–surround receptive fields.

Question 48

What do amacrine cells do?

Answer 48

Regulate timing and motion detection between bipolar and ganglion cells.

Question 49

What is a receptive field?

Answer 49

The area of the retina where light changes a neuron’s activity.

Question 50

What is center–surround organization?

Answer 50

Center and surround regions respond oppositely to light → detects contrast and edges.

Question 51

ON-Center

Answer 51

mGluR6 (metabotropic) Depolarize in light Glutamate ↓ → inhibition released

Question 52

OFF-Center

Answer 52

AMPA/Kainate (ionotropic) Hyperpolarize in light Glutamate ↓ → less excitation

Question 53

What do bipolar cells detect?

Answer 53

Contrast, not uniform brightness.

Question 54

What do ganglion cells do?

Answer 54

Receive from bipolars → generate action potentials → send signals via optic nerve.

Question 55

Do ganglion cells also have center–surround receptive fields?

Answer 55

Yes, for contrast and edge detection

Question 56

Why is spatial resolution better in the fovea?

Answer 56

1:1 receptor-to-bipolar-to-ganglion mapping (low convergence).

Question 57

Why is the periphery more light-sensitive?

Answer 57

High convergence of many rods → greater sensitivity but less detail.

Question 58

M (Magnocellular) Ganglion Cell

Answer 58

Motion detection Phasic

Question 59

P (Parvocellular)

Answer 59

Form & fine detail Tonic

Question 60

Melanopsin

Answer 60

Circadian rhythms Photosensitive ganglion cells

Question 61

What do the optic nerves carry?

Answer 61

Axons of ganglion cells (cranial nerve II).

Question 62

What happens at the optic chiasm?

Answer 62

Nasal fibers cross; temporal fibers remain on the same side.

Question 63

Why do nasal fibers cross?

Answer 63

So each hemisphere processes the opposite visual hemifield.

Question 64

What are optic tracts?

Answer 64

Post-chiasm pathways carrying hemifield info to the thalamus (LGN).

Question 65

Where do optic tracts project?

Answer 65

Lateral geniculate nucleus (LGN) → primary visual cortex (V1) via optic radiations.

Question 66

Where is V1 located?

Answer 66

In the occipital lobe, around the calcarine sulcus.

Question 67

What does “retinotopic organization” mean?

Answer 67

Adjacent areas in the retina project to adjacent neurons in the brain. That is, neurons close to each other in the brain get information from close-together parts of the retina. This arrangement is found in the lateral geniculate nuclei, V1, and many higher visual processing areas.

Question 68

Why does the fovea occupy a large cortical area?

Answer 68

It has high receptor density and provides detailed information.

Question 69

Anopia

Answer 69

Total vision loss

Question 70

Scotoma

Answer 70

Localized blind spot

Question 71

Monocular

Answer 71

One eye only

Question 72

Binocular

Answer 72

Both eyes

Question 73

Hemianopia

Answer 73

Loss of half the visual field

Question 74

Quadrantanopia

Answer 74

Loss of a quarter

Question 75

Homonymous

Answer 75

Same side in both eyes

Question 76

Bitemporal

Answer 76

Outer halves lost (optic chiasm lesion)

Question 77

Contralateral

Answer 77

Lesion opposite to lost field

Question 78

Information moves from chiasm to thalamus and then to cortex

Answer 78

The nerve bundles emerging from the chiasm are called the optic tracts. They end in the 2 lateral geniculate nuclei (LGN) in the thalamus, which project via the optic radiations to primary visual cortex, V1.

Question 79

Why do the fibers cross?

Answer 79

In the eye, the right side of the scene (the right visual hemifield) projects onto the left side of each retina, i.e. onto the nasal side of the right retina and the temporal side of the left retina. Because the nasal fibers cross, all the information from the right hemifield comes together in the left cerebral hemisphere, and vice versa.