Retina

PSYCH 333 Exam 1

What is the full visual processing flow?

Light → eye → retina → photoreceptors → retinal processing → brain

What does this chapter focus on?

How vision begins in the retina

What are the two natures of light?

Wave and particle (photons)

What is wavelength?

Distance between peaks of a wave

What does wavelength determine?

Perceived color

What can photons do?

• Absorbed

• Reflected

• Refracted

What is the retinal image?

The image formed on the retina (proximal stimulus)

What provides most focusing power?

Cornea (~80%)

What provides adjustable focusing?

Lens (~20%)

What is accommodation?

Lens changes shape to focus

Myopia?

Nearsighted

Hyperopia?

Farsighted

Presbyopia?

Aging lens loses flexibility

What is acuity?

Ability to see fine detail

How does aging affect vision?

• Lens yellows (less blue light)

• Cataracts (clouding blocks light)

What is the fovea?

Center of vision, highest acuity, only cones

What is the optic disk?

Blind spot (no receptors)

How does light travel in the retina?

Through layers → photoreceptors

How do neural signals travel?

Photoreceptors → bipolar → ganglion → brain (reverse direction)

Shape difference?

Rods = cylindrical, Cones = tapered

Which are more sensitive?

Rods (work in dim light)

Which need bright light?

Cones

Which recover faster?

Cones (~5 min)

Which recover slower?

Rods (~20 min)

Where are cones concentrated?

Fovea

Where are rods concentrated?

Periphery

Rod convergence?

High → high sensitivity, low detail

Cone convergence?

Low → high detail

Rod peak sensitivity?

~500 nm

Cone types?

• S ≈ 419 nm

• M ≈ 532 nm

• L ≈ 558 nm

What are the two parts of visual pigment?

Opsin + retinal

What happens when a photon is absorbed?

Retinal changes shape (isomerization)

What does this trigger?

Chemical cascade → changes membrane potential

Why are rods very sensitive?

One photon can trigger a large response

Do photoreceptors use action potentials?

No, graded potentials

What does light do to photoreceptors?

Hyperpolarizes them

What happens to neurotransmitter release?

Decreases

What does each photoreceptor encode?

Number of photons absorbed

How is an image represented?

Pattern of activation across retina

Pathway?

Photoreceptors → bipolar → ganglion → brain

Function?

• Convergence

• Summation

• Center of receptive field

Cells involved?

• Horizontal cells

• Amacrine cells

Function?

Lateral inhibition

What does it form?

Surround of receptive field

Approx numbers?

~100 million receptors → ~1 million ganglion cells

Why do rods have high sensitivity?

High convergence

Why do cones have high detail?

Low convergence (especially fovea)

What causes the blind spot?

No receptors where optic nerve exits

Why don’t you notice it?

• Other eye compensates

• Brain fills in missing info