Vision

Cornea

Cornea

Outer cover where light first passes through

• helps focus light rays

Pupil

Small adjustable opening in the iris

Iris

muscles that dilate/restrict pupil

• responds to light intensity or emotions

Lens

focuses light into an image projected onto retina

• image is reversed (but is later unreversed in the brain)

Retina

• the light-sensitive inner surface of eye

• 3 layers of nerve cells (photoreceptors, interneurons, afferent cells)

• transduces light into neural signals

Fovea

Tiny pit filled with cones

• responsible for sharp vision

Optic Nerve

thick rope of intertwined ganglion axons

• carries messages to brain

Blind Spot

area without visual receptors because it's where blood vessels and nerves connect to eyeball

1. Photoreceptor Cells

• First layer of retina

• rods and cones

• transduces light (distal stimulus) into neural signal (proximal stimulus)

Rods

• a type of photoreceptor cell

• detect black and white

• peripheral vision

• dim light (nighttime)

Rods

What kind of photoreceptor cell works best at night?

Rods

what kind of photoreceptor cell is responsible for peripheral vision?

Cones

• a type of photoreceptor cell

• sensitive to red, green, or blue

• works best in daytime

• detailed vision

Cones

What kind of photoreceptor cells work best at daytime/in bright light?

Cones

What kind of photoreceptor cells are responsible for detailed vision?

2. Bipolar Cells

• Second layer of retina

• specialized interneurons

• connect photorecptors to ganglion cells

3. Ganglion Cells

• third layer of retina

• specialized afferent neurons

• have long axons that intertwine

Visual Cortex in Occipital Lobe

Final Destination of visual neural message?

Trichromatic Theory

“there are only 3 types of photoreceptors, so we can only see red, green, and blue”

BUT

“when combined, these photoreceptors allow us to see ALL colors of light”

Opponent-Process Theory

“2 sets of firing neurons that work in opposite ways”

ex. see blue → blue cones fire → neurons connected to those blue cones fire (excitatory messages) while orange gets suppressed (inhibitory messages)

evidence: afterimage (seeing opposite colors after staring at a color for awhile)

when excitatory signal disappears, the inhibitory briefly overshoots (out of balance)

Light Adaptation

adjusting to brighter light

• squinting and our pupils constricting

Dark Adaptation

adjusting to dimmer light

• pupils dilate quickly

Near-sightedness

people can only see what’s near their eyes

Far-sightedness

distant objects are clearer and near objects are blurry

Cataracts

cloudy lenses

• easily removed with surgery

Glaucoma

excess fluid in eye presses on optical nerve so it can’t fire

• pressure

Color blindness

• usually in males

• red-green

blindness

can be result of damage to eye, neurons, or visual cortex