The Visual System: The Nature of Light

Electromagnetic Energy

• An energy spectrum that includes X-rays, radar, and radio waves

• A small portion of the spectrum includes light visible to the human eye

• Light can be described as both a particle and a wave

Electromagnetic Spectrum of Light

Gammarays

X-raysUltra-violetrays

Infraredrays

RadarBroadcastbands

ACcircuits

Visible light

PrismWhitelight

400 500 600 700

10 -5 10 -3 10 -1 10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 15 10 17

Wavelength in nanometers (billionths of a meter)

Hue• Hue is the color of

light as determined by the wavelength of the light energy

• Includes: red, orange, yellow, green, blue, indigo and violet (ROY G. BIV)

• The eye can detect 7 million separate hues

Amplitude• The brightness of light as determined

by height of the wave

• The taller the wave, the brighter the color

The Visual System: The Structure of the

Visual System

Vision

Purpose of the visual system– Transduction: transform light energy into an

electro-chemical neural response (action potential & synaptic transmission)

– represent characteristics of objects in our environment such as size, color, shape, and location

Parts of the Eye – Cornea

Cornea

• The clear bulge on the front of the eyeball

• Begins to focus the light by bending it toward a central focal point

• Protects the eye

Parts of the Eye - Iris

Iris

• A ring of muscle tissue that forms the colored portion of the eye; creates a hole in the center of the iris (pupil)

• Regulates the size of the pupil by changing its size--allowing more or less light to enter the eye

Parts of the Eye - Pupil

Pupil• The adjustable opening in the center

of the eye that controls the amount of light entering the eye (surrounded by the iris)

• In bright conditions the iris expands, making the pupil smaller.

• In dark conditions the iris contracts, making the pupil larger.

Parts of the Eye - Lens

Lens• A transparent structure behind the pupil; focuses

the image on the back of the eye (retina)• Muscles that change the thickness of the lens

change how the light is bent thereby focusing the image (Accommodation)

• Glasses or contacts correct problems in the lens’ ability to focus.

The Cosmic Flower

Does this picture seem to pulsate? Because the lens of your eye is not perfectly round some parts of what you look at are blurry. Your eyes make micro movements to try to put this entire picture into focus and this creates the pulsation.

The lens correctly focuses the image onto the back of the eye (retina).

(Myopia)

Misshapen eye focuses light rays from a distant

object in front of the retina. Can see near but not far.

The lens correctly focuses the image onto the back of the eye (retina).

(Myopia)

(Hyperopia)

Misshapen eye focuses light rays from near objects past the retina. Can see far but not near.

Misshapen eye focuses light rays from a distant object in front of the retina. Can see

near but not far.

The lens correctly focuses the image onto the back of the eye (retina).

Other Causes of Poor VisionAstigmatism – Uneven curvature of the cornea

causes multiple focus points/images on the retina resulting in blurry vision.

Other Causes of Poor Vision• Presbyopia – form of

farsightedness caused when lens becomes brittle & inflexible. Usually starts to happen in your early 40’s

Reading glasses

will correct this.

Nearsightedness Farsightedness

Parts of the Eye - Retina

Retina• Light-sensitive surface with cells that convert light

energy to nerve impulses• At the back of the eyeball• Made up of three layers of cells

– Receptor cells (Rods & Cones)– Bipolar cells– Ganglion cells

Parts of the Eye - Fovea

Fovea

• The central focal point of the retina

• The spot where vision is best (most detailed – visual acuity)

• Only cones are found in the Fovea

Receptor Cells(Rods & Cones)

• These cells are present in every sensory system to change (transduce) some other form of energy into neural impulses.

• In sight they change light into neural impulses the brain can understand.

• Visual system has two types of receptor cells – rods and cones

Rods

• Visual receptor cells located in the retina

• Can only detect black and white

• Respond to less light than do cones

• Located around the fovea. (remember pen-top demo from class)

Cones

• Visual receptor cells located in the retina

• Can detect sharp images and color• Need more light than the rods• Many cones are clustered in the fovea

at the center of the retina. (remember pen-top demo from class)

Rods

Cones

Watch Blue Man Group’s Rods & Cones Performance

Distribution of Rods and Cones

• Cones—concentrated in center of eye (fovea)– approx. 6 million

• Rods—concentrated in periphery – approx. 120 million

• Stare at a word and you’ll notice the others around it become blurred. (Clear word seen with cones, blurry area seen with Rods)

• Blind spot—region with no rods or cones (pg. 98 in book is a demo)

The Hermann Grid

Are there gray dots between the squares? Rods in the periphery are responsible for this. When you look at an area directly there is no dot because you are using your cones but the periphery has dots because the rods are trying to do two things, show you there is a dark area and a light area.

Optic Nerve/Blind Spot/Optic Disc• The nerve that carries visual information from

the eye to the thalamus then on to the occipital lobes of the brain

Parts of the Eye – Optic Nerve

Parts of the Eye – Blind Spot

Blind Spot• The point at which the optic nerve travels

through the retina to exit the eye (Optic Disk)• There are no rods and cones at this point, so

there is a small blind spot in vision. (do demo on page 98)

• We don’t notice our blind spot because each eye compensates for the other or your brain “fills in” the missing background info. (Top-down process & Gestalt Theory)

Cover your right eye and stare at the can as you move closer to the screen. Notice the spider disappear in your peripheral vision?

Visual Processing in the Retina

Processing Visual Information

• Rods & Cones transform light into action potential/synaptic transmission.

• Bipolar cells—neurons that connect rods and cones to the ganglion cells

• Ganglion cells—neurons that connect to the bipolar cells, their axons form the optic nerve

• Optic chiasm—point in the brain where the optic nerves from each eye meet and partly crossover to opposite sides of the brain

Bipolar Cells

• Cells that form the middle layer in the retina

• Gather information from the rods and cones and pass it on to the ganglion cells

• Hundreds of Rods feed into 1 Bipolar Cell.

• 1 to 2 Cones feed into 1 Bipolar Cell.

• This is why Cones have better visual acuity.

Ganglion Cells

• Pass the information from the bipolar cells through their axons

• Together these cells form the optic nerve.

• The top layer of the cells in the retina

Receptive Fields and Rod vs. Cone Visual Acuity

• Cones—in the fovea, one cone often synapse onto only a single ganglion cell

• Rods—the axons of many rods synapse onto one ganglion cell

• This allows rods to be more sensitive in dim light, but it also reduces visual acuity

Visual Processing in the Retina

Visual Processing in the Retina

Visual Processing in the Retina

Visual PathwayFrom the eye to the brain

Light travels through…

Cornea – Pupil – Lens – Fovea (retina) – Rods/Cones – Bipolar Cells – Ganglion cells (movement & light /color & detail) – Optic Nerve (blind spot) – Optic Chiasm (crossover point) – Thalamus – Occipital Lobe (Primary Visual Cortex)

How Can I Possibly Remember All of That in Order?

Cool = CorneaPeople = PupilLike = LensFrosties = Fovea (Rods & Cones)Because = Bipolar CellsGangsters = Ganglion CellsNever = Optic NerveCheat = Optic ChiasmThe = ThalamusOfficers = Occipital LobeDon’t like this one? Create Your Own!!!

Primary Visual Pathway: Thalamus processes info about form, color, brightness & depth

Secondary Visual Pathway: Midbrain processes info about the location of an object

– feature detectors respond to things like angles, edges, lines & movement

Visual Adaptation

(a) A projector mounted on a contact lens makes the projected image move with the eye. (b) Initially the person sees the stabilized image, but soon she sees fragments fading and reappearing.

Visual Impairment

• Play “Smart Glasses” (8:13) Segment #9 from Scientific American Frontiers: Video Collection for Introductory Psychology (2nd edition).

Watch in Class