Sensation and Perception:Vision

Mr. Callens

Psychology

1. Sensation vs. Perception

• Sensation– The process through which the senses pick up

information from our surroundings and transmit it to the brain to interpret.

• Perception– The process by which that sensory information is actively

organized and interpreted by the brain.

Three Definitions:

2. Transduction

• Process where the receptors change or convert the sensory stimulation into electrical signals the brain can process.

3. Adaptation• The process of becoming less sensitive to an unchanging

sensory stimulus over time• Allows you to shift your attention to what is most important

at any given moment

Process of Sensation• Absolute threshold

– The minimum amount of sensory stimulation that can be detected 50% of the time

• Difference threshold– The smallest increase or decrease in a stimulus required

for sensation to be noticed 50% of the time– “Just noticeable difference” (JND)

Process of Sensation• Ernst Weber (1795-1878)

– Observed that the JND for all the senses depends on a proportion or percentage of change rather than a fixed amount of change

Vision

• People can only perceive a small part of the total electromagnetic spectrum.

Vision

1. Reverse Image

• The eye perceives all images upside down in the back of the retina because of the way the light is reflected.

• The brain later turns these images right side up for you.

2. Cornea

• Thin rounded covering over the eye that focuses broad light waves into a narrower beam.

3. Pupil

• A round opening at the front of the eye that allows light waves to pass to the eye’s interior.

4. Iris

• The circular muscle that surrounds the pupil and controls the amount of light entering the eye.

• Iris also contains the pigment that gives your eye its color.

• HD Eyeball

5. Lens

• An oval structure that focuses the beam from the cornea and pupil even more narrow.

• Works almost in tandem with the iris.

6. Retina

• A thin film at the back of the eye that contains cells that are sensitive to light.

• These cells begin the process of transduction by absorbing light waves.

Vision

What happens in Lasik Eye Surgery?

Vision• Rods

– Allow humans to see in black, white, and shades of gray in dim light

– Take 20 – 30 minutes to fully adapt to darkness

• Cones– Enable humans to see color and

fine detail in adequate light, but that do not function in dim light

– Adapt fully to darkness in 2 – 3 minutes

Vision

• Trichromatic theory– First proposed by Thomas Young in 1802– The theory of color vision suggesting that there are three

types of cones, which are maximally sensitive to red, green, or blue, and that varying levels of activity in these receptors can produce all of the colors

S -Co n es(S en sit iv e to b lue)

M -Co n es(S en sit iv e to G ree n)

L -C on es(S en sit iv e to R e d)

T h ree T y p es o f C on es

Vision

• Opponent-process theory– The theory that the eye and the thalamus only respond

to the combination of red/green and yellow/blue. – Cells will respond to one color versus the other in each

pair based on stimuli.– Based upon the studies of afterimages.

Afterimages

Color Blindness

• Unable to distinguish two or more shades of colors• Monochromats: total color blindness (world looks

like black and white movies).• Dichromats: red/green color blind (results in them

seeing much more in blue and yellow)

Dichromats

Optical Illusions

Optical Illusions• A perceptual experience that preys upon the mind’s

need to interpret depth, shape, and size, and create an image that in reality cannot or does not actually exist:

Optical Illusions

• Percept– Stimuli that create our visual experience (what

your eyes see, even when you don’t know you see it)

• Illusion– A percept that fails to agree with real world

measurements

Two Reasons Illusions Occur in Our Mind:

1. Our mind tries to figure out the most useful aspect of what we are seeing, and sometimes it gets it wrong. (biological failure)

Example: Crazy Nuts

Two Reasons Illusions Occur in Our Mind:

2. Our mind tries to cheat by creating a list of common experiences and uses visual stimuli to quickly figure out what experience it matches and how to guide our behavior. (prediction error)

Example: Lionel Train

Optical Illusions

• What do you see?

Optical Illusions

• Sometimes, we see objects we recognize, but not how we usually see them.

• Our mind has no construct in which to place these objects, and we usually call them “junk” or “art”.

• It may not be until we see them in a different way that we understand their meaning.

Shigeo Fukuda

• Japanese sculptor, died 2009, specialized in optical illusions:

• Mirror Image

• Forks, Knives, Gasoline