Introduction to Sensation and

Perception: Vision

Module 11

Sensation & PerceptionHow do we construct our representations of

the external world?

To represent the world, we must detect physical energy (a stimulus) from the

environment and convert it into neural signals. This is a process called sensation.

When we select, organize, and interpret our sensations, the process is called perception.

Bottom-up Processing

Analysis of the stimulus begins with the sense receptors and works up to the level of

the brain and mind.

Letter “A” is really a black blotch broken down into features by the brain that we perceive as an

“A.”

Top-Down ProcessingInformation processing guided by higher-level mental processes as we construct perceptions, drawing on our experience

and expectations.

THE CHT

Top Down Processing

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Likelihood Principle• Herman von Helmholtz• We will perceive the object that is most

likely to be the cause of our sensory stimulation. That is, if a number of different objects could have caused a specific pattern of light and dark on the retina, we will perceive the object that is most likely to occur in that particular situation.

Our sensory and perceptual processes work together to help us sort out complex

images.

Making Sense of Complexity

“The Forest Has Eyes,” Bev Doolittle

Sensing the WorldSenses are nature’s gift that suit an

organism’s needs.

A frog feeds on flying insects; a male silkworm moth is sensitive to female sex-attractant odor; and we as human beings are sensitive to sound frequencies that

represent the range of the human voice.

Exploring the Senses

What stimuli cross our threshold for conscious awareness?

PsychophysicsA study of the relationship between

physical characteristics of stimuli and our psychological experience with them.

Physical World Psychological World

Light Brightness

Sound Volume

Pressure Weight

Sugar Sweet

ThresholdsAbsolute Threshold: Minimum stimulation needed to detect a particular stimulus 50%

of the time.

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0 5 10 15 20 25 Stimulus Intensity (lumens)

ABSOLUTE THRESHOLDS• Vision: a single candle flame from 30

miles on a dark, clear night.• Hearing: The tick of a watch from 20 feet

in total quiet.• Smell: 1 drop of perfume in a 3 room apt.• Taste: 1 tsp. sugar in two gallons of water.• Touch: The wing of a bee on your cheek,

dropped from 1 cm.

Subliminal ThresholdSubliminal Threshold:

When stimuli are below one’s absolute threshold

for conscious awareness.

Kurt Scholz/ Superstock

http://www.scentair.com/index.html

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Signal Detection Theory

• Predicts when we will detect new weak signals.

• Why do people respond differently to the same stimuli?

• You’re driving a bus with 12 passengers. At your first stop, 6 passengers get off. At the second stop, 3 get off. At the third stop, 2 more get off the but 3 get on. What color are the bus driver’s eyes?

Weber’s LawTwo stimuli must differ by a constant minimum percentage (rather than a constant amount), to be perceived as

different. Weber fraction: k = dI/I.• Difference thresholds grow with the magnitude of the stimulus.

• (quarter example)

Sensory AdaptationDiminished sensitivity as a consequence of

constant stimulation.

Put a band aid on your arm and after awhileyou don’t sense it.

We perceive the world not exactly as it is, but as it is useful for us to perceive it.

Now you see, now you don’t

VISION

• What is the energy that we see as visible light?

• Transduction: In sensation, the transformation of stimulus energy (sights, sounds, smells) into neural impulses.

ElectroMagneticSpectrum

The Stimulus Input: Light Energy

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The Stimulus Input: Light Energy

• What strikes our eye is not color but pulses of electromagnetic energy that our visual system perceives as color.

Physical Characteristics of Light that help our

determine our sensory experience of them.

1. Wavelength (hue/color): distance from one wave peak to the next

2. Intensity (brightness)-the amount of energy in the light wave –determined by the height or magnitude.

Wavelength (Hue)

Hue (color) is the dimension

of color determined by the wavelength

of the light.

Wavelength is the distance

from the peak of one wave to the peak of the

next.

Wavelength (Hue)

Different wavelengths of light resultin different colors.

400 nm 700 nmLong wavelengthsShort wavelengths

Violet Indigo Blue Green Yellow Orange Red

Intensity (Brightness)

Intensity: Amount of energy in a

wave determined

by the amplitude. It is related to perceived

brightness.

Intensity (Brightness)

Blue color with varying levels of intensity.As intensity increases or decreases, blue color

looks more “washed out” or “darkened.”

The Eye

Parts of the eye1. Cornea: Transparent tissue where light

enters the eye.2. Iris: Muscle that expands and contracts

to change the size of the opening (pupil) for light.

3. Lens: Focuses the light rays on the retina.

4. Retina: Contains sensory receptors that process visual information and sends it to the brain.

The LensLens: Transparent

structure behind the pupil that changes

shape to focus images on the retina.

Accommodation: The process by which the eye’s lens changes shape to help focus

near or far objects on the retina.

RetinaRetina: The light-

sensitive inner surface of the

eye, containing receptor rods and cones in

addition to layers of other neurons (bipolar, ganglion

cells) that process visual information.

Optic Nerve, Blind Spot & Fovea

http://www.bergen.org

Optic nerve: Carries neural impulses from the eye to the brain. Blind Spot: Point where the optic nerve leaves the eye because there are no receptor cells located there. Fovea: Central point in the retina around which the eye’s cones cluster.

Test your Blind SpotUse your textbook. Close your left eye, and fixate your right eye on the black dot. Move the page towards your eye and away from

your eye. At some point the car on the right will disappear due to a blind spot.

Photoreceptors

E.R. Lewis, Y.Y. Zeevi, F.S Werblin, 1969

Bipolar & Ganglion CellsBipolar cells receive messages from photoreceptors and transmit them to

ganglion cells, which converge to form the optic nerve.

Visual Information ProcessingOptic nerves connect to the thalamus in the

middle of the brain, and the thalamus connects to the visual cortex.

Feature DetectionFeature detector nerve cells in the visual cortex of the

occipital lobe respond to specific features, such as edges, angles, and movement. They then pass the info. on to supercells in other cortical areas that respond to more complex problems. Ex. Temporal Lobe-enables you to

perceive faces.

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Shape Detection

Specific combinations of temporal lobe activity occur as people look at shoes,

faces, chairs and houses.

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Visual Information ProcessingProcessing of several aspects of the stimulus

simultaneously is called parallel processing. The brain divides a visual scene into subdivisions such

as color, depth, form, movement, etc. (unlike a computer that does “Serial processing”)

Motion Aftereffects

• http://www.michaelbach.de/ot/mot_adaptSpiral/index.html

• Are caused by the adaptation of motion-specific detectors that are tuned to the direction of the movement of the stimuli being viewed.

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Color VisionTrichromatic theory: Young and von

Helmholtz suggested that the eye must contain three receptors that are sensitive to

red, blue and green colors. http://phet.colorado.edu/simulations/sims.php?sim=Color_Vision#topics

Blue Green Red

Medium LowMax

Standard stimulus

Comparison stimulus

• Any color can be created by combining the lightwaves of 3 primary colors –red, blue, green.

• When we stimulate combinations of these cones, we see other colors.

• For example, there are no receptors especially sensitive to yellow—when both red and yellow cones are stimulated-we see yellow.

Color Blindness

Ishihara Test

Genetic disorder in which people are blind to green or red colors. This supports the

Trichromatic theory.

COLOR BLINDNESS• Most people with

color –deficient vision are not color blind. They simply lack functioning red or green sensitive cones.

OPPONENT PROCESS THEORY

• Trichromatic Theory does not solve all issues with color vision.

• For example, we see yellow when mixing red and green light.

• BUT those blind to red and green can often still see yellow?

• Clue found by Hering (a physiologist) when looking at afterimages.

Opponent Colors

Gaze at the middle of the flag for about 30Seconds. When it disappears, stare at the dot

and reportwhether or not you see Britain's flag.

WHY?

• When you stare at a green square for a while, then look at a white sheet of paper—you see RED—its opponent color.

• Said there must be two additional processes-one responsible for green vs. red and one responsible for blue vs. yellow (later also white and black).