• The nervous system’s job is to coordinate us with our environment.
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Electric-chemical process
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We are exposed to an enormous amount of stimuli.
–
To deal with this, our perceptions can be biased.

•
How does physical energy from the environment get encoded as neural signals?
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1. Sensation : process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.

• 2. Sensation vs. Perception
•
Sensation is not all we require to make sense of world (“to see the bear”)
•
Sensation: detecting physical energy....
• Perception
: How we select, organize, and interpret the information we sense.
– Active process, involves imposing order on stimuli
– Sensation provides “raw” information (stimuli) that is selected, organized, etc.

• 1. 5 senses
•
Seeing
•
Hearing
•
Smelling
•
Tasting
•
Touching

Sensation involves converting one type of energy into another.
- Energy from environment – to neural impulses.
• i. External Stimulus (energy) – big, furry, smelly bear
• ii. Stimulus takes different energy forms...
– see bear: light waves...
• iii. That energy interpreted by receptors.
– see bear: light waves received by photoreceptors in retina
• iv. Convert that energy into form brain can understand.
2. Transduction : Stimulus is converted into neural impulses

• We do not detect all of the stimuli that are present.
Examples?
•
Senses are limited or restricted.
• 1. Absolute Threshold
: The minimum stimulation necessary to detect a particular stimulus.
(usually 50% of time)

• How do we determine absolute threshold?
•
2. Signal Detection Theory :
Used to predict how & when we will detect a stimulus.
Considers:
– Strength of signal
Absolute thresholds vary – not inherent to the stimulus.
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Situational differences (expectations, motivation, fatigue)
– Individual differences (experience)

• 3. Sensing the difference between 2 stimuli:
• Difference threshold
(just noticeable difference):
Minimum difference a person can detect between any two stimuli (50% of the time)
–
How to detect the JND?
• right or wrong
• adjustment
– The JND increases with the magnitude of the stimulus.

• 4. Can we ever detect stimuli that are below threshold?
Subliminal : below one’s absolute threshold for conscious awareness.
•
How do we test for this?
– Yes – can detect stimuli under threshold.
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Yes – can have subtle, fleeting influence on thinking.
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No – does not have powerful, enduring effect on behavior.

• 4. What else influences our sensitivity to stimuli?
•
Sensory Adaptation : diminishing sensitivity to an unchanged stimulus.
- after constant exposure to a stimulus, our nerve cells fire less frequently.
•
But...

•
Why?
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Our eyes are always quivering just enough to maintain stimulation of neurons.

D. VISION
• Review the basic process:
• 
Stimulus input
(“bear” or beautiful sunset)
• 
Input as light waves
• 
Received by receptors in eye.
• 
Light waves transformed into neural information – impulses ( transduction ).
• 
Messages go to brain to be organized/interpreted - to where in brain?

D. VISION
•
1. What is the stimulus input?
a. Light waves or energy .
- Pulses of electromagnetic energy that our visual system experiences as color.
- Do we see all possible light waves?

D. VISION
• 1. What is the stimulus input?
What determines the characteristics of the colors we see?
– a. Wavelength : Distance from one wave peak to another.
– Determines “hue” or color.
– b. Amplitude : Wave height.
– Determines amount of energy in light wave or intensity/brightness.

D. VISION
• 2. The process of light energy becoming vision .
a. Structure of the eye – key are the receptors.

D. VISION
• a. Important Structures:
• cornea : transparent protector.
•  pupil : adjustable opening, determines how much light is let into eye.
•  lens : focuses incoming rays into an image on retina.
•  retina : light sensitive tissue - receptors.

D. VISION
• b. Accommodation
Process by which lens changes shape to focus the image of objects on retina.
c. Receptors in retina
- When image focused onto retina by lens: upside down.
- Key to vision: light energy
 neural impulses
Light strikes receptors in retina
 produces chemical changes (photopigments that break down)
 trigger neural impulses.

D. VISION
• c. Receptors (2 types):
Rods : located in peripheral area of retina.
– Highly sensitive to light.
– Enables black and white vision.
Cones
: located in fovea (retina’s central point of focus).
– Each cone has cell that relays messages directly to visual cortex
– Detects fine detail from light energy.
– Enables us to see color.


Retina processes some info before gets to brain (encodes and analyzes it)

Chemical reaction – activates bipolar cells
– eventually activates ganglion cells that make up the optic nerve .

Info. sent to brain through optic nerve brain rearranges image to right side-up.

D. VISION
• 
When info. reaches visual cortex, processed by feature detectors.
• d. Feature Detectors :
Neurons in brain that respond to specific features of the stimulus (shape, angles, movement).
• Importance of “brain” in vision:
– “parallel processor”
• e. Comparing the vision process to other senses...

• 3. Color Vision
• Light rays themselves aren’t “colored”
• Color of an object is the wavelength “rejected” or reflected
(versus the others that are absorbed)
• a. Young-Helmotz Trichomatic Theory
•
Retina - cones that are sensitive to 3 colors:
– red, green, blue
– each contain different photopigment
– fires differently depending on wavelength struck by
– relationship to colorblindness?

• 3.
Color Vision
•
After-images – why?
• b. Opponent-Process Theory
• Neurons are sensitive to “pair of opponent” colors:
– red/green, blue/yellow, black/white
– stare at green – remove green stimulus – cell is fatigued
– leaves only “opponent” color part of cell to fire – red
– also explains why color blind people can see yellow

D. VISION
•
4. Why do some people have poor vision?
a. Acuity : sharpness of vision.
Poor vision: Caused by small distortions in shape of eye ball.
b. Nearsightedness : eyeball is longer than normal in relation to lens.
b. Farsightedness : eyeball is shorter than normal in relation to lens.

• Sensation:
- haven’t touched on organizing/interpreting that material (perception)
- “raw” material for perception
- started at “entry level”, data driven
“bottom-up processing”
• Perception: “top-down processing”
- concept driven, use preexisting knowledge to interpret information.