Transcript SENSATION

VI. SENSATION
Two pieces of the puzzle....
• The nervous system’s job is to coordinate us
with our environment.
– Electric-chemical process
• We are exposed to an enormous amount of
stimuli.
– To deal with this, our perceptions can be biased.
A. What is sensation?
• How does physical energy from the
environment get encoded as neural
signals?
• 1. Sensation: process by which our sensory
receptors and nervous system receive and
represent stimulus energies from our environment.
A. What is sensation?
• 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.
B. Basics of Sensation:
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1. 5 senses
Seeing
Hearing
Smelling
Tasting
Touching
B. Basics of Sensation:
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...
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iii. That energy interpreted by receptors.
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see bear: light waves...
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
C. What do we sense/detect from
the environment?
• 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)
C. What do we sense from the
environment?
• 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.
– Situational differences (expectations, motivation, fatigue)
– Individual differences (experience)
C. What do we sense from the
environment?
• 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.
C. What do we sense from the
environment?
• 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.
– Yes – can have subtle, fleeting influence on thinking.
– No – does not have powerful, enduring effect on behavior.
C. What do we sense from the
environment?
• 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?
• Our eyes are always quivering just enough
to maintain stimulation of neurons.
D. VISION
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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...
D. Vision
• 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:
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red, green, blue
each contain different photopigment
fires differently depending on wavelength struck by
relationship to colorblindness?
D. Vision
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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.