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:

• 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...

– 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

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

• 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:

– red, green, blue– each contain different photopigment– fires differently depending on wavelength struck by– relationship to colorblindness?

D. Vision

• 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.