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Sensation and Sensation and PerceptionPerception

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Sensation & PerceptionSensation & Perception

How do we construct our representations of How do we construct our representations of the external world?the external world?

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

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

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

perceptionperception..

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Bottom-up ProcessingBottom-up Processing

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

the brain and mind.the brain and mind.

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

“A.”“A.”

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Top-Down ProcessingTop-Down Processing

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

and expectations.and expectations.

TTHHE CE CHHTT

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

Making Sense of Making Sense of ComplexityComplexity

“The Forest Has Eyes,” Bev Doolittle

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TransductionTransduction

In sensation, the transformation of In sensation, the transformation of stimulus energy into neural stimulus energy into neural

impulses.impulses.

Example:Example:

Phototransduction:Phototransduction: Conversion of Conversion of light energy into neural impulses light energy into neural impulses

that the brain can understand.that the brain can understand.

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PsychophysicsPsychophysics

A study of the relationship between A study of the relationship between physical characteristics of stimuli and physical characteristics of stimuli and

our psychological experience with them.our psychological experience with them.

Physical WorldPhysical World Psychological Psychological WorldWorld

LightLight BrightnessBrightness

SoundSound VolumeVolume

PressurePressure WeightWeight

SugarSugar SweetSweet

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No

DetectionDetection

Intensity

AbsoluteThreshold

Detected

YesYesNo No

Observer’s Response

Tell when you (the observer) detect the light.

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ThresholdsThresholds

Absolute Threshold:Absolute Threshold: Minimum stimulation needed Minimum stimulation needed to detect a particular stimulus 50% of the time.to detect a particular stimulus 50% of the time.

Pro

port

ion

of

“Yes”

Resp

on

ses

0.0

0

0

.50

1.0

0

0 5 10 15 20 25 Stimulus Intensity (lumens)

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Difference ThresholdDifference Threshold

Difference Threshold:Difference Threshold: Minimum Minimum difference between two stimuli required difference between two stimuli required for detection 50% of the time, also called for detection 50% of the time, also called

just noticeable difference (JND).just noticeable difference (JND).DifferenceThreshold

Tell when you (observer) detect a difference in the light.

No

Observer’s Response

No Yes

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Weber’s LawWeber’s Law

Two stimuli must differ by a constant Two stimuli must differ by a constant minimum percentage (rather than a minimum percentage (rather than a constant amount), to be perceived as constant amount), to be perceived as

different. Weber fraction: k = different. Weber fraction: k = I/I.I/I.

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Signal Detection Theory Signal Detection Theory (SDT)(SDT)

Predicts how and when we detect the Predicts how and when we detect the presence of a faint stimulus (signal) amid presence of a faint stimulus (signal) amid

background noise (other stimulation). SDT background noise (other stimulation). SDT assumes that there is no single absolute assumes that there is no single absolute

threshold and detection depends on:threshold and detection depends on:

Person’s experienceExpectationsMotivationLevel of fatigue

Carol L

ee/ Tony Stone Im

ages

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Signal to Noise RatioSignal to Noise Ratio

Signal/NoiseSignal/Noise

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SDT MatrixSDT Matrix

DecisionDecision

YesYes NoNo

SignalSignal

PresentPresent HitHit MissMiss

AbsentAbsentFalseFalseAlarmAlarm

Correct Correct RejectionRejection

The observer decides whether she hears the tone or not, based on the signal being present or not. This translates into four

outcomes.

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Sensory AdaptationSensory Adaptation

Diminished sensitivity as a consequence of Diminished sensitivity as a consequence of constant stimulation.constant stimulation.

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

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Subliminal PerceptionSubliminal Perception

sensing without awarenesssensing without awareness

is it possible?is it possible?

can it be used for persuasion?can it be used for persuasion?

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RED SOX TICKETSRED SOX TICKETS

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Perception in BrainPerception in Brain

Our perceptions are a combination of sensory (bottom-up) and cognitive (top-

down) processes.

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Other Important SensesOther Important Senses

The sense of touch is a mix of four distinct The sense of touch is a mix of four distinct skin senses—pressure, warmth, cold, and skin senses—pressure, warmth, cold, and

pain.pain.

Bru

ce A

yers

/ Sto

ne/ G

etty

Im

ages

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Skin SensesSkin Senses

Only pressure has identifiable receptors. All Only pressure has identifiable receptors. All other skin sensations are variations of other skin sensations are variations of

pressure, warmth, cold and pain.pressure, warmth, cold and pain.

Burning hot

Pressure Vibration Vibration

Cold, warmth and pain

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PainPain

Pain tells the body that something has gone Pain tells the body that something has gone wrong. Usually pain results from damage to the wrong. Usually pain results from damage to the skin and other tissues. A rare disease exists in skin and other tissues. A rare disease exists in

which the afflicted person feels no pain.which the afflicted person feels no pain.

Ashley Blocker (right) feels neither painnor extreme hot or cold.

AP Photo/ Stephen M

orton

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Biopsychosocial InfluencesBiopsychosocial Influences

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Gate-Control TheoryGate-Control Theory

Melzak and Wall (1965, 1983) proposed that Melzak and Wall (1965, 1983) proposed that our spinal cord contains neurological “gates” our spinal cord contains neurological “gates” that either block pain or allow it to be sensed.that either block pain or allow it to be sensed.

Gary C

omer/ PhototakeU

SA.com

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Pain ControlPain Control

Pain can be controlled by a number of therapies Pain can be controlled by a number of therapies including, drugs, surgery, acupuncture, exercise, including, drugs, surgery, acupuncture, exercise,

hypnosis, and even thought distraction.hypnosis, and even thought distraction.

Todd R

ichards and Aric V

ills, U.W

. ©

Hunter H

offman, w

ww

.vrpain.com