Sensation and Perception 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.

Sensation vs. Perception Psychophysics: The study of the relationship between physical

stimuli and our experience of them.

Sensation The experience of sensory stimulation “I see…hear…feel…smell…taste something”

Perception Creating meaningful patterns from raw sensory information “I see a cat;” “I hear footsteps” etc.

Bottom-Up Processing Beginning with stimulation of our senses, we interpret sensory

information with our brains I see a furry, 4-legged creature with a tail and identify this as a dog

Top-Down Processing Using our schemas and past experiences, we interpret sensory

information to construct deeper meaning The dog is growling and foaming at the mouth and I realize it may have

rabies so I will not approach it

Sensation:

Definition:

bottom up processing: raw sensory information comes in and then gets interpreted by brain

Perception:

Definition:

Top down processing: Based upon expectations, motivations, past experiences

Say What???Aoccdrnig to rscheearch at Cmabrigde Uinervtisy,

it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a total mses and you can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

Selective Attention and Change Blindness

Selective attention is purposely focusing your conscious awareness onto a specific stimulus. This means that if you are in a noisy place with lots of people and you purposely pay attention to the person you are speaking with, you are engaging in selective attention.

Selective AttentionPerceptions about objects change from moment to moment. We can perceive different forms of

the Necker cube; however, we can only pay attention to one aspect of the object at a time.

6Necker Cube

Selective attention Experiment!

Human Earphones

We need four volunteers!

And two textbooks

Dichotic listening

Field-dependent or Independent?

Herman Witkin devised the Embedded Figures Test(EFT) to measure field dependence–independence and has studied this for more than 30 years)•Field Dependent: Difficulty locating hidden figures within complex surroundings (prefer social sciences and education)•Field Independent: Easily locatinghidden figures (tend to favor natural sciences, math and engineering)•Scores on field dependence–independence arerelatively stable over time.

How fast can you find the 12 items?

Selective Inattention

Because of selective attention, we are consciously “blind” to most visual stimuli in the environment:

2 famous examples:

Inattentional blindess: Gorillas in our midst basketball experiment.

Change Blindness: Asking for directions and the intruding door experiment.

Inattentional BlindnessInattentional blindness refers to the inability to see

an object or a person in our midst. Simmons & Chabris (1999) showed that half of the observers failed to see the gorilla-suited assistant in a ball

passing game.

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Change BlindnessChange blindness is a form of inattentional blindness in which two-thirds of individuals giving directions failed to notice a change in

the individual asking for directions.

12© 1998 Psychonomic Society Inc. Image provided courtesy of Daniel J. Simmons.

Absolute Threshold

One formal definition is that absolute threshold is the smallest intensity of a stimulus that has to be present for the stimulus to be detected 50% of the time.

Absolute Threshold Example

Think of an electric burner on a stove. Imagine turning that burner on and then placing your hand directly on it.

At first you won't feel much heat because is takes time for the burner to heat up. But at some point it will get hot enough for you to detect…meaning, there is some temperature that is just hot enough for you to notice it.

This isn't the point at which you get burned, but the point at which it is just hot enough for you to detect the presence of the heat.

Sensory Thresholds Absolute threshold (Demonstration #1)

The minimum amount of energy that can be detected 50% of the time

E.g. At what point can you hear the presence of a sound?

More examples for humans: Taste: 1 gram (.0356 ounce) of table salt in 500 liters (529

quarts) of water Smell: 1 drop of perfume diffused throughout a three-room

apartment Touch: the wing of a bee falling on your cheek from a height

of 1cm (.39 inch) Hearing: the tick of a watch from 6 meters (20 feet) in very

quiet conditions Vision: a candle flame seen from 50km (30 miles) on a

clear, dark night

A Challenge to Sensory Thresholds?

Signal Detection Theory challenges the notion of the absolute threshold.

SDT is a mathematical model that predicts how and when we will detect the presence of a faint stimulus or signalThere is NO single absolute thresholdDetection of a stimulus depends on a person’s

experience, expectations, motivations, and fatigue.

Difference ThresholdThe difference threshold,

a.k.a. the just noticeable difference (jnd), is the minimum difference in stimulation needed so that a person can detect the difference between two stimuli.

Difference Threshold Example

For example, let's say I asked you to put your hand out and in it I placed a pile of sand. Then, I add tiny amounts of sand to your hand and ask you to tell me when you notice any change in the overall weight.

As soon as you can detect any change in the weight, that difference between the weight of the sand before I added that last bit of sand and the amount of sand after I added it, is the difference threshold.

Sensory ThresholdsDifference threshold (Demonstration #2)

The smallest change in stimulation that can be detected 50% of the time

Also called the just noticeable difference or JND e.g. At what point can you tell that the TV volume has been

raised? Weber’s Law

States that the difference threshold is detected by a constant minimum percentage of the stimulus, not a constant amount

e.g. to detect a difference in weight, the change must be 2% of the original stimulus’ weight

Weber’s LawPsychophysics

Weber-Fechner law  Originated by the German physiologist Ernst Heinrich Weber (1795–1878) in 1834 and elaborated by his student Gustav Theodor Fechner.

The law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. It was later shown not to hold for extremes of stimulation.

Weber’s Law- ExampleWeber’s Law – If we can distinguish a Just-

Noticeable Difference in sound intensity between 100 hand bells and 110 hand bells, we may argue that we may be able to distinguish the difference between 10 and 11 hand bells, or between 300 and 330 hand bells: a 10% difference in each case. So argued the German physiologist E H Weber.

Our thresholds for detecting differences are a roughly constant proportion to the size of the original stimulus. (approx. 10%)

Sensory AdaptationWe get used to things. This goes for lots

of things in life including smells, sounds, sights, games, people, situations…seems like after a while we get used to everything.

One reason we get used to everything is because of sensory adaptation, which is reduced sensitivity to stimulation that results from repeated or prolonged presentations of that stimulation.

Sensory Adaptation Example

For example, my car was in for service recently and the dealer gave me a rental to use while the car was being serviced. As soon as I got into the car I was overwhelmed by the smell of smoke (even though I asked for a non-smoking car). It stunk!

But after driving the car for 30 minutes or so, I didn't really notice the smell. I got used to it because I was immersed in it. I experienced sensory adaptation.

Subliminal Subliminal: As humans, we have great abilities to perceive

things (e.g., to see things happening far away, to hear sounds at a distance, etc.). However, we also have limitations. For example, eagles can see much, much better than humans. When we are presented with some information that is just below our conscious awareness but still reaches our brains, it is a subliminal message - meaning, the information is getting into our systems and to our brains without us truly being aware of it.

You may be familiar with the idea of subliminal messages is advertising in which a message is flashed so quickly that we don't "think" we saw it (containing the message the advertiser wants us to get) but our brains actually processed it. The idea being that we will still respond to that message even though we didn't realize we saw it.

Camel Cigarettes

Subliminal Perception?We know that below threshold (subliminal) stimuli bombard

us regularly…

BUT…do we respond to these stimuli that are below our level of awareness? Research shows that the effect only occurs in controlled

laboratory studies Research outside the laboratory shows no significant, lasting

effect of subliminal information Priming

The often unconscious activation of certain associations for the purpose of altering perception, memory or response

Individuals flashed a pleasant or unpleasant image before viewing a photo of a person were influenced to judge the person positively is they saw a pleasant picture and negatively if unpleasant

Subliminal Research at Duke University Subliminal Advertising Experiment (Demo #3) Subliminal McDonald's Advertisement