Invitation to Psychology

190 Chapter 6 Sensation and Perception

than the one you currently lug around? We often

need to be able to compare two stimuli and deter-

mine whether they are the same or different on

some dimension.

Suppose you’re in the gym, pressing a barbell

loaded with 100 pounds overhead, and one of your

practical-joker friends adds weight to it when you

are not looking. What is the smallest amount of

weight she can add before you think to yourself,

“Hey, this bar feels heavier”? If you and your

friend tried to answer this question systematically,

you would probably determine that the answer is

another 2 pounds. In other words, 2 pounds is the

smallest difference in the weight of two barbells

that you would reliably detect, where again, “reli-

ably” means half of the time. Scientists call this

point the difference threshold or just noticeable dif-

ference (jnd).

Simulate the Experiment Weber’s Law

at MyPsychLab

Now suppose that after you lift a small

1-pound dumbbell overhead, your friend tries

the same prank. Would she have to add 2 pounds

to that little dumbbell before you would notice?

Unlikely—that would triple the weight of the

dumbbell. Way back in the nineteenth century,

German scientist Ernst Weber determined that

for people to detect a difference between two

stimuli, such as two weights, those stimuli must

differ by a certain fixed proportion (such as 2 per-

cent), not a certain amount (such as 2 pounds or 2

ounces). Different properties of stimuli have their

own constant percentages: For two weights, it is

2 percent; for the brightness of two lights or the

saltiness of two liquids, it is 8 percent; and for the

loudness of two noises, it is 5 percent.

difference threshold
The smallest difference
in stimulation that can be
reliably detected by an
observer when two stimuli
are compared; also called
just noticeable difference
(jnd).

dark room and look at a wall or screen. You would

then be shown flashes of light varying in bright-

ness, one flash at a time. Your task would be to

say whether you noticed a flash. Some flashes you

would never see. Some you would always see. And

sometimes you would miss seeing a flash, even

though you had noticed one of equal brightness

on other trials. Such errors seem to occur in part

because of random firing of cells in the nervous

system, which produces fluctuating background

noise, something like the static in a radio trans-

mission that is slightly out of range.

By studying absolute thresholds, psycholo-

gists have found that our senses are sharp indeed.

If you have normal sensory abilities, you can see a

candle flame on a clear, dark night from 30 miles

away. You can taste a teaspoon of sugar diluted

in two gallons of water, smell a drop of perfume

diffused through a three-room apartment, and

feel the wing of a bee falling on your cheek from

a height of only one centimeter (Galanter, 1962).

Yet despite these impressive skills, our senses

are tuned in to only a narrow band of physical en-

ergies. We are visually sensitive to just a tiny frac-

tion of the electromagnetic energy that surrounds

us; we do not see radio waves, infrared waves, or

microwaves (see Figure 6.1). Many other species

can pick up signals that we cannot. Dogs can de-

tect high-frequency sound waves that are beyond

our range, and bees can see ultraviolet light, which

merely gives human beings a sunburn.

Difference Thresholds. Does an expensive

toothpaste really make your teeth noticeably

brighter? How about that new laptop you have

your eye on; can you really tell that it’s lighter

Wavelength THE ELECTROMAGNETIC SPECTRUM

Wavelength in nanometers

Infrared Visible spectrum Ultraviolet

1500 1000 700 600 500 400 300

3000 mi. 1 mi. 100 ft. 1 ft. .01 ft. .0001 ft. 10 nm. 1 nm. .001 nm. .00001 nm.

Radio TV Microwaves Infrared U-V X-rays Gamma

rays

Cosmic

rays

Figure 6.1 The Visible Spectrum of electromagnetic energy

Our visual system detects only a small fraction of the electromagnetic energy around us.