Measuring Sensation (OLI)

Humans possess powerful sensory capacities that allow us to sense the kaleidoscope of sights, sounds, smells, and tastes that surround us. Our eyes detect light energy and our ears pick up sound waves. Our skin senses touch, pressure, hot, and cold. Our tongues react to the molecules of the foods we eat, and our noses detect scents in the air. The human perceptual system is wired for accuracy, and people are exceedingly good at making use of the wide variety of information available to them.

In many ways our senses are quite remarkable. The human eye can detect the equivalent of a single candle flame burning 30 miles away and can distinguish among more than 300,000 different colors. The human ear can detect sounds as low as 20 hertz (vibrations per second) and as high as 20,000 hertz, and it can hear the tick of a clock about 20 feet away in a quiet room. We can taste a teaspoon of sugar dissolved in 2 gallons of water, and we are able to smell one drop of perfume diffused in a three-room apartment. We can feel the wing of a bee on our cheek dropped from 1 centimeter above.

Although there is much that we do sense, there is even more that we do not. Dogs, bats, whales, and some rodents all have much better hearing than we do, and many animals have a far richer sense of smell. Birds are able to see the ultraviolet light that we cannot (see the figure below) and can also sense the pull of the earth’s magnetic field. Cats have an extremely sensitive and sophisticated sense of touch, and they are able to navigate in complete darkness using their whiskers. The fact that different organisms have different sensations is part of their evolutionary adaptation. Each species is adapted to sensing the things that are most important to them, while being blissfully unaware of the things that don’t matter.

Ultraviolet Light and Bird Vision
Because birds can see ultraviolet light but humans cannot, what looks to us like a plain black bird looks much different to a bird.Because birds can see ultraviolet light but humans cannot, what looks to us like a plain black bird looks much different to a bird. From Flat World Knowledge, Introduction to Psychology, v1.0: Adapted from Fatal Light Awareness Program. (2008). Our research program. Retrieved from


Psychophysics is the branch of psychology that studies the effects of physical stimuli on sensory perceptions and mental states. The field of psychophysics was founded by the German psychologist Gustav Fechner (1801–1887), who was the first to study the relationship between the strength of a stimulus and a person’s ability to detect the stimulus.

The measurement techniques developed by Fechner and his colleagues are designed in part to help determine the limits of human sensation. One important criterion is the ability to detect very faint stimuli. The absolute threshold of a sensation is the intensity of a stimulus that allows an organism to just barely detect it.

In a typical psychophysics experiment, an individual is presented with a series of trials in which a signal is sometimes presented and sometimes not, or in which two stimuli are presented that are either the same or different. Imagine, for instance, that you were asked to take a hearing test. On each of the trials, your task is to indicate either yes if you heard a sound or no if you did not. The signals are purposefully made to be very faint, making accurate judgments difficult.

The problem for you is that the very faint signals create uncertainty. Because our ears are constantly sending background information to the brain, you will sometimes think that you heard a sound when no sound was made, and you will sometimes fail to detect a sound that was made. Your must determine whether the neural activity that you are experiencing is due to the background noise alone or is a result of a signal within the noise.

Signal Detection Analysis

The responses you give on the hearing test can be analyzed using signal detection analysis. Signal detection analysis is a technique used to determine the ability of the perceiver to separate true signals from background noise. [1] [2] As you can see in the figure below, each judgment trial creates four possible outcomes: A hit occurs when you, as the listener, correctly say yes when there was a sound. A false alarmoccurs when you respond yes to no signal. In the other two cases, you respond no—either a miss (saying no when there was a signal) or a correct rejection (saying no when there was in fact no signal).

Outcomes of a Signal Detection Analysis
Our ability to accurately detect stimuli is measured using signal detection analysis. Two of the possible decisions (hits and correct rejections) are accurate; the other two (misses and false alarms) are errors.Our ability to accurately detect stimuli is measured using signal detection analysis. Two of the possible decisions (hits and correct rejections) are accurate; the other two (misses and false alarms) are errors. From Flat World Knowledge,Introduction to Psychology, v1.0, CC-BY-NC-SA.

The analysis of the data from a psychophysics experiment creates two measures. One measure, known as sensitivity, refers to the true ability of the individual to detect the presence or absence of signals. People who have better hearing will have higher sensitivity than will those with poorer hearing. The other measure, response bias, refers to a behavioral tendency to respond “yes” to the trials, which is independent of sensitivity.

Imagine for instance that rather than taking a hearing test, you are a soldier on guard duty, and your job is to detect the very faint sound of the breaking of a branch that indicates that an enemy is nearby. You can see that in this case making a false alarm by alerting the other soldiers to the sound might not be as costly as a miss (a failure to report the sound), which could be deadly. Therefore, you might well adopt a very lenient response bias in which whenever you are at all unsure, you send a warning signal. In this case, your responses may not be very accurate (your sensitivity may be low because you are making many false alarms) and yet the extreme response bias can save lives.

Another application of signal detection occurs when medical technicians study body images for the presence of cancerous tumors. Again, a miss (in which the technician incorrectly determines that there is no tumor) can be very costly, but false alarms (referring patients who do not have tumors to further testing) also have costs. The ultimate decisions that the technicians make are based on the quality of the signal (clarity of the image), their experience and training (the ability to recognize certain shapes and textures of tumors), and their best guesses about the relative costs of misses versus false alarms.

Just Noticeable Difference (JND)

Although we have focused to this point on the absolute threshold, a second important criterion concerns the ability to assess differences between stimuli. The difference threshold (or just noticeable difference [JND]), refers to the change in a stimulus that can just barely be detected by the organism
The German physiologist Ernst Weber (1795–1878) made an important discovery about the JND: that the ability to detect differences depends not so much on the size of the difference but on the size of the difference in relationship to the absolute size of the stimulus. Weber’s law maintains that the just noticeable difference of a stimulus is a constant proportion of the original intensity of the timulus. As an example, if you have a cup of coffee that has only a very little bit of sugar in it (say, 1 teaspoon), adding another teaspoon of sugar will make a big difference in taste. But if you added that same teaspoon to a cup of coffee that already had 5 teaspoons of sugar in it, then you probably wouldn’t taste the difference as much (in fact, according to Weber’s law, you would have to add 5 more teaspoons to make the same difference in taste).

One interesting application of Weber’s law is in our everyday shopping behavior. Our tendency to perceive cost differences between products is dependent not only on the amount of money we will spend or save but also on the amount of money saved relative to the price of the purchase. I would venture to say that if you were about to buy a soda or candy bar in a convenience store and the price of the items ranged from $1 to $3, you would think that the $3 item cost a lot more than the $1 item. But now imagine that you were comparing two music systems, one that cost $397 and one that cost $399. Probably you would think that the cost of the two systems was about the same even though buying the cheaper one would still save you $2.
Weber’s law states that our ability to detect the difference between two stimuli is proportional to the magnitude of the stimuli. This may sound difficult, but consider this example. Imagine that you have a 1-pound weight in one hand. I put a 2-pound weight in your other hand. Do you think you could tell the difference? Probably so. These weights are light (low magnitude) so a difference of 1 pound is very easily detected. Now I put a 50-pound weight in one hand and a 51-pound weight in the other hand. Now do you think you could tell the difference? Probably not. When the weight is heavy (high magnitude), the 1-pound difference is not so easily detected.

Weber’s law focuses on one of the oldest variables in psychology, the JND. These letters stand for just noticeable difference, which is the smallest difference between two stimuli that you can reliably detect. Using this term, Weber’s law says that the size of the JND will increase as the magnitude of the stimulus increases. In the weight example, the JND when you have a 50-pound weight in your hand is much greater (2 pounds? 5 pounds? 1pounds?) than when you have a 1-pound weight in your hand (1 pound? ½ pound? ¼ pound?).

Did I get this

A JND is _____________________.
  • the greatest detectable difference between two stimuli
  • the smallest detectable difference between two stimuli
  • the lowest possible intensity of a stimulus that can be detected
JND stands for Just Noticeable Difference, the smallest difference between two stimuli that you can reliably detect.

Weber’s law states that the JND for high magnitude stimuli is ____________ the JND for low magnitude stimuli.
  • smaller than
  • the same as
  • grater than
Weber’s law states that the JND for a high magnitude stimulus is greater than the JND for a low magnitude stimulus.

In signal detection theory, a false alarm occurs when the target is __________ and you respond that it is __________.
  • absent; present
  • present; absent
  • present; present
  • absent; absent
When the target is absent and you say it is present, you have a FALSE ALARM.


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