A threshold is an entrance; in psychology, it is the intensity a stimulus must achieve to be sensed, either by a receptor or in our awareness. Of course, thresholds are different for every kind of sense–and sensory attribute–that you can imagine.
Psychology recognizes two kinds of threshold, absolute and difference (or relative). The absolute threshold is the weakest intensity that can be detected.
Remember from the previous discussion topic that what we see is not always what is there physically and vice versa. That may make it easier to understand why the absolute threshold is defined as the fifty-percent level of detection. To experience the reason for yourself, go outside late on a clear night and look for the dimmest star you can see. Does it seem to be sometimes there and sometimes not?
Some people will say they see a star on the slightest evidence, while other people will wait to report one until they are certain. If you can find the cluster of stars called the Pleiades, count the number of stars you can see in it. Some will see four, others seven. In fact, there are more than 20 stars in the Pleiades. Why does the count vary? Obviously, some people see better than others; but some will not wait to confirm a pinpoint of light before counting it, while others will want to make sure they can always see a star before counting it. This is typical of absolute threshold tests. Near the threshold, the subject is never sure whether a light–or any other stimulus in the test–is there or not. Responses vary and make it appear that the idea of an absolute threshold is vague.
Signal Detection Theory (SDT)
Most psychologists have dropped the concept of an absolute threshold in favor of a measure from signal detection theory called d’ (read d-prime). Signal detection theory separates a subject’s response criterion from sensory detection so that both may be measured. The response criterion is a person’s willingness to report detection, while detection reflects the observer’s sensitivity to a stimulus such as light or sound. In the process of doing this, however, the simple notion of an absolute threshold is lost.
SDT is a useful technique. It is used in psychology, medicine, and engineering. Our resources avoid mathematical calculations, but the whole thing rests on a solid foundation of statistics. (Maybe only psychologists think of statistics as a solid foundation.) You can learn more about it starting on p. 5 of this presentation.
The difference threshold is also called a difference limen or relative threshold. It is the smallest change in a stimulus that can just be detected. You can’t see the motion of a clock’s hour hand; you may see motion in the minute hand of a large clock; and you can certainly see the second hand move. The second hand exceeds the difference threshold for motion detection.
An absolute threshold is typically defined against the condition of no stimulation at all. A difference threshold is always a change in existing or repeated stimulation. Both are defined as a 50% level of detection.
A Personal Experiment. A less obvious result can appear if you test someone’s two-point discrimination threshold for touch. How far apart must two points be before a subject can feel the two points separately? Bend a paper clip so that the two ends lie close together, about two millimeters apart–the width of two grains of table salt. Can you find any part of a friend’s body where two points rather than one can be felt? Now bend the paper clip so that the clip ends lie 25 millimeters apart, roughly an inch. Touch the clip ends to the shoulder, forehead, thigh, and back. What do you find? Even if your data are fairly dull, you should now be on very friendly terms with your experimental partner. Time for a cup of coffee. Next, we’ll look at how to measure difference thresholds using Weber’s law.
Psychological magnitudes often don’t go up and down the same way that physical magnitudes do. Actually, even scales in the physical world can vary in how they measure the same stimulus. Temperature is measured differently on the Fahrenheit and Celsius scales. Compare marginal tax rate with income level: The scales are very different. Your first $20,000 of earnings isn’t taxed at the same rate as your neighbor’s top $200,000, is it? Musical scales don’t indicate sound frequency very well, either. Playing eight notes up the scale only doubles the sound frequency (or you can see the difference graphically here.) In fact, the subjective psychological impression called pitch increases at a different rate from sound frequency. Response expansion (pain of shock) and compression (brightness): They reflect the mismatch between one scale and another.
This mismatch is what makes Weber’s law seem complicated. Our sense organs, and therefore our subjective awareness, change at a different rate than the physical stimulus does.
Here’s the problem: Our sense organs can only respond with action potentials, and action potentials aren’t normally fired off more than a few hundred times per second by a neuron*. That’s a problem because the intensity of light goes up to at least ten billion (i.e., ten thousand million if you’re British; 1010 if you’re an engineer) times absolute threshold; that is, the most intense light we see is about ten billion times the weakest light. We can’t generate action potentials ten billion times per second, or even a million times per second. How does the eye signal changes in light intensity to the brain?
How Weber’s Law Works
That’s where Weber’s law comes in: The retina signals percentage increases to the brain. If the difference threshold for light intensity is 0.3, or 30%, the retina will signal an uptick in light intensity when there’s a 30% increase. In that way, the visual system doesn’t max out its response before the light is near its maximum intensity.
Fechner’s Law and Stevens’ Law
Weber’s law was replaced by the more accurate Fechner’s law; it in turn gave way to Stevens’ power law, which uses a technique called magnitude estimation. This difference often appears in psychophysical discussions, so if it’s not clear, ask a question.
Magnitude estimation yields different results for different kinds of sensory measurements. For example, little
increases in shock intensity tend to produce large increases in our pain, while big changes in light intensity may produce only small increases in brightness. It’s important not to confuse the scale of our psychological impressions (brightness, loudness, pitch, pain, warmth, cold) with the physical stimulus scale (light intensity, sound intensity, sound frequency, shock intensity, and temperature, respectively).
You can find another explanation of Stevens’ power law here. This website illustrates why Stevens’ law is called a power law: The magnitude of a subjective experience (S) is related to the physical magnitude (I) raised to some power, a. If it’s not clear how magnitude estimation works, please raise a question.
Question: Please respond to either question 1 or question 2:
1. It’s not always easy to tell absolute and difference thresholds apart. Briefly explain your answer to each of the following examples:
· Someone who takes an opioid drug like Oxycontin or fentanyl for pain relief may find themselves taking more and more to reach the same level of pain control. Would you say that the absolute threshold for the drug effect is decreasing or the difference threshold is increasing? .
· Entering a dark theater, you cannot see the seats and have to wait a moment for them to become visible. When the seats are not visible are they below your absolute threshold for visual sensitivity or below your difference threshold?
· In story of the princess and the pea, how did the princess differ from ordinary young women? Did she have a lower absolute threshold for tactile pressure or a higher difference threshold?
· If you walk to the kitchen to get an apple, the trip back to your seat won’t seem different in length from the trip to the kitchen. But if you travel to another city for the first time and return home by the same route, you will think the return trip felt shorter. Does this return trip effect illustrate an absolute threshold or a difference threshold for perception of the trip distance?
2. How would you actually use psychophysics?
Here’s an example from the magical world of spaghetti sauce. (It lasts 17 ½ minutes but it offers an interesting look at psychophysics.)
Sweetness is an important factor in tomato taste that was bred out of the plant in an effort to make them redder at market time. How about replacing the lost sweetness in sauces with artificial sweeteners? But then how do you decide how much to add without going bankrupt?