Chapter 78. Absolute Thresholds and Difference Thresholds

Learning Objectives

absolute threshold

smallest amount of stimulus energy that is detectable 50 percent of the time

sensation

detecting stimulus energy from the outside world and sending that information to the brain for interpretation

brightness

the apparent intensity of a light; whether the light seems dim (weak) or bright (intense)

sensory receptors

specialized cells in the sensory systems that can capture energy or information from a stimulus and convert that information into neural impulses

difference threshold

smallest amount of change in stimulus energy that is detectable 50 percent of the time; also called the just noticeable difference (JND)

sensory system

all the body cells involved in one of the senses, such as hearing

just noticeable difference (JND)

the minimum change in a stimulus that is detectable 50 percent of the time; also called the difference threshold

stimulus

an external event, such as a light or sound, detected by our senses

loudness

the apparent intensity of a sound; whether a sound seems soft (weak) or loud (intense)

Weber’s law

psychological principle stating that the smallest change that can be detected in a stimulus is a constant ratio of the original stimulus intensity

perception

organizing and interpreting information from the senses to understand its meaning

experiment

a method of research that manipulates an independent variable to measure its effect on a dependent variable

Absolute Thresholds and Difference Thresholds

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Learning Objectives:

Describe the role of the absolute threshold in detecting a stimulus.

Contrast the absolute threshold with the difference threshold.

Explain how Weber’s Law predicts the size of the just noticeable difference (JND).

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The figure is a series of sensory receptors for the various senses. Olfactory cells represent smell. Taste buds represent taste. Skin receptors represent touch and pain. Auditory cells in the cochlea represent hearing. Rods and cones in the retina represent vision.

1. The first step in perception—understanding what is happening in the world around us—is to become aware that some form of stimulus energy is striking our sensory receptors. This detection process is called sensation.

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The figure is a line graph with one line. The Y Axis is labeled “Sound energy from mosquito wings”. The X axis is not labeled but represents time. The line for the X axis is near the top of the Y axis and represents the absolute threshold. The line in the graph starts at the bottom left at a low sound energy and goes up and down below the absolute threshold. Halfway along the X axis the line crosses the absolute threshold before coming back down below it, where it cycles up and down again. The time when the line was above the absolute threshold is when sound is detected. All of the time when the line is below the absolute threshold is when sound is not detected.

2. How much stimulus energy is necessary before we can sense that something is happening in our environment? The minimum amount of stimulus energy that is detectable on average is called the absolute threshold. A threshold is a “cross-over” point, such as a doorway separating one room from another. When a mosquito is flying a few feet away, the buzzing sound may be too soft to hear. But, as the mosquito gets close to our ear, the sound energy crosses our absolute threshold, and we become aware of the buzz.

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The figure is a line graph with one line. The X axis is labeled “Stimulus Intensity” and ranges from 0 to 5 in increments of 1. A series of 7 light bulbs appear at the top of the graph, ranging from off to bright light with small variations in the light emitted from the bulb. This corresponds to the stimulus intensity on the X axis as the bulb becomes brighter as the intensity increases. The Y axis is labeled “Percent detected” and ranges from 0 to 100 in increments of 50. A dotted vertical line in the middle of the graph is at 50% detectability. The line in the graph starts at 0 intensity and 0 detectability, and the light bulb is off. The line slowly rises to about 20% detectability at a intensity level of 2. There is some light in the light bulb. The line then rises sharply to an intensity level of 2.5. This is when the line reaches the 50% detectability level. This is labeled as the “absolute threshold”. The line continues to increase sharply before leveling off at around an intensity level of 3.5. This is at almost 100% detectability and is when the light bulb is very bright.

3. How is the absolute threshold measured? If we flash a light briefly 100 times, randomly varying the brightness of the light on each trial, the results will look like the blue curve on this graph. As the stimulus energy increases, the likelihood that a person will detect the stimulus increases. The absolute threshold, shown as a red dot, is the amount of light energy that is needed in order for this particular person to have a 50 percent chance of seeing the light.

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The figure is a line graph with one line. The X axis is labeled “Stimulus Intensity” and ranges from 0 to 5 in increments of 1. A series of 7 light bulbs appear at the top of the graph, ranging from off to bright light with small variations in the light emitted from the bulb. This corresponds to the stimulus intensity on the X axis as the bulb becomes brighter as the intensity increases. The Y axis is labeled “Percent detected” and ranges from 0 to 100 in increments of 50. A dotted vertical line in the middle of the graph is at 50% detectability. The line in the graph starts at 0 intensity and 0 detectability, and the light bulb is off. The line slowly rises to about 20% detectability at a intensity level of 2. There is some light in the light bulb. The line then rises sharply to an intensity level of 2.5. This is when the line reaches the 50% detectability level. This is labeled as the “absolute threshold”. The line continues to increase sharply before leveling off at around an intensity level of 3.5. This is at almost 100% detectability and is when the light bulb is very bright. The area immediately to the left of the absolute threshold and below the 50% detectability line is labeled as “greater than 0%”. The area immediately to the right of the absolute threshold, and above the 50% detectability line is labeled as “less than 100%”. These areas represent times when the absolute threshold may be detected when the stimulus is less than expected for detectability, and may not be detected when the stimulus is greater than expected for detectability.

4. Because there is random activity (“background noise”) within each sensory system, sometimes a light that is slightly above the absolute threshold will not be detected. Likewise, sometimes the person will detect a light that is slightly below the absolute threshold.

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5. Once a stimulus is above our absolute threshold, how much additional energy is needed in order for us to detect that the stimulus has changed? This is called the difference threshold, and the minimum amount of energy change is called the just noticeable difference (JND). Imagine that you are turning up the volume on a sound system. How much would the physical intensity of the sound need to increase before you could hear that the sound had become louder?

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6. Weber’s law tells us that the just noticeable difference (JND) is not a constant amount of stimulus energy, but instead is a constant percentage of the current stimulus energy. If the JND for loudness of a sound is 5 percent (or 1/20), increasing the sound system’s volume setting from 20 units to 21 units should, on average, be just enough to produce a noticeably louder sound. But if the setting is already at 40 units, we would need to add 2 units in order to hear the difference in loudness.

Practice 1: Testing the Absolute Threshold

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Practice 1: Testing the Absolute Threshold

On each trial, drag the slider to one of the 10 different levels of stimulus energy, and select the PLAY SOUND button. The participant will make a response, and we’ll plot the results. There will only be 10 trials, but each trial will simulate 4 additional trials at the same intensity level.

We are going to simulate an experiment on testing the absolute threshold for this participant. You will control the intensity of a sound stimulus, and this participant will indicate whether he can detect a sound at each level of intensity. When you have tested all 10 levels of stimulus energy (and viewed the completed response curve), we will calculate the absolute threshold for this individual.

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The absolute threshold for this participant seems to be about 40 units of sound intensity. On average, if the stimulus had at least that much intensity, he could detect it. Now, select the NEXT button and move to Practice 2.

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Practice 2: Testing the Difference Threshold

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Practice 2: Testing the Difference Threshold

On each trial, drag the slider to one of the 8 different levels of stimulus energy, and select the PLAY SOUND button. The participant will hear two sounds, and will indicate whether the sounds have the same or different loudness. Then, we’ll plot the results. There will only be 8 trials, but each trial will simulate 4 additional trials at the same intensity level.

Now we will simulate an experiment on testing the difference threshold for this participant. We will begin each trial by playing a “standard” sound of 60 units of sound intensity, which is well above this participant’s absolute threshold. This will be followed by a second “comparison” sound of varying intensity (ranging from 61 to 68 units), which you will control. The participant will indicate whether he can detect a difference in the loudness of the sound at each level of intensity. When you have tested all 8 levels of stimulus energy (and viewed the completed response curve), we will calculate the difference threshold for this individual.

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The difference threshold—or just noticeable difference (JND)—for this participant seems to be about 3 units of sound intensity. On average, if the stimulus had changed by at least that much intensity from the original intensity of 60 units, he could detect the change in loudness. The ratio of the JND to the original intensity was 3/60 or 1/20 (5 percent). Now, select the NEXT button and move to Practice 3.

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Practice 3: Applying Weber’s Law

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Practice 3: Applying Weber’s Law

Select each of the buttons to see what happens.

Now, let’s put Weber’s law into action. This research participant is in a completely dark room, illuminated only by some candles burning on a table behind her. If we added a new candle, would she notice that the room had become brighter? Weber’s Law states that the just noticeable difference (JND) is a constant ratio of the original stimulus energy. From earlier research, we know that the “Weber ratio” for detecting a change in the brightness of a light is about 1/12, or 8 percent. Let’s test this by varying the number of original candles, and then adding one new candle.

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The change in stimulus intensity is 1/4, or 25 percent. Because this change is greater than the JND of 8 percent, this participant easily detected the difference in brightness.

The change in stimulus intensity is 1/20, or 5 percent. Because this change is smaller than the JND of 8 percent, this participant could not detect the difference in brightness.

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Quiz 1

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Quiz 1

Match the terms with their descriptions by dragging each colored circle to the appropriate gray circle. When all the circles have been placed, select the CHECK ANSWER button.

Perhaps you should go back to review the two types of sensory thresholds.

Select the NEXT button and move to Quiz 2.

absolute threshold

difference threshold

Weber’s Law

smallest amount of change in stimulus energy that is detectable 50 percent of the time

smallest amount of stimulus energy that is detectable 50 percent of the time

smallest change that can be detected in a stimulus is a constant ratio of the original stimulus intensity

Quiz 2

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0, 1, 0, 1, 1, 0

Quiz 2

For each statement, select one of the buttons to indicate whether the statement is True or False. When a response has been placed for all statements, select the CHECK ANSWER button.

Select the NEXT button and move to the Conclusion.

Perhaps you should go back to review the two types of sensory thresholds.

True	False
		If a new sound is played at an intensity level that is above a participant’s absolute threshold for sound, there is a 100 percent chance that the participant will hear the sound.
		Seventy-six trombones are playing a single note. If we add one more trombone, a typical listener will be able to tell that the sound has gotten louder.
		In our daily lives, many external stimuli stimulate our sensory systems at levels below our absolute threshold, so we don’t detect those events.

Conclusion

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