Chapter 1. Absolute Threshold

1.1 Title slide

Demonstration 1.3
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Absolute Threshold

Determine your absolute threshold for visual stimuli, using the method of constant stimuli and the staircase method.

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Thresholds of perceptual experience are the points at which there are perceptual transitions, such as the transition from not seeing anything in a foggy background to seeing a tree or a person walking a dog.
Photo: © M.Sobreira / Alamy

How Are Psychophysical Methods Used to Measure the Absolute Threshold?

The absolute threshold is the minimum intensity of a physical stimulus that can just be detected. Three methods used to measure absolute thresholds are the method of adjustment, the method of constant stimuli, and the staircase method. (The method of adjustment isn't covered in this demonstration.)

In the method of adjustment, the stimulus is presented at an intensity well below or above the presumed threshold, and the participant adjusts the intensity until the stimulus is just detectable or just undetectable. The intensity at which each adjustment ends is taken as an estimate of the absolute threshold. The average of these estimates is taken as the final estimate of the absolute threshold. However, the method of adjustment is rarely used because the results tend to vary quite a bit.

The method of constant stimuli gives more consistent and reliable results. This method uses stimuli covering a range of intensity values likely to include the absolute threshold. The stimuli are presented in random order, and the participant indicates whether each stimulus was detected or not. The frequency with which each intensity was detected is plotted on a graph, and the resulting curve is used to estimate the absolute threshold (often defined as the intensity at which the participant detected the stimulus 50% of the time).

The staircase method is a more efficient version of the method of constant stimuli, also using stimuli covering a range of intensity values likely to include the absolute threshold. But in the staircase method, if a stimulus is detected, the intensity of the next stimulus is one step down; and if a stimulus isn't detected, the next stimulus is one step up. This means that, after the first few trials, the intensities of the presented stimuli tend to be near the absolute threshold. The mean of the intensities at which the participant's judgments "reverse" (i.e., change from detected to not detected or vice versa) is taken as an estimate of the absolute threshold.

1.2 Explain - custom

test_activity_1_3_slide_2
You must adjust the brightness level of your screen by dragging the slider. If the brightness level isn't properly adjusted, you won't be able to determine your absolute threshold accurately.
In this demonstration, you'll determine your absolute threshold for detecting a spot of blue light against a black background,
using two psychophysical methods: the method of constant stimuli and the staircase method.
Before you begin:► Orient your screen to minimize glare.► Adjust the brightness level of your screen by dragging the slider below to the left or right, until the spot of blue light in the box is at the dimmest level at which you can still see it. (If you can't see the spot now, start by dragging the slider to the right to make the spot brighter.)
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1.3 Intro Experement 1

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Method of Constant Stimuli
In this part of the demonstration, you'll use the method of constant stimuli to determine your
absolute threshold for detecting a spot of blue light against a black background.
In each trial Press
In each trial Press
if you see the spot.
if you can't see the spot.
When you're ready, press the space bar to start trial 1.

1.4 Experement 1

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Method of Constant Stimuli
In this part of the demonstration, you'll use the method of constant stimuli to determine your
absolute threshold for detecting a spot of blue light against a black background.
Press the space bar to continue.
If you can see the spot, press
Y
If you can't see the spot, press
N
Trial 1 of 30

1.5 Experement 1 Result

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Method of Constant Stimuli
Results
Percentage of "Y" responses
Your results cannot be displayed because you didn't complete all the trials.
To see your results, go back and complete a new sequences of trials.
Relative intensity of stimuli

Estimated absolute threshold: n/a

Stimuli with relative intensities of 1, 3, 5, 7, and 9 were presented 6 times each in random order. The plotted points show the percentage of 'Y' responses for each intensity. The curve is the psychometric function that best fits the plotted points. By definition, the estimated absolute threshold is the intensity at which you would be expected to detect the stimulus 50% of the time - that is, the intensity at which the

psychometric function is at 50%, as indicated by the green arrow.
Note: In laboratory experiments using the method of constant stimuli, participants' responses typically fit an S-shaped psychometric function like the one shown here. In this demonstration, done under much less controlled conditions, your responses are less likely to fit a smooth S-shaped curve.

1.6 Intro Experement 2

activity_1_3_slide_6
Staircase Method
In this part of the demonstration, you'll use the staircase method to determine your
absolute threshold for detecting a spot of blue light against a black background.
In each trial Press
In each trial Press
if you see the spot.
if you can't see the spot.
When you're ready, press the space bar to start the trials.

1.7 Experement 2

You haven't completed the activity on this screen. If you leave this screen now, any work you've done on this screen will be lost. Are you sure you want to leave?
activity_1_3_slide_7
Staircase Method
In this part of the demonstration, you'll use the staircase method to determine your
absolute threshold for detecting a spot of blue light against a black background.
Press the space bar to continue.
If you can see the spot, press
Y
If you can't see the spot, press
N
Trial 1

1.8 Experement 2 Result

You must select an answer and then click SUBMIT before you can click proceed to the next screen.
activity_1_3_slide_8
Staircase Method
Results
Your results cannot be displayed because you didn't complete all the trials.
To see your results, go back and complete a new sequences of trials.
Trials
Relative intencity of stimuli
"Y" response
"N" response
Reversal

Estimated absolute threshold: n/a

Stimuli were presented at a range of intensities. Each time you responded Y the intensity was reduced, and each time you responded N the intensity was increased. After 30 trials or 8 Y/N reversals, the run of trials ended. The estimated absolute threshold is the average of the intensities at the n/a reversal points.

1.9 Explain

explain_text

How Are Psychophysical Methods Used to Measure the Absolute Threshold?

The absolute threshold is the minimum intensity of a physical stimulus that can just be detected. Three methods used to measure absolute thresholds are the method of adjustment, the method of constant stimuli, and the staircase method. (The method of adjustment isn't covered in this demonstration.)

In the method of adjustment, the stimulus is presented at an intensity well below or above the presumed threshold, and the participant adjusts the intensity until the stimulus is just detectable or just undetectable. The intensity at which each adjustment ends is taken as an estimate of the absolute threshold. The average of these estimates is taken as the final estimate of the absolute threshold. However, the method of adjustment is rarely used because the results tend to vary quite a bit.

The method of constant stimuli gives more consistent and reliable results. This method uses stimuli covering a range of intensity values likely to include the absolute threshold. The stimuli are presented in random order, and the participant indicates whether each stimulus was detected or not. The frequency with which each intensity was detected is plotted on a graph, and the resulting curve is used to estimate the absolute threshold (often defined as the intensity at which the participant detected the stimulus 50% of the time).

The staircase method is a more efficient version of the method of constant stimuli, also using stimuli covering a range of intensity values likely to include the absolute threshold. But in the staircase method, if a stimulus is detected, the intensity of the next stimulus is one step down; and if a stimulus isn't detected, the next stimulus is one step up. This means that, after the first few trials, the intensities of the presented stimuli tend to be near the absolute threshold. The mean of the intensities at which the participant's judgments "reverse" (i.e., change from detected to not detected or vice versa) is taken as an estimate of the absolute threshold.

1.10 Test - single choice

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E663rk8IVI9+xpLA+mwY4+MgJY6/90EH/Cf6WRxWbrmsZ6BwqBJ7+J7JhdSXEK/DQa6PyPzqvpx51iu977cci7VriKEeAmQiFnDk0T/y9iVE2bCbajJiY4qNZ3nvx+W781YUWL3ESmsdErlyrCWjsdl4jv3Z4fKWCQFuIZswPp+NdxUhATz74cPf2HZEOly05zhuKqV7lLT0YRUGWz3NERR6hUQ8mkXHr0CLvSVgK6zNt7grX7s8tBu0RP6muPTwxFlfPofDXl+3GiUiTJTKS9NSgoPeQBQnJARSofUyldUlu/WmBqjm1bDiOsLC9fai80VOMsTmGkSQDUwSfTFfdyQ4JIc=
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The correct answer is B.
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1.11 Test - single choice

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z5E+cCRWk+G1WIAlIzsF8fr2ECcyk4Dg0JR6XrB+b2xOQEnVduWVx5JG3Y83KVRHseapmotQiiH99XGFlAJlV/xCl6P24ztAV7y+whAfBtzZT210aMSGyqybgmxWOCImvSsK2/tE+OsrR5RA9iLsKwEtc06Qvp3KgOfwHmcQIBhP++u3zD0bNukNoQHzHXSgoacSse6pxt9VntkhAseps4PM3ma9NVlKxRWJ/Ti1ZIpTIrsF/HlU+PHnxGn4BRWsTzfdVvMh/q+NmWsTtgpWazY1+tECzTf6wgXNObrdxu5FHVKq4A5Wv+V4V8YNWw1oPGn8R6MZzfTxJP1I8gqXOu1+p1wfHK161fX9iCEOYR/a9xlqpKRM+sWHXO453wwKQJI+MyUD4Mb2oQYi
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The correct answer is A.
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1.12 Test - single choice

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Select your answer to the question below. Then click SUBMIT.

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The correct answer is B.
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1.13 Test - single choice

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Select your answer to the question below. Then click SUBMIT.

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8TT+nlrbqDqUpzhPEzHkz31gRRj99ylG4rrQhGEbGUXA/kgDUaAmT/lVJlLHNLnBZMfAfn0V4KOHxDbCoJJ/alYi75eC7p5f8XWP3hn6bPTF9zWNCMtSTtk8O00YxjZoCI1D6SKSIB2475vzQu3GEpUdo5iPCeoeph8gEE1ERlaiFU3IbPz2bK2ZO5WjFNMKJgjiz8dbCdlymvXuXZtvqjBhCyZX/HMpHdyN5bukqpEWf/6RoBXauFpyG+kO4N31yp5DMHtmPGINCHD4eSi14lwHedS3Vyq6pPU4659Mmqk5cEl47D3xV95t38h83abhz5p4OFrY4J7JupbVdficGQE7et473Cem2i6IkhonayyblHHn0KOLekhyKGjNFVoOMYM397HdXZ3lpNdO4J8mTA5Kj5RCp/wVzi+ZwHtl2nUDiAwispSXg6/wUosztpR1eqPB/P+VMWjEEDB7JU68WSfEt/OhhKb/uZJ8UbYhTqSdFvJ2hFAj1NNEZii6bwpCGGzwpaIhSrtuDezXC/DmOT84Y08tcvCtglXqdHNMYZ4GVxaHkxlxGv6692dZ6vcYLStE9w==
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