Absolute Threshold of Hearing - Psychophysical Methods For Measuring Thresholds - Classical Methods

Classical Methods

Classical methods date back to the 19th century and were first described by Gustav Theodor Fechner in his work Elements of Psychophysics. Three methods are traditionally used for testing a subject's perception of a stimulus: the method of limits, the method of constant stimuli, and the method of adjustment.

Method of limits
In the method of limits, the tester controls the level of the stimuli. Single-interval “yes”/”no” paradigm’ is used, but there are no catch trials.
There are several series of descending and ascending runs.
The trial starts with the descending run, where a stimulus is presented at a level well above the expected threshold. When the subject responds correctly to the stimulus, the level of intensity of the sound is decreased by a specific amount and presented again. The same pattern is repeated until the subject stops responding to the stimuli, at which point the descending run is finished.
In the ascending run which comes after, the stimulus is first presented well below the threshold and then gradually increased in two decibel (dB) steps until the subject responds. As there are no clear margins to ‘hearing’ and ‘not hearing’, the threshold for each run is determined as the midpoint between the last audible and first inaudible level.
The subject's absolute hearing threshold is calculated as the mean of all obtained thresholds in both ascending and descending runs.
There are several issues related to the method of limits. First is anticipation, which is caused by the subject's awareness that the turn-points determine a change in response. Anticipation produces better ascending thresholds and worse descending thresholds.
Habituation creates completely opposite effect, and occurs when the subject becomes accustomed to responding either “yes” in the descending runs and/or “no” in the ascending runs. For this reason, thresholds are raised in ascending runs and improved in descending runs.
Another problem may be related to step size. Too large a step compromises accuracy of the measurement as the actual threshold may be just between two stimulus levels.
Finally, since the tone is always present, “yes” is always the correct answer.
Method of constant stimuli
In the method of constant stimuli, the tester sets the level of stimuli and presents them at completely random order.
Thus, there are no ascending or descending trials.
The subject responds “yes”/”no” after each presentation.
The stimuli are presented many times at each level and the threshold is defined as the stimulus level at which the subject scored 50% correct. “Catch” trials may be included in this method.
Method of constant stimuli has several advantages over the method of limits. Firstly, the random order of stimuli means that the correct answer cannot be predicted by the listener. Secondarily, as the tone may be absent (catch trial), “yes” is not always the correct answer. Finally, catch trials help to detect the amount of a listener's guessing.
The main disadvantage lies in the large number of trials which are needed to obtain the data and therefore long time required to complete the testing.
Method of adjustment
Method of adjustment shares some features with the method of limits, but differs in others. There are descending and ascending runs and the listener knows that the stimulus is always present.
However, unlike in the method of limits, here the stimulus is controlled by the listener. The subject reduces the level of the tone until it cannot be detected anymore, or increases until it can be heard again.
The stimulus level is varied continuously via a dial and the stimulus level is measured by the tester at the end. The threshold is the mean of the just audible and just inaudible levels.
Also this method can produce several biases. In order to avoid giving cues about the actual stimulus level, the dial must be unlabeled. Apart from already mentioned anticipation and habituation, stimulus persistence (preservation) could influence the result from the method of adjustment.
In the descending runs, the subject may continue to reduce the level of the sound as if the sound was still audible, even though the stimulus is already well below the actual hearing threshold.
In contrast, in the ascending runs, the subject may have persistence of the absence of the stimulus until the hearing threshold is passed by certain amount.

Read more about this topic:  Absolute Threshold Of Hearing, Psychophysical Methods For Measuring Thresholds

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