Il metodo della scalinata per trovare la soglia percettiva
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Overview
Fonte: Laboratorio di Jonathan Flombaum—Johns Hopkins University
Psicofisica è il nome di un insieme di metodi in psicologia percettiva progettati per mettere in relazione l’effettiva intensità degli stimoli con la loro intensità percettiva. Un aspetto importante della psicofisica riguarda la misurazione delle soglie percettive: quanto deve essere luminosa una luce affinché una persona sia in grado di rilevarla? Quanta poca pressione applicata sulla pelle è rilevabile? Quanto può essere morbido un suono ed essere ancora ascoltato? In altre parole, quali sono le più piccole quantità di stimolazione che gli esseri umani possono percepire? La procedura della scala è una tecnica efficace per identificare la soglia percettiva di una persona.
Questo video dimostrerà i metodi standard per applicare la procedura della scala al fine di identificare la soglia uditiva di una persona, cioè il volume minimo necessario per percepire un tono.
Procedure
Results
The aim of the staircase procedure is to bring the participant to a volume at which they can just barely hear a tone. This is achieved by prompting a series of 'No' responses in the first few trials. Once a 'Yes' response is produced, the goal is to keep the volume played close to the one that elicited the first 'Yes'. This is done by lowering the volume whenever a 'Yes' response is given. This produces a pattern in which the volume rises steadily in the first few trials, and then plateaus, remaining in a narrow range until the end of the experiment, as seen in Figure 3. The central tendency of this narrow range is a measure of the threshold. In Figure 3, it is clear that the threshold is reached at around 6 dB. A common way to calculate the threshold is to compute the average of the volumes played during the last 10 trials of the experiments. In the case of Figure 3, that average works out to 6.1 dB.
With results obtained for six tones of different frequencies, one can see that perceptibility thresholds vary by frequency (what is often called pitch). Higher pitched sounds are harder to hear than lower pitched ones. To see this graphically, plot the volume threshold for each of the six tones tested in the experiment, just as done for the 1 kHz tone-as shown in Figure 4. The data shown are for a single participant, 20 yrs old. The main pattern is that the low frequency tones are easier to hear than high frequency tones. This is a fact of human hearing that arises because of the structure of the auditory system, starting with nature of the vibrating filaments and bones inside the human ear.
Figure 4. Volume threshold as a function of frequency. Data shown are for a single participant, age 20 yrs. Because of the structure of the human auditory system, sounds with lower frequencies-what are colloquially called lower pitched or deeper-are easier to hear than high frequency (high-pitched) sounds. It takes a larger volume to make a high frequency sound audible.
Indeed, as people age, the disparity between low and high frequency sounds increases. Figure 5 graphs auditory thresholds for the 20-yr-old subject shown in Figure 4, along with thresholds for a 40-yr-old and a 60-yr-old. In general, thresholds increase as people get older. But in addition, higher frequency tones become considerably harder to hear than low frequency tones.
Figure 5. Volume thresholds as a function of frequency and age. In general, volume thresholds increase as people age. In addition, the disparity between low and high frequency sounds grows. To be audible to someone aged around 60 yrs, a high frequency sound needs be almost four times as loud as it would have been to be audible by someone aged 20 yrs.
Applications and Summary
One of the primary applications of the auditory staircase procedure is to assess hearing impairment. Beyond normal aging, hearing impairments can be caused by damage to the inner ear, brain damage, and disease. Often, hearing impairment affects particular frequencies more than others. The staircase method can be used to determine whether someone possesses especially poor hearing within a narrow frequency range, which would suggest hearing impairment caused by more than normal aging. Figure 6 graphs auditory thresholds for a hearing impaired 60-yr-old compared with an unimpaired 60-yr-old. The impaired individual suffers hearing loss at 4 and 5 kHz, as indicated by very high auditory thresholds at those frequencies. Otherwise, the impaired individual performs similarly to an age matched control.
Figure 6. Volume thresholds for a hearing impaired individual (60 yrs) compared with an unimpaired age match. Hearing impairment often affects only a portion of frequency space. The impaired individual shown here suffers severe impairment-very high thresholds-at 4 and 5 kHz, but appears otherwise normal compared with an age-matched control.
This approach can also be used to assess the consequences of various types of experiences on the auditory system. For example, studies have used a threshold approach to evaluate the effects of hearing loud heavy-metal music in a concert.1 Scientists tested people just before attending a concert, and a half an hour after. Perhaps unsurprisingly, heavy metal increased the volume threshold for sounds, especially in the range of 6Hz. Rock music can make you hard of hearing!
References
- Drake-Lee, A. B. (1992). Beyond music: auditory temporary threshold shift in rock musicians after a heavy metal concert. Journal of the royal society of medicine, 85(10), 617-619.
Transcript
One branch of perceptual psychology—psychophysics—is concerned with relating a stimulus’s actual compared to its perceived intensity.
Just like actual levels, perceptual ones can be measured: how bright a light must be for it to be observed, or how soft a sound can be for it to be heard.
For example, someone waiting for dinner to be served may not hear that it’s ready if they are at the base of the stairs; they have to climb a few steps before they hear anything, and maybe even a few more to Interpret the sounds.
This dynamic adjustment is the concept behind the staircase procedure, where the minimum noticed intensity can be reliably determined by stepping up or down the amount of stimulation.
This video demonstrates how to design and implement the staircase procedure, specifically to measure auditory thresholds—the minimal volume necessary for a tone to be perceived.
In this experiment, tones are presented through headphones at six different frequencies or pitches: 1–6 kHz—all within the human hearing range.
Given that our thresholds are not the same across all frequencies, six blocks are used to test each one independently. In each block, the frequency is briefly presented for 200 ms at volumes ranging from 2–40 dB.
The first tone is played at the lowest volume of 2 dB, a level that the participant should not perceive. If that’s the case, the volume on the next trial is increased by a step, 1 dB.
On the other hand, if it is noticeable, the volume is decreased by one. This procedure is repeated for 30 trials—resulting in staircase-like changes in volume.
The dependent variable is the participants’ responses—whether they heard the tone or not. This information is then combined with the volume intensity data to determine the perceptual volume threshold at each frequency.
To begin the experiment, greet the participant in the lab and have them sit comfortably in front of the computer. Explain the task instructions: In each trial, the computer will play a tone through the headphones, after which you will be prompted to press the ‘Y’ key if you heard the tone or ‘N’ if you did not.
Allow the participant to put on the headphones, start the trials associated with the 1 kHz tone, and then leave the room.
After the first block of six frequencies is completed, return to the room and ask the participant to remove the headphones. Answer any questions they may have and give them a 2-min break.
When time is up, have the participant put the headphones back on to begin the trials related to the next tone. Repeat the steps until all six tones have been tested.
To analyze the results, generate a separate data table for each of the tones tested, with a column for trial number, volume level, and the participant’s responses.
During the first few trials, verify that they responded with a series of no’s, indicating that tones were inaudible at the start, which then should have prompted volume increases until the auditory threshold was reached.
Following verification, graph the volume played on each trial of every block as shown here for 1 kHz.
When the auditory threshold was reached, notice that the participant moved back and forth between ‘No’ and ‘Yes’ responses, which allows for the identification of what sounds first became detectable. The central tendency of this narrow range is a measure of the threshold.
To calculate the volume threshold at each tone, average the last 10 trials of every block and graph the results. Observe how this tended to increase as the frequency increased; in other words, low-pitched tones were easier to hear than high-pitched ones, which is due to the vibration properties of the filaments and bones of the ear.
Now that you are familiar with this efficient method for finding perceptual thresholds, let’s look at how it’s used to examine sensory decline in normal aging and with exposure to loud performances.
The staircase procedure has been used by researchers to examine how hearing thresholds change as humans age. In general, they found that volume thresholds increased as people get older. That is, for individuals aged 60, a high-frequency sound needed to be four times as loud as it would to be audible by those who are 20 years old.
Using similar methods, researchers also compared volume thresholds of people with normal hearing to those with impairments to identify the nature of the deficits. Specific frequencies were affected, such as at 4 and 5 kHz, whereas others were normal, suggesting that disease or damage is the cause, not aging.
In addition, the approach can be used to assess the consequences of various types of experiences on the auditory system. For example, studies have used a threshold approach to evaluate the effects of hearing loud heavy-metal music during a concert.
When researchers tested people just before attending a concert, and a half an hour after, they found that heavy metal increased the volume threshold for sounds. Thus, rock music can make you hard of hearing!
You’ve just watched JoVE’s video on the staircase procedure. Now you should have a good understanding of how to design a perceptual threshold task and run the experiment, as well as analyze and assess the results.
Thanks for watching!