Psychoacoustics
- Psychophysics - how we perceive they physical stimuli impinging on our senses
- the study of the relationship between the physiological and physical aspects of a stimulus
- how does the perception relate to the actual stimulus
- the study of the relationship between the physiological and physical aspects of a stimulus
- Psychoacoustics - the branch of psychophysics that deals with the perception of sound
- the science concerned with the psychological correlates of the physical parameters of sound
- aka auditory perception
- aka auditory perception
- making a sharp distinction between the physical stimulus and the psychological response to it
- includes coding of sounds, processing sound, integrating sound and responding to sound
- the science concerned with the psychological correlates of the physical parameters of sound
- Pitch is the perception related to sound frequency; pitch gets higher as frequency increases
Psychoacoustics & Clinical Audiology
- It is important to understand how the normal auditory system functions
- then we can begin to consider how auditory pathology may affect the system
- it also provides information to develop models of the normal system to predict how the system operates
- it provides information for the development of methods and devices for remediation of hearing loss
- Many of the diagnostic tests used clinically are based on our understanding of how the ear functions
- e.g. clinical masking based on concept of the critical band
- e.g. clinical masking based on concept of the critical band
- Audiologic equipment incorporates information about normal hearing auditory processing
Conducting Hearing Research
- What is an experiment?
- It is a collection of research designs which use manipulation and controlled testing to understand causal processes
- Generally, one or more variables are manipulated to determine their effect on a dependent variable
- The experimental method is a systematic and scientific approach to research in which the researcher manipulates one or more variables and controls and measures any change in other variables
- It is a collection of research designs which use manipulation and controlled testing to understand causal processes
- What is an independent variable?
- a variable (often denoted by x ) whose variation does not depend on that of another.
- a variable (often denoted by x ) whose variation does not depend on that of another.
- What is a dependent variable?
- a variable (often denoted by y ) whose value depends on that of another
- a variable (often denoted by y ) whose value depends on that of another
- What do you want to determine?
- DV = f(IV) the functional relationship between the two variables
- looking at how the DV changes when the IV is manipulated
- e.g. change in frequency difference between 2 sounds (IV) and measure the ability to perceive a difference in pitch (DV) or change the intensity difference between 2 sounds (IV) and measure the ability to perceive a difference in loudness (DV)
- e.g. change in frequency difference between 2 sounds (IV) and measure the ability to perceive a difference in pitch (DV) or change the intensity difference between 2 sounds (IV) and measure the ability to perceive a difference in loudness (DV)
- A problem is that in most cases there are other variables in addition to the IV that may affect the DV
- It is important to know about these variables especially if they covary with the IV
- imagine you are interested in the relationship between food and fluid intake
- you think that the more a person drinks the less they will eat
- you think that the more a person drinks the less they will eat
- no suppose that the amount of physical activity is increased to increase the amount of fluid the person drank
- then fluid intake and physical activity covary together
- then fluid intake and physical activity covary together
- if you are really interested in just the relationship between fluid intake and food intake then the fact that fluid intake and physical activity covary confounds the results
- so physical activity is a confounding variable (CV) in this experiment
- to conduct a scientifically sound experiment, need to control the CVs
- imagine you are interested in the relationship between food and fluid intake
- DV = f(IV) the functional relationship between the two variables
- One of the most important aspects of any experiment is presenting the findings
Methods in Psychoacoustics
- Two general approaches to studying the relationship between the psychological and physical aspects of a stimulus
- discrimination
- are the stimuli the same or different
- indirect means of determining what a subject can respond to
- scaling procedures
- ask the subject directly about the stimuli
- direct estimate of what subject responds to
- discrimination
Scales of Measurement
- Our ability to properly analyze what we find and to arrive at valid interpretations depends on knowing the properties of the measurements made and the qualities of the resulting data
- Nominal scales are the least restrictive in the sense that the observations are simply assigned to groups
- makes no assumptions about the order among the classes; thus it is the least restrictive and least informative of the levels of scaling
- example: male vs female; type of subcompact car (toyota, ford, chevrolet, etc)
- Allows us to express the mode of the data
- which category contains more cases than any other
- lowest order scale
- Ordinal scales imply that the observations have values, which can be rank-ordered so that one class is greater or lesser than another with respect to the parameter of interest
- it does not tell how far apart they are
- gives rank of the order of the categories but does not specify the distances between them
- a>b>c>d>e
- ordinal scales permit the use of the median
- the value with the same number above and below it
- observations can be rank ordered
- Interval scale specifies the order among categories and the fixed distances among them
- the distance between any two successive categories is equal to the distance between any other successive pair
- do not imply a true zero point
- fixed distance and specific order among categories
- Ratio scales include all the properties of interval scales as well as an inherent zero point
- permits values to be expressed as ratios or in decibels and the use of mathematical operations
- give the most information about the data and their interrelationships
- like interval scale but with a true zero
Summary
- In summary, nominal variables are used to “name,” or label a series of values.
- Ordinal scales provide good information about the order of choices, such as in a customer satisfaction survey.
- Intervalscales give us the order of values + the ability to quantify the difference between each one.
- Finally, Ratio scales give us the ultimate–order, interval values, plus the ability to calculate ratios since a “true zero” can be defined.
Psychophysical Procedures
- two general classes of methodology
- classical methods
- signal detection theory procedures
- responses are also affected by response proclivity or bias
- classical methods cannot entirely control or eliminate bias, so procedures incorporating signal-detection theory are often used
- the procedures we will discuss attempt to measure sensitivity without the effects of response bias
Terminology
- Stimulus - an auditory presentation of a sound
- Parameter - characteristics of the stimulus -- intensity, frequency, duration
- Run - a number of individual presentations of the stimulus
- varies with the type of experiment and stimuli
- Trial - each presentation of the stimulus or group of stimuli for which a response is required
- Block of trials - in some experiments a number of trials, 1 or more, is considered together as a block for purposes of evaluating the results
- Step Size - increment by which the stimulus is changed from trial to trial
- Catch trial - aka blank trial - a trial in which either no stimulus is presented or stimuli that are exactly the same are presented
- Reversal - in an adaptive method this refers to when the direction of change goes from increasing to decreasing or decreasing to increasing
- Interval - aka token - part or all of a trial depending on the type of measurement
- Threshold (nominal definitions)
- Absolute threshold - the lowest intensity at which a stimulus can be detected
- Discrimination threshold - The minimal amount of change in a stimulus that can be detected
- often called JND (just noticeable difference) or DL (difference limen)
- often called JND (just noticeable difference) or DL (difference limen)
- Absolute threshold - the lowest intensity at which a stimulus can be detected
Measurement Methods
- Establishing relationships between the sound presented and how the subject perceives it is a primary goal
- The stimulus and response are clearly specified and then some aspect of the stimulus is manipulated
- The subject's task is to respond in a predetermined manner so that the investigator can get an unambiguous idea of what was heard
- absolute sensitivity - lowest level at which the sound is heard
- differential sensitivity - the just noticeable difference
- absolute sensitivity - lowest level at which the sound is heard
- We must also distinguish between what the subject actually hears and the manner in which he responds
- sensory capability - former (for the most part what we are interested in)
- response proclivity - latter (reflects bias and criteria that affects response)
- sensory capability - former (for the most part what we are interested in)
Classical Methods of Measurement
Method of Limits
Method of Adjustment
Method of Constant Stimuli
- The stimulus is under the investigators control and the subject simply responds after each presentation
- determine the stimulus intensity at which the observer can either no longer detect the stimulus as its intensity is decreased from a high level, or begin to detect the stimulus as its intensity is increased from a low level
- experimenter controls the stimulus in discrete steps
- stimuli are presented in ascending or descending order
- subject says present/absent or same/different after each presentation
- do a number of runs (individual estimates) and get an average - this is threshold or DL
- method is also affected by HABITUATION; the subject doesn't change response till the threshold is exceeded
- descending runs - a stimulus is lowered until impersceptible
- ascending runs - a stimulus is raised until persceptible
- "threshold" for each series is taken as the halfway point between them
- "threshold" is obtained by averaging the threshold levels across runs
- it is customary to define a threshold at which the sound is heard 50% of the time
- several response bias are associated; method is limited because it's affected by RESPONSE BIAS
- the subject can anticipate when the response should change
- anticipation results in a lower (better) ascending threshold
- the subject can habituate
- does not change their response until threshold is exceeded by a few trials
- the subject can anticipate when the response should change
- limited in terms of step size
- too large of a step size reduces accuracy
- may place a level within 0% probability of a response
- a smaller step size permits a more precise estimate of threshold
- too large of a step size reduces accuracy
- psychometric function shows the probability of responses for different stimulus levels
- the method of limits with an appropriate step size is still a popular
- particularly true in pilot experiments and in clinical evaluations
- may be used to determine differential thresholds
- two stimuli presented in each trial
- to minimize bias
- do both ascending and descending runs
- vary the starting level
- don't tell the listener if you are ascending or descending
- present a different number of stimuli in each series
- do both ascending and descending runs
- Examples of Method of Limits
- two point discrimination test; observable distance of two objects on the skin
- ascending trial; distance gets bigger
- descending trial; distance gets smaller
- ascending trial; distance gets bigger
- two point discrimination test; observable distance of two objects on the skin
Method of Adjustment
- Differs from the method of limits in two ways
- the stimulus is controlled by the subject instead of by the investigator
- the level of the stimulus is varied instead of in discrete steps
- The observer, not the experimenter adjusts the stimulus intensity until the observer can either no longer detect the stimulus or just begin to detect it
- the subject is told to bracket the threshold, go below and then above, etc.
- problems associated
- the subject can change criterion during a run
- the subject can change criterion during a run
Method of Constant Stimuli
- sort of a tabulation of the method of limits
- the stimulus is presented a specified number of times at various levels in random order
- equal number of trials at each level
- trials are presented non-sequentially - not ascending or descending
- subject indicates if the signal is present/absent or same/different on each trial
- Involves presentation of various stimulus levels to the subject in random order
- it is a non-sequential procedure
- the stimuli are not presented in an ascending or descending manner
- a range of intensities is selected which, based upon previous experience or a pilot experiment, encompasses the threshold level
- a step size is selected and the stimuli are then presented to the subject in random order
- enables the investigator to include "catch" trials over the course of the experiment
- these are intervals during which the subject is asked whether a tone was heard, when no tone was really presented
- after all trials completed, plot a PSYCHOMETRIC FUNCTION
- a graph indicating the % correct as a function of the stimulus parameter being changed
- relates a dependent and independent variable
- if step size is too large, decreases accuracy, too small, waste trials
- Estimate of threshold of DL is taken from the psychometric function
- experimenter decides apriori what level of performance they want to measure
- after the psychometric function is plotted can read value off the curve
- experimenter decides apriori what level of performance they want to measure
- a graph indicating the % correct as a function of the stimulus parameter being changed
- may end up with a lot of wasted trials at the top and bottom of the function where all correct or all incorrect
- can estimate bias by including "catch" trials
- no stimulus is presented in some trials
- this will reflect subject bias
- no stimulus is presented in some trials
- procedure is more precise than method of limits or adjustment but less efficient because so many trials are needed
Variation in Sensitivity
- The absolute threshold is not a rigidly fixed value
- sensitivity fluctuates irregularly so that a given stimulus level may trigger a response at one time but not another
- the threshold is usually defined statisticall
- sensitivity is also subject to systemic variation, either of the the permanent kind encountered in aging or in pathology of the sensory tissues or of a temporary kind observed
- these and many other systemic changes in sensitivity are frequently expressed as alterations of the absolute threshold
Adaptive Procedures
- Designed to take advantage of signal detection theory
- all adaptive procedures converge on threshold and make most of the measurements near threshold
- procedures maximize precision and efficiency by not wasting trials at ends of underlying psychometric function
- many different adaptive procedures in use in psychophysical experiments
- number of intervals used varies as well as the number of trials in a block
- stimulus level depends on previous response
- Beskesy tracking
- tracking is fast and fairly precise, can adjust rate at which threshold is tracked
- if attenuation rate is too fast, reaction time may become a factor and the precision of the measure may decrease
- up down staircase method
- converges on 50% correct on psychometric function
- Designed to control for bias in a response
- focus measurement on actual sensitivity
- All adaptive procedures converge on threshold and make most of the measurements near threshold
- procedures maximize precision and efficiency by not wasting trials at ends of underlying psychometric function
- many different adaptive procedures in use in psychophysical experiements
- number of intervals used varies as well as the number of trials in a block
- stimulus level depends on previous response
Adaptive Procedures - Transformed - What if we want 70% correct instead of 50%?
- modification of simple up-down procedure
- the level of the stimulus changes after a certain sequence of responses
- can target any point on psychometric fx you want
- target level can be changed by setting criteria for increasing the stimulus level after a negative response or a positive response followed by a negative one and lower the stimulus level after two successive positives.
- in other words we have established the following rules for changing stimulus level
- up rule: - or +,-
- down rule +,+
- then, you have to use math using formula
- up rule: - or +,-
- target level can be changed by setting criteria for increasing the stimulus level after a negative response or a positive response followed by a negative one and lower the stimulus level after two successive positives.
Psychometric Function
- The plot of proportion of correct detections as a function of level is called the psychometric function
- for each level, the proportion of times that the subject correctly heard the sound is determined
- a psychometric function for frequency discrimination would show the proportion of times the listener could tell that the frequency of a tone had changed as a function of the size of the frequency change
- there is no common sense threshold
- there is no level above which the listener always hears the sound and below which he never hears the sound
- rather there is a gradual improvement in detection with level
- we define the threshold as the level at which the listener achieves some arbitrary proportion of correct detections
- THE METHOD OF CONSTANT STIMULI
- provides complete picture of sensitivity
- easy to administer
- have to know what the threshold is going to be (approximately) before you start
- if you just want a threshold, a lot of trials and time are wasted
Response Bias
- several forms of response biases
- anticipation
- habituation
- persistance/perseveration
- When the sound is near threshold, it is hard to tell the difference between "sound" and "no sound"
- people use the evidence their ears and brain provide but whether they say "yes" or "no" depends on
- how much they like to say "yes"
- other things they know about the stiuation
- people use the evidence their ears and brain provide but whether they say "yes" or "no" depends on
Two alternative forced choice 2AFC
- if the listener listens to a signal and to a "no signal" and then chooses the one that was the signal, then bias applies equally to the two observations and is cancelled out
Conclusions
- Psychophysical methods vary from inefficient and limited in scope to efficient and complete
- Bias always affects the results of a psychophysical experiment; methods to control bias are available
- The psychometric function provides information about attentiveness and memory
- The most popular means of studying hearing sensitivity is an adaptive 2AFC method
- bias free thresholds can be estimated efficiently using this combination