Evoked Potential

Psychoacoustic Methods

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Psychoacoustics is concerned with how we perceive the physical stimuli impinging on our senses; the perception of sound
  • 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
  • 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
    • 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
  • 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
  • ​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
  • What is an independent variable?
    • ​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
  • ​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)
    • ​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
      • ​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
      • ​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
  • ​​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

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)
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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
  • ​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)

Classical Methods of Measurement

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METHOD OF LIMITS!
Method of Limits
  • 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
  • ​​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
  • ​​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
  • ​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

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


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
  • ​​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
  • ​​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 
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the numbers indicate reversals; an example of simple up-down method; 50% of performance is being targeted; threshold is calculated by average of the midpoints of the runs; or as the average of the peaks and troughs

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

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
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