RESPONSE BIAS & AUDIBILITY CURVE

RESPONSE BIAS & AUDIBILITY CURVE Lab 5

PURPOSE The purpose of this lab is to demonstrate how response bias can affect threshold measurements and how the frequency of sounds affects their audibility.

EQUIPMENT • Audiometer and booth • CD player and Auditory Demonstrations CD: • Demonstration 6: Frequency Response of the Ear (tracks 17 and 18)

ORIENTATION POINTS • Response bias and its clinical relevance • Terms: hit, miss, false alarm, and correct rejection. • ROC curves • Response bias experiment • Audibility curve • Demonstration 6: Frequency Response of the Ear (Track 17 & 18)

RESPONSE BIAS • Behavior is influenced by factors other than sensitivity • Response bias (the listener’s tendency to say “Yes, I heard that”) • Attention • Memory • Motivation (rather small effect on threshold)

RESPONSE BIAS • 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 situation (e.g., sounds are presented on 90% of the trials).

RESPONSE BIAS AND CATCH TRIALS Present “no-tone” sometimes If the person says “yes” a lot, to the no-tone trials, conclude that the person has a liberal response bias If the person does not respond very much/at all, to the no-tone trials, conclude that the person has a conservative bias In reality, an unbiased listener should say “yes” on some no-tone trials

WHAT IS THE CLINICAL APPLICATION?

RESPONSE BIAS: HIT AND FALSE ALARM RATES

ROC CURVES: • A receiver operating characteristic (ROC), or simply ROC curve, is a graphical plot of the sensitivity vs. specificity as its discrimination threshold is varied • The ROC can also be represented equivalently by plotting the fraction of true positives (true positive rate) vs. the fraction of false positives (false positive rate) Hit/

SENSITIVITY VS. SPECIFICITY • Sensitivity and specificity are statistical measures of performance • Sensitivity: measures the proportion of actual positives which are correctly identified • e.g. the percentage of people with hearing loss who are identified as having hearing loss • Specificity: measures the proportion of negatives which are correctly identified • e.g. the percentage of people who do not have hearing loss and are identified as not having hearing loss

LAB ASSIGNMENT: RESPONSE BIAS • Divide into groups of four • Select one person to be the subject • One person to operate the audiometer • One person to alert the subject of the beginning and the end of the interval (signal start and finish in some) • One person to record the subject’s responses

LAB ASSIGNMENT: RESPONSE BIAS (CONT.) • Determine the subject’s threshold for a 1000 Hz tone • Setup the audiometer to present the stimulus at 2 dB SL/Sensation Level (2 dB above the persons threshold) • For the first run, instruct the subject to say "Yes" even if s/he is not sure that a tone has been presented (Liberal)

LAB ASSIGNMENT: RESPONSE BIAS (CONT.) • Select one of the randomized lists • For each interval: • Alert the subject of the beginning of the interval • Present the tone or no-tone • Alert the subject of the end of the interval • Record the response as: • Hit (Subject heard the tone and responded correctly) • Miss (Subject did not hear the tone) • False alarm (No-tone was presented but subject said “yes”) • Correct rejection (Subject say “no” when a no-tone was presented)

LAB ASSIGNMENT: RESPONSE BIAS (CONT.) • For the second run, instruct the subject to say "Yes" only if s/he is absolutely certain a tone was presented • Select another of the randomized lists • Present the intervals and record the responses as described before

LAB ASSIGNMENT: RESPONSE BIAS (CONT.) • For the third run, instruct the subject to say "Yes" in an unbiased manner • Select the last randomized list • Present the intervals and record the responses as described before

LAB ASSIGNMENT: RESPONSE BIAS (CONT.) • For each run fill in the corresponding table with • percent hit • percent miss • percent false alarm • percent correct rejection • Remember that: • percent hit + percent miss = 1 • percent false alarm + percent correct rejection = 1 = 1 = 1

LAB ASSIGNMENT: RESPONSE BIAS (CONT.) • Plot the hit rate (y-axis) as a function of false alarm rate • EG Conservative (red): Hit Rate = 0.7 or 70% False Alarm Rate = 0 Unbiased (blue dashed): Hit Rate = 1.0 or 100% False Alarm Rate = 0 • Describe your results and discuss what you think they show 100 (or 1.0) 70 Hit Rate % (Sensitivity) 0 False Alarm Rate 100 (or 1.0)

AUDIBILITY CURVE • Minimum audibility curve is a standardized graph of the threshold of hearing versus frequency for an average human • Used as the reference level when measuring hearing loss with an audiometer as shown on an audiogram

AUDIBILITY CURVE: NUMBER OF TONES HEARD AS A FUNCTION OF FREQUENCY Frequency 0 1 2 3 4 5 6 7 8 Number of tones heard

FACTORS RESPONSIBLE FOR THE EFFECTS OF FREQUENCY ON ABSOLUTE SENSITIVITY External and middle ear transfer functions are primarily responsible for the shape of the audibility curve. From Pickles (1988)

AUDIBILITY CURVE: NUMBER OF TONES HEARD AS A FUNCTION OF FREQUENCY Frequency 8 7 6 5 4 3 2 1 0 Number of tones heard

LAB ASSIGNMENT: THE AUDIBILITY CURVE • Adjust the level of the calibration tone so that it is just audible • Divide into groups of three or four • For each frequency played (frequencies of 125, 250, 500, 1000, 2000, 4000, and 8000 Hz): • Count and record the number of tones you hear • Note that each frequency staircase is presented twice

LAB ASSIGNMENT: THE AUDIBILITY CURVE (CONT.) • Plot each group member’s audibility curve • Discuss the shape of each subject’s audibility curve relative to the expected shape • Are there differences between your subjects in threshold or in the shape of the curve? • Plot the group’s average audibility curve • Indicate the range of thresholds you obtained • Is the average curve more or less similar to the expected shape?

RESPONSE BIAS & AUDIBILITY CURVE