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LESSON 164

Behavioral observation audiometry

Year 2
Spring semester
Pediatric Audiology
Pediatric assessment
Duration: 10:14

This lesson covers behavioral observation audiometry (BOA), used with infants who are developmentally younger than six months. We explain how infants respond to sound, stimulus choice, infant state, observer bias and habituation, the minimal response level rather than a threshold, and the clinical role of BOA.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn behavioral observation audiometry, or BOA, used with the youngest infants.
We will cover how infants respond to sound, the limitations of the test, and the minimal response level rather than a threshold.
Behavioral observation audiometry, or BOA, watches the changes in an infant's behavior in response to sound.
Unlike visual reinforcement audiometry, or VRA, the infant is not conditioned and correct responses are not reinforced with a reward.
The method is used with infants who are developmentally younger than about six months.
At this age the infant is not yet ready to learn to turn toward sound, so the responses are mostly reflexive.
The lowest response level found with BOA is called not a threshold but a minimal response level, or MRL.
This distinction is the key to interpreting BOA correctly.
The way infants respond to sound changes with age.
The most obvious response is the startle reflex; the infant's arms fling out and the whole body startles.
Blinking of the eyelids to a sudden sound is called the auropalpebral reflex.
A sleeping infant opening the eyes or stirring can also be accepted as a response.
In a calm infant, a sudden stop in ongoing movement can also be a response to sound.
In an infant sucking on a pacifier or bottle, sucking that stops or speeds up is also observed.
From about four months, the infant begins to turn the head toward the sound, and this response gradually becomes clearer.
BOA is usually done in a sound-treated room, with the infant on the mother's or father's lap.
Stimuli are most often presented through a loudspeaker, that is, in the sound field.
At least one observer sits facing the infant or in the control room.
If possible, two observers decide together whether a response is real.
The parent is asked to stay calm and give the infant no cues at all.
The infant's state, that is, whether asleep, calm or fussy, is recorded throughout the test.
Different stimuli can be used in BOA, and the infant's response varies with the stimulus.
Infants respond more easily to speech and broadband sounds than to pure or frequency-modulated tones.
However, speech does not give frequency-specific information.
Noisemaker toys attract attention, but the level and frequency of their sound must be measured in advance.
Frequency-specific stimuli such as warble tones give more information, but infants respond to them more weakly.
Which stimulus was given at which level is always recorded, because it is essential for interpreting the result.
An infant in deep sleep responds very little to sound, so an absent response is misleading in this state.
In light sleep, responses such as arousal and stirring are easier to observe.
In a calm, alert infant, stilling, changes in sucking and shifts of gaze can be observed.
Reliable observation is not possible in a crying or fussy infant; the test resumes once the infant settles.
So the infant's state is noted on every trial.
Before each trial, the infant's current behavior is watched for a few seconds; a response can only be recognized against this baseline.
The stimulus usually starts at a low level and is raised step by step until a response is seen.
In this way, the lowest response level is sought before the infant habituates to loud sounds.
Only a change that appears within a short time window right after the stimulus counts as a response.
Control trials with no sound show how often the infant moves spontaneously.
Each response is recorded with the stimulus type, level, the infant's state and the form of the response.
When interpreting BOA results, it is important to keep its major limitations in mind.
An observer who knows when the sound was presented tends to take every movement as a response; this is called observer bias.
After hearing the same sound a few times, the infant quickly stops responding; this is called habituation.
Responses vary greatly from infant to infant, and in the same infant from one state to another.
Because stimuli are usually presented in the sound field, the result reflects the better-hearing ear; a unilateral loss is not seen.
So BOA alone is not enough to show that hearing is normal or to show the degree of a loss.
The minimal response level is the lowest stimulus level at which a change in the infant's behavior is observed.
A normally hearing infant's MRL can be well above the true hearing threshold, because infants do not always respond to sound with behavior.
As the infant grows and develops, responses begin to appear at lower levels.
So an infant with a mild or moderate loss may be thought to hear normally because they respond to loud sounds.
Conversely, the absence of a response in a drowsy or uninterested infant does not mean there is a hearing loss.
This uncertainty in both directions explains why a BOA result is not reported as a threshold.
If possible, two observers decide on a response together, and only a change seen by both counts as a response.
Bias is reduced if the observer next to the infant cannot hear when the sound is given; some centers mask the observer through headphones.
Control trials with no sound show how often false responses occur.
Which behavior will count as a response is clearly defined before the test.
Video recording of the infant allows the responses to be reviewed again later.
Despite these precautions, BOA remains by nature a subjective method.
Despite its limitations, BOA has a place in the clinic.
It can be used to see how the infant reacts to sounds in daily function and to share this with the family.
The results are cross-checked with objective tests such as the auditory brainstem response, or ABR, and otoacoustic emissions, or OAE.
It can also provide observational information in older children who cannot do VRA for developmental reasons.
By seeing together how their baby responds to sounds, the family can take a more active part in the process.
However, at this age guidelines rely not on BOA but on objective tests such as frequency-specific ABR for threshold estimation and fitting.
Now let us look at the findings of a purely example three-month-old infant.
The family says the baby reacts to sounds such as a door slamming.
On BOA, the infant clearly startles to high-intensity sounds.
However, thresholds estimated with frequency-specific ABR are about seventy decibels in both ears.
So although the infant responds to loud sounds, there is a moderately severe hearing loss.
This example reminds us that reacting to loud sounds does not show that an infant's hearing is normal.
A clinical tip: do not say an infant hears normally because the infant startles to loud sounds.
In the report, write BOA results not as thresholds but as minimal response levels, together with the stimulus used.
At this age, plan objective tests for ear-specific and frequency-specific threshold estimation.
If the family is concerned, take this concern seriously whatever the BOA finding, and complete the assessment.
BOA watches spontaneous changes in behavior to sound in infants developmentally younger than six months.
The value found is not a threshold but a minimal response level, and it can be high even in a normally hearing infant.
Observer bias, habituation and sound field testing are the main limitations of the method.
At this age, threshold estimation and amplification decisions rely on frequency-specific objective tests.
Test yourself: what is the lowest level at which a response is seen with BOA called?
The correct answer is B: the minimal response level, or MRL.
Because this level can be above the true threshold even in normally hearing infants, it is not reported as a threshold.
In the next lesson we will learn visual reinforcement audiometry, or VRA, used with infants older than six months.
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Prerequisites

LESSON 163 Introduction to pediatric audiology

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