The ultimate goal of pediatric aural habilitation is age-appropriate spoken development.Three main steps are involved:Providing the best possible hearing capacity.Developing the best possible hearing skills.Ensuring that maximum contribution of hearing to the development of cognition, social cognition, and languageThis presentation deals with the first two steps – capacity and skill.Specifically, it deals with their assessment.
First let me explain how I distinguish capacity and skill.The sense of hearing has three main components:1. First is the conductive component, consisting of the outer and middle ears.2. Second is the sensorineural component, consisting of the cochlea and the auditory nerve pathways.3. Third is the central component – that is the brain. The brain processes information coming from the ears.One can think of the whole hearing system as a team:1. The conductive mechanism delivers sound patterns. 2. The sensorineural mechanism translates the language of sound into the language of the brain and delivers the results.3. The brain analyzes the resulting patterns, makes decisions about their source, and decides how to respond.Hearing capacity is determined by the conductive and sensorineural mechanisms.Hearing skill resides in the brain. In other words, hearing capacity is what the ear sends to the brain and hearing skill is what the brain does with it.The distinction, however, is not absolute. The hearing pathways help with translation and analysis and we are learning that their performance can be modified by feedback from higher level brain centers as hearing skill develops.
The development of hearing skill obviously depends on hearing capacity but it also depends on perceptual and cognitive development. It is as if perception and cognition multiply capacity to create skill. Note the multiplication sign. Skill requires both the basic capacity and its incorporation into perceptual and cognitive development. If either is zero, skill is zero.Each of these three factors has many components:1. The most basic aspect of hearing capacity is sensitivity, usually expressed in terms of pure-tone threshold. 2. Second is resolution - the preservation of details of sounds in the resulting patterns of nerve activation. 3. Third is transmission – preservation of those details as the patterns are carried to higher brain centers.Note, again, the multiplication signs. If any of these components is missing, capacity is zero, regardless of the condition of the other two. The most basic aspect of skill is detection, which involves awareness and arousal. It is the first skill to develop.Second is attention – listening to sounds and ignoring interference. Third is recognition – in which the sound-producing event is identified – along with the objects that were involved. Not that the term incorporates cognition. The child cannot recognize something until it has a place in his internal model of the world. Fourth is localization – determining where the sound came from. Actually this is an early skill that develops along with detection and attention.Finally, there is interpretation, or comprehension. Events usually have some significance. Footsteps may mean that someone is approaching. And the events of speech have complex meanings.Perception has many levels. The two most important areDiscrimination – the awareness of differences, and Association – connecting sound patterns to other aspects of sensation.Cognition can be thought of as an internal, conceptual model of the world. As the child develops, she acquires concepts ofSpace and time – here, there, up, down, before, after – represented in language by prepositions.Objects – which occupy space and are represented in language by nouns.Events – which involve objects and occupy time and are represented in language by verbs.Properties of space, time, objects, and events and are represented in language by adjectives and adverbs.Finally, there is the concept of cause and effect. This happened because of something else. If I do this then that will happen.Physicians, surgeons, and audiologists seek to provide the best capacity.Interventionists seek to enhance perception, cognition, and skill. At the same time they play a role in ensuring the use and maintenance of hearing assistance.
But my focus here is on assessment. Why do we need to assess hearing capacity and hearing skill?It is because the management of childhood hearing loss is best guided by knowledge. 1. The audiologist requires knowledge about the underlying hearing capacity – which is needed for planning assistance, assessing outcome, and planning habilitation.2. Similarly, the interventionist requires knowledge about capacity and current skill – which is needed for planning habilitation and measuring the results. 3. At the same time, the audiologist requires feedback from interventionists about emerging skills – which can provide valuable information about the outcome of sensory assistance and, possibly, the need for change.
There are basically two ways to approach assessment – behavioral and electrophysiological. Behavioral approaches take advantage of several sound-related responses. Younger children can make involuntary, voluntary or conditioned responses to indicate detection, alerting, listening, and localizing.Older children can be trained to perform some action in response to sound. They may be asked to imitation or repeat speech.Or they may follow directions and answer questions. Alerting and acting can involve speech or non-speech sounds but imitation and answering obviously require speech skills.
An important limitation of behavioral testing is that it does not measure skill or capacity directly. Instead, the results the results are used to make inferences about the underlying skill and capacity.With properly designed and administered tests, positive results can be used to confirm the presence of skill and capacity.Unfortunately, negative results are open to several interpretations:The child lacks the capacity and has not, therefore developed the skill.The child has the capacity but has not yet developed the skill.The child has the capacity, but physiological or developmental factors have limited the development of skill.The child has both capacity and skill but task-related, or behavioral problems prevented their demonstration. Pediatric audiology calls for special expertise in the administration and interpretation of tests behavioral tests.When alerting behaviors and actions are present, they confirm the presence of arousal and attending skills and tell us something about sensitivity.. In fact, behavioral approaches to assessing decibel hearing loss in children aged 6 months to 3 years of age have a long and successful history. Their limitation, however, is that responses to a change from silence to sound do not tell us much about resolution or the transmission of detail. I shall return in a moment, however, to ways in which this problem might be overcome.
The presence of the more advanced behaviors such as speech imitation involve recognition of speech sounds and/or words. In addition to providing information about perception and cognition, positive results confirm the presence of resolution, and the preservation of detail during neural transmission in addition to sensitivity. Imitation and repetition have a long history in audiology as indices of both sensitivity and resolution. As with alerting, however, negative results can be interpreted in a variety of ways. And negative results become increasingly likely in very young children. This is a population in which many non-hearing factors can limit behavior, even when the capacity and skill are present.
Hearing capacity places an upper limit on the development of hearing skill. But only when this limit is reached can we use current skill as an indicator of the underlying capacity.This problem is particularly serious for children with hearing loss in whom the development of hearing skills may not begin until the child is fitted with appropriate hearing assistance. In other words we may need to think in terms of “hearing age” rather than actual age.In the early days of pediatric cochlear implants it was observed that skills related to imitation and comprehension continued to improve for several years after implantation. A feature of early amplification or implantation is that it reduces the difference between hearing age and actual age and allows development to more closely follow a natural time course.
Imitation is a natural behavior for young children and provides an excellent way of probing the capacity of resolution. It is impossible for a child to imitate speech sounds and words unless she has the sensitivity to detect them and the resolution and transmission to permit recognition. At the House Ear Institute, we have developed an imitative task that is designed for assessing resolution in young children.
In order to determine the lowest age at which performance on this test should be expected to provide a valid index of resolution, we carried out a study of children with normal hearing and examined maturation rate. The underlying assumption was that a normally hearing child has normal sensitivity and transmission and should, therefore be able to perform with close to 100% accuracy. We tested 31 children, aged 2 through 7 years. Of 22 children aged 3 years or more, all co-operated and passed the test with a score between 90 and 100%.Of 9 children aged between 2 and 3 years. Five co-operated but only 3 of them met the 90% criterion. The other 4 children did not co-operate.Clearly, this test, in its present form, is not suitable for children below 3 years of age. The data serve to illustrate the high level of non-co-operation in the younger children. Performance can be limited by immaturity and behavioral issues as well as by incompletely developed phonology and speech skills.
Because imitation is not a viable option for assessing resolution in children between 6 months and three years of age, we need an easier task on which performance can be expected to approach the limits of capacity at an early age.
Before leaving the topic of behavioral assessment, let me summarize.When we try to assess capacity from performance we can be confident in the results when they are positive. But negative results can mean either that the skill or capacity is absent or that other factors prevent their demonstration. Uncertainties over the interpretation of negative results can be reduced if we can be sure there has been adequate opportunity for learning.If the child is cooperating, if there are no signs on other developmental delays, and if there is no change in performance over time, we can be more certain that performance is being limited by poor capacity.Unfortunately delays in reliable assessment seriously limit the usefulness of the results for planning sensory and habilitative intervention.
The limitations of behavioral testing can be avoided with electrophysiological approaches which focus on capacity and do not require overt behavior on the part of the child.The auditory-brain-stem response (ABR) occurs at very low levels in the auditory pathways and provides objective information about sensitivity and the early stages of transmission. This approach is widely used for assessment of sensitivity in very young children.
Responses at the level of the auditory cortex can also provide information about sensitivity and transmission. This response was popular in the early days of work on this topic but has been largely replaced by the earlier brain-stem response. There has, however, been increasing interest its potential value for examining responses to speech.
Some years ago it was demonstrated that cortical responses could be elicited not just by a change from silence to sound but also by change from one sound to another. My colleagues and I refer to this response as the acoustic-change complex (ACC) and demonstrated that it could be elicited by changes of amplitude and spectrum. To examine its potential for assessment of resolution among speech sounds we generated synthetic stimuli that could be perceived as containing a change from on vowel to another.
Although group data with normally hearing adults can be interesting, application in pediatric audiology requires the ACC can be used with individual children, including those with hearing loss.Our current research is providing evidence that this condition is met.
This graph shows the group mean response to the change from oo to ee in 5 normally hearing children aged between 2 and 4. The peak-to-peak amplitude is around 9 microvolts and is well above the background noise.These data confirm that the response is elicited in children – at least down to the age of 2 years.
The next figure shows individual tracings for 6 children with hearing loss, who were wearing hearing aids. Ages ranged from 2year 3 months to 6 yrs 3 months. The mean tracing for normally hearing children is shown for comparison in black. Except for the one child shown in blue, these children gave responses very similar to those found with hearing children. The data confirm that the response can be elicited in children with hearing loss and can be demonstrated in single subjects.
The largest amplitude was obtained from a child aged 2 years and 7 months. Unfortunately, the poorest result was found for the youngest child, aged only 2 years and 3 months. The problem in this case, however, was difficulty attaching the electrodes with a low enough impedance. This reminds us that task-related variables can still be a problem when using electrophysiological techniques with young children. Skilled pediatric audiologists are essential for this work.
The last set of data, from a five-year-old child, confirm the possibility of using the Acoustic Change Complex to confirm that hearing assistance is not just improving sensitivity but also resolution. The aided response to the change from oo to aa is shown in green and response to the change from oo to ee is shown in brown. When the aid was turned off, the oo/ee response vanished – as shown by the black dotted line.
At this point, it appears that the Acoustic Change Complex can be used as an objective indicator of resolution capacity. Its presence also provides information about transmission from cochlea to cortex. We have demonstrated that it can be obtained in children younger than 3 years but we do not yet know whether it can be used with children between 9 months and 2 years of age.The ACC can contribute to outcome assessment following the provision of hearing aids or cochlear implants.The technique, however, still experimental and considerably more research will be required if it is to find its way into pediatric audiology practice.
To summarize:Hearing-related behavior depends on hearing skill which, in turn, depends on hearing capacity.Behavioral tests can be used to make inferences about skill and capacity.The demonstration of orienting or attending responses to sound can provide quite good indices of sensitivity.Responses to differences between sounds can provide information about resolution, but negative responses are difficult to interpret.Imitation is a good indicator of resolution but it requires advanced knowledge and skill and is seldom applicable below age 3.Electrophysiology has proved its value in providing objective measures of sensitivity in very young children.Whether it can do the same for assessment of resolution remains to be seen.
Assessing hearing capacity and hearing skill in infants 2
Assessing Hearing Capacity and HearingSkill in Children aged 6 months to 3 years Misura dell’udito e delle abilità uditive nei bambini da 6 mesi a 3 anni Arthur Boothroyd Presented to short course on Childhood Deafness and Language Amplifon Center for Research and Study Milan, October 2012 Director: Edoardo Arslan University of Padua
AcknowledgementsLaurie Eisenberg, House Research Institute Amy Martinez, House Research Institute NIDCD grant numbers: DC 006238 and DC 004433
Pediatric Aural Habilitation1. Best hearing capacity2. Best hearing skill3. Best use of hearing for: Cognitive development Social development Language development