Question
What is the consensus on pediatric behavioral hearing testing (4 months+) regarding the upper limit of normal? In other words, what is defined as a "Normal range" of response for young infants for Behavioral Observational Audiometry? I have read conflicting literature on this, some of which suggests that you should expect infants to respond similarly to adults if they have normal hearing, at 0-15 dB. Elsewhere, I have read that you should expect elevated thresholds for normal hearing infants.
Answer
The consensus in pediatric audiology is that normal hearing level for the audiometric frequencies and speech awareness threshold for young infants is < 15 dB HL. For young infants there are two methods for behavioral hearing assessment. The first, and less reliable/valid, is behavioral observation audiometry (BOA). For BOA, stimuli are presented and trained observers watch for a time-locked response to the sound, such as eye-widening or cessation of activity. BOA responses are not conditioned and will occur at varying sensation levels above threshold. Since the advent of auditory brainstem response audiometry (ABR), this technique is rarely used in clinic to attempt to define hearing loss.
The second technique is visual reinforcement audiometry (VRA). For VRA, the infant is conditioned through operant conditioning techniques to turn toward the sound stimulus. When the child turns toward the sound stimulus, the visual reward is typically the lighting of a three dimensional toy animal or the lighting and animation of the toy animal. Much research has gone into VRA and if the infant is well conditioned (i.e., good stimulus-response control), responses may be obtained at levels very close to true threshold (within 5 to 10 dB). To obtain the maximum number of threshold searches within a clinical session, most audiologists do not test below either 10 or 15 dB HL There is no point in spending valuable attention span jockeying for a threshold response of 0 or 5 dB HL. If the response is normal for the test stimulus (i.e., < 15 dB HL), it is better to move to another frequency or stimulus to map out a greater portion of the audiogram.
With respect to your question about age, most infants who are at least 6 months of age with good head/neck/trunk control and normal cognition can be trained to perform VRA. As you reduce age level to 5 or 4 months of age, fewer normally developing infants can be successfully conditioned and/or can maintain performance levels long enough to produce sufficient threshold searches to justify a clinical test session. When stimulus-response control is not fully achieved or maintained, the infant will not respond to threshold levels. In these cases the audiologist records a minimum response level (MRL), which represents the “best” response obtained to the stimulus, rather than a threshold estimate. In our experience, many infants are interested in speech as a stimulus and readily respond down to a level of 15 dB HL or close to true threshold (in the case of hearing loss).
However, when the audiologist presents pure tones, narrow-band noise or warble tones, the stimulus is intrinsically less interesting and they may respond only to elevated sensation levels.
For infants with special needs, either delayed cognition or motor control, the age at which they can be successfully conditioned for VRA increases. Thompson, Wilson and Moore (1979) revealed that VRA was successful for 88% of low-functioning infants and children over 9 months of age. Wilson, Folsom and Widen (1983) found that 80% of children with Down syndrome were testable via VRA by 12 months of age.
Successful conditioning and maintenance of conditioning has been studied and the principles of VRA administration need to be integrated into the test session to obtain optimal results. The following references are selected to assist the reader in establishing the best practices for VRA.
Selected References
Dievendorf, Allen O. and Gravel, Judith S.(1996) Behavioral Observation and Visual Reinforcement Audiometry. In S.E. Gerber (ed.), The Handbook of Pediatric Audiology, chapter 4, 55-83.Gallaudet University Press.
Renshaw, J.J. and Diefendorf, A. O. (1998) Adapting the Test Battery for the Child with Special Needs. In F. H. Bess (ed.), Children with Hearing Impairment: Contemporary Trends, chapter 7, 83-104, Vanderbilt Bill Wilderson Center Press.
Thompson, G. Wilson, R.W. and Moore, J.M. (1979) Application of visual reinforcement audiometry (VRA) to low-functioning children. Journal of Speech and Hearing Disorders, 44(2), 80-90.
Wilson, W.R., Folsom, R.C., and Widen, J.E. (1983) Hearing impairment in Down’s syndrome children. In G. Mencher and S. Gerger (eds), The multiply handicapped hearing impaired child. New York: Grune and Stratton.