Annette Hurley

Contralateral suppression of transient-evoked otoacoustic emissions in humans: intensity effects

Transient evoked otoacoustic emissions (TEOAEs) were recorded to clicks presented at peak sound pressures of 50, 55, 60, 65 and 70 dB while continuous contralateral white noise was varied from 10 dB below to 10 dB above the click level. Suppression increased predictably with suppressor noise level for any given click level. However, when the suppressor noise level was held constant, suppression was greater for lower click levels. This observation is consistent with the association of suppression of otoacoustic emissions with active cochlear processes and efferent function at low intensity levels.

Contralateral suppression of otoacoustic emissions: An index of the function of the medial olivocochlear system

We can now distinguish, in part, between nerve deafness and hair cell deafness through the use of otoacoustic emissions. We can also assess the efferent system by carefully quantifying the effects of contralateral stimulation on these same otoacoustic emissions. The suppression of transient evoked emissions by continuous contralateral white noise is an ostensibly small effect of 2 or 3 dB when studied over a 20-msec window. However, when subjected to microstructural analysis, the effect can exceed 6 to 8 dB in the zones from 10 to 20 msec after the stimulus has subsided. Temporal and spectral analyses reveal robust effects of contralateral lateral stimulation, although in any given normal subject it may be difficult to separate middle ear effects from efferent effects. Evidence is strong that the efferent effect is mediated in part by cholinergic — primarily nicotinic — receptors in the outer hair cell. However, a unique type of patient, who shows nearly normal pure-tone audiograms and absent ABRs, shows virtually no contralateral suppression of transient evoked emissions. Some other patients, with symptoms of Charcot-Marie-Tooth disease, may paradoxically show extremely poor audiograms, but perfectly normal evoked emissions along with absent contralateral suppression. The ABR, along with middle ear muscle reflexes and masking level differences, are all absent in these patients; we therefore think they have a disorder that desynchronizes most of their primary auditory nerve fibers and thereby disconnects them from any efferent activity or masking cancellation. The existence of such an auditory disorder, characterized by severe dysfunction in speech comprehension — especially when listening in noise—suggests that what appears to be a “central auditory imperception” might stem instead from a systemic peripheral primary neuropathy.

The effect of midline petrous apex lesions on tests of afferent and efferent auditory function.

Objectives Historically, audiological procedures have focused on the assessment of the afferent (ascending) cochlear-VIIIth nerve system and have, for the most part, ignored the efferent (descending) auditory system. We report afferent and efferent auditory test results for two cases with a cholesterol cyst of the right petrous apex; one lesion involves the afferent segment of the auditory system, and the second lesion involves both the afferent and efferent segments of the auditory system. These “natural experiments” provide a unique opportunity to study the effect of a space-occupying lesion of the petrous apex on afferent and efferent function of the auditory system. Design Transient evoked otoacoustic emission (TEOAE) suppression studies were performed to assess the effect of the cholesterol cyst on the efferent system of the two cases. In addition, three complementary afferent tests of brain stem auditory function were administered: 1) acoustic reflex thresholds (ARTs); 2) masking level difference (MLD); and 3) auditory brain stem response (ABR). These tests are complementary because the superior olivary complex (SOC) is involved not only in the mediation of the sound evoked efferent reflex assessed in TEOAE suppression, but in the mediation of the ARTs, the MLD, and the ABR. Results The two cases with midline petrous apex lesions, one not involving the VIII-cochlear efferent auditory system, differed from each other with regards to TEOAEs suppression, and ARTs. Specifically, the case with only afferent involvement produced normal TEOAE suppression, a normal MLD, normal ARTs, and abnormal waves III and V of the ABR, whereas the case with both afferent and efferent involvement produced abnormal TEOAE suppression, a normal MLD, abnormal ARTs, and abnormal waves III and V of the ABR. Conclusions These cases illustrate that although several auditory tests can be mediated within the same or adjacent anatomical structures, i.e., the SOC, they may not be equally affected by the same lesion due to different physiology. Further, the TEOAE suppression paradigm is a clinically relevant test to assay the sound evoked efferent reflex that is mediated by the medial olivocochlear system of the SOC.

Informal auditory therapy: Whatʼs in the toy box?

“Can you help him listen?” But recently I was asked this by a friend who is a parent of a boisterous 4-year-old. My friend had concerns about her son’s hearing, but an audiologic assessment revealed normal peripheral activity. The mother, an informed, educated parent who is also an elementary school teacher, was aware of her son’s predisposition to otitis media and the family history of dyslexia and ADHD. Her next few questions were not unexpected: “Does he have auditory processing problems?” “What can you do to help him?” “What can I do to help?” It is important to know the difference between hearing and listening. Hearing is the reception of an acoustic event. Listening is an active process involving the reception and utilization of acoustic information, and it incorporates auditory and language processing skills. Unfortunately, a diagnostic assessment for (central) auditory processing disorder, (C) APD, at an early age is not possible. Diagnostic assessment is usually not performed until a child is at least 7, and although some screening tests may be administered as early as 5, results should be interpreted with caution. Audiologists can implement early auditory training to improve listening comprehension and communication processing for children with predisposing risk factors for (C)APD. This includes localization and lateralization, sequencing of sounds, phoneme/syllable discrimination, auditory memory, and temporal processing.