Highly reverberant environments: A commentary on an interesting case study

Frank Musiek, Ph.D.

 It is well-known that highly reverberant rooms make communication difficult for all people.  This is especially the case for individuals with hearing loss and those with central auditory dysfunction. Highly reverberant rooms or hallways are often termed “echoey” by the lay public. Schools have recognized   this problem and do their best to reduce reverberation time in the classroom. Civil/acoustical engineers who construct auditoriums, music halls, and recording studios are obviously sensitive to the issue of reverberation time and hearing.

 Long reverberation times serve to contaminate speech signals so that the understanding of these signals are indeed difficult. But what is a long reverberation time?  The classic way reverberation time is measured considers the elapsed time that it takes a given signal to decrease 60 dB (RT60) once the sound source is abated. I’m sure there are formal standards for reverberation times in various kinds of rooms and buildings.  However, my own rule of thumb is that reverberation times that are near one second or greater can be very troublesome. Given this backdrop of reverberation time and echoey rooms I would like to proceed to informally discuss an interesting case and the circumstances that surround it.

This case is one that I saw a number of years ago but remember it well. It involved a young business person. This individual was involved in an athletic activity one afternoon and suffered what turned out to be a stroke. After a short stay in a local hospital, the patient made good progress and was released with a recommendation of physical therapy for motor problems and weakness on the left side. This therapy was to be carried out in the medical center in which I worked. Specifically, the therapy was to include water therapy in pool.

The first day of therapy started with a hospital attendant meeting the patient at the front entrance of the hospital. The attendant put the patient in a wheel chair and started a rather long trek to the swimming pool area. On their way to the pool area the attendant and the patient struck up a conversation. As they opened the door to the pool area and entered an interesting thing happened. The patient became quite alarmed because they could no longer understand what the attendant was saying ! After pausing for a short time, the attendant wheeled the patient out of pool area and kept talking to the patient. Once they left the pool area the patient could once again understand what the attendant was saying. Both the patient and attendant agreed that hearing was a factor in the “pool experience”.  Shortly after this incident I was contacted about this patient with a message relating something to the effect that this patient had an unusual hearing problem.

I saw this patient for a hearing evaluation. In brief, the history was essentially reiterated as mentioned above. Overall, they seemed to be doing ok auditorily, however, I am not sure they had been in many if any difficult listening situations after the stroke (other than the pool experience). We sent for previous evaluations including radiology. The radiology (CT scan) was revealing in that it showed that the stroke had affected a moderate portion of the insula on the right side. There may have been some contamination also of the internal capsule. There clearly was no involvement of the auditory cortex or other auditory regions of the brain

The pure tone audiogram was normal bilaterally 250 – 8000 Hz. In fact, most thresholds were between 0 and 10 dB HL and bilaterally symmetrical. Speech recognition in quiet was excellent bilaterally. This was not a surprise given the patient’s age and history. At this point we entertained doing both electrophysiological and behavioral central tests. Our test battery selection called for dichotic digits, frequency patterns, dichotic rhyme and probably the ABR-MLR combination. In retrospect, I am puzzled that we did not select compressed speech with reverberation – as it would have been logical and ecological to do. To our surprise, both the dichotic digits and frequency pattern tests revealed bilaterally normal performance. The dichotic rhyme test however showed a marked deficit for the left ear – contralateral to the lesioned hemisphere. Scores for the left ear on 2 trials of the dichotic rhyme were e 0% and 10%.

The right ear scores were about 80% better than those of the left ear—yielding a striking laterality effect. The ABR-MLR was also revealing in that the ABR was normal for both ears but the MLR was highly abnormal for the Na – Pa waveform. The recording for the electrode over the right temporal cortex (T4) was approximately 1/4 to 1/3 the amplitude compared to that which was recorded over the left temporal cortex (T-3). The diagnostic summary revealed normal peripheral hearing bilaterally and two central auditory tests that were normal and two that were markedly abnormal.

 

In retrospect, this case seems to demand some comments. The pool area was clearly an environment where the reverberation time would well exceed 1 second. It was truly amazing to witness the devastating effect this highly reverberant area had on the patient’s communication. Though the patient did not have major complaints in other acoustic environments we are quite sure if they had to listen in noisy surroundings it would have been problematic. I assumed that when we saw this patient they had somewhat restricted everyday activities and were likely in quiet surroundings after the stroke, therefore auditory complaints were minimal. The test results also deserve comment. Why would one dichotic test yield normal results and the other extremely low scores ? Are these tests though similar, assessing different processes ? At least in this case it does seem likely. The dichotic rhyme test requires dichotic fusion and the digits requires a dichotic integration process (see Musiek et al. 1989). So clearly there are task differences in these tests which, may in turn require quite different underlying processes that became evident with this patient’s pathology. The ABR-MLR helped objectively confirm the central deficit created by the insular lesion. As relatively recent but often overlooked research has shown, the insula is a major player in higher auditory processing (Bamiou et al., 2006). This is important to keep in mind. Another aspect of this case worthy attention is the fact that hearing complaints should be carefully investigated in all patients. Patients are most grateful for explanations of their symptoms and what intervention(s) may be helpful. Finally, I think this case reminds us to ask our patients about hearing in highly reverberant environments – their response may provide interesting insights !

 

Relevant Readings

  1. Bamiou, D.-E., Musiek, F.E., Stow, I., Stevens, J., Cipolotti, L., Brown, M.M., & Luxon, L.M.  (2006). Auditory Temporal Processing Deficits in Patients with Insular Stroke, Neurology, 67, 614-619.
  2. Musiek, F., Kurdziel-Schwan, S., Kibbe, K., Gollegly, K., Baran, J. & Rintelmann, W.  (1989). The Dichotic Rhyme Task: Results in Split Brain Patients. Ear and Hearing, 10, 33-39.

About Pathways

Pathways is both a column that covers topics related to CAPD and Neuroaudiology and a society for people interested in central auditory disorders that regularly meets to discuss these issues.

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