A Test for the Diagnosis of Dead Regions in the Cochlea

Abstract

Hearing impairment may sometimes be associated with complete loss of inner hair cells (IHCs) over a certain region of the basilar membrane. We call this a 'dead region'. Amplification (using a hearing aid) over a frequency range corresponding to a dead region may not be beneficial and may even impair speech intelligibility. However, diagnosis of dead regions is not easily done from the audiogram. This paper reports the design and evaluation of a method for detecting and delimiting dead regions. A noise, called 'threshold equalizing noise' (TEN), was spectrally shaped so that, for normally hearing subjects, it would give equal masked thresholds for pure tone signals at all frequencies within the range 250-10,000 Hz. Its level is specified as the level in a one-ERB (132 Hz) wide band centred at 1000 Hz. Measurements obtained from 22 normal-hearing subjects and TEN levels of 30, 50 and 70 dB/ERB confirmed that the signal level at masked threshold was approximately equal to the noise level/ERB and was almost independent of signal frequency. Masked thresholds were measured for 20 ears of 14 subjects with sensorineural hearing loss, using TEN levels of 30, 50 and 70 dB/ERB. Psychophysical tuning curves (PTCs) were measured for the same subjects. When there are surviving IHCs corresponding to a frequency region with elevated absolute thresholds, a signal in that frequency region is detected via IHCs with characteristic frequencies (CFs) close to that region. In such a case, threshold in the TEN is close to that for normal-hearing listeners, provided that the noise intensity is sufficient to produce significant masking. Also, the tip of the PTC lies close to the signal frequency. When a dead region is present, the signal is detected via IHCs with CFs different from that of the signal frequency. In such a case, threshold in the TEN is markedly higher than normal, and the tip of the PTC is shifted away from the signal frequency. Generally, there was a very good correspondence between the results obtained using the TEN and the PTCs. We conclude that the measurement of masked thresholds in TEN provides a quick and simple method for the diagnosis of dead regions.

Compressão de frequências no reconhecimento de fala de idosos com possíveis zonas mortas na cóclea

“…This technique 10 showed very strong agreement with the above criteria indicative of possible ZMC 25 and showed that white noise has an effect similar to the threshold equalizing noise (TEN) 27 , which is internationally recognized as a simple and efficient method for identifying possible ZMC. Therefore, the masking test by white noise is considered as a reliable alternative for identifying the possible presence of ZMC.…”

“…When the masked thresholds obtained were at least 10 dB above the absolute threshold and the masking level used, the result was considered indicative of the presence of ZMC 27 .…”

High-frequency tinnitus without hearing loss does not mean absence of deafferentation

“…Dead regions. Following the pitch scaling experiment dead region diagnostics were undertaken using a the Threshold Equalizing Noise (TEN) test (Moore et al, 2000). This test was the last in the sequence of test to exclude the bias that could occur when choosing upper anchors for the pitch scaling experiment after knowledge of possible dead regions.…”

“…For this purpose, we utilized the Threshold Equalizing Noise (TEN) test of Moore et al (2000). The main feature of this diagnostic screening tool is to present a broad-band noise (at different levels), that is designed in such a way that equal noise power (calibrated in SPL) is contained in all cochlear auditory output filters.…”

“…In case of our tinnitus subjects in which highfrequency tinnitus was prevalent, putatively indicating high-frequency hearing receptors damage, a shift towards lower frequencies would be the most likely effect, resulting in slopes larger 1. This idea is based, e.g., on data from psychoacoustical tuning curves, which show that when a signal frequency falls into a dead region, the tip of the tuning curve shifts to low frequencies, indicating that off-frequency listening is taking place (Moore et al, 2000). The rationale is schematically displayed in Fig.…”