Dead region audiometry Sensorineural hearing loss

1 dead region audiometry

1.1 pure tone audiometry (pta)
1.2 psychoacoustic tuning curves (ptc) , threshold equalizing noise (ten) tests
1.3 perceptual consequences of dead region

dead region audiometry
pure tone audiometry (pta)

dead regions affect audiometric results, perhaps not in way expected. example, may expected thresholds not obtained @ frequencies within dead region, obtained @ frequencies adjacent dead region. therefore, assuming normal hearing exists around dead region, produce audiogram has dramatically steep slope between frequency threshold obtained, , frequency threshold cannot obtained due dead region.

figure 7: response of basilar membrane pure tone.



figure 8: response of basilar membrane pure tone, when there dead region.

however, appears not case. dead regions cannot found via pta audiograms. may because although neurons innervating dead region, cannot react vibration @ characteristic frequency. if basilar membrane vibration large enough, neurons tuned different characteristic frequencies such adjacent dead region, stimulated due spread of excitation. therefore, response patient @ test frequency obtained. referred “off-place listening”, , known ‘off-frequency listening’. lead false threshold being found. thus, appears person has better hearing do, resulting in dead region being missed. therefore, using pta alone, impossible identify extent of dead region (see figure 7 , 8).

consequently, how audiometric threshold affected tone frequency within dead region? depends on location of dead region. thresholds @ low frequency dead regions, more inaccurate @ higher frequency dead regions. has been attributed fact excitation due vibration of basilar membrane spreads upwards apical regions of basilar membrane, more excitation spreads downwards higher frequency basal regions of cochlea. pattern of spread of excitation similar ‘upward spread of masking’ phenomenon. if tone sufficiently loud produce enough excitation @ functioning area of cochlea, above areas threshold. tone detected, due off-frequency listening results in misleading threshold.

to overcome issue of pta producing inaccurate thresholds within dead regions, masking of area beyond dead region being stimulated can used. means threshold of responding area sufficiently raised, cannot detect spread of excitation tone. technique has led suggestion low frequency dead region may related loss of 40-50 db. however, 1 of aims of pta determine whether or not there dead region, may difficult assess frequencies mask without use of other tests.

based on research has been suggested low frequency dead region may produce relatively flat loss, or gradually sloping loss towards higher frequencies. dead region less detectable due upward spread of excitation. whereas, there may more obvious steeply sloping loss @ high frequencies high frequency dead region. although slope represents less pronounced downward spread of excitation, rather accurate thresholds frequencies non-functioning hair cells. mid-frequency dead regions, small range, appear have less effect on patient’s ability hear in everyday life, , may produce notch in pta thresholds. although clear pta not best test identify dead region.

psychoacoustic tuning curves (ptc) , threshold equalizing noise (ten) tests

although debate continues regarding reliability of such tests, has been suggested psychoacoustic tuning curves (ptcs) , threshold-equalising noise (ten) results may useful in detecting dead regions, rather pta. ptcs similar neural tuning curves. illustrate level of masker (db spl) tone @ threshold, function of deviation center frequency (hz). measured presenting fixed low intensity pure tone while presenting narrow-band masker, varying center frequency. masker level varied, level of masker needed mask test signal found masker @ each center frequency. tip of ptc masker level needed mask test signal lowest. normal hearing people when masker center frequency closest frequency of test signal (see figure 9).

in case of dead regions, when test signal lies within boundaries of dead region, tip of ptc shifted edge of dead region, area still functioning , detecting spread of excitation signal. in case of low frequency dead region, tip shifted upwards indicating low frequency dead region starting @ tip of curve. high frequency dead region, tip shifted downwards signal frequency functioning area below dead region. however, traditional method of obtaining ptcs not practical clinical use, , has been argued tens not accurate enough. fast method finding ptcs has been developed , may provide solution. however, more research validate method required, before can accepted clinically.

perceptual consequences of dead region

audiogram configurations not indicators of how dead region affect person functionally, due individual differences. example, sloping audiogram present dead region, due spread of excitation. however, individual may affected differently corresponding sloped audiogram caused partial damage hair cells rather dead region. perceive sounds differently, yet audiogram suggests have same degree of loss. huss , moore investigated how hearing impaired patients perceive pure tones, , found perceive tones noisy , distorted, more (on average) person without hearing impairment. however, found perception of tones being noise, not directly related frequencies within dead regions, , therefore not indicator of dead region. therefore suggests audiograms, , poor representation of dead regions, inaccurate predictors of patient’s perception of pure tone quality.

research kluk , moore has shown dead regions may affect patient’s perception of frequencies beyond dead regions. there enhancement in ability distinguish between tones differ in frequency, in regions beyond dead regions compared tones further away. explanation may cortical re-mapping has occurred. whereby, neurons stimulated dead region, have been reassigned respond functioning areas near it. leads over-representation of these areas, resulting in increased perceptual sensitivity small frequency differences in tones.