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Psychoacoustic and physiological reflections of hearing loss: C315/A6 Andrew Faulkner: Department of Phonetics & Linguistics University College London Where can hearing loss arise? Conductive hearing loss in the outer ear • Outer and middle ears – conductive hearing loss – a problem in the conduction of sound from air to the inner ear • Cochlea plus auditory nerve – sensori-neural or cochlear hearing loss • Auditory nerve and beyond – retro-cochlear hearing loss (beyond the ear) – not discussed here Conductive losses in the middle ear An audiogram completely captures the attenuation of sound in conductive losses Eardrum damage Otitis Media Otosclerosis 1 Conductive hearing loss: Summary • Outer ear (blockages) – foreign bodies, infections, congenital malformations • Middle ear problems – ruptured eardrums – otitis media – otosclerosis • Equivalent to an attenuation of sound – helped by surgery and/or amplification Age related hearing loss presbyacousis Sensori-neural hearing loss • A problem in the inner ear (cochlea), usually involving the hair cells – can be caused by • genetic factors • mumps or meningitis • a bang on the head • age — ‘presbyacusis’ • noise, even ‘leisure noise’ Psychoacoustic concomitants of sensori-neural (cochlear) hearing loss • Loss of sensitivity • Abnormal loudness growth (recruitment) – Intense sounds appear to have normal loudness while less intense ones are abnormally quiet or inaudible. • Reduction of dynamic range (related to recruitment) • Impaired frequency selectivity threshold → ← dynamic range → ULL ------------------- An auditory area in sensori-neural loss ULL ------------------- ←—-—→ A normal auditory area threshold → 2 Auditory filtering in normal hearing Impaired frequency selectivity Auditory filtering in impaired hearing 35 dB loss at 2 kHz Impaired frequency selectivity • Broadened auditory filters effectively smooth the spectrum of speech leading to … – a reduced ability to resolve spectral detail – greater disruption from background noise Physiological background 3 Normal peripheral hearing depends upon ... • Inner Hair Cells for sensory transduction of basilar membrane movements to spikes on the auditory nerve. and ... • Outer Hair Cells which feed energy back into the basilar membrane amplifying its movement at relatively low sound levels. OHC activity is responsible for ... • Low absolute thresholds. • Level-dependent amplification of basilar membrane motion that increases dynamic range of hearing • Fine frequency selectivity of the basilar membrane (place coding). Consider the frequency response of a single place on the basilar membrane What goes wrong in sensori-neural hearing loss? Functional loss results from hair cell damage 4 Damage to haircells can cover extensive areas of the cochlear spiral Outer Hair Cells are relatively vulnerable to damage, leading to ... • Decreases in basilar membrane movement and hence increased thresholds to sound – hearing loss • Loss of level-dependent amplification – reduced dynamic range – loudness recruitment Relation of Hair Cell loss to audiogram Inner Hair Cell (IHC) damage ... • Leads to a more sparse representation of all auditory information passed on to higher auditory centres. • There are possibly even regions of the cochlea without any IHCs — so-called dead regions. • Loss of frequency tuning - widened filters degraded frequency selectivity Sensori-neural losses: Summary Effects of Organ of Corti damage on tuning in the auditory nerve • Three main consequences of cochlear hearing loss – Reduced audibility: unable to hear the quieter parts of speech. – Loudness recruitment: abnormally rapid growth of loudness and reduced dynamic range. – Reduced frequency selectivity: spectral smearing of speech signal. 5 Summary • All three appear to arise from damage to normal OHC function. • Loss of sensitivity and reduced dynamic range can be helped by amplification and compression in hearing aids • Loss of frequency selectivity cannot be corrected The End • Caveat – haven’t discussed effects on temporal processing abilities … – but these appear to be relatively robust in cochlear hearing loss. 6