HSC Exam Practice

Sample response. Sensorineural hearing loss is hearing loss caused by damage to the cochlear hair cells (mechanosensory cells in the organ of Corti) or to the auditory nerve (cranial nerve VIII). The structure primarily damaged is the cochlear hair cells, which are unable to transduce basilar membrane vibration into electrical signals.

Marking notes. 1 mark for defining as hearing loss caused by damage to cochlear hair cells / auditory nerve (inner ear pathology). 1 mark for identifying cochlear hair cells as the damaged structure (accept “inner ear” with specification that it is the hair cells).

Sample response. Conductive hearing loss involves a problem in the outer or middle ear (e.g. fluid in the middle ear, perforated eardrum, ossicle damage) that prevents sound waves from reaching the cochlea. The cochlear hair cells are intact and functional — if sound reaches the cochlea by any route, transduction occurs normally. Sensorineural hearing loss involves damage to the cochlear hair cells or auditory nerve in the inner ear. The hair cells are non-functional or absent and cannot transduce basilar membrane vibration into action potentials, regardless of how much sound reaches the cochlea.

Marking notes. 1 mark — site: conductive = outer/middle ear; sensorineural = inner ear / cochlear hair cells or auditory nerve. 1 mark — cochlear hair cells: conductive = intact and functional; sensorineural = damaged/absent/non-functional. 1 mark — functional consequence stated for each (conductive: if sound reaches cochlea, transduction is normal; sensorineural: transduction fails regardless of sound intensity).

Sample response. A hearing aid amplifies incoming sound waves and delivers a louder acoustic signal to the ear. For this to produce hearing, cochlear hair cells must be present and functional to transduce the amplified sound into electrical signals. In profound sensorineural hearing loss, cochlear hair cells are absent or severely damaged, so they cannot respond to the amplified signal regardless of the volume. There are no cells to transduce the louder sound, so amplification provides no benefit.

Marking notes. 1 mark — hearing aid mechanism is amplification (making sound louder / larger amplitude). 1 mark — in profound SNHL, cochlear hair cells cannot transduce even amplified sound, so amplification provides no functional benefit.

Sample response. Tonotopic organisation is the spatial arrangement of the cochlea in which different sound frequencies cause maximum basilar membrane vibration at different positions: high-frequency sounds maximally stimulate hair cells near the cochlear base, and low-frequency sounds near the apex. This allows the cochlea to discriminate pitch — the brain interprets which cochlear region is activated as a specific frequency. A cochlear implant uses this same spatial map: its 12–22 electrodes are positioned along the cochlea at positions corresponding to specific frequency regions. Each electrode fires in response to the coded signal for its frequency band, stimulating auditory nerve fibres at that tonotopic position and thereby encoding pitch information.

Marking notes. 1 mark — defines tonotopic organisation: different frequencies detected at different positions along cochlea / basilar membrane. 1 mark — explains functional role: allows pitch discrimination / frequency analysis. 1 mark — links to cochlear implant: electrodes positioned along the tonotopic map fire at positions corresponding to different frequency bands, encoding pitch information.

Sample response. BAHA is most appropriate for conductive hearing loss (or single-sided deafness). BAHA transmits sound vibrations directly through the skull bone to the cochlea, bypassing the outer and middle ear. It requires a functional cochlea with intact hair cells and auditory nerve to convert those vibrations into electrical signals. In a patient with profound sensorineural hearing loss, the cochlear hair cells are non-functional. BAHA would deliver vibrations to a cochlea that cannot transduce them — the transducer (hair cells) is broken — so no usable auditory signal is produced.

Marking notes. 1 mark — identifies conductive hearing loss (or single-sided deafness) as the primary indication. 1 mark — explains the mechanism of BAHA (bone conduction bypasses outer/middle ear, vibration reaches cochlea). 1 mark — explains why unsuitable for profound SNHL: requires functional cochlear hair cells to transduce vibrations, which are non-functional in SNHL, so the signal cannot be processed.

Sample response. Speech recognition relies primarily on temporal envelope cues (patterns of amplitude change over time) and broad spectral features called formants, which determine vowel and consonant distinctions. These broad acoustic features can be adequately encoded by 12–22 cochlear implant electrode channels, allowing most CI users to achieve reasonable speech understanding in quiet with training. Music appreciation requires fine frequency resolution to discriminate pitch differences between musical notes and to perceive the harmonic overtones that give instruments their characteristic timbre. The normal cochlea encodes pitch via ~3,500 discrete hair cell positions. With only 12–22 cochlear implant channels, each representing a broad frequency band, adjacent notes may be indistinguishable, harmonics are lost, and instrument timbres cannot be differentiated. The coarse spectral resolution of a cochlear implant is sufficient for speech but insufficient for the fine pitch discrimination that music requires.

Marking notes. 1 mark — speech relies on temporal / broad spectral features encodable in 12–22 channels (temporal envelope / formants). 1 mark — music requires fine pitch resolution to distinguish notes and harmonic overtones, not achievable with 12–22 channels. 1 mark — connects channel number to frequency resolution: ~3,500 hair cell positions vs 12–22 channels; coarser map insufficient for pitch discrimination required by music.

Sample response (a). In quiet, the 22-channel CI produced a word recognition score of 87%, compared with 98% for the normal cochlea — a difference of 11 percentage points. Both values are high, suggesting that 22 channels is sufficient for reasonable word recognition in quiet listening conditions. [2 marks: 1 for identifying the values and difference, 1 for interpreting the practical implication]

Sample response (b). In quiet, the difference between the 22-channel CI (87%) and normal cochlea (98%) is 11 percentage points. In background noise (+5 dB SNR), this gap widens substantially: the 22-channel CI scores 61% versus 91% for the normal cochlea, a difference of 30 percentage points. This larger gap in noise occurs because speech understanding in noisy environments requires finer spectral resolution to separate the target speech signal from background noise. With only 22 channels, the CI cannot resolve overlapping frequency information from multiple competing sounds as effectively as the ~3,500-position normal cochlea, so performance degrades disproportionately in noise. [3 marks: 1 for data comparison in quiet with values; 1 for data comparison in noise with values showing the widened gap; 1 for biological explanation using channel number and spectral resolution in noise]

Sample response (c). A 4-channel CI scores only 52% in quiet — below the threshold for functional communication (approximately 70% is typically considered adequate for conversational speech). This demonstrates that 4 channels provides insufficient spectral information for reliable word recognition. Increasing electrode number beyond 4 channels substantially improves outcomes: the 22-channel CI achieves 87% in quiet. Engineering development has focused on increasing electrode number because more channels encode more frequency bands, providing finer spectral resolution and improving word recognition, particularly in challenging listening conditions. [2 marks: 1 for interpreting 52% as functionally inadequate / identifying that more channels = better spectral resolution; 1 for explaining engineering goal of increasing channels in terms of improved spectral encoding]

Sample response. Three technologies assist people with hearing loss: hearing aids, cochlear implants, and bone-anchored hearing aids. Each targets a different point in the auditory pathway and is matched to a specific type of hearing loss and patient profile.

Hearing aids amplify incoming sound electronically and deliver a louder acoustic signal into the ear canal. This amplified signal travels through the outer and middle ear to the cochlea, where residual cochlear hair cells transduce it. Hearing aids are effective for conductive hearing loss (where the cochlea is intact but outer/middle ear conduction is impaired) and mild-to-moderate sensorineural hearing loss (where enough functional hair cells remain to respond to amplified sound). Benefits include non-invasive, reversible application, adjustable digital programming, and subsidisation under the Australian Government Hearing Services Program. Limitations include dependence on residual hair cell function (ineffective for profound sensorineural hearing loss), amplification of background noise, and inability to restore natural hearing quality.

Cochlear implants surgically bypass non-functional cochlear hair cells by threading an electrode array (12–22 electrodes) into the scala tympani of the cochlea. An external sound processor captures and frequency-analyses incoming sound, transmitting coded signals electromagnetically through intact skin to an internal receiver-stimulator. The electrode array delivers precisely timed electrical pulses to auditory nerve fibres at tonotopic positions, directly depolarising them and generating action potentials to the auditory cortex. Cochlear implants are indicated for severe-to-profound sensorineural hearing loss where hearing aid benefit is inadequate and the auditory nerve is intact. Benefits include providing access to sound when no other technology is effective, and — when implanted before 12–18 months during the critical period of auditory cortex development — enabling speech and language outcomes approaching hearing peers (Dettman et al., Melbourne CI Program data). Limitations include irreversibility (destroys residual hair cells), failure to restore natural hearing (only 12–22 frequency channels vs ~3,500 hair cell positions — poor spectral resolution particularly for music and in noise), requirement for extensive auditory rehabilitation, anaesthetic risk, and ethical questions around infant consent to an irreversible procedure. Cochlear implants are also ineffective if the auditory nerve is damaged.

BAHA uses bone conduction: a titanium screw osseointegrated into the mastoid bone anchors an external processor that converts sound to vibrations transmitted directly through skull bone to the cochlea, bypassing the outer and middle ear. BAHA is appropriate for conductive hearing loss where surgical correction is not possible or desired (e.g. otosclerosis, bilateral atresia/microtia as in Sophie from the lesson), and for single-sided deafness where routing sound to an intact contralateral cochlea is beneficial. It requires a functional cochlea — BAHA is entirely unsuitable for sensorineural hearing loss because it still requires hair cells to transduce the bone-conducted vibrations. Benefits include more natural sound quality than a cochlear implant (intact cochlea performs normal tonotopic transduction), and a removable external processor. Limitations include requirement for minor surgery (osseointegration), skin irritation around the abutment, and minimum age restrictions.

Patient-specific factors determine technology selection: (1) type of hearing loss — conductive vs sensorineural; (2) severity — mild-moderate vs severe-profound; (3) cochlear hair cell status — any residual function vs absent; (4) auditory nerve integrity — must be intact for cochlear implant; (5) age — early implantation maximises critical-period benefit; (6) reversibility preferences and ethical considerations. No single technology is appropriate for all patients — the correct choice matches the technology’s mechanism to the specific anatomical site and degree of pathology.

Marking notes (9 marks).

1 — Hearing aid mechanism correct (amplification, delivers louder sound to ear, requires residual hair cells). 1 — Hearing aid indication correct (conductive or mild-moderate sensorineural with residual hearing) plus one benefit and one limitation. 1 — Cochlear implant mechanism correct (electrode array bypasses hair cells, directly stimulates auditory nerve, tonotopic positioning). 1 — Cochlear implant indication correct (severe-profound SNHL, intact auditory nerve) plus benefit of early implantation with reference to critical period. 1 — Cochlear implant limitation correct: does not restore natural hearing (channel number / spectral resolution difference stated). 1 — BAHA mechanism correct (bone conduction, skull vibration, bypasses outer/middle ear, requires functional cochlea). 1 — BAHA indication correct (conductive hearing loss / single-sided deafness) and correctly identifies why BAHA is unsuitable for SNHL (requires functional cochlear hair cells). 1 — At least three patient-specific factors named and explained as determinants of technology selection (type of hearing loss, severity, residual hair cell function, auditory nerve status, age, reversibility). 1 — Overall evaluative judgement: explicitly states that no single technology is universally appropriate and that selection must match mechanism to type and site of hearing pathology, using HSC terminology throughout.