Hearing Loss and Speech Perception Week 2

Effects of Hearing Loss on Communication

Hearing loss impacts communication through several key variables:

• Degree of hearing loss: The severity of the loss across various frequencies.
• Configuration of hearing loss: The specific shape or pattern of the hearing loss across the frequency spectrum.
• Age of onset: Whether the loss occurred before (pre-lingual), during (peri-lingual), or after (post-lingual) the development of speech and language.

Primary vs. Secondary Effects of Hearing Loss

Primary Effects

Primary effects are the direct result of the hearing loss itself. They originate from the physiological damage to the auditory system.

• Loss of sensitivity: Reduced ability to detect sounds at normal volume levels.
• Loss of audibility: Specific sounds, particularly those relevant to speech, become impossible to hear.
• Distortion of the signal: Even when sounds are made loud enough to hear, they may be unclear. This is often attributed to:
  • Loss of inner hair cells.
  • Damage to auditory nerve fibers (often inferred rather than directly measured).

Secondary Effects

Secondary effects are the indirect consequences of the hearing loss combined with other environmental or developmental factors.

• Delays in speech and language development: Because children cannot hear the full spectrum of spoken language, their own production and comprehension are hindered.
• Educational achievement implications: Language delays often lead to difficulties in academic settings.
• Social implications: Difficulty communicating can affect social integration and peer relationships.

Estimating Speech Perception and Intelligibility

Speech perception is not equally distributed across all frequencies. Certain ranges contribute more to intelligibility than others.

• Frequency Importance: Frequencies between $1000\,Hz$ and $2000\,Hz$ are significantly more important for speech intelligibility than very high frequencies such as $10,000\,Hz$ to $20,000\,Hz$.
• Weighted Estimates: It is possible to calculate a weighted estimate of how much speech is intelligible to a listener.
  • Articulation Index (AI) or Speech Intelligibility Index (SII): These metrics represent intelligibility as a number ranging from $0.0$ to $1.0$.
  • Scale: $0.0$ indicates speech is completely unintelligible; $1.0$ indicates speech is completely intelligible.
• Signal Levels: Calculations for AI or SII can be performed separately for different signal levels:
  • Soft speech.
  • Medium speech.
  • Loud speech.

Speech Recognition Variability

There is significant variability in speech recognition scores among individuals with similar hearing thresholds, particularly in cases of Neural Hearing Loss or Auditory Neuropathy/Dys-synchrony.

• Open-Set Word Scores: Research involving 46 children with auditory neuropathy/dys-synchrony shows scores ranging from $0\%$ to nearly $100\%$ despite overlapping 3-frequency average thresholds (measured in $\text{dBHL}$).
• Factors in Variability:
  • Distortion: This may be the primary factor when speech recognition is significantly poorer than what audibility alone would predict.
  • Linguistic Context Cues: Some individuals are more adept at using surrounding words to fill in the gaps of missing sounds.
  • World Knowledge: Using general knowledge to decode partially inaudible or distorted signals.

Requirements for Audibility

The amount of audibility required to understand speech depends on both the stimulus and the listener.

• Stimuli Redundancy:
  • High Context: Simple, redundant sentences require less audibility because meaning can be inferred.
  • Low Context: Nonsense syllables (e.g., $/w\Lambda rp/$) require high audibility because there are no linguistic cues to assist the listener.
• Listener Age: Children require higher levels of audibility than adults to achieve the same percentage of correct speech perception.
  • Younger children ($5-8$ years old) require more audibility than older children ($9-12$ years old).
  • Adults require the least audibility to process speech effectively.

Clinical Speech Audiometry Examples

Example 1

• Ear: Right (R) - $60\,dB$ level, $100\%$ Recognition; Left (L) - $55\,dB$ level, $80\%$ Recognition ($90\,dB$ level, $90\%$ Recognition).
• Material: NU-6(S).

Example 2

• Ear: Right (R) - SRT $75\,dB$, SAT $75\,dB$, $20\,dB$ Level, $68\%$ Recognition; Left (L) - SRT $60\,dB$, SAT $75\,dB$, $20\,dB$ Level, $76\%$ Recognition.
• Material: NU-6.

Example 3

• Ear: Right (R) - SRT $50\,dB$, SAT $5\,dB$, $100\%$ Recognition; Left (L) - SRT $50\,dB$, SAT $10\,dB$, $100\%$ Recognition.
• Material: PBK-50.

Example 4

• Ear: Right (R) - SAT $90\,dB$, $45\,dB$ Level, $88\%$ Recognition; Left (L) - SAT $90\,dB$, $45\,dB$ Level, $76\%$ Recognition.
• Material: NU-6(1/2).

Example 5

• Ear: Right (R) - SRT $45\,dB$, SAT $15\,dB$, $92\%$ Recognition; Left (L) - Did Not Test (DNT).
• Material: PBK-50.

Example 6

• Ear: Right (R) - DNT; Left (L) - DNT.

Factors in Oral Language Development

Children develop oral language based solely on what they can hear. This is influenced by:

• Hearing Thresholds: The quietest sounds a child can hear at different frequencies.
• Sound Level and Location: How loud the speech is and where the source is located relative to the listener (Modifiable).
• Noise: Background interference that can mask speech (Modifiable).
• Device: The presence and quality of hearing aids or cochlear implants (Modifiable).
• Secondary Factors: Cognitive skills and the communication environment (Modifiable).

Arthur Boothroyd’s Hearing Loss Groups

Arthur Boothroyd developed a classification system for children with sensory hearing loss based on audibility and auditory resolution. This system is used to predict outcomes and plan rehabilitation.

Group I: Mild Hearing Loss ($15$ to $45\,dB$)

• Audibility: Children retain significant audibility of speech even without hearing aids.
• Language: Oral language typically develops spontaneously without hearing aids.
• Amplification: Hearing aids can restore full audibility. Auditory resolution is generally good.
• Weakness: Highly susceptible to interference from noise and reverberation. There may still be significant educational and communicative effects.

Group II: Moderate Hearing Loss ($50$ to $70\,dB$)

• Audibility: Partial audibility of conversational speech without help.
• Language: Children can develop oral language spontaneously, but there will be noticeable delays and differences compared to peers.
• Amplification: Hearing aids can restore full audibility. Auditory resolution is usually good.
• Weakness: Susceptible to noise and reverberation.

Group III: Severe Hearing Loss ($70$ to $90\,dB$)

• Audibility: Cannot hear conversational speech without amplification.
• Language: Oral language does not develop spontaneously.
• Amplification: Hearing aids can provide full audibility, but auditory resolution varies from good to poor.
• Speech Issues: Even when aided, these individuals often struggle with voice quality, place of articulation, and general perception/production problems.
• Candidacy: May be a candidate for a Cochlear Implant (CI).

Group IV: Profound Hearing Loss ($90$ to $110\,dB$)

• Audibility: Cannot hear conversational speech unaided. With hearing aids, only partial audibility is restored.
• Capacity: Very limited speech discrimination capacity even when aided. Perception is often limited to prosodic features (rhythm, pitch, intonation) supplemented by speechreading.
• Language: Visual language is likely to be the primary mode of communication.
• Candidacy: Likely a candidate for a Cochlear Implant (CI).

Group V: Total Hearing Loss ($110\,dB$ or more)

• Audibility: No conversational speech is heard, with or without hearing aids. Sound is perceived via the sense of touch (vibrotactile).
• Language: Without a CI, oral language acquisition depends entirely on visual input and intensive habilitation. Visual language is typically primary.
• Candidacy: Likely a candidate for a Cochlear Implant (CI).

The Role of Technology and Environment

As the degree of hearing loss increases, the capacity for aided auditory perception decreases because both pure tone sensitivity and auditory resolution suffer from increased cochlear damage.

• Retained Capacity: Those with loss up to approximately $90\,dB$ can still retain considerable auditory capacity if aided.
• Critical Factors for Child Health and Habilitation (CHH):
  • Age of identification (ID).
  • Consistency of device use.
  • Access to early intervention.
  • Non-verbal IQ.
  • Managing the communication environment.