Auditory Steady-State Response (ASSR)

A Comprehensive Review for 50-Mark RGUHS Examination

1. INTRODUCTION & DEFINITION

2. HISTORICAL BACKGROUND

3. PHYSICS & STIMULUS CHARACTERISTICS

Stimulus Composition

ASSR Stimulus = Carrier Frequency (CF) × Modulation Frequency (MF)

CF = 500, 1000, 2000, 4000 Hz (test frequencies)
MF = 40 Hz (awake) or 70–110 Hz (sleep/sedation)

S(t) = A [1 + m·cos(2π·fm·t)] · cos(2π·fc·t)

Where: A = amplitude, m = modulation depth (typically 100%), fm = modulation frequency, fc = carrier frequency

Types of Modulation

1. Amplitude Modulation (AM) — variation in amplitude at MF rate
2. Frequency Modulation (FM) — variation in frequency at MF rate
3. Mixed AM+FM (most commonly used clinically) — superior SNR

4. NEUROPHYSIOLOGICAL BASIS

┌─────────────────────────────────────────────────────────────┐
│               NEURAL GENERATORS OF ASSR                     │
├──────────────────┬──────────────────────────────────────────┤
│ 40 Hz ASSR       │ Primary Auditory Cortex (A1)              │
│                  │ Middle Latency Response generators        │
│                  │ Thalamo-cortical loops                    │
├──────────────────┼──────────────────────────────────────────┤
│ 80 Hz ASSR       │ Brainstem (Inferior Colliculus)           │
│                  │ Cochlear Nucleus                          │
│                  │ Superior Olivary Complex                  │
│                  │ Auditory Nerve                            │
└──────────────────┴──────────────────────────────────────────┘

"The part of the cochlea stimulated by the carrier frequency (e.g., 1000 Hz) must be intact for the cochlea to respond to the modulation rate (e.g., 80 Hz), producing an ASSR."

• CF determines cochlear place (tonotopic specificity)
• MF determines the neural following rate (temporal coding)
• When the cochlea at CF region is intact → sustained neural discharge phase-locked to MF → measurable ASSR

5. THE 40 Hz vs 80 Hz DICHOTOMY

6. FLOWCHART: HOW ASSR IS PERFORMED

                    ┌──────────────────────────┐
                    │   PATIENT PREPARATION    │
                    │  (Sleep/Sedation for     │
                    │   infants; awake adults) │
                    └────────────┬─────────────┘
                                 │
                    ┌────────────▼─────────────┐
                    │  ELECTRODE PLACEMENT     │
                    │  • Active: Cz (vertex)   │
                    │  • Reference: mastoid    │
                    │  • Ground: forehead      │
                    └────────────┬─────────────┘
                                 │
                    ┌────────────▼─────────────┐
                    │  STIMULUS DELIVERY       │
                    │  AM tones via insert     │
                    │  earphones               │
                    │  CF: 500,1000,2000,4000Hz│
                    │  MF: 70-100 Hz           │
                    └────────────┬─────────────┘
                                 │
                    ┌────────────▼─────────────┐
                    │  SIGNAL ACQUISITION      │
                    │  EEG recorded from scalp │
                    │  electrodes              │
                    └────────────┬─────────────┘
                                 │
                    ┌────────────▼─────────────┐
                    │  FOURIER ANALYSIS (FFT)  │
                    │  Transform EEG to        │
                    │  frequency domain        │
                    └────────────┬─────────────┘
                                 │
                    ┌────────────▼─────────────┐
                    │  STATISTICAL DETECTION   │
                    │  F-test or phase         │
                    │  coherence algorithm     │
                    │  (p < 0.05 = PRESENT)    │
                    └────────────┬─────────────┘
                                 │
              ┌──────────────────┴──────────────────┐
              │                                      │
  ┌───────────▼──────────┐              ┌────────────▼──────────┐
  │  RESPONSE PRESENT    │              │  RESPONSE ABSENT      │
  │  Reduce intensity    │              │  Increase intensity   │
  │  (10 dB steps)       │              │  (10 dB steps)        │
  └───────────┬──────────┘              └────────────┬──────────┘
              │                                      │
              └────────────────┬─────────────────────┘
                               │
                  ┌────────────▼─────────────┐
                  │  THRESHOLD DETERMINATION │
                  │  Lowest level with       │
                  │  significant response    │
                  └────────────┬─────────────┘
                               │
                  ┌────────────▼─────────────┐
                  │  CORRECTION FACTOR       │
                  │  Added to convert ASSR   │
                  │  threshold → Behavioral  │
                  │  threshold equivalent    │
                  └──────────────────────────┘

7. STATISTICAL RESPONSE DETECTION

Methods of Detection:

1. F-ratio test (ANOVA-based) — compares the energy at the MF to background EEG noise
   • F = Signal variance / Noise variance
   • Significant if F > critical value at p < 0.05
2. Phase Coherence (Rayleigh test) — measures consistency of phase across sweeps
   • More robust at low SNR conditions
3. Weighted averaging — used in MASTER system

    FFT of EEG
         │
    ─────┴─────
    │         │
Energy at   Energy at
 MF (Hz)  adjacent bins
(signal)    (noise)
    │         │
    └────┬────┘
         │
      F-ratio
         │
      p < 0.05?
      ┌───┴───┐
     YES      NO
      │        │
   RESPONSE  NO RESPONSE
   PRESENT    PRESENT

8. SIMULTANEOUS MULTI-FREQUENCY TESTING (MASTER)

┌─────────────────────────────────────────────────────────┐
│              MASTER SYSTEM PROTOCOL                      │
│                                                          │
│  RIGHT EAR           │  LEFT EAR                        │
│  500 Hz CF @ 77 Hz MF│  500 Hz CF @ 85 Hz MF            │
│ 1000 Hz CF @ 85 Hz MF│ 1000 Hz CF @ 93 Hz MF            │
│ 2000 Hz CF @ 93 Hz MF│ 2000 Hz CF @ 101 Hz MF           │
│ 4000 Hz CF @ 101 Hz MF│ 4000 Hz CF @ 109 Hz MF          │
│                                                          │
│  → 8 frequencies tested SIMULTANEOUSLY (both ears)      │
│  → Each frequency uses UNIQUE modulation frequency       │
│  → FFT separates responses at different MF peaks        │
└─────────────────────────────────────────────────────────┘

9. CLINICAL APPLICATIONS

A. Threshold Estimation in Neonates/Infants

• Gold standard complementary test alongside click ABR in newborn hearing screening
• Provides frequency-specific thresholds at 500, 1000, 2000, 4000 Hz
• Particularly valuable when behavioral audiometry is unreliable (< 6 months developmental age)

B. Severe-to-Profound Hearing Loss

• Critical advantage over ABR: ASSR can differentiate between severe (70–90 dB) and profound (>90 dB) hearing loss
• ABR saturates at ~90–100 dB nHL; ASSR systems can test up to 120 dB SPL
• Essential before cochlear implant candidacy selection

C. Confirmation of ABR Findings

• ASSR and ABR thresholds show moderate-to-strong correlation (varies by study)
• Cummings (2019): "ASSR is often used to confirm ABR results"
• ASSR is complementary, not a replacement for ABR

D. Bone Conduction ASSR

• Can estimate bone conduction thresholds
• Helps determine air-bone gap in conductive hearing loss (0.5–4 kHz)
• Data validation limited; masking is complex

E. Pseudohypacusis (K.J. Lee)

• Because it is an objective test, ASSR/ABR is powerful for detecting functional/non-organic hearing loss
• Cannot be voluntarily suppressed by the patient

F. Pre-fitting of Hearing Aids

• Provides frequency-specific audiometric data in patients who cannot perform behavioral audiometry
• Guides hearing aid selection and programming

10. ASSR THRESHOLD vs BEHAVIORAL THRESHOLD

Estimated Behavioral Threshold = ASSR Threshold − Correction Factor

11. ABR vs ASSR — COMPARATIVE TABLE

12. ADVANTAGES OF ASSR

1. Estimates severe-to-profound hearing loss — information unavailable from click or tone-burst ABR
2. Reasonably frequency-specific — pure-tone audiogram equivalent
3. Automated statistical analysis — removes examiner subjectivity
4. Objective for both subject and examiner
5. Records simultaneous responses — faster assessment (MASTER)
6. High-level testing up to 120 dB SPL
7. Does not rely on subjective waveform interpretation
8. Useful in malingering/pseudohypacusis

13. DISADVANTAGES OF ASSR

1. Requires quiet patient state — sleep or sedation needed in infants/toddlers
2. Possible artifactual response — cardiac, myogenic, or electrical artifacts can mimic ASSR
3. Limited anatomic site of lesion information — cannot differentiate cochlear from retrocochlear pathology
4. Difficult with bone conduction — may require masking
5. Overestimation of thresholds at near-normal hearing levels
6. Cannot distinguish profound hearing loss from auditory neuropathy
7. More research needed — normative data limited; sedative effects on results incompletely characterized
8. Varying threshold correlation with ABR across studies

14. FLOWCHART: ASSR IN NEONATAL HEARING ASSESSMENT PATHWAY

         NEWBORN
            │
            ▼
   ┌─────────────────┐
   │  NHS SCREENING  │
   │ OAE / AABR      │
   └────────┬────────┘
            │
    ┌───────┴───────┐
  PASS             REFER
    │                │
    ▼                ▼
 Discharge    Diagnostic ABR
              (click + tone burst)
                    │
         ┌──────────┴──────────┐
       NORMAL              ABNORMAL
         │                     │
       PASS           ┌────────▼────────┐
                      │  ASSR testing   │
                      │ (frequency      │
                      │  specific       │
                      │  thresholds)    │
                      └────────┬────────┘
                               │
              ┌────────────────┤
              │                │
       Near-normal      Severe/Profound
       thresholds        thresholds
              │                │
    Hearing aids/        Cochlear implant
    monitoring           candidacy
                         evaluation

15. ASSR IN RELATION TO OTHER AEPs

CLASSIFICATION OF AUDITORY EVOKED POTENTIALS
│
├── EARLY (< 10 ms) — Brainstem
│     ├── ECochG (Electrocochleography)
│     └── ABR (Waves I–V)
│
├── MIDDLE LATENCY (10–50 ms) — Thalamocortical
│     ├── Middle Latency Response (MLR) — Na, Pa, Nb, Pb
│     └── 40 Hz ASSR (overlapping MLR generators)
│
├── LATE / CORTICAL (> 50 ms)
│     ├── N1-P2 (CERA)
│     └── P300 / MMN (cognitive)
│
└── STEADY-STATE (frequency domain)
      ├── 40 Hz ASSR — cortical generator
      └── 80 Hz ASSR — brainstem generator ← CLINICALLY PREFERRED

• Auditory Steady-State Evoked Potential (ASSEP)
• 40-Hz response
• Steady-State Evoked Potential (SSEP)
• Amplitude Modulation Following Response (AMFR)
• Envelope Following Response (EFR)
• Frequency Following Response (FFR)

16. CLINICAL IMAGES

ASSR + ABR Waveforms in Unilateral Hearing Loss

ASSR Temporal Modulation Transfer Functions (TMTF)

17. RECENT ADVANCES

A. Bone Conduction ASSR

• Provides frequency-specific BC thresholds, useful in conductive/mixed hearing loss
• Limitation: masking difficulties; inter-device variability
• Studies show BC-ASSR correlates reasonably with BC-ABR at 0.5–4 kHz (Cummings, 2019)

B. Cortical ASSR (40 Hz) in Auditory Processing

• 40 Hz ASSR is being investigated as a biomarker for schizophrenia and autism spectrum disorder — gamma-band (40 Hz) ASSR is reduced in schizophrenia (Kaplan & Sadock)
• May reflect thalamocortical connectivity

C. ASSR with Narrow-Band CE-Chirp Stimuli

• CE-chirp stimulus compensates for cochlear traveling wave delay
• Results in larger amplitude ASSR, better SNR, shorter test time compared to pure tone AM stimuli

D. Multiple Auditory Steady-State Response (MASTER)

• Simultaneous bilateral, multi-frequency testing
• Reduces test time by ~60–70% vs sequential ABR
• Particularly valuable in sedated neonates (limited sedation window)

E. Automated ASSR (A-ASSR)

• Integrated into newborn screening protocols in some centres
• Automated pass/refer output without audiologist interpretation
• Challenges: higher refer rates than AABR

F. ASSR in Auditory Neuropathy Spectrum Disorder (ANSD)

• ASSR is absent or severely degraded in ANSD (unlike OAEs which are present)
• Helps differentiate cochlear from neural pathology

G. Objective Audiogram via ASSR

• Concept of generating a predicted audiogram from ASSR thresholds with frequency-specific correction factors
• Being incorporated into AI-aided audiological assessment platforms

18. LIMITATIONS SPECIFIC TO RGUHS EXAMINATION EMPHASIS

1. ASSR cannot replace ABR as the gold standard in neonatal hearing assessment — it is complementary (Cummings)
2. The 80 Hz ASSR is preferred over 40 Hz in sleeping infants because 40 Hz responses are state-dependent and suppressed during sleep
3. ASSR provides the only objective means of estimating thresholds in the severe-to-profound range beyond ABR's upper limit
4. The correction factor is essential to convert ASSR thresholds to estimated behavioral thresholds
5. ASSR cannot localize the site of lesion (cochlear vs retrocochlear)

19. SUMMARY TABLE

REFERENCES (Standard Textbooks for RGUHS)

1. Cummings Otolaryngology – Head and Neck Surgery, 7th Ed — Chapter on Pediatric Audiology (ASSR as complementary to ABR, threshold estimation in severe-profound HL)
2. K.J. Lee's Essential Otolaryngology, 11th Ed — Chapter on Audiologic Tests (Auditory Steady-State Evoked Potentials, Advantages/Disadvantages)
3. Scott Brown's Otorhinolaryngology, Head and Neck Surgery — Paediatric Audiology chapter (threshold estimation, neonatal hearing)
4. Stell and Maran's Head and Neck Surgery — Audiological assessment section
5. Dhingra's Diseases of Ear, Nose and Throat, 7th Ed — ERA/Objective Audiometry chapter
6. Hazarika's Textbook of ENT and Head & Neck Surgery — Electrophysiological tests chapter
7. Zakir Hussain's Principles of Audiology — ASSR principles, clinical applications
8. Picton et al. (2003) — "Human auditory steady-state responses" — Ear and Hearing — foundational reference for MASTER system
9. Lins et al. (1996) — Comparison of ASSR and ABR thresholds — Ear and Hearing
10. Celik et al. (2016) — ASSR vs ABR threshold correlation study (cited in Cummings)