5.3 The Plateau Method, Overmasking & The Masking Dilemma

5.3 The Plateau Method, Overmasking & The Masking Dilemma

[!NOTE] Introducing masking noise is only the initial step in obtaining ear-specific data. To confirm that the recorded threshold is neither an undermasked shadow response nor an artificially elevated overmasked artifact, the clinician must perform a dynamic threshold search using the Hood Plateau Method. Recognizing the boundaries of the plateau—and knowing how to manage when that plateau collapses into a masking dilemma—is a hallmark of advanced clinical competence.

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The Hood Plateau Method & The Concept of Effective Masking

Introduced in 1960 by British audiologist J.D. Hood, the Plateau Method provides a standardized psychophysical protocol for identifying true sensory thresholds in the presence of contralateral masking noise.

Understanding Effective Masking (EM)

Modern audiometers calibrate masking noise in Decibels of Effective Masking (dB EM) rather than Sound Pressure Level (SPL):

• $0\text{ dB EM}$: The exact intensity of narrow-band noise required to just mask a pure tone presented at $0\text{ dB HL}$ in the same ear.
• $40\text{ dB EM}$: Sufficient noise to just mask a $40\text{ dB HL}$ pure tone.
• Under an effective masking calibration, each $1\text{ dB}$ increase in masking noise shifts the threshold of that ear by exactly $1\text{ dB}$.

The Three Psychophysical Masking Zones

When masking noise in the non-test ear is plotted on the horizontal axis against the test ear tone threshold on the vertical axis, three distinct behavioral zones emerge:

                      THE HOOD MASKING PLATEAU CURVE

    Test Ear Threshold (dB HL)
          ▲
          │                                            / Overmasking Zone
          │                                           /  (Slope = 1:1)
          │                                          /   EM crosses skull to TE
          │                      ┌──────────────────┐
          │                      │   PLATEAU ZONE   │
          │                      │  (Slope = 0:1)   │
          │                      │  True Threshold  │
          │                     ┌┴──────────────────┴┐
          │                    / │ 15 - 20 dB Width │
          │                   /  └──────────────────┘
          │                  / Undermasking Zone
          │                 /  (Slope = 1:1)
          │                /   NTE still hearing tone
          │               / 
          └──────────────┴───────────────────────────┴──────────────►
          0             IML                     Overmask    Masking Level
                                                Threshold   in NTE (dB EM)

1. Undermasking Zone: The masking noise presented to the non-test ear is insufficient to prevent the non-test cochlea from detecting the crossed-over tone. As the masking noise is increased by $10\text{ dB}$, the apparent threshold of the test ear shifts upward by $10\text{ dB}$ in a direct 1:1 ratio. The patient is still responding with their non-test ear.
2. Plateau Zone: The masking noise in the non-test ear is sufficiently intense to prevent the non-test cochlea from hearing the crossed tone, but not yet intense enough to cross back to the test cochlea. Across this range (typically $15\text{ to }20\text{ dB}$ wide), increasing the masking noise produces no change whatsoever in the test ear threshold (slope = 0). This unchanging response level represents the true threshold of the test ear.
3. Overmasking Zone: The masking noise in the non-test ear reaches such high sound pressure levels that it crosses the skull via bone conduction, entering the test ear cochlea and artificially driving up the test ear's true threshold in a 1:1 ratio.

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Step-by-Step Clinical Plateau Protocol

To establish a valid plateau, the hearing instrument specialist executes a disciplined four-phase clinical sequence:

                 STEP-BY-STEP PLATEAU PROTOCOL

     [Phase 1: Set Initial Masking Level (IML) in NTE]
     (AC: AC_NTE + 10 dB; BC: AC_NTE + OE + 10 dB)
                           │
                           ▼
     [Phase 2: Re-establish Test Tone Threshold in TE]
     (Present tone at original unmasked threshold level)
                           │
            ┌──────────────┴──────────────┐
            ▼                             ▼
      [Tone Heard?]                 [Tone NOT Heard?]
            │                             │
            │                             ▼
            │                   [Raise Tone in 5 dB Steps]
            │                   (Hughson-Westlake search until heard)
            │                             │
            └──────────────┬──────────────┘
                           │
                           ▼
     [Phase 3: The 5-dB Incremental Masking Search]
     (Raise masking noise in NTE by 5 dB; re-present tone)
                           │
            ┌──────────────┴──────────────┐
            ▼                             ▼
      [Tone Heard?]                 [Tone NOT Heard?]
            │                             │
            │                             ▼
            │                   [Raise Tone in 5 dB Steps]
            │                   (Until response is restored)
            │                             │
            └──────────────┬──────────────┘
                           │
                           ▼
     [Phase 4: Confirm Plateau Verification]
     (Must achieve 3 consecutive 5-dB masking increases
      with ZERO shift in test tone threshold = 15 dB plateau)
                           │
                           ▼
     [Record True Masked Threshold with Appropriate Symbol]

Clinical Nuance: Distinguishing Plateau Shifts from Central Masking

During Phase 3, when the initial masking noise is introduced, a threshold shift of $5\text{ dB}$ is frequently observed even when no cross-hearing was occurring. This mild shift is caused by central masking—a normal neurophysiological phenomenon where low-level noise in one ear inhibits sound perception in the contralateral auditory cortex by approximately $5\text{ dB}$. Clinicians should anticipate a $5\text{ dB}$ shift as a normal physiological reaction rather than immediate cross-hearing.

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Undermasking vs. Overmasking Mechanics

Undermasking

• Definition: Delivering a masking level in the non-test ear that falls below the intensity of the crossed-over tone ($Noise_{NTE} < Signal_{TE} - IA$).
• Clinical Danger: The clinician records a false, artificially sensitive threshold (the shadow curve). In unilateral profound deafness, undermasking leads to the tragic error of declaring the patient a candidate for standard amplification in a dead ear rather than directing them toward a CROS fitting, bone-anchored device, or ENT referral.

Overmasking

• Definition: The masking noise presented to the non-test ear exceeds the interaural attenuation and stimulates the test ear's cochlea via bone conduction.
• Mathematical Trigger for Overmasking: Overmasking occurs at the exact threshold where the effective masking level in the non-test ear equals or exceeds the bone-conduction threshold of the test ear plus interaural attenuation:

$\mathbf{EM_{NTE} \ge BC_{TE} + IA}$

Where:

• $EM_{NTE}$ is the effective masking level delivered to the non-test ear.
• $BC_{TE}$ is the true bone-conduction threshold of the test ear.
• $IA$ is the interaural attenuation ($40\text{ dB}$ for supra-aural, $60\text{ dB}$ for inserts).

Mathematical Demonstration of Overmasking

Suppose a patient has a left ear bone conduction threshold of $BC_{TE} = 20\text{ dB HL}$, and testing is conducted with supra-aural earphones ($IA = 40\text{ dB}$): $\text{Overmasking Threshold} = BC_{TE} (20) + IA (40) = \mathbf{60\text{ dB EM}}$ If the clinician raises the masking noise in the right ear to $60\text{ dB EM}$ or higher, the noise itself crosses the skull via bone conduction, enters the left cochlea at $20\text{ dB HL}$, and begins elevating the left ear's pure-tone threshold. Any threshold recorded with masking $\ge 60\text{ dB EM}$ is contaminated by overmasking.

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The Masking Dilemma: Etiology & Clinical Gridlock

What is the Masking Dilemma?

The Masking Dilemma is an insurmountable audiometric gridlock occurring when the Initial Masking Level ($IML$) required to eliminate cross-hearing in the non-test ear is equal to or greater than the overmasking threshold ($EM_{NTE} \ge BC_{TE} + IA$).

When this occurs, the plateau width shrinks to zero decibels. There is no intensity level at which masking is both sufficient to prevent cross-hearing and low enough to avoid overmasking:

$\mathbf{IML_{NTE} \ge BC_{TE} + IA \implies \text{Plateau Width } \le 0\text{ dB}}$

                      ANATOMY OF THE MASKING DILEMMA

     Acoustic Decibel Scale (dB HL / EM)

     100 ──┐
      90 ──┤
      80 ──┤ ◄── Initial Masking Level Required (IML = AC_NTE + 10 = 80 dB)
      70 ──┤
      60 ──┤
      50 ──┤ ◄── Overmasking Threshold (BC_TE + IA = 10 + 40 = 50 dB)
      40 ──┤
      30 ──┤     ═══════════════════════════════════════════════════════
      20 ──┤     CRITICAL GRIDLOCK:
      10 ──┤     IML (80 dB) is 30 dB HIGHER than Overmasking (50 dB)!
       0 ──┘     Plateau is completely obliterated (-30 dB width).
                 ═══════════════════════════════════════════════════════

Clinical Etiology: Bilateral Conductive/Mixed Losses

The masking dilemma occurs almost exclusively in cases of bilateral severe conductive or mixed hearing losses, such as:

• Advanced bilateral otosclerosis with stapedial footplate fixation.
• Chronic bilateral otitis media with extensive ossicular chain discontinuity and effusion.
• Bilateral congenital aural atresia or severe middle ear malformations.

Quantitative Proof of Plateau Obliteration

Consider a patient with bilateral otosclerosis evaluated with supra-aural earphones ($IA = 40\text{ dB}$):

• Right Ear (TE): $AC = 70\text{ dB HL}$, $BC = 10\text{ dB HL}$ (Air-Bone Gap = $60\text{ dB}$)
• Left Ear (NTE): $AC = 70\text{ dB HL}$, $BC = 10\text{ dB HL}$ (Air-Bone Gap = $60\text{ dB}$)

1. Calculate Air Conduction IML in Left Ear (NTE): $IML = AC_{NTE} + 10 = 70 + 10 = \mathbf{80\text{ dB EM}}$
2. Calculate Overmasking Threshold for Right Ear (TE): $\text{Overmasking Threshold} = BC_{TE} + IA = 10 + 40 = \mathbf{50\text{ dB EM}}$
3. Analyze the Difference: To prevent cross-hearing, the clinician must present at least $80\text{ dB EM}$. But introducing anything over $50\text{ dB EM}$ immediately overmasks the test ear! The clinician is caught in an inescapable trap: presenting $80\text{ dB EM}$ overmasks by $30\text{ dB}$, while presenting $< 80\text{ dB EM}$ undermasks.

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Clinical Management of the Masking Dilemma

When confronted with a masking dilemma, the specialist cannot simply guess at bone-conduction thresholds. Several validated clinical strategies must be employed:

                    MASKING DILEMMA RESOLUTION STRATEGIES

                             [Masking Dilemma Identified]
                             (IML ≥ Overmasking Threshold)
                                          │
           ┌──────────────────────────────┼──────────────────────────────┐
           ▼                              ▼                              ▼
   [Transducer Conversion]       [SAL Technique]             [Diagnostic Cross-Check]
   Switch from Supra-Aural       Sensorineural Acuity        Immittance Audiometry
   (IA = 40) to Inserts          Level Test                  Type B Tympanograms,
   (IA = 60)                     Forehead BC Noise +         Absent Acoustic Reflexes
   Widens Window by 20-30 dB     Air-Conducted Tones         Confirms Conductive Loss
           │                              │                              │
           └──────────────────────────────┼──────────────────────────────┘
                                          │
                                          ▼
                        [Otologic Medical Referral (ENT)]
                        Surgical Candidacy (Stapedectomy)
                        Bone-Conduction Auditory Brainstem Response

1. Transducer Conversion to Insert Earphones (Primary Intervention)

The immediate, most powerful clinical intervention is replacing supra-aural earphones with deeply seated insert earphones (ER-3A/ER-3C):

• IA increases from $40\text{ dB}$ to $60\text{ dB}$ (a $20\text{ dB}$ boost).
• The Occlusion Effect is drastically reduced from $20\text{ dB}$ down to $0\text{-}5\text{ dB}$ at low frequencies, lowering the required $IML$.
• In our otosclerosis example, raising IA to $60\text{ dB}$ shifts the overmasking threshold from $50\text{ dB}$ to $70\text{ dB EM}$ ($10 + 60 = 70$). Combined with the elimination of the occlusion effect, this intervention frequently reclaims a usable $10\text{ to }20\text{ dB}$ plateau where none existed.

2. The Sensorineural Acuity Level (SAL) Technique

Developed by James Jerger and Tom Tillman in 1960, the SAL test circumvents the masking dilemma entirely:

1. Pure-tone air-conduction thresholds are measured in both ears in the unmasked state.
2. A bone-conduction oscillator is placed on the center of the forehead, delivering a fixed, maximum level of narrow-band or thermal noise.
3. Air-conduction thresholds are re-measured while the noise is playing through the bone oscillator.
4. The shift in air-conduction threshold produced by the bone-conducted noise is compared against normative threshold shifts established in normal-hearing listeners.
5. If the patient has a pure sensorineural loss, the noise shifts their air threshold dramatically. If the patient has a severe conductive loss, the bone-conducted noise shifts the threshold by the exact normative amount, permitting calculation of true sensorineural sensitivity without earphone masking.

3. Immittance & Acoustic Reflex Cross-Checks

When behavioral masking fails, the clinician relies on objective immittance testing (the Jerger Cross-Check Principle):

• Flat Type B tympanograms with normal canal volume indicate middle ear effusion.
• Type $A_s$ (shallow) tympanograms with completely absent acoustic reflexes (probe-ear and sound-ear) corroborate ossicular fixation (otosclerosis).
• Absent acoustic reflexes in the presence of normal tympanometry strongly support a bilateral conductive lesion, validating that the unmasked air-bone gaps are physiological rather than artifacts.

4. Electrophysiology and Otologic Medical Referral

If a definitive sensorineural reserve cannot be established behaviorally:

• The patient must be referred to an Otolaryngologist (ENT) for medical evaluation and high-resolution temporal bone CT imaging.
• Diagnostic verification can be achieved via Bone-Conduction Auditory Brainstem Response (BC-ABR) testing, which objectively identifies wave V latencies to establish cochlear thresholds without behavioral masking plateaus.