Central Auditory Processing Assessment: Dichotic, Temporal, Spatial & Low-Redundancy Tests

Central Auditory Processing Assessment: Dichotic, Temporal, Spatial & Low-Redundancy Tests

Central auditory processing disorder (CAPD or APD) is a deficit in how the central auditory nervous system processes sound. People with APD often have normal pure-tone thresholds but struggle to understand speech in noise, follow rapid speech, or localize sounds. The Praxis Audiology blueprint lists assessment of functional hearing ability and auditory processing, including speech-in-noise, spatial, and temporal testing. This section explains who to test, which test categories to use, and how to interpret results. Key professional documents are ASHA's 2005 technical report on (central) auditory processing disorders and the American Academy of Audiology's 2010 clinical practice guidelines.

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Before Testing: Candidacy and Confounds

• Peripheral hearing first: complete pure-tone, speech, immittance, and OAE testing. Peripheral hearing loss, especially asymmetric loss, can produce abnormal central test results.
• Age: most central tests are normed for children about 7 years and older; results from younger children are hard to interpret because the central auditory system is still maturing.
• Language, attention, and cognition: many tests depend on language skills, memory, and attention. Test in the person's native language, choose tests with low language demands (tones, digits) when needed, and work with speech-language pathologists, psychologists, and teachers.
• Screening tools: SCAN-3 (for children and for adolescents and adults) and questionnaires such as the Children's Auditory Performance Scale identify who needs a full battery.
• Common reasons for referral: difficulty listening in noise, frequent requests for repetition, trouble following multi-step directions, poor attention to spoken information, and difficulty localizing sound.

APD can coexist with, or be confused with, attention-deficit/hyperactivity disorder, language disorders, learning disabilities, and autism. A multidisciplinary approach helps separate a primary auditory deficit from these conditions.

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The Test Battery by Process

Dichotic Listening (Binaural Separation and Integration)

Different signals are presented to the two ears at the same time.

• Dichotic Digits Test: two digits to each ear simultaneously; the listener repeats all four (binaural integration). Normal adults score about 90% or better in each ear; children score lower and often show a larger right-ear advantage.
• Staggered Spondaic Word (SSW) test: spondee words overlap partly between ears.
• Competing Sentences and dichotic consonant-vowel tests: the listener attends to one ear (binaural separation) or both.
• Interpretation: a small right-ear advantage is normal because the right ear has strong crossed pathways to the language-dominant left hemisphere. A left-ear deficit is common in young children and in people with corpus callosum (interhemispheric) dysfunction. A temporal lobe lesion typically lowers scores in the ear opposite the lesion.

Temporal Processing

• Temporal resolution (gap detection): the Gaps-in-Noise (GIN) test presents noise segments with silent gaps of different lengths; normal approximate gap thresholds are about 5 ms in adults and in children 7 years and older. The Random Gap Detection Test uses tone or click pairs; thresholds above about 20 ms are considered abnormal. Long gap thresholds suggest poor temporal resolution, which affects perception of consonant timing (for example, voice onset time).
• Temporal patterning (ordering): the Frequency Pattern Test presents three-tone sequences of high and low tones (such as "high-low-high"), and the Duration Pattern Test uses long and short tones. Labeling the pattern verbally requires the right hemisphere to recognize the contour and the left hemisphere to name it, with transfer through the corpus callosum. Normal humming with poor verbal labeling suggests an interhemispheric transfer deficit. Bilateral abnormal patterning results are typical of cortical involvement on either side.

Monaural Low-Redundancy Speech

These tests remove redundancy from the speech signal so that subtle central deficits show up.

• Low-pass filtered speech (removes high-frequency cues).
• Time-compressed speech (for example, 45% or 65% compression), sometimes with added reverberation.
• Speech in noise or competing speech in the same ear, such as the Synthetic Sentence Identification test with an ipsilateral competing message.
• Speech-in-noise tests (QuickSIN, BKB-SIN, Words-in-Noise) also measure functional hearing in noise and are covered in the speech-in-noise section.

Binaural Interaction

• Masking level difference (MLD): detection of a 500-Hz tone in noise improves when the tone is inverted in one ear (SπN0) compared with in phase (S0N0). Normal MLDs are roughly 10 to 15 dB; a reduced MLD suggests lower brainstem dysfunction, although peripheral hearing loss can also reduce it.
• Localization and lateralization, binaural fusion, and rapidly alternating speech tests.

Spatial Processing

• The Listening in Spatialized Noise-Sentences (LiSN-S) test measures how much a listener benefits when distracting speech comes from a different direction (spatial advantage) or a different talker (talker advantage). A reduced spatial advantage indicates a spatial processing disorder, which is linked to difficulty in noisy classrooms and can be treated with targeted auditory training (LiSN & Learn).

Electrophysiologic Measures

ABR, middle latency response (for example, an electrode effect with smaller responses over one hemisphere), the P300, and mismatch negativity can support behavioral findings, especially when behavioral testing is not reliable.

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Interpreting the Battery

• Use tests that assess different processes, because APD is not a single deficit.
• A commonly used diagnostic criterion is performance at least 2 standard deviations below the mean on at least two tests, or at least 3 standard deviations below the mean on one test together with documented functional difficulties. Large interaural asymmetries also count.
• Check that the pattern makes sense: abnormal results across every test, including tones, digits, and sentences, may reflect attention, cognition, or motivation rather than a specific auditory deficit.
• Describe which processes are affected (for example, binaural integration and temporal patterning) rather than only stating "APD present," because management targets the specific deficit.

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Linking Results to Management

• Environmental changes: remote microphone systems, preferential seating, better classroom acoustics, and visual supports.
• Compensatory strategies: active listening, asking for clarification, note-taking, and using context.
• Deficit-specific auditory training: for example, dichotic interaural intensity difference training for a dichotic ear asymmetry, temporal training for gap or pattern deficits, and LiSN & Learn for spatial processing deficits.
• Collaboration: share results with speech-language pathologists, teachers, and the IEP or Section 504 team.