Glossary

The steady-state response evoked at a modulation rate near 40 Hz. Weighted toward cortical and thalamic generators, it is large in awake adults but degraded by sleep, sedation, and an immature nervous system.

Variation of the carrier tone's level over time. AM is the most common ASSR modulation because of its good frequency specificity; modulation depth is expressed as a percentage, with high depths of about 90–100% typical.

The lowest stimulus intensity at which the response remains statistically present for a given carrier. It is found by stepping intensity downward and is consistently poorer than the true behavioural threshold.

A transient auditory evoked potential — a response to discrete clicks or brief tone-bursts — read by visually identifying a waveform. ASSR and ABR are complementary objective tests of hearing.

A disorder in which sound is detected by the cochlea but the signal is not transmitted normally to the brain. ASSR cannot diagnose ANSD; near-normal ASSR thresholds have been reported in ANSD with no recordable ABR.

An auditory evoked potential produced by rapid, periodic stimulation — typically a continuous modulated tone — whose amplitude and phase stay essentially constant over time. Used to estimate hearing thresholds objectively.

The hearing threshold obtained by behavioural audiometry — the standard against which an ASSR estimate is compared. ASSR thresholds overestimate hearing loss relative to behavioural thresholds.

The audiometric frequency of the tone being tested. The carrier determines which region of the cochlea is activated; it is analogous to the test frequency in pure-tone audiometry. The standard set is 500, 1000, 2000, and 4000 Hz.

A frequency-specific value subtracted from a recorded ASSR threshold to estimate the behavioural threshold. An alternative is a regression formula. Correction is device- and population-specific.

A statistical test used to decide whether an ASSR is present. The frequency bin at the modulation frequency is compared with neighbouring noise bins, producing a p-value tested against a chosen significance level (commonly p < 0.05).

An abnormally rapid growth of response amplitude with stimulus intensity above threshold, seen in impaired ears. Recruitment is part of why severe losses give tighter ASSR threshold estimates.

A stimulus combining amplitude and frequency modulation. Mixed modulation generally produces a larger response than AM or FM alone, which is why it is often preferred.

The rate at which the carrier tone is varied. The modulation frequency relates to the neural generators of the recorded response — roughly 40 Hz is weighted toward the midbrain and cortex, while 80–90 Hz is brainstem-weighted.

The neural structures whose summed activity produces the scalp-recorded ASSR. The contributing generators shift with modulation rate: brainstem nuclei at high rates, midbrain and cortex at lower rates.

The level of ongoing EEG and muscle noise against which an ASSR must be detected. A response is typically tens of nanovolts; a settled adult's residual noise may be on the order of tens of nanovolts too — hence the need for statistical detection.

A family of detection methods that test whether the phase of the response is consistent across recording segments. Comparisons have found phase-coherence and F-test approaches detect responses with broadly similar efficiency.

The property that a carrier tone excites a restricted region of the basilar membrane. Place specificity is what allows several carriers to be tested at once and read as separate cochlear places — the basis of an estimated audiogram.

The additional spectral energy introduced on either side of a carrier tone whenever it is modulated. Deeper or faster modulation spreads this energy further, eroding the frequency specificity the carrier was chosen for.

The rules that decide when to end a recording channel: a response is declared present once significance is sustained, absent once the noise floor is low enough that a real response would have been seen, and a maximum time caps the test.