Performing the Test
Naming conventions, the choice of eliciting stimulus, how the threshold is found, the response criterion, and the reflex-decay procedure.
Naming: probe ear versus stimulus ear
Two ears are involved in every measurement, and confusing them is the commonest source of error in interpretation. By convention, the reflex is named for the probe ear — the ear in which the admittance change is recorded — not the ear that receives the eliciting sound.
In the ipsilateral condition the probe and stimulus are in the same ear. In the contralateralcondition the stimulus is delivered to one ear and the probe records from the other. A “right contralateral reflex” therefore means the probe is in the right ear and the stimulus in the left.
Choosing the stimulus
The reflex can be elicited with pure tones — conventionally 500, 1000, 2000 and 4000 Hz — or with broadband noise. The two behave differently. Broadband noise elicits the reflex at a level roughly 20 dB lower than a pure tone, because the auditory system integrates energy across frequency[2].
For tonal stimuli the normal acoustic reflex threshold lies roughly between 70 and 100 dB HL. The exact value depends on frequency and on the individual, which is why the pattern across conditions usually matters more than any single absolute number.
The response criterion
A reflex is recorded when the eliciting sound produces a deflection in admittance of at least a criterion magnitude — conventionally 0.02 mmho. The reflex threshold is then defined as the lowest stimulus level that reliably produces a criterion response, usually bracketed in 5 dB steps.
Anything smaller than the criterion, or a deflection that cannot be repeated, is reported as no response. A clean, repeatable, time-locked deflection at and above threshold is the signature of a present reflex.
Reflex decay
Reflex decay testing asks a different question: not whether a reflex can be elicited, but whether it can be sustained. A tone is presented continuously for 10 seconds at 10 dB above the reflex threshold (10 dB SL), and the amplitude of the response is watched.
A normal reflex is well maintained. A positive (abnormal) result is one in which the amplitude falls to 50% or less of its initial value within the 10-second hold[7]. Crucially, decay is tested only at 500 and 1000 Hz: even normal ears show appreciable decay at 2000 Hz and above, so testing there would generate false-positive results.