Physiology of the Reflex
What the stapedius contraction actually does, why the reflex appears in both ears at once, and what — and what not — it protects against.
The admittance change
Admittance is a measure of how readily acoustic energy flows into the middle ear. When the stapedius contracts, it stiffens the ossicular chain. A stiffer system accepts less energy, so admittance falls. The immittance meter, holding a probe tone in the ear canal, records this fall as a brief downward deflection that begins shortly after the eliciting sound starts and recovers after it stops[2].
The deflection is small — a criterion change of just 0.02 mmho is enough to count as a reflex — but it is reliable and repeatable, and its onset and offset track the eliciting stimulus closely.
Use the simulator below to see the admittance dip. Raising the stimulus level above the reflex threshold deepens the response; lowering it below threshold abolishes it.
Drag the on and off markers to change when the eliciting tone starts and stops.
The consensual reflex
Stimulating one ear produces a reflex in both middle ears simultaneously. This bilateral, or consensual, character follows directly from the anatomy: signals from one cochlea reach the facial nuclei on both sides of the brainstem through the crossed and uncrossed pathways[1].
Clinically, the consensual reflex is what allows the test to be run in two configurations. In the ipsilateral configuration the stimulus and probe are in the same ear; in the contralateral configuration the stimulus is in one ear and the probe in the other. Recording both gives the four-cell grid that underpins pattern interpretation.
The protective role and its limits
The reflex has long been described as protective: by stiffening the chain it attenuates the energy transmitted to the cochlea. But the protection is real only within strict limits[1]. The attenuation is greatest for low-frequency sound and modest for high frequencies. And the reflex has a latency of tens of milliseconds — far too slow to guard against a sudden impulsive sound such as a gunshot.
The reflex is therefore better understood as a slow gain-control for sustained loud sound — perhaps helping to reduce the masking of speech by one’s own voice — than as a shield against acoustic trauma.