The acoustic reflex (also known as <!-- in order of usage -->the stapedius reflex, stapedial reflex, auditory reflex, middle-ear-muscle reflex (MEM reflex, MEMR), attenuation reflex, cochleostapedial reflex or intra-aural reflex The stapedius stiffens the ossicular chain by pulling the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the tympanic membrane when it pulls the malleus (hammer) in toward the middle ear. The reflex decreases the transmission of vibrational energy to the cochlea, where it is converted into electrical impulses to be processed by the brain.

Acoustic reflex threshold

The acoustic reflex threshold (ART) is the sound pressure level (SPL) from which a sound stimulus with a given frequency will trigger the acoustic reflex. The ART is a function of sound pressure level and frequency.

People with normal hearing have an acoustic reflex threshold (ART) around 70–100&nbsp;dB SPL. People with conductive hearing loss (i.e., bad transmission in the middle ear) may have a greater or absent acoustic reflex threshold.

The acoustic reflex threshold can be decreased by the simultaneous presentation of a second tone (facilitator). The facilitator tone can be presented to either ear. This facilitation effect tends to be greater when the facilitator tone has a frequency lower than the frequency of the elicitor (i.e. the sound used to trigger the acoustic reflex).

Characteristics and effects

  • For most animals, the acoustic reflex is the contraction of both middle ear muscles: the stapedius and tensor tympani muscles. However, in humans, the acoustic reflex only involves the contraction of the stapedius muscle, not the tensor tympani.
  • The contraction of the stapedius muscle occurs bilaterally in normal ears, no matter which ear was exposed to the loud sound stimulation.
  • The prevalence of bilateral acoustic reflexes in persons 18–30 years old is 85.3% (82.9%, 87.4%) 95th percentile confidence interval N = 3280 and in all persons 74.6% (73.2%, 75.9%) N = 15,106.
  • The acoustic reflex mostly protects against low frequency sounds.
  • When triggered by sounds 20&nbsp;dB above the reflex threshold, the stapedius reflex decreases the intensity of the sound transmitted to the cochlea by around 15&nbsp;dB.
  • The acoustic reflex is also invoked when a person vocalizes. In humans, the vocalization-induced stapedius reflex reduces sound intensities reaching the inner ear by approximately 20&nbsp;decibels. The reflex is triggered in anticipation of the onset of vocalization.

Hypothesized function

The main hypothesized function of the acoustic reflex is the protection of the organ of Corti against excessive stimulation (especially that of the lower frequencies). This protection has been demonstrated both in humans and animals, but with limited effects.

Because of this latency, the acoustic reflex cannot protect against sudden intense noises.

Moreover, the full tension of the stapedius muscle cannot be maintained in response to continued stimulation. Indeed, the tension drops to about 50% of its maximum value after a few seconds.

Measurement

Most of the time, the stapedius reflex is tested with tympanometry. The contraction of the stapedius muscle stiffens the middle-ear, thus decreasing middle-ear admittance; this can be measured thanks to tympanometry.

See also

  • Tensor tympani
  • Otoacoustic emission
  • Equal-loudness contours
  • Audiometry
  • Hyperacusis
  • Stapedius muscle
  • Tympanometry

References