specific level (e.g. 134 dB). Recent measurements of the acoustic reflex with a group of 50 subjects found that only 2 of the subjects exhibited any pre-activation of the reflex in the warned (countdown) or volitional control of the eliciting stimulus. An alternative hypothesis for the role of the acoustic reflex is the prevention of auditory masking of high-frequencies by low-frequencies, which are predominant in natural sounds. 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. The acoustic stapedius reflex can also be recorded by means of extratympanic manometry (ETM). The stapedial reflex can be measured with laser Doppler velocimetry. Jones et al. focused a laser on the light reflex of the manubrium in awake human subjects. The amplitude of a 500 Hz probe tone was used to monitor the vibrations of the tympanic membrane. Various elicitors were presented to the subjects: 1000 Hz tone-burst for 0.5 s at 100 dB SPL, recorded .22 caliber gunshot noise with a peak level of 110 dB SPL. The amplitude of the 500 Hz probe tone was reduced in response to the eliciting stimuli. Time constants for the rate of onset and recovery were measured to be about 113 ms for the tone and 60-69 ms for the gunshot recordings. As the stapedius muscle is innervated by the facial nerve, a measurement of the reflex can be used to locate the injury on the nerve. If the injury is distal to the stapedius muscle, the reflex is still functional. A measurement of the reflex can also be used to suggest a retrocochlear lesion (e.g., vestibular schwannoma). The acoustic reflex normally occurs only at relatively high intensities; contraction of middle ear muscles for quieter sounds can indicate ear dysfunction (e.g. tonic tensor tympani syndrome -TTTS). The pathway involved in the acoustic reflex is complex and can involve the ossicular chain (malleus, incus and stapes), the cochlea (organ of hearing), the auditory nerve, brain stem, facial nerve, superior olivary complex, and cochlear nucleus. Consequently, the absence of an acoustic reflex, by itself, may not be conclusive in identifying the source of the problem. See also Tensor tympani Otoacoustic emission Equal-loudness contours Audiometry Hyperacusis Stapedius muscle Tympanometry References Reflexes Auditory system Audiology