Exploring the middle ear function in patients with a cluster of symptoms including tinnitus, hyperacusis, ear fullness and/or pain.

Middle ear muscle (MEM) abnormalities have been proposed to be involved in the development of ear-related symptoms such as tinnitus, hyperacusis, ear fullness, dizziness and/or otalgia. This cluster of symptoms have been called the Tonic Tensor Tympani Syndrome (TTTS) because of the supposed involvement of the tensor tympani muscle (TTM). However, the putative link between MEM dysfunction and the symptoms has not been proven yet and the detailed mechanisms (the causal chain) of TTTS are still elusive. It has been speculated that sudden loud sound (acoustic shock) may impair the functioning of the MEM, specifically the TTM, after an excessive contraction. This would result in inflammatory processes, activation of the trigeminal nerve and a change of the MEMs state into a hypersensitive one, that may be associated to the cluster of symptoms listed above. The goal of this study is to provide further insights into the mechanisms of TTTS. The middle ear function of 11 patients who reported TTTS symptoms has been investigated using either admittancemetry and/or measurement of air pressure in the sealed external auditory canal. While the former method measured the middle ear stiffness the latter provides an estimate of the tympanic membrane displacement. Most patients displayed results consistent with phasic contractions of the TTM (n = 9) and/or Eustachian Tube (ET) dysfunction (n = 6). The MEM contraction or ET dysfunction could be evoked by acoustic stimulation (n = 3), somatic maneuvers (n = 3), or pressure changes in the ear canal (n = 3). Spontaneous TTM contraction (n = 1) or ET opening (n = 1) could also be observed. Finally, voluntary contraction of MEM was also reported (n = 5). On the other hand, tonic contraction of the TTM could not be observed in any patient. The implications of these results for the mechanisms of TTTS are discussed.

Contraction of the stapedius and tensor tympani muscles explored by tympanometry and pressure measurement in the external auditory canal.

It has been suggested that tensor tympani muscle (TTM) contraction may be involved in the development of ear-related pathologies such as tinnitus, hyperacusis and otalgia, called the tonic tensor tympani syndrome (TTTS). However, as there is no precise measure of TTM function under normal and pathological states, its involvement remains speculative. When the TTM or the stapedius muscle (SM) contracts, they both generate an increase of middle ear stiffness that can be measured through middle ear admittance. However, this technique cannot differentiate the contraction between the two muscles. On the other hand, the air pressure measured in a sealed external auditory canal can provide a measure of the eardrum displacement that may be able to differentiate SM from TTM contraction. TTM is attached to the malleus, and its contraction causes a retraction of the eardrum inside the middle ear cavity, while SM can have a small but reversed effect on TTM displacement. To investigate this issue, we compared the middle ear admittance and air pressure in a sealed external ear canal upon auditory stimulation (sMEMC) and voluntary middle ear muscle contraction (vMEMC). In addition, we assessed the perceptual effect of vMEMC, including pitch and loudness matching of the fluttering noise produced by vMEMC and the threshold shifts, were measured. Out of the 14 ears tested, sMEMC was associated with a decrease of admittance in 93% (mean peak average: -0.06 ml, SD:0.04) and an increase of air pressure in 29% of ears (mean peak average: 8.1 Pa, SD:5.1). No decrease in air pressure was found upon sMEMC. For vMEMC (n = 8 ears), decreases were found for both admittance and air pressure in 100% and 88%, with a mean peak average of -0.38 ml, SD: 0.54 and -149 Pa, SD:156, for admittance and pressure respectively. These results suggest that SM and TTM are involved in sMEMC and vMEMC, respectively. In addition, vMEMC was associated with perceptual effects including a low-frequency sound, pitch-matched at ∼30 Hz (>15 dB SL), and a low-frequency hearing loss of at least 10 dB between 20 and 200 Hz. In conclusion, admittance and air pressure recordings provide useful and complementary information on middle ear muscle contraction and can be used to explore the middle ear function.

Active tympanic tuning facilitates sound localization in animals with internally coupled ears.

Earlier studies have reported that numerous vertebrate taxa have skeletal muscle(s) attaching directly, or indirectly, onto the tympanic membrane. The present study links these prior studies by quantitatively modeling the influence of skeletal muscle contraction on tympanic tension, tympanic dampening, and, ultimately, the fundamental frequency. In this way, the efficacy of these tympanic muscles to dynamically alter the sensory response of the vertebrate ear is quantified. Changing the tension modifies the eardrum's fundamental frequency, a key notion in understanding hearing through internally coupled ears (ICE) as used by the majority of terrestrial vertebrates. Tympanic tension can also be modulated by altering the pressure acting on the deep (medial) surface of the tympanum. Herein we use the monitor lizard Varanus as an example to demonstrate how active modulation of the pharyngeal volume permits tuning of an ICE auditory system. The present contribution offers a behaviorally and biologically realistic perspective on the ICE system, by demonstrating how an organism can dynamically alter its morphology to tune the auditory response. Through quantification of the relationships between tympanic surface tension, damping, membrane fundamental frequency, and auditory cavity volume, it can be shown that an ICE system affords a biologically relevant range of tuning.

Audiometric findings with voluntary tensor tympani contraction.

BACKGROUND: Tensor tympani contraction may have a "signature" audiogram. This study demonstrates audiometric findings during voluntary tensor tympani contraction. METHODS: Five volunteers possessing the ability to voluntarily contract their tensor tympani muscles were identified and enrolled. Tensor tympani contraction was confirmed with characteristic tympanometry findings. Study subjects underwent conventional audiometry. Air conduction and bone conduction threshold testing was performed with and without voluntary tensor tympani contraction. MAIN OUTCOME MEASURE: Changes in air conduction and bone conduction thresholds during voluntary tensor tympani contraction. RESULTS: Audiometric results demonstrate a low frequency mixed hearing loss resulting from tensor tympani contraction. Specifically, at 250 Hz, air conduction thresholds increased by 22 dB and bone conduction thresholds increased by 10 dB. CONCLUSIONS: Previous research has demonstrated a low frequency conductive hearing loss in the setting of tensor tympanic contraction. This is the first study to demonstrate a low frequency mixed hearing loss associated with tensor tympani contraction. This finding may aid in the diagnosis of disorders resulting from abnormal tensor tympani function. Tensor tympani contraction should be included on the differential for low frequency mixed hearing loss.

Fetal Tendinous Connection Between the Tensor Tympani and Tensor Veli Palatini Muscles: A Single Digastric Muscle Acting for Morphogenesis of the Cranial Base.

Some researchers contend that in adults the tensor tympani muscle (TT) connects with the tensor veli palatini muscle (TVP) by an intermediate tendon, in disagreement with the other researchers. To resolve this controversy, we examined serial sections of 50 human embryos and fetuses at 6-17 weeks of development. At 6 weeks, in the first pharyngeal arch, a mesenchymal connection was found first to divide a single anlage into the TT and TVP. At and after 7 weeks, the TT was connected continuously with the TVP by a definite tendinous tissue mediolaterally crossing the pharyngotympanic tube. At 11 weeks another fascia was visible covering the cranial and lateral sides of the tube. This "gonial fascia" had two thickened borders: the superior one corresponded to a part of the connecting tendon between the TT and TVP; the inferior one was a fibrous band ending at the os goniale near the lateral end of the TVP. In association with the gonial fascia, the fetal TT and TVP seemed to provide a functional complex. The TT-TVP complex might first help elevate the palatal shelves in association with the developing tongue. Next, the tubal passage, maintained by contraction of the muscle complex, seems to facilitate the removal of loose mesenchymal tissues from the tympanic cavity. Third, the muscle complex most likely determined the final morphology of the pterygoid process. Consequently, despite the controversial morphologies in adults, the TT and TVP seemed to make a single digastric muscle acting for the morphogenesis of the cranial base.

The endoscopic study of human middle ear mucociliary transport.

The mucociliary clearance (MCC) is an important defence mechanism of the middle ear. The mucociliary transport (MCT) is a part of MCC. We measured the duration of MCT and visualised its routes in middle ears of 31 patients (mean age 45 years; range 7-61 years; SD 11.6) with intact tympanic membrane, with ventilated middle ears and without a history of prolonged otitis media. The transition time of indigo carmine dye from the promontory mucosa to the middle ear orifice of the Eustachian tube (ET) was observed with a rigid 30°, 1.7-mm-diameter tympanoscope. The dye took an average of 7 min (range 4.5-15 min; SD 3.4; median 4.5) to reach the ET orifice in 25 (81 %) patients. Three main ciliary pathways were detected: (1) below and parallel to the tensor tympani muscle; (2) downwards, anterior to the round window, and then ascending to the ET; and (3) straight across the promontory.

Voluntary eardrum movement: a marker for tensor tympani contraction?

HYPOTHESIS: Voluntary eardrum movement (VEM) and resultant tympanometric changes reflect tensor tympani (TT) contraction. BACKGROUND: TT contraction has been hypothesized to cause symptoms of aural fullness, tinnitus, clicking, and even vertigo despite the lack of understanding of how it functions or what causes it to contract. Identifying tympanometric changes unique to TT contraction can provide a diagnostic tool for identifying its role in pathologic conditions. METHODS: Various tympanometric measurements were performed on human subjects who could voluntarily move their eardrums. These were compared with similar tympanometric measurements performed on cadaveric temporal bones while manually tensing the TT and stapedius muscles individually. RESULTS: Eight subjects (14 ears) who could cause VEM were identified. Compared with baseline, VEM resulted in significantly decreased middle ear compliance (p < 0.01) and middle ear pressure (p < 0.01) measurements. The compliance changes seen with VEM were larger than those seen with acoustically stimulated stapedius contraction. Finally, the direction of compliance change with VEM was dependent on the pressure applied to the external auditory canal (EAC), with compliance increasing with positive EAC pressure. This was not seen with stapedius contraction. These findings were reproduced using the cadaveric temporal bone model: larger compliance changes with pull on TT as compared with stapedius with neutral EAC probe pressure; change in direction of compliance changes with varying EAC probe pressure with TT pull, not with stapedius pull. CONCLUSION: TT contraction produces distinctive tympanometric findings that can be used to support its abnormal contraction in ears with symptoms compatible with TT syndrome.

Effects of tensor tympani muscle contraction on the middle ear and markers of a contracted muscle.

OBJECTIVES/HYPOTHESIS: Many otologic disorders have been attributed to dysfunction of the tensor tympani muscle, including tinnitus, otalgia, Meniere's disease and sensorineural hearing loss. The objective of this study was to determine adequate stimuli for tensor tympani contraction in humans and determine markers of the hypercontracted state that could be used to detect this process in otologic disease. STUDY DESIGN: Multiple types of studies. METHODS: Studies included 1) measuring middle ear impedance changes in response to orbital puffs of air, facial stroking, and self-vocalization; 2) measuring changes in stapes and eardrum vibrations and middle ear acoustic impedance in response to force loading of the tensor tympani in fresh human cadaveric temporal bones; 3) measuring changes in acoustic impedance in two subjects who could voluntarily contract their tensor tympani, and performing an audiogram with the muscle contracted in one of these subjects; and 4) developing a lumped parameter computer model of the middle ear while simulating various levels of tensor tympani contraction. RESULTS: Orbital jets of air are the most effective stimuli for eliciting tensor tympani contraction. As markers for tensor tympani contraction, all investigations indicate that tensor tympani hypercontraction should result in a low-frequency hearing loss, predominantly conductive, with a decrease in middle ear compliance. CONCLUSIONS: These markers should be searched for in otologic pathology states where the tensor tympani is suspected of being hypercontracted.

De visione, voce et auditu: the contribution of Hieronymous Fabricius to our understanding of tensor tympani function.

STUDY DESIGN: This work reviews the literature concerning the life and scientific contributions of Hieronymous Fabricius (1533-1619). We also analyze the translated Latin text that focuses on the middle ear muscles from a first edition of Fabricius' book entitled 'De Visione, Voce et Auditu' (The Vision, Voice and Hearing, 1600). RESULTS: Hieronymous Fabricius was one of the first scientists to introduce structure-function relationships in anatomic research. Better known for his descriptions of venous valves and human fetal development, his major contribution to otology was his theory on tensor tympani (TT) function. The TT is 1 of 2 middle ear muscles whose function has undergone a long period of speculation since its discovery by Vesalius (1514-1564) and description by Eustachius (1510-1574). Fabricius' theory of TT physiology was described in his treatise 'De Visione, Voce et Auditu' (1600). He wrote that the TT "protected" the tympanic membrane and helped in middle ear ventilation. In the 20th century, the development of the acoustic impedance measuring bridge by Otto Metz (1905-1993) allowed for the first objective measurements of middle ear function. Experiments on human subjects with various ear pathologies confirmed that the stapedius was the dominant sound evoked middle ear muscle. However, Fabricius' original theory on the TT's role in middle ear ventilation persists to this day based on recent physiologic, embryologic, and histologic studies. CONCLUSION: Hieronymous Fabricius was a pioneer in approaching anatomy from a structure-function relationship, and was an active proponent for maximizing the learning environment for students. Fabricius' writings provided the foundation for contemporary theories on the role of the TT, and he proposed those ideas during an era when great strides were being made to increase our understanding of ear anatomy and physiology.

Topography of the chorda tympani nerve and the tensor tympani muscle in carnivores provides a new synapomorphy for Herpestidae (Carnivora, Mammalia).

The topographical relationship of the chorda tympani nerve (chorda tympani) to the tensor tympani muscle in the middle ear of carnivores provides new phylogenetic information. The examination of histological serial sections of 16 carnivore species representing most families revealed two distinct character states concerning the course of the chorda tympani: a hypotensoric state with the nerve running below the insertion tendon of the tensor tympani muscle, and an epitensoric state with the nerve running above the tendon. The shift from the plesiomorphic hypotensoric chorda tympani to the apomorphic epitensoric condition occurred once in carnivore phylogeny: Only in the herpestid species under study does the chorda tympani cross above the tensor tympani muscle. Therefore, we introduce the epitensoric pattern as a new synapomorphy for herpestids. Within the herpestids we find the following structural distinctions: Herpestes javanicus and Galerella sanguinea have a chorda tympani running in a sulcus directly above the insertion of the tensor tympani muscle, whereas in the eusocial herpestid species Suricata suricatta and Mungos mungo the chorda tympani lies far above the insertion of the muscle.

[Tensor veli palatini and tensor tympani muscles: anatomical, functional and symptomatic links]. / Tensores del velo del paladar y del martillo: vínculos anatómicos, funcionales y sintomáticos.

INTRODUCTION AND OBJECTIVES: Temporomandibular disorders are associated with symptoms such as tinnitus, vertigo, sensation of hearing loss, ear fullness and otalgia. The connection and dysfunction of the tensor tympani and tensor veli palatini muscles seems to be associated with the aforementioned symptoms. We seek to demonstrate and explain this connection through the morphometry of these structures. METHODS: We studied 22 paired blocks and 1 left side of human temporal bone. Digital measurements were made of the tensor tympani muscles and stapes. RESULTS: The average length of the stapedial muscle was 5.8 mm SD 0.61, and that of the tensor tympani was 19.69 mm SD 1.07. Anatomical connections were found in all the samples between the tensor veli palatini muscles through a common tendon. CONCLUSIONS: There is a need for an interdisciplinary management between physician and specialized dentist in cases of craniofacial pain.

Short-term habituation of eye-movement responses induced by galvanic vestibular stimulation (GVS) in the alert guinea pig.

In a recent study, we showed that primary afferent neurons innervating all vestibular end organs were sensitive to galvanic vestibular stimulation (GVS) in guinea pigs. In order to determine the three-dimensional character of eye movements induced by GVS, changes in eye position were recorded using digital video oculography during delivery of bilateral GVS ranging in intensity between 20 and 80 microA. Pulses were delivered in repetitive trains in order to also ascertain the involvement of vestibular habituation. At low intensities of GVS (up to 40 microA), maintained changes in eye position were induced toward the anode and away from the cathode. These eye movements were predominantly vertical with some horizontal eye movement and very little or no torsion. At higher intensities of GVS (>40 microA), horizontal nystagmus was initially observed, as well as an overall deviation of the beating field toward the anode. Nystagmus was found to habituate rapidly over successive presentations of GVS, whereas the tonic deviation of the eye remained consistent without any detectable habituation. The direction of eye movements induced by GVS was similar to that observed in humans during trans-mastoidal GVS, and the threshold differences between tonic and phasic components for GVS were also similar to previous human GVS studies. The observed habituation appears to be more specific to the phasic VOR component in quadrupedal animals such as guinea pigs, and this may reflect a considerable emphasis placed on otolithic stimulation in these animals during complex locomotor activities.

Experimental measurement and modeling analysis on mechanical properties of tensor tympani tendon.

In this paper, we report mechanical properties of the tensor tympani tendon of human ear measured from uniaxial tensile, stress relaxation and failure tests. The hyperelastic Ogden model and digital image correlation method were employed to analyze experimental data. The constitutive equation of the tendon was derived through data iteration processes, and Young's modulus was presented as a function of stress. The viscoelastic property of the tendon was described by stress relaxation function and hysteresis. Furthermore, three-dimensional finite element analysis was carried out on five tendon models to investigate relationship between the structure and properties. The dimensions of the tendon were also measured by image processing techniques and presented with statistic significance. The structure and properties of the tensor tympani tendon reported in this study add new data into the study of ear tissue biomechanics.

Neurons in the cochlear nuclei controlling the tensor tympani muscle in the rat: a study using pseudorabies virus.

The middle ear muscle reflex has been implicated in modulation of auditory input and protection of the inner ear from acoustic trauma. However, the identification of neurons in the cochlear nuclei participating in this reflex has not been fully elucidated. In the present study, we injected the retrograde transynaptic tracer pseudorabies virus into single tensor tympani (TT) muscles, and identified transynaptically labeled cochlear nucleus neurons at multiple survival times. Motoneurons controlling TT were located ventral to the ipsilateral motor trigeminal nucleus and extended rostrally towards the medial aspect of the lateral lemniscus. Transynaptically labeled neurons were observed bilaterally in the dorsal and dorso-medial parts of ventral cochlear nuclei as early as 48 h after virus injection, and had morphological features of radiate multipolar cells. After >or=69 h, labeled cells of different types were observed in all cochlear nuclei. At those times, labeling was also detected bilaterally in the medial nucleus of the trapezoid body and periolivary cell groups in the superior olivary complex. Based on the temporal course of viral replication, our data strongly suggest the presence of a direct projection of neurons from the ventral cochlear nuclei bilaterally to the TT motoneuron pool in rats. The influence of neurons in the cochlear nuclei upon TT activity through direct and indirect pathways may account for multifunctional roles of this muscle in auditory functions.

Human efferent adaptation of DPOAEs in the L1,L2 space.

The adaptive properties of distortion product otoacoustic emissions (DPOAEs) at 2f(1)-f2 were investigated in 12 ears of normally hearing adults aged 18-30 years using long-lasting 1-s primary-tone on-times. In this manner, DPOAE adaptation at a single f2 of 1.55 kHz (f2/f1=1.21) was evaluated as a function of the levels of the primary tones in a matrix of L1, L2 settings, which varied from 45 to 80 dB SPL, in 5-dB steps. DPOAEs were elicited under both monaural and binaural stimulus-presentation conditions. Adaptation was defined as the difference in DPOAE levels between the initial 92-ms baseline measure using a standard protocol and one obtained during the final 92 ms of the prolonged 1-s primary-tones. These differences were averaged across subjects to create contour plots of mean adaptation in the L1,L2 space. The 2f(1)-f2 DPOAE revealed consistent regions of suppression (-0.5 dB difference) or enhancement (+0.5 dB difference) with respect to baseline measures within the L(1),L(2) matrix for both acoustic-stimulation conditions. Specifically, 2f(1)-f2 DPOAE suppressions of 1-2 dB occurred for both monaural and binaural presentations, typically at level combinations in which L1>L2. In contrast, larger 2f(1)-f2 DPOAE enhancements of 3-4 dB occurred for only the binaural condition, at primary-tone level combinations where L1

Do the tensor tympani and tensor veli palatini muscles of man form a functional unit? A histochemical investigation of their putative connections.

The discussion among anatomists and otolaryngologists about the muscles originating from the Eustachian tube and the connections between the tensor tympani and tensor veli palatini muscles started in the 1860s. From then on, a considerable number of contradictory hypotheses and data have been presented. However, before discussing whether or not these two muscles form a functional unit, interest should focus on the question of whether it is even possible. The cartilaginous portion of the Eustachian tube with all muscles originating from it, including the whole tensor tympani muscle, was dissected from five perfusion-fixed cadavers and removed in toto. Complete longitudinal serial sections of 10 microm were made in the axis of the tensor tympani muscle. Sections were alternatingly stained according to Cason's and Maskar's techniques. The macroscopic aspect (under the operating microscope) of a tendinous connection between the two muscles under consideration could be proven by the histochemical methods used in all cases. Based on our findings and the literature reviewed we are convinced that the tensor tympani and tensor veli palatini muscles of man constitute a functional unit. This represents an important step forward towards the understanding of the possible functions the tensor tympani muscle serve in man.

Observations on simultaneous perilymphatic motions and cochlear microphonics suppression.

Very low frequencies interfere in the intact cochlea with higher frequencies and suppress these depending on the vibration phase of the low-frequency sound. Physiological functions of the body, mediated, for example, by the eardrum or perilymph coupling with the cerebrospinal fluid, cause a low-frequency pressure modulation of the perilymph, which generates a synchronous perilymphatic motion resulting from the unevenly distributed compliances in the cochlea. This slow streaming causes a displacement of the entire basilar membrane, with as a consequence a postponement of the operating point of the mechanoelectrical transducer as a result of the pressure drop in the helicotrema and the narrow apical cochlear turn. In this contribution, interference phenomena are described, which are caused by spontaneous contractions of the tensor tympani muscle and by respiration-synchronous perilymphatic flow. These two test signals have trapezoidal and triangular impulse functions. In both cases, as suppression pattern of the cochlear microphonics level-time function, the second derivative of the pressure-time function was observed. The suppression is found to lie between 1 and 2 dB. It depends on the level of the suppressed sound and shows a compressive nonlinearity.

Observations on the number, distribution and morphological peculiarities of muscle spindles in the tensor tympani and stapedius muscle of man.

Although the middle ear muscles have been described for the first time more than four hundred years ago their role in modulation and transmission of sound is not yet fully understood. Surprisingly very little is known about proprioceptors in these muscles, especially in man, although this seems to be the key to the understanding of their various functions. Therefore, the question for proprioceptive sensory organs in these muscles is still relevant. The tensor tympani and stapedius muscles of four women who had donated their bodies to our institute were taken. Complete serial sections of these muscles were made which were either impregnated with silver, stained with ferric oxide for acidic polysaccharides or incubated with antibodies against S-100 protein. Thereby four to eight (mean five) muscle spindles distributed along the whole muscle could be detected in the tensor tympani muscles. These spindles contain one to three intrafusal muscle fibres and their length ranges from 140 to 4270 microm (mean 1492.8 microm). Furthermore, in three stapedius muscles one to two (mean 1.7) muscle spindles were found. They were from 350 to 500 microm (mean 482 microm) long and contained only one intrafusal muscle fiber. Regarding the diameter of intrafusal muscle fibers in both, the tensor tympani as well as the stapedius muscle, no difference to extrafusal muscle fibers of these muscles could be detected. The structure of these spindles differs considerably from those found in skeletal muscles. The morphological findings presented strongly suggest that muscle spindles occur regularly in both middle ear muscles. The results presented herein are consistent with clinical findings obtained from electromyographic studies and may help to elucidate all functions the middle ear muscles might serve in man.

An anatomic study of the tensor veli palatini and dilatator tubae muscles in relation to eustachian tube and velar function.

In a gross anatomic study of 20 sides in 16 human head specimens, the tensor veli palatini, the dilatator tubae, and the tensor tympani muscles were studied. The tensor veli palatini was observed to insert onto the anterior one-third of the pterygoid hamulus, whereas the dilatator tubae rounded the middle one-third of the pterygoid hamulus without an insertion. Thus, the dilatator tubae, not the tensor veli palatini, could serve to tense the anterior velum. An insertion from the superior pharyngeal constrictor muscle onto the posterior one-third of the hamulus could provide a curbing function for the dilatator tubae muscle. Adipose tissue, located at the hamulus, could provide lubrication for the tendinous fibers of the dilatator tubae as they round the hamulus. The dilatator tubae was observed to attach to the hook of the eustachian tube and is accepted as the tubal dilator. Observed on 13 of 20 sides in 11 specimens, the bulk of the dilatator tubae remained distinct from the tensor veli palatini despite a connective tissue alliance and intermingling of some muscle fibers. On 5 of 20 sides in 5 specimens, fibers of the dilatator tubae intermingled extensively with the tensor veli palatini. Of the 20 dilatator tubae muscles dissected, 2 were observed to be deficient. The tensor veli palatini was observed to be continuous with the tensor tympani. Full color versions of the figures are available at the following website: http://www.shc.uiowa.edu/papers/tensor/.

Frequency summation observed in the human acoustic reflex.

It is known that the threshold of an acoustically induced middle-ear-muscle (MEM) reflex can be lowered by the simultaneous presentation of a second tone (facilitator), which is presented to the ipsilateral or contralateral ear at a level below the acoustic reflex threshold (ART) of the facilitator itself (Sesterhenn and Breuninger, 1976; Blood and Greenberg, 1981). In the present study, a primary elicitor and a facilitator were presented to the ear contralateral to that used for measurement of the acoustic reflex (AR), and the effects of changing frequencies and sound levels of the facilitator were investigated in human subjects with normal ears. The sound levels of facilitators, which caused a significant reduction of ART for the primary elicitors (facilitation thresholds), showed an asymmetrical pattern as a function of frequency of the facilitators. The facilitation thresholds tended to be lower when a facilitator with a frequency lower than the frequency of the elicitor (1 kHz) was used. In addition, effects of the elicitor on the masked thresholds of the facilitator were examined to observe the possible interaction between elicitor and facilitator from the viewpoint of 'spread of excitation'. The underlying mechanism of summation effects of two tones are discussed based on the possible input mechanism involved in the acoustically induced MEM reflex are.