Electrocochleography

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Electrocochleography (abbreviated ECochG or ECOG) is a technique of recording electrical potentials generated in the inner ear and auditory nerve in response to sound stimulation, using an electrode placed in the ear canal or tympanic membrane. [1] The test is performed by an otologist or audiologist with specialized training, and is used for detection of elevated inner ear pressure (endolymphatic hydrops) or for the testing and monitoring of inner ear and auditory nerve function during surgery. [2]

Contents

Clinical applications

The most common clinical applications of electrocochleography include:

Cochlear physiology

Human ear anatomy, with the cochlea "uncoiled" showing frequency mapping to different regions of the basilar membrane. Uncoiled cochlea with basilar membrane.png
Human ear anatomy, with the cochlea "uncoiled" showing frequency mapping to different regions of the basilar membrane.
Cross-sectional view of the organ of Corti within the cochlea. The basilar membrane is labeled "basilar fiber." Organ of corti.svg
Cross-sectional view of the organ of Corti within the cochlea. The basilar membrane is labeled "basilar fiber."

The basilar membrane and the hair cells of the cochlea function as a sharply tuned frequency analyzer. [3] Sound is transmitted to the inner ear via vibration of the tympanic membrane, leading to movement of the middle ear bones (malleus, incus, and stapes). Movement of the stapes on the oval window generates a pressure wave in the perilymph within the cochlea, causing the basilar membrane to vibrate. Sounds of different frequencies vibrate different parts of the basilar membrane, and the point of maximal vibration amplitude depends on the sound frequency. [4]

As the basilar membrane vibrates, the hair cells attached to this membrane are rhythmically pushed up against the tectorial membrane, bending the hair cell stereocilia. This opens mechanically gated ion channels on the hair cell, allowing influx of potassium (K+) and calcium (Ca2+) ions. The flow of ions generates an AC current through the hair cell surface, at the same frequency as the acoustic stimulus. This measurable AC voltage is called the cochlear microphonic (CM), which mimics the stimulus. The hair cells function as a transducer, converting the mechanical movement of the basilar membrane into electrical voltage, in a process requiring ATP from the stria vascularis as an energy source.

The depolarized hair cell releases neurotransmitters across a synapse to primary auditory neurons of the spiral ganglion. Upon reaching receptors on the postsynaptic spiral ganglion neurons, the neurotransmitters induce a postsynaptic potential or generator potential in the neuronal projections. When a certain threshold potential is reached, the spiral ganglion neuron fires an action potential, which enters the auditory processing pathway of the brain.

Cochlear potentials

A resting endolymphatic potential of a normal cochlea is + 80 mV. There are at least 3 other potentials generated upon cochlear stimulation:

As described above, the cochlear microphonic (CM) is an alternating current (AC) voltage that mirrors the waveform of the acoustic stimulus. It is dominated by the outer hair cells of the organ of Corti. The magnitude of the recording is dependent on the proximity of the recording electrodes to the hair cells. The CM is proportional to the displacement of the basilar membrane. [4] A fourth potential, the auditory nerve neurophonic, is sometimes dissociated from the CM. The neurophonic represents the neural part (auditory nerve spikes) phased-locked to the stimulus and is similar to the Frequency following response. [5]

The summating potential (SP), first described by Tasaki et al. in 1954, represents the direct current (DC) response of the hair cells as they move in conjunction with the basilar membrane, [6] as well as the DC response from dendritic and axonal potentials of the auditory nerve. [7] The SP is the stimulus-related potential of the cochlea. Although historically it has been the least studied, renewed interest has surfaced due to changes in the SP reported in cases of endolymphatic hydrops or Ménière's disease.

The auditory nerve action potential, also called the compound action potential (CAP), is the most widely studied component in ECochG. The AP represents the summed response of the synchronous firing of the nerve fibers. It also appears as an AC voltage. The first and largest wave (N1) is identical to wave I of auditory brainstem response (ABR). Following this is N2, which is identical to wave II of the ABR. The magnitude of the action potential reflects the number of fibers that are firing. The latency of the AP is measured as the time between the onset and the peak of the N1 wave.

The CAP is considered to have low sensitivity to changes in stimulus polarity, in contrast to the CM which follows the polarity of the stimulation. As a result, researchers often use the sum (or difference) of responses to stimuli of alternating polarity to dissociate the CAP from CM.

Procedure and recording parameters

ECochG can be performed with either invasive or non-invasive electrodes. Invasive electrodes, such as transtympanic (TT) needles, give clearer, more robust electrical responses (with larger amplitudes) since the electrodes are very close to the voltage generators. The needle is placed on the promontory wall of the middle ear and the round window. Non-invasive, or extratympanic (ET), electrodes have the advantage of not causing pain or discomfort to the patient. Unlike with invasive electrodes, there is no need for sedation, anesthesia, or medical supervision. The responses, however, are smaller in magnitude.

Auditory stimuli in the form of broadband clicks 100 microseconds in duration are used. The stimulus polarity can be rarefaction polarity, condensation polarity, or alternating polarity. Signals are recorded from a primary recording (non-inverted) electrode located in the ear canal, tympanic membrane, or promontory (depending on type of electrode used). Reference (inverting) electrodes can be placed on the contralateral earlobe, mastoid, or ear canal.

The signal is processed, including signal amplification (by as much as a factor 100000 for extratympanic electrode recordings), noise filtration, and signal averaging. A band-pass filter from 10 Hz to 1.5 kHz is often used.

Interpretation of results

The CM, SP, and AP are all used in the diagnosis of endolymphatic hydrops and Ménière's disease. In particular, abnormally high SP and a high SP:AP ratio are signs of Ménière's disease. An SP:AP ratio of 0.45 or greater is considered abnormal.

History

The CM was first discovered in 1930 by Ernest Wever and Charles Bray in cats. [8] Wever and Bray mistakenly concluded that this recording was generated by the auditory nerve. They named the discovery the "Wever-Bray effect". Hallowell Davis and A.J. Derbyshire from Harvard replicated the study and concluded that the waves were in fact cochlear origin and not from the auditory nerve. [9]

Fromm et al. were the first investigators to employ the ECochG technique in humans by inserting a wire electrode through the tympanic membrane and recording the CM from the niche of the round window and cochlear promontory. Their first measurement of the CM in humans was in 1935. [10] They also discovered the N1, N2, and N3 waves following the CM, but it was Tasaki who identified these waves as auditory nerve action potentials.

Fisch and Ruben were the first to record the compound action potentials from both the round window and the eighth cranial nerve (CN VIII) in cats and mice. [11] Ruben was also the first person to use CM and AP clinically.

The summating potential, a stimulus-related hair cell potential, was first described by Tasaki and colleagues in 1954. [6] Ernest J. Moore was the first investigator to record the CM from surface electrodes. In 1971, Moore conducted five experiments in which he recorded CM and AP from 38 human subjects using surface electrodes. The purpose of the experiment was to establish the validity of the responses and to develop an artifact-free earphone system. [12] Unfortunately, bulk of his work was never published.

See also

Related Research Articles

<span class="mw-page-title-main">Inner ear</span> Innermost part of the vertebrate ear

The inner ear is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:

<span class="mw-page-title-main">Cochlea</span> Snail-shaped part of inner ear involved in hearing

The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the organ of Corti, the sensory organ of hearing, which is distributed along the partition separating the fluid chambers in the coiled tapered tube of the cochlea.

<span class="mw-page-title-main">Vestibulocochlear nerve</span> Cranial nerve VIII, for hearing and balance

The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the brain. Through olivocochlear fibers, it also transmits motor and modulatory information from the superior olivary complex in the brainstem to the cochlea.

<span class="mw-page-title-main">Basilar membrane</span> Stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes

The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in response to incoming sound waves, which are converted to traveling waves on the basilar membrane.

<span class="mw-page-title-main">Organ of Corti</span> Receptor organ for hearing

The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transduction occurs through vibrations of structures in the inner ear causing displacement of cochlear fluid and movement of hair cells at the organ of Corti to produce electrochemical signals.

<span class="mw-page-title-main">Auditory system</span> Sensory system used for hearing

The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs and the auditory parts of the sensory system.

<span class="mw-page-title-main">Hair cell</span> Auditory sensory receptor nerve cells

Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment.

<span class="mw-page-title-main">Endolymph</span> Inner ear fluid

Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. It is also called Scarpa's fluid, after Antonio Scarpa.

<span class="mw-page-title-main">Audiometry</span> Branch of audiology measuring hearing sensitivity

Audiometry is a branch of audiology and the science of measuring hearing acuity for variations in sound intensity and pitch and for tonal purity, involving thresholds and differing frequencies. Typically, audiometric tests determine a subject's hearing levels with the help of an audiometer, but may also measure ability to discriminate between different sound intensities, recognize pitch, or distinguish speech from background noise. Acoustic reflex and otoacoustic emissions may also be measured. Results of audiometric tests are used to diagnose hearing loss or diseases of the ear, and often make use of an audiogram.

Presbycusis, or age-related hearing loss, is the cumulative effect of aging on hearing. It is a progressive and irreversible bilateral symmetrical age-related sensorineural hearing loss resulting from degeneration of the cochlea or associated structures of the inner ear or auditory nerves. The hearing loss is most marked at higher frequencies. Hearing loss that accumulates with age but is caused by factors other than normal aging is not presbycusis, although differentiating the individual effects of distinct causes of hearing loss can be difficult.

<span class="mw-page-title-main">Volley theory</span>

Volley theory states that groups of neurons of the auditory system respond to a sound by firing action potentials slightly out of phase with one another so that when combined, a greater frequency of sound can be encoded and sent to the brain to be analyzed. The theory was proposed by Ernest Wever and Charles Bray in 1930 as a supplement to the frequency theory of hearing. It was later discovered that this only occurs in response to sounds that are about 500 Hz to 5000 Hz.

The temporal theory of hearing, also called frequency theory or timing theory, states that human perception of sound depends on temporal patterns with which neurons respond to sound in the cochlea. Therefore, in this theory, the pitch of a pure tone is determined by the period of neuron firing patterns—either of single neurons, or groups as described by the volley theory. Temporal theory competes with the place theory of hearing, which instead states that pitch is signaled according to the locations of vibrations along the basilar membrane.

<span class="mw-page-title-main">Tympanic duct</span>

The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of humans. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as vestibular duct.

Computational auditory scene analysis (CASA) is the study of auditory scene analysis by computational means. In essence, CASA systems are "machine listening" systems that aim to separate mixtures of sound sources in the same way that human listeners do. CASA differs from the field of blind signal separation in that it is based on the mechanisms of the human auditory system, and thus uses no more than two microphone recordings of an acoustic environment. It is related to the cocktail party problem.

<span class="mw-page-title-main">Hearing</span> Sensory perception of sound by living organisms

Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is auditory science.

The endocochlear potential is the positive voltage of 80-100mV seen in the cochlear endolymphatic spaces. Within the cochlea the EP varies in the magnitude all along its length. When a sound is presented, the endocochlear potential changes either positive or negative in the endolymph, depending on the stimulus. The change in the potential is called the summating potential.

Bone-conduction auditory brainstem response or BCABR is a type of auditory evoked response that records neural response from EEG with stimulus transmitted through bone conduction.

The frequency following response (FFR), also referred to as frequency following potential (FFP) or envelope following response (EFR), is an evoked potential generated by periodic or nearly-periodic auditory stimuli. Part of the auditory brainstem response (ABR), the FFR reflects sustained neural activity integrated over a population of neural elements: "the brainstem response...can be divided into transient and sustained portions, namely the onset response and the frequency-following response (FFR)". It is often phase-locked to the individual cycles of the stimulus waveform and/or the envelope of the periodic stimuli. It has not been well studied with respect to its clinical utility, although it can be used as part of a test battery for helping to diagnose auditory neuropathy. This may be in conjunction with, or as a replacement for, otoacoustic emissions.

Cochlea is Latin for “snail, shell or screw” and originates from the Greek word κοχλίας kokhlias. The modern definition, the auditory portion of the inner ear, originated in the late 17th century. Within the mammalian cochlea exists the organ of Corti, which contains hair cells that are responsible for translating the vibrations it receives from surrounding fluid-filled ducts into electrical impulses that are sent to the brain to process sound.

Cochlear hydrops is a condition of the inner ear involving a pathological increase of fluid affecting the cochlea. This results in swelling that can lead to hearing loss or changes in hearing perception. It is a form of endolymphatic hydrops and related to Ménière's disease. Cochlear hydrops refers to a case of inner-ear hydrops that only involves auditory symptoms and does not cause vestibular issues.

References

  1. ↑ Gibson, William P. (2017-05-19). "The Clinical Uses of Electrocochleography". Frontiers in Neuroscience. 11: 274. doi: 10.3389/fnins.2017.00274 . ISSN   1662-453X. PMC   5437168 . PMID   28634435.
  2. 1 2 Ferraro, John A. (November 15, 2000). "Clinical Electrocochleography: Overview of Theories, Techniques and Applications". Audiology Online. Retrieved 15 September 2014.
  3. ↑ Kohlloffel LUE (1972). "A study of basilar membrane vibrations III: The basilar membrane frequency response curve in the living guinea pig". Acustica. 27: 82.
  4. 1 2 Eggermont JJ (1974). "Basic Principles for Electrocochleography". Acta Oto-Laryngologica Supplementum. 316: 7–16. doi:10.1080/16512251.1974.11675742. PMID   4525558.
  5. ↑ Snyder RL, Schreiner CE (1984). "The auditory neurophonic: basic properties". Hearing Research. 15 (3): 261–80. doi:10.1016/0378-5955(84)90033-9. PMID   6501114. S2CID   41111768.
  6. 1 2 Tasaki I, et al. (1954). "Exploration of cochlear potentials in guinea pigs with a micro-electrode". Journal of the Acoustical Society of America . 26 (5): 765. Bibcode:1954ASAJ...26..765T. doi:10.1121/1.1907415.
  7. ↑ Lutz BT; et al. (2022). "Neural Contributions to the Cochlear Summating Potential: Spiking and Dendritic Components". JARO. 23 (3): 351–363. doi:10.1007/s10162-022-00842-6. PMC   9085993 . PMID   35254541. S2CID   247252398.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ↑ Wever EG, Bray CW (1930). "Auditory Nerve Impulses". Science. 71 (1834): 215. doi:10.1126/science.71.1834.215. PMID   17818230.
  9. ↑ Moore EJ (1983). Bases of auditory brain-stem evoked responses. Grune & Stratton, Inc.
  10. ↑ Fromm B, et al. (1934–1935). "Studies in the mechanism of the Wever-Bray effect". Acta Oto-Laryngologica. 22 (3): 477–486. doi:10.3109/00016483509118125.
  11. ↑ Fisch UP, Ruben RJ (1962). "Electrical acoustical response to click stimulation after section of the eighth nerve". Acta Oto-Laryngologica. 54 (1–6): 532–42. doi:10.3109/00016486209126971. PMID   13893094.
  12. ↑ Moore EJ (1971). Human cochlear microphonics and auditory nerve action potentials from surface electrodes. Unpublished Ph.D. dissertation, University of Wisconsin. Madison, Wisconsin.
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