Elliot Kim
The vestibulo-ocular reflex (VOR) is a reflex that contributes to maintaining clear vision when the head is in motion. It is one of the reflexes of the vestibular system, which is responsible for sensing the position of the head in space and changes in the direction of head movement. Sound and vibration can evoke a short-latency eye movement or sound-evoked vestibulo-ocular reflex (VOR) and an infraorbital surface potential: the ocular vestibular-evoked myogenic potential (OVEMP). Research has been conducted on the sound-evoked vestibulo-ocular reflex in rats and monkeys, and it has been found that sound activation in the extraocular muscles can cause the eye to rotate upward or downward. In humans, the sound-evoked vestibulo-ocular reflex has been studied using subjects with superior semicircular canal dehiscence (SCD) and normal controls, and it has been found that the OVEMP and VOR magnitudes are larger in SCD compared to controls.
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What You'll Learn
Sound and vibration evoke short-latency eye movement
Sound and vibration can evoke short-latency eye movement, also known as the "sound-evoked vestibulo-ocular reflex" (VOR). This phenomenon is often accompanied by an infraorbital surface potential called the "ocular vestibular-evoked myogenic potential" (OVEMP).
The relationship between VOR and OVEMP was examined by measuring the modulation of both responses by gaze and stimulus parameters. The study involved seven subjects with superior semicircular canal dehiscence (SCD) and six controls. The sound-evoked VOR was measured in 3D using scleral search coils, while OVEMPs were recorded simultaneously using surface electromyography.
Results showed that eye movement onset (11.6 ± 0.8 ms) coincided with the OVEMP peak (12.1 ± 0.35 ms). OVEMP and VOR magnitudes were significantly larger in SCD compared to controls, with a 5-15 times increase. OVEMP amplitudes were at their maximum on up-gaze and disappeared on down-gaze, while VOR magnitudes remained unchanged.
When the stimulus type was switched from sound to vibration, OVEMP and VOR changed concordantly: increasing in controls and decreasing in SCD. These findings suggest a strong correlation between sound and vibration stimuli and their respective effects on eye movement and muscle potential.
In summary, sound and vibration can indeed evoke short-latency eye movements, providing valuable insights into the complex relationship between auditory stimuli, eye movements, and underlying physiological processes.
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Ocular Vestibular-Evoked Myogenic Potential (OVEMP)
Sound and vibration can evoke a short-latency eye movement, known as the "sound-evoked vestibulo-ocular reflex" (VOR). This phenomenon is often accompanied by an infraorbital surface potential called the "ocular vestibular-evoked myogenic potential" (OVEMP). OVEMP is a neurophysiological assessment technique that helps determine the function of the inner ear's otolithic organs, specifically the utricle and saccule.
OVEMP testing involves placing electrodes near the eyes to record the function of the utricular system and the superior vestibular nerve pathway. The test is typically performed with the eyes closed, and the results can be influenced by factors such as gaze direction, stimulus type, and frequency.
OVEMP responses are typically larger in the eye contralateral to the stimulus. When recorded during an upgaze, OVEMP amplifies, indicating a potential extraocular muscle response arising from the inferior oblique muscle. This response is distinct from the R1 component of the blink reflex, as it is abolished in vestibulopathy but preserved in facial palsy.
OVEMP and VOR are amplified in individuals with superior canal dehiscence (SCD), making it a useful tool for studying the temporal relationship between eye movement and muscle potential. The combination of OVEMP and cervical VEMP (cVEMP) provides complementary information on otolith pathways to the neck and eyes.
Research has shown that sound-evoked VOR and OVEMP are correlated, with eye movement onset coinciding with the OVEMP peak. Changing the stimulus type from sound to vibration results in concordant changes in both OVEMP and VOR, either increasing in controls or decreasing in SCD.
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Superior semicircular-canal dehiscence (SCD)
SCD induces Superior Canal Dehiscence Syndrome (SCDS), which is characterised by specific sets of hearing and balance symptoms. People with SCDS may experience autophony, where they hear their own voice, pulse, stomach noises, or even eyeball movements abnormally loudly in the affected ear. This is caused by the opening in the superior semicircular canal, allowing sounds from the body to enter the inner ear.
SCDS can affect one or both ears, and symptoms can be constant or intermittent. It can cause dizziness or vertigo due to the dysfunction of the superior semicircular canal. The sound pressure from loud sounds can also cause abnormal movements in the fluid of the inner ear, leading to a sense of motion. These symptoms can be triggered or worsened by activities that change the pressure in the brain or ear, such as coughing, sneezing, or loud noises.
The presence of SCD can be detected using advanced imaging techniques like high-definition coronal CT scans. Treatment options include management techniques and, in some cases, surgery. Surgical repairs aim to plug the opening using bone or connective tissue or resurfacing techniques that cover the opening without completely closing it.
Regarding the impact of sound on the vestibulo-ocular reflex, studies have indeed examined the relationship between sound and the vestibulo-ocular reflex in individuals with SCD. Sound and vibration can evoke short-latency eye movements, known as the sound-evoked vestibulo-ocular reflex (VOR) and the ocular vestibular-evoked myogenic potential (OVEMP). In individuals with SCD, the magnitudes of OVEMP and VOR were found to be significantly larger compared to controls. These findings suggest that sound can have a pronounced effect on eye movements and vestibular function in individuals with SCD.
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Vestibulo-ocular reflex pathways in rats
The vestibulo-ocular reflex (VOR) is a reflex that involves three parts: a peripheral sensory apparatus, semicircular canals, and otolith organs. The semicircular canals detect head rotation due to their sensitivity to angular acceleration, while the otolith organs detect the position of the head relative to gravity and head translation as they are sensitive to linear acceleration. The VOR can be tested using the caloric reflex test, where cold or warm water is poured into the patient's external auditory canal to induce nystagmus.
Research has been conducted on the sound-evoked vestibulo-ocular reflex pathways in rats. One study used air-conducted clicks or tone bursts delivered to the ears of Sprague-Dawley or Long-Evans rats to examine sound-evoked responses of the vestibular nucleus neurons and the abducens neurons. Another study described a technique for accurate long-term monitoring of eye movements in rats using permanently implanted scleral search coils, which can be used to study the VOR.
In terms of sound-evoked responses, studies have shown that sound evokes conjugate horizontal eye movements in both rats and monkeys. However, there are also differences in the sound-evoked responses between the two species. For example, sound evoked disjunctive vertical eye movements in rats, but it evoked conjugate vertical eye movements in one monkey and disjunctive vertical eye movements in another. Additionally, the amplitudes of tone-evoked vertical eye movements in rats are about 20 times larger than in monkeys.
Overall, the research on vestibulo-ocular reflex pathways in rats has provided valuable insights into the neural mechanisms underlying the VOR and its responses to sound stimuli.
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Vestibular disorders and impaired VOR
Vestibular disorders are a group of conditions that affect an individual's sense of balance. These disorders can be broadly categorized into peripheral and central vestibular disorders. Peripheral vestibular disorders (PVD) affect the inner ear or the nerve that carries balance signals to the brain (vestibular nerve). On the other hand, central vestibular disorders (CVD) impact the parts of the brain that process balance signals from the peripheral vestibular system.
Benign paroxysmal positional vertigo (BPPV) is the most common form of acute peripheral vestibular dysfunction. It occurs when tiny crystals (otoconia) in the inner ear become displaced and travel to the semicircular canals, resulting in vertigo, dizziness, and balance issues. Vestibular neuritis, another type of PVD, involves inflammation of the vestibular nerve, leading to severe vertigo, dizziness, balance problems, nausea, and vomiting.
Ménière's disease, a central vestibular disorder, can produce drop attacks caused by a loss of tone in postural muscles mediated by an impaired vestibulospinal reflex. Additionally, conditions such as stroke and demyelinating diseases can lead to central vestibular dysfunction by damaging the protective covering of nerves.
Vestibular disorders can cause a range of symptoms, including dizziness, vertigo, and balance issues. In some cases, individuals may experience episodes of symptoms followed by periods of remission. Triggers for these episodes can include environmental changes, sudden head movements, certain foods and drinks, and lack of sleep.
While vestibular disorders primarily affect balance, they can also impact other aspects of an individual's health and well-being. The specific symptoms and severity can vary depending on the underlying cause and the affected structures within the vestibular system. Treatment options typically involve managing symptoms and may include medications or vestibular rehabilitation therapy.
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Frequently asked questions
The vestibulo-ocular reflex (VOR) involves three parts: a peripheral sensory apparatus, the central nervous system (CNS), and the vestibulospinal reflex (VSR). The VOR helps maintain clear vision during head movement.
Sound and vibration evoke a short-latency eye movement or "sound-evoked vestibulo-ocular reflex" (VOR) and an infraorbital surface potential: the "ocular vestibular-evoked myogenic potential" (OVEMP).
The sound-evoked VOR was measured in 3D using scleral search coils in seven subjects with superior semicircular canal dehiscence (SCD) and six controls. OVEMPs were recorded simultaneously using surface electromyography.
The vestibulo-ocular reflex can be tested using the Halmagyi-Curthoys test, also known as the rapid impulse test, or the caloric reflex test.
The vestibulo-ocular reflex (VOR) plays a critical role in maintaining eye stability during head motion, known as gaze stability. A disrupted or impaired VOR can result in abnormal eye movements and blurred or double vision.
