The Vestibular System

Fundamentals of the Vestibular System

The vestibular system is a crucial sense, contrary to its brief coverage in textbooks (approximately 370 words). It plays a significant role in various functions, making it a fascinating area of research.

Overview of Lecture Topics

  1. Fundamentals:

    • Anatomy

    • Functions

    • Brain regions

    • Reflexes

  2. Research methodologies for the vestibular system

  3. Higher-level functions:

    • Bodily self-consciousness

    • Self-motion perception

    • Gravity perception

  4. Clinical aspects:

    • Conditions involving vestibular dysfunction

    • Symptoms experienced by patients

Anatomy of the Vestibular System

The vestibular system is located in the inner ear, adjacent to the cochlea. The speaker brought a model to show what it looks like more closely.

  • The inner ear contains the vestibular system and the cochlea (responsible for hearing).

  • The vestibular system resides deep inside the ear. The speaker showed a preserved human vestibular labyrinth and cochlea.

  • There are two vestibular labyrinths, one on each side of the head.

Each labyrinth includes:

  • Three semicircular canals

  • Two otolith organs

Semicircular Canals

Semicircular canals detect angular acceleration of the head, such as when nodding or shaking the head.

  • The anterior semicircular canal detects pitch movements (nodding).

  • The posterior semicircular canal detects roll movements (ear to shoulder).

  • The horizontal semicircular canal detects yaw movements (shaking head left and right).

The semicircular canals function based on fluid movement. Fluid inside the canals shifts when the head rotates, deflecting cristae filled with hair cells.

These hair cells generate signals to the brain, indicating the head's movement direction and acceleration. Cristae detect changes in acceleration; constant speed rotation may feel like stillness because the fluid catches up with the head's motion.

Otolith Organs

The otolith organs detect linear acceleration of the head and gravity like forwards, backwards, left, right, up, and down movements.

  • The utricle detects movements in the horizontal plane.

  • The saccule detects movements in the vertical plane and gravity.

The otoliths are considered an old sensory system (possibly over 500 million years old), though this is debated, since some say that touch is the oldest sensory system.

Otolith organs operate differently than semicircular canals.

  • They use crystals of calcium carbonate called otoconia.

  • These crystals sit on a gel-like layer encasing hair cells.

  • Head movements cause the otoconia to shift, deflecting the gel layer and triggering hair cells.

  • This process signals to the brain how the head moves relative to gravity.

Brain Projections and Regions

The vestibular labyrinths project to numerous brain regions.

  1. Signals first go to the vestibular nerve.

  2. Then to the cerebellum and vestibular nuclei in the medulla and pons.

  3. Vestibular nuclei send signals to:

    • Spinal cord

    • Eye muscles

    • Cerebellum (additional projections)

    • Thalamus (then to the cortex and hippocampus)

Unlike other senses with dedicated processing regions (e.g., occipital lobe for vision), the vestibular system lacks a single, exclusive processing area:

  • Vestibular stimulation activates projections across the entire brain and cortex.

  • Core regions exist in the parietal lobe, including the parietoinsular vestibular cortex (PIVC) in monkeys.

  • In humans, the parietal operculum is considered the equivalent of the PIVC.

The vestibular system interacts with many different senses.

Vestibular Reflexes

Vestibulo-Ocular Reflex (VOR)

The vestibulo-ocular reflex (VOR) stabilizes gaze during head movements. When the head moves, the vestibular system signals the eyes to move in the opposite direction, maintaining stable vision.

  • VOR involves a three-neuron arc from the vestibular labyrinth to the vestibular nuclei, interneurons, and eye muscles.

  • Dysfunction of the VOR leads to oscillopsia, causing the world to appear unstable and jumpy during head movements

Vestibulo-Spinal Reflex

The vestibulospinal reflex stabilizes body posture in response to head movements, maintaining the center of gravity.

  • This reflex involves the vestibular nuclei projecting to the spinal cord (specifically the lateral vestibulospinal tract).

  • It causes extensor effects on the side toward the head tilt and flexor effects on the opposite side.

Vestibulo-Colic Reflex

The vestibulocolic reflex stabilizes the head in response to unexpected body movements. Neck muscles activate to keep the head steady.

  • Neurons involved are suppressed during active head movements.

  • This reflex is evident in birds, where the head remains stable despite body movement.

Researching the Vestibular System

Studying the vestibular system presents challenges:

  • The vestibular system is always active which means there is no option to turn it off in an ethical way to study it.

  • Motion is essential but difficult to study in neuroimaging.

Instead of turning it off, modulation of vestibular activity must happen, changing activity instead of switching it off, while maintaining experimental control.

Turntables or Motion Platforms

These devices rotate participants at predetermined accelerations in specific planes.

  • This is a passive vestibular stimulation, differing from active movement.

For example, Zurich University uses a rotating chair to induce the Coriolis effect to study possible effects astronauts might feel in training.

Motion platforms can investigate:

  • Responses to rotation in general

  • Eye movements

  • Effects of rotation on specific tasks

Example Study: The Mental Number Line

Researchers investigated if physical motion affects number processing.

  • Participants moved left, right, up, down, forwards, and backwards while generating random numbers.

  • Leftward and downward movements produced smaller numbers, whereas rightward and upward movements produced bigger numbers.

  • The study was extended to see if moving along the mental number line influenced the perception of movement.

Findings from both studies suggest an interaction between mental representation of numbers and processing of space and physical movements.

Caloric Vestibular Stimulation (CVS)

Caloric vestibular stimulation uses temperature to modulate fluid movement in the semicircular canals, primarily stimulating the horizontal canal.

  • Cold stimulation involves water at 0-30 degrees Celsius.

  • Warm stimulation involves water at 44-50 degrees Celsius.

Water irrigation as a method injects water into the ear canal. Cold stimulation causes the eyes to move away from the stimulated ear, and warm stimulation causes the eyes to move toward the stimulated ear. Mnemonic: COWS (Cold Opposite, Warm Same).

Challenges of CVS include varying onset and offset, aftereffects, and limited stimulation periods, as well as side effects like motion sickness and dizziness.

Clinically, caloric vestibular stimulation is used to detect brain stem function.

Example Study: Body Schema Modification

Researchers explored if vestibular stimulation could modify body schema using caloric vestibular stimulation.

  • Participants pointed at landmarks on their hands during CVS or sham stimulation.

  • During CVS, the perceived size of the hand was elongated (wider and longer).

  • This shows that body perception changes depending on senses and signals.

Galvanic Vestibular Stimulation (GVS)

Galvanic vestibular stimulation uses electrical currents to directly modulate the activity of the vestibular nerve. Electrodes are placed on the mastoid bone behind the ears.

  • A binaural bipolar setup involves a positive electrode (anode) that hyperpolarizes and decreases nerve activity and a negative electrode (cathode) that depolarizes and increases nerve activity.

  • Participants feel as if they are rolling toward the cathode.

GVS is gaining popularity outside the lab and useful for researchers to change vestibular activity.

However, GVS parameters are well-controlled, responses vary widely among individuals (related to head size, bone thickness, skin sensitivity, and vestibular sensitivity).

Example Demonstration: Remote-Controlled Human

GVS electrodes were attached to a person, and a remote control was used to steer their movements by sending signals to their vestibular nerve.

Example Study: Perception of Verticality

Researchers investigated how GVS modulated the perception of verticality.

  • Participants performed visual, haptic, and postural vertical tasks during and after GVS.

  • Results showed a bias toward the anode during GVS for visual and haptic tasks.

  • This method is also used for studying the perception of gravity.

Higher Level Functions of the Vestibular System

Because of vast brain projections, the vestibular system is integrated with different senses, influencing how we perceive the world. Other than balance, the vestibular system could play a role in a massive range of cognitive and perceptual functions.

Bodily Self-Consciousness

Bodily self-consciousness includes body ownership, first-person perspective, self-location, body schema, and sense of agency.

  • Vestibular signals influence body schema, body ownership, and self-location.

Body Schema

Body schema refers to the size and shape of the body. So for example, changing size with the help of caloric stimulation. Specifically for those with vestibular conditions tend to also have issues with their body schema.

  • Patients with vestibular damage may report elongated feet or swollen necks.

  • There is also evidence suggests that patients with anorexia might have a greater sensitivity to vestibular cues.

  • There also might be phantom limb sensations in amputees modulated from applying vestibular stimulations.

The role of the vestibular system in detecting gravity means considering otoliths in also detecting linear acceleration and gravity.

Body Ownership

Body ownership is the idea that my body belongs to me. For example, the rubber hand illusion. Vestibular Stimulation can decrease ownership and can even decrease the ownership of a fake hand

We can use virtual reality to look into the full body illusion to get people to feel as if they're embodying an avatar or mannequin.

Interesting that we feel greater ownership in the mannequin but less about the fake hand. But what these exactly indicate is unknown as a question for future research.

Self Location

Self-location is the idea of where I may be located in space, which is thought to be the most fundamental part of self.

  • When something goes wrong with this location of self, this can cause out of body experience. These experiences can also be induced with the help of virtual reality.

  • Those with vestibular difficulties tend to have high occurrences of out of body experiences.

Neuroimaging studies have also found a massive amount of overlap between those that process vestibular information and areas that have out of body experiences.

The idea is that these signals (tactile, proprioceptive, and visual cues) can be integrated and therefore be integrated to construct this experience, so if integration breaks down this can cause a breakdown of these senses.

Self Motion Perception

More reliable signals are given a higher weighting, however visual vestibular integration doesn't entirely follow the same trajectory and we estimate we give the vestibular cues more reliability.

The region in the brain that plays a role in this processing is the Dorsal Middle Superior Temporal Region (MST)

Can cause cyber sickness (a condition that causes people to have sickness when in virtual reality).

Can use GVS to match with visual vestibular cues to prevent sensory conflict. There are cases where randomly deliver the patterns so that the brain weights the signal less.

Issue with not weighting as much is the after effects, where this means changes to coordination and altered reflexes could occur. Potentially there can be less sensitivity to vestibular clues even.

Gravity Perception

Gravity is a signal around and doesn't change as a reference for the brain.

  • Downward acceleration of 9.8 meters per second squared caused by the mass of the earth

There can be different perceptions and ways gravity shows up.

  • Verticality can be another, ex. Uber effect is a phenomenon where holding something in your hand while tilted to vertical you may be off.

  • Another is Proprioception, where joints usually have an upward bias to calculate for gravitational force.

  • Behavior can also be a factor. If you have reduced gravity or supine, there's reduced exploration as there may not be a way to account for the gravitation signal anymore.

Microgravity is where the calcium crystals don't sit on the membrane anymore and float around because otoliths can’t signal where gravity is which leads to motion sickness.

Under lunar gravity, we can't drive even if we tried as Apollo astronauts didn't have that feeling under lunar gravity.

Clinical Factors

Vestibular Disorders

Vestibular disorders are classified as:

  • Peripheral, referring to vestibular labyrinths or nerves

  • Central, referring to the central nervous system

Symptoms of peripheral vestibular conditions include oscillopsia, instability, or imbalance, and vertigo. Some of the most common include:

Benign paroxysmal positional vertigo (BPPV)

  • The otaconia escapes and is located inside the semicircular canal, where it disrupts fluid and causes horrible symptoms.

  • Causes recurring vertigo.

  • Is easily curable most of the time with physical therapy called the Epley Remover, which is similar to a water puzzle where you try to physically move the otaconia

Meniere's Disease

  • Best guess for root cause is too much endolymph fluid in the inner ear.

  • Causes damaging effects to vestibular nerve can cause vertigo, nausea, and hearing loss.

Vestibular Neuritis

  • Inflammation of the vestibular nerve caused by viral infection.

  • Extremely similar to labyrinthitis but labyrinthitis affects cochlea; inflammation in all these regions

  • Causes vertigo, imbalance, and nausea.

Central Vestibular Conditions

  • Regions impacting include vestibular nuclei and the midbrain as a source of the issue.

  • Not only includes vertigo but also disorientation and perceptual disturbances

Most specifically includes:

Vestibular Migraine

  • Migraine that affects the vestibular system.

  • It is related from the Trigeminal nerve to vestibular networks.

Symptoms often include: Photophobia, phonophobia, and visual aura.

Stroke

  • Strokes come from artery blockages to certain areas for the inner ear.

  • Depending on where the stroke happen symptoms range from vertigo, nystagmus, disturbance to gait.

Vestibular Dysfunction

  • Dysfunction is when you can't get a diagnosis or something is generally wrong causing disruptions.

  • Can have impacts greater than just vestibular symptoms

Symptoms of patients can vary but the most common include difficulty concentrating, attention, blur vision, and out of body experiences.