Chapter 12: Spatial Orientation and the Vestibular System

Rotation Perception

-At first, constant rotation (in the dark) is perceived accurately. -Soon, however, subjects feel as if they are slowing down. -After 30 seconds, they no longer feel as if they are rotating. Time course of habituation for perceived velocity is slower than time course of habituation for velocity neurons: "velocity storage." -When rotation stops, subjects feel as if they are rotating in the opposite direction.

Coding of Amplitude in Semicircular Canals

-In the absence of any rotation, many afferent neurons from the semicircular canals have a resting firing rate of about 100 spikes per second. -This firing rate is high relative to nerve fibers in other sensory systems. -High firing rate allows canal neurons to code amplitude by decreasing their firing rate, as well as increasing it. -Changes in firing rate are proportional to angular velocity of the head aligned with the canal the neuron is in.

Coding of Amplitude in Otolith Organs

-Larger accelerations (or larger gravitational shear forces) move to otolith organ's otoconia more. -This leads to greater deflection of the hair cell bundles. -Change in receptor potential is proportional to magnitude of linear acceleration or gravitational shear.

Semicircular Canal Dynamics

-Neural activity in semicircular canals is sensitive to changes in rotation velocity. -Constant rotation leads to decreased responding from the canal neurons after a few seconds. -Canal afferent neurons are sensitive to back and forth rotations of the head. -Greatest sensitivity to rotations at 1 Hz or less. Faster rotations than 1 Hz would be dangerous. -Firing rate goes up and down as the head rotates back and forth. -The overall normalized amplitude of the canal neuron response scales with head rotation frequency.

Tilt Perception

-We are very accurate when perceiving tilt for angles between 0 degrees (upright) and 90 degrees (lying down). -Illusion: If you roll tilt your head to the left or right while looking at a vertical streak of light, the light appears to tilt in the opposite direction.

Translation Perception

-When people are passively translated in the dark, they are able to use a joystick to reproduce the distance they traveled quite accurately. -Interestingly, they also reproduce the velocity of the passive-motion trajectory. -This implies that the brain remembers and replicates the velocity trajectory. The otolith organs register acceleration, and our brains mathematically integrate the acceleration and turn it into the perception of linear velocity.

spatial orientation perception

3 experimental paradigms used to investigate ____________ _______________ _______________: -Threshold estimation -Magnitude estimation -Matching

Direction in Semicircular Canals

3 semicircular canals in each ear. Each canal is oriented in a different plane. Each canal is maximally sensitive to rotations perpendicular to the canal plane.

Otoconia

A gelatinous structure that contains calcium carbonate crystals that provide inertial mass for the otolith organs, enabling them to sense gravity and linear acceleration. Hair cells are encased in this structure.

Vertigo

A sensation of rotation or spinning.

Spatial Orientation

A sense comprised of 3 interacting sensory modalities: our senses of linear motion, angular motion, and tilt.

Macula

A specialized detector of linear acceleration and gravity. Each otolith organ contains one. Each is roughly planar and sensitive to shear forces.

Saccule

A type of otolith organ. A saclike structure that contains the saccular macula. Contains about 16,000 hair cells.

Utricle

A type of otolith organ. A saclike structure that contains the utricular macula. Contains about 30,000 hair cells.

Vestibulo-Spinal Responses

A whole family of reflexes that work together to keep us from falling over. Without these responses, we would be unable to stand up in the dark. -Comparison of postural responses of subjects with normal vestibular function to responses of patients suffering severe bilateral vestibular loss: gain greater for vestibular-loss patients.

Ampulla

An expansion of each semicircular-canal duct that includes that canal's cupula, crista, and hair cells, where transduction occurs. Cross section of each semicircular canal swells substantially near where the canals join in the vestibule. Within the endolymph space of each ampulla is the crista.

Vection

An illusory sense of self motion produced when you are not in fact moving. Part of visual-vestibular integration. -Ex.: The feeling of flying while watching an IMAX movie. -Ex.: Being stopped in your car at a light next to a semi. The semi begins to roll forward and you press on the brake because you feel as if you are rolling backwards.

Spatial Disorientation

Any impairment of spatial orientation. Most common sensation resulting from problems with the vestibular system.

Coding of Direction in Otolith Organs

Arises in part from the anatomical orientation of the organs. Utricular macula and saccular macula.

Vestibulo-Ocular Reflexes (VORs)

Counter-rotating the eyes to counteract head movements and maintain fixation on a target. Accomplished by 6 oculomotor muscles that rotate the eyeball.

No

Do we have a vestibular cortex? -Areas of cortex respond to vestibular input, but they tend to respond to visual input as well. No need to have cortex for processing vestibular info in isolation if visual info is also available. -Vestibular info reaches cortex via thalamo-cortical pathways. -Areas of cortex that receive projections from the vestibular system also project back to the vestibular nuclei. Knowledge and expectations can influence perception of tilt and motion.

amplitude; direction

Each spatial orientation modality can change in terms of its ________________ and ______________.

Endolymph

Fluid that the inner membrane of the semicircular canals are filled with. Has a high amount of potassium.

Perilymph

Fluid that the outer part of the semicircular canals are filled with.

Cupula

Forms an elastic dam extending to the opposite ampulla wall, with endolymph on both sides of the dam.

hyperpolarization

Hair cells bending away from the kinocilium leads to __________________: lower firing > inhibition

depolarization

Hair cells bending towards the kinocilium leads to _______________: higher firing > excitation.

Push-Pull Symmetry

Hair cells in opposite ears respond in a complementary fashion to each other. When hair cells in the left ear depolarize, those in the analogous structure in the right ear hyperpolarize.

Tilt

Head orientation with respect to gravity.

Utricular Macula

Horizontal plane. Sensitive to horizontal linear acceleration and gravity.

Yaw Rotation Thresholds

Humans are so sensitive to yaw rotation that we can detect movements of less than 1 degree per second. At this rate, it would take 6 minutes to turn completely around. As yaw rotation frequency decreases, it takes faster movement to be detected.

constant

In the absence of stimulation, hair cells release neurotransmitters at a _____________ (base) rate.

The Spins

Ingesting a great deal of alcohol in a short amount of time causes alcohol to quickly enter the bloodstream. The alcohol causes the cupula to become lighter than the surrounding endolymph. This causes the system to become sensitive to gravity in addition to rotational acceleration.

Hair Cells

Mechanoreceptors that support the stereocilia that transduce mechanical movement in the vestibular labyrinth into neural activity sent to the brain stem. -Like the hair cells involved in hearing, vestibular hair cells act as the mechanoreceptors in each of the 5 vestibular organs. -Head motion causes hair cell stereocilia to deflect, causing a change in hair cell voltage and altering neurotransmitter release.

Dizziness

Nonspecific spatial disorientation.

rotational

Observers looking at a rotating display report ____________ vection. Subjects have the illusion of tilt but do not feel as if they turn upside-down. Why don't people feel as if they are turning upside-down? -The vestibular system's sense of gravity stops the illusion. -Astronauts without gravity feels as if they are tumbling under these circumstances.

Matching

Participants are tilted and then orient a line with the direction of gravity. This is done in a dark room with only the line visible to avoid any visual cues to orientation.

Magnitude Estimation

Participants report how much (e.g., how many degrees) they think they tilted, rotated, or translated.

x-axis

Points forward and backward (positive is forward, i.e., in the facing direction).

y-axis

Points laterally (positive is out of the left ear).

z-axis

Points vertically (positive is out of the top of the head).

Problems with Vestibular System

Problems with the vestibular system can lead to peculiar sensations: -Spatial disorientation -Dizziness -Vertigo -Imbalance -Blurred vision -Illusory self-motion -Cognitive problems

Motion Sickness

Results when there is a disagreement between the motion and orientation signals provided by the semicircular canals, otolith organs, and vision. Could be an evolutionary response to being poisoned. Vestibulo-autonomic response.

Roll

Rotation around the x-axis.

Pitch

Rotation around the y-axis.

Yaw

Rotation around the z-axis.

Angular Motion

Rotational motion like the rotation of a spinning top or swinging saloon doors that rotate back and forth. Can be sensed when rotating head rotates in place.

Linear Motion

Sensed when the head accelerates or decelerates in a line. Movements represented in terms of changes in the x-, y-, and z-axes. Any arbitrary linear motion can be represented as a change along these 3 axes.

Mechanoreceptors

Sensory receptors that are responsive to mechanical stimulation (pressure, vibration, movement).

Meniere's Syndrome

Sudden experience of dizziness, imbalance, and spatial disorientation. Can cause sudden falling down and/or repeated vomiting from severe motion sickness. Possible treatments: meds, implanted devices, or sometimes removal of the vestibular apparatus itself.

Mal de Debarquement Syndrome

Swaying, rocking, or tilting perceptions felt after spending time on a boat or in the ocean. Aftereffect of adapting to the rocking motion of the ocean. Usually goes away after a few hours, but some people experience it continuously.

Kinocilium

Tallest stereocilium.

Semicircular Canals

The 3 toroidal tubes in the vestibular system that sense angular acceleration, a change in angular velocity. Source of our sense of angular motion.

Direction

The line along which one faces or moves, with reference to the point or region toward which one is facing or moving.

Otolith Organs

The mechanical structures in the vestibular system that sense both linear acceleration and gravity. Source of our sense of linear velocity and gravity. Sense acceleration and tilt. 2 in each ear: utricle and saccule.

Angular VOR

The most well-studied VOR. When the head turns to the left, the eyeballs are rotated to the right to partially counteract this motion.

Autonomic Nervous System

The part of the nervous system innervating glands, heart, digestive system, etc., and responsible for regulation of many involuntary actions.

Sensory Integration

The process of combining different sensory signals. Typically leads to more accurate information than can be obtained from individual senses alone.

Vestibular Organs

The set of 5 organs--3 semicircular canals and 2 otolith organs--located in each inner ear that sense head motion and head orientation with respect to gravity. -Also called the vestibular labyrinth or vestibular system.

Amplitude

The size (increase or decrease) of a head movement (e.g., angular velocity, linear acceleration, tilt).

Cristae

The specialized detectors of angular motion located in each semicircular canal in the ampulla. Each crista has about 7000 hair cells, associated supporting cells, and nerve fibers. Cilia of hair cells project into jelly-like cupula.

acceleration

The vestibular organs do not respond to constant velocity. They only respond to changes in velocity, aka:

spatial orientation

The vestibular organs provide us with a sense of ____________ ______________ consisting of linear motion, angular motion, and tilt.

True

True or false: Blood pressure is regulated by vestibulo-autonomic responses.

True

True or false: Gravity and acceleration share a deep connection and can be considered equivalent.

True

True or false: Hair cells increase firing to rotation in one direction and decrease firing to rotation in the opposite direction.

True

True or false: Most motions involve a combination of angular motion, linear motion, and tilt.

True

True or false: Patients with vestibular loss overcompensate for body sway.

Saccular Macula

Vertical plane. Sensitive to vertical linear acceleration and gravity.

Visual-Vestibular Integration

Vestibular information is combined with visual information to yield a consensus about our sense of spatial orientation.

Threshold Estimation

What is the minimum motion needed to correctly perceive motion direction?

deflection

When hair cell bundles bend, change in hair voltage is proportional to the amount of _______________. -Bending toward tallest stereocilium (kinocilium): depolarization (higher firing > excitation). -Bending away from the kinocilium: hyperpolarization (lower firing > inhibition).

Torsional Eye Movements

When the head is rolled about the x-axis, the eyeballs can be rotated a few degrees in the opposite direction to compensate.

modalities

Why considered different _____________? -Sensing linear motion, angular motion, and tilt involves different receptors and/or different stimulation energy. Involve different anatomy/pathways.