Chapter 12: Spatial Orientation and the Vestibular System

Question 1

Vestibular organs

Answer 1

Set of 5 organs
— 3 semicircular canals
— 2 otolith organs

Located in each inner ear.

Sense head motion and head orientation with respect to gravity

Also called the “vestibular labyrinth” or the “vestibular system”

An often overlooked sense:
The vestibular “sixth sense”
Evolutionarily very old

Question 2

The vestibular organs help us in many ways:
Provide a sense of spatial
Allow for the vestibulo-ocular reflex
Stabilizes visual input by counter rotating the eyes to compensate for head movement

Answer 2

Provide a sense of spatial orientation, consisting of

• Linear motion
• Angular motion
• Tilt

Allows for the Vestibulo-Ocular Reflex:
Stabilizes visual input by counter rotating the eyes to compensate for head movement

Question 3

vestibulo-ocular reflex

Answer 3

Stabilizes visual input by counter rotating the eyes to compensate for head movement

Question 4

Semicircular canals

Answer 4

The 3 toroidal tubes in the vestibular system that sense angular acceleration, a change in angular velocity

Question 5

Otolith organs

Answer 5

The mechanical structures in the vestibular system that sense both linear acceleration and gravity.

Source of our sense of linear velocity and gravity

Question 6

The vestibular organs do not respond to constant velocity

Answer 6

They only respond to changes in velocity, aka acceleration

Question 7

Push-pull symmetry

Answer 7

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

Question 8

Coding of direction in the semicircular canals

Answer 8

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

Question 9

Hair cell responses

Answer 9

In the absence of stimulation, hair cells release neurotransmitter at a constant rate

When hair cell bundles bend, change in hair cell voltage is proportional to the amount of deflection

Bending toward tallest stereocilia: Depolarization

Bending away from tallest stereocilia: Hyperpolarization

Hair cells increase firing to rotation in one direction and decrease firing to rotation in the opposite direction

Question 10

Semicircular canals

Answer 10

Each one is about 3/4 of a toroid (donut) shape, measuring 15mm long and 1.5mm in diameter

Canals are filled with a fluid called perilymph

A second, smaller toroid is found inside the larger toroid, measuring 0.3mm in diameter

Formed by a membrane filled with fluid called endolymph

Cross section of each canal swells substantially near where the canals join the vestibule: Ampulla

Question 11

Ampulla

Answer 11

Cross section of each canal swells substantially near where the canals join the vestibule

Question 12

Semicircular canals (cont’d)

Answer 12

Within the endolymph space of each ampulla is the crista

When the head rotates, the inertia of the endolymph causes it to lag behind, leading to tiny deflections of the hair cells

Question 13

Cristae

Answer 13

The specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla

Each crista has about 7000 hair cells, associated supporting cells, and nerve fibers

Cilia of hair cells project into jellylike cupula which forms an elastic dam extending to the opposite ampulla wall, with endolymph on both sides of dam.

Question 14

Semicircular canal dynamics

Answer 14

Canal afferent neurons are sensitive to back and forth rotations of the head, as well

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

Question 15

Otolith

Answer 15

Otolith organs sense acceleration and tilt

There are 2 otolith organs in each ear:
Utricle: Contains about 30,000 hair cells
Saccule: Contains about 16,000 hair cells

Each organ contains a macula: A specialized detector of linear acceleration and gravity

Question 16

Utricle

Answer 16

Contains about 30,000 hair cells

Question 17

Saccule

Answer 17

Contains about 16,000 hair cells

Question 18

macula

Answer 18

Each Otolith organ contains a macula:
A specialized detector of linear acceleration and gravity.

Each macula is roughly planar and sensitive primarily to shear forces

Question 19

Hair cells

Answer 19

encased in a gelatinous structure that contains calcium carbonate crystals called otoconia (“ear stones” in Greek)

Question 20

Coding of amplitude in the otolith organs

Answer 20

Larger accelerations (or larger gravitational shear forces) move the 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

Question 21

3 experimental paradigms are typically used to investigate spatial orientation perception:

Answer 21

Threshold estimation:
What is the minimum motion needed to correctly perceive motion direction?

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

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.

Question 22

Threshold estimation:

Answer 22

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

Question 23

Magnitude estimation:

Answer 23

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

Question 24

Matching:

Answer 24

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.

Question 25

Rotation perception

Answer 25

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

Question 26

Vestibulo-ocular reflexes (VORs):

Answer 26

Counter-rotating the eyes to counteract head movements and maintain fixation on a target

VORs are accomplished by 6 oculomotor muscles that rotate the eyeball

Question 27

Angular VOR:

Answer 27

The most well-studied VOR

Example: When the head turns to the left, the eyeballs are rotated to the right to partially counteract this motion

Question 28

Torsional eye movements:

Answer 28

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

Question 29

“The Spins”

Answer 29

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.

Question 30

Visual–vestibular integration

Answer 30

Vection: An illusory sense of self motion produced when you are not, in fact, moving

Example: The feeling of flying while watching an IMAX movie

Example: 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

Sometimes a mismatch between visual and vestibular information can cause motion sickness

Question 31

Since the vestibular system has such a widespread influence, what happens when it fails?

Answer 31

Possible problems:

Spatial disorientation
Imbalance
Distorted vision unless head is held perfectly still
Motion sickness
Cognitive problems