Perception Ch.12: Spatial orientation and vestibular system

Vestibulo-spinal responses

-a whole fam of reflexes that work together to keep us from falling over-messages are sent to muscles in your legs and back to "adjust" balance and posture for any tilting or falling; messages are sent to the brain to adjust blood pressure to adequately monitor blood (oxygen) supply to the brain, they send signals to your stomach make you nauseous or vomit indicating motion sickness

Rotation perception

-at first, constant rotation is perceived accurately -soon, however, subjects feel as if they are slowing down -after 30 seconds, they are no longer feel as if they are rotating -thus, when rotation stops, subjects feel as if they are rotating in the opposite direction

Vestibulo-autonomic Responses

-autonomic nervous system: the part of the nervous system innervating glands, heart, digestive system, etc, and responsible for regulation of many involuntary actions -blood pressure is regulated by vestibulo-autonomic responses (think about what happens if you suddenly stand up from a prone position and what would you feel if these systems didn't work together)

Suppose you have damage to your vestibular labyrinth, it'll probably affect:

-balance -posture -blood pressure -nausea -dizziness -orientation -vision

Vertigo

-dizziness, is a symptom, not a disease -refers to the sensation of spinning or whirling that occurs as a result of a disturbance in balance (equilibrium) -may be used to describe feelings of dizziness, lightheadedness, faintness, and unsteadiness -about 40% of USA population report having one of these

Push-pull symmetry

-hair cells in opposite ears response 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 -the brain then reads these neural signals from all six pair to help calculate motion

It's tough to stand up straight, what you are asking your brain to do is balance 4 key areas...

-head -torso -thighs -calves all hinged areas, of varying weights, lengths, and sizes, capable of swaying on two small ankle joints.

Yaw (tilt) rotation thresholds

-humans are so sensitive to yaw (tilt) rotation that we can detect movements of less than 1 degree per second (at this rate, it would take 6 mins to turn completely around) -as yaw rotation frequency decreases, it takes faster movement to be detected

Coding of amplitude in the semicircular canals

-in the absence of any rotation, many afferent neurons from the semicircular canals have a resting firing rate of about 100 spike/s (this firing rate is high relative to nerve fibers in other sensory systems) -changes in firing rate are proportional to angular velocity of the head aligned with the canal the neuron is in

Hair cell reponses

-in the absence of stimulation, hair cells release neurotransmitter at a constant rate -its detection of change that gets them firing -when hair cell bundles bend, change in hair cell voltage is proportional to the amount of deflection -hair cells increase firing to rotation in one direction and decrease firing to rotation in the opposite direction

So why does motion sickness make us vomit or feel nauseous?

-it might be an old evolutionary defense against some classes of poisons (if you've been poisoned, you want to get rid of it by vomit before it kills you). normally if you move, your visual system and your vestibular system will both register that fact. if the vestibular system says one thing and the visual system says another, the digestive system will decide it is time to rid the body of a possible cause of the disagreement. -then again, perhaps getting sick is your body's way of telling you to stop whatever you are doing that is causing internal sensory chaos.

Coding of amplitude in the otolith organs

-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

Some disorders of the vestibular system

-meniere's syndrome -mal de debarquement syndrome -vertigo

Linear Motion

-movements represented in terms of changes in the x, y, and z-axes 1. x-axis projects from your nose out the back of your head 2. y-axis projects from each ear 3. z-axis projects straight up from top of your head down through your feet

Meniere's Syndrome

-named after Prosper Meniere, a French physician who first described the syndrome in 1861 -patients experience sudden dizziness, imbalance and spatial disorientation so severe that they either have to lie down or fall down. -that motion sickness ensues so severely that it leads to repeated vomiting -these symptoms occur at any time

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

three directions for sense of ROTATION

-roll (2): rotation around x-axis -pitch (3): rotation around y-axis -yaw (1): rotation around z-axis

Mal de Debarquement Syndrome

-symptoms of spatial disorientation, imbalance, and rocking after disembarking a boat last a month or more -in extreme cases, the symptoms can last for years and can be very debilitating. -why some individuals are unable to readapt to the normal situation of standing on firm ground remains a mystery -"disembarking sickness"

All kinds of visio-spatial illusions

-the leans -graveyard spiral -graveyard spin -coriolis illusion -inversion illusion -head-up illusion -head-down illusion -low altitude illusion -high altitude illusion -vection illusion -black hole illusion

Coding of direction in the semicircular canals

-three 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

Balance is a combo of...

-vision and sight cues -force of gravity -kinesthetics -vestibular system -proprioception

Tilt Perception

-we are very accurate when perceiving tilt for angles b/t 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, but our brains turn this into a perception of linear velocity)

It's the moving head that triggers a response

-when the head rotates, the inertia of the endolymph (fluid) causes it to lag behind, leading to tiny deflections of the hair cells -think of it as a moving head causing the endolymph in these canals to slosh around, displacing the crista and thereby bending the tiny hair cells which in turn fire!

The three spatial orientation modalities

1. Angular motion 2. Linear motion 3. tilt

Coordinate system for classifying direction

1. x-axis: points forward, in the direction the person is facing 2. y-axis: points laterally, out of the person's left ear 3. z-axis: points vertically, out of the top of the head -axes are defined relative to the person, not relative to gravity

Spatial Orientation

A sense comprised of three interacting sensory modalities: linear motion, angular motion, and tilt

Why is it that when you shake your head the object you are starring at doesn't appear to move?

Because the vestibular system helps us see clearly by reflexively rotating the eyeballs in the sockets to compensate for head rotation, thereby helping to keep visual images stable on the retina. -so, the vestibular system works with visual system through the VESTIBULO-OCULAR REFLEX. This is another ex of 2 sensory systems working together for perception.

Bending AWAY from tallest stereocilia:

Hyperpolarization

Dramamine helps with motion sickness

Mechanism of Action: along with its actions as an antagonist at H1 receptors, meclizine also possesses anticholinergic, central nervous system depressant, and local anesthetic effects. dramamine depresses labyrinth excitability and vestibular stimulation and may affect the medullary chemoreceptor trigger zone

In addition to the semicircular cans we have two structures in vestibular system, which are?

The otolith organs (a pair in each hemisphere): -utricule (or utriculus) -saccule (or sacculus) designed to detecting gravity and linear acceleration

Two otolith organs in each ear:

Utricle: contains about 30,000 hair cells Saccule: contains about 16,000 hair cells -each organ contains a macula

Visual-vestibular integration

Vection: an illusory sense of self motion produced when you are not, in fact, moving -induced motion: you are sitting stationary, but the room stars to spin around you...you'll get the feeling as if you are in motion

macula

a specialized detector of linear acceleration and gravity -each macula is roughly planar and sensitive primarily to shear forces

Otolith organs sense

acceleration and tilt

Any arbitrary linear motion can be represented as a change along these three axes, and it really doesn't matter whether you are standing up straight or laying down, these angular movements are...

always calculated with reference to how they project from your head

Relying only on your vision and vestibular sense can get you into...

big time especially in bad weather conditions or night flying. Learning to use the instrument panel-and trusting it even when your "senses" don't agree is even more difficult

tilt

can be sensed when nodding head up and down as if to say "yes"

angular motion

can be sensed when rotating head from side to side as if to say "no"

What is velocity?

changes in rate and direction

Vestibulo-ocular reflexes (VORs)

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

Bending TOWARD tallest (kinocilium) stereocilia:

depolarization

semicircular canals

each one is a fluid filled canal about three-fourths of a toroid (donut) shape, measuring 15 mm long and 1.5 mm in diameter -a second, smaller toroid is found inside the larger toroid, formed by a membrane filled with fluid called endolymph -cross section of each canal swells substantially near where the canals join the vestibule: ampulla -within the endolymph space of each ampulla is the crista

Otoconia ("ear stones" in Greek)

hair cells are encased in a gelatinous structure that contains calcium carbonate crystals

what does a high firing rate allow?

it allows canal neurons to code amplitude by decreasing their firing rate, as well as increasing it

What does space flight do to these systems?

lose complete balance

Translation

movement in a direction WITHOUT rotation or tilting

so after lift off do astronauts "feel" they are moving through space?

no

Motion sickness

results when there is a disagreement between the motion and orientation signals provided by the semicircular canals, otolith organs, and vision (too many conflicting signals).

linear motion

sensed when accelerating or decelerating in a car (it's motion in a specific direction)

Why considered different "modalities"?

sensing linear motion, angular motion, and tilt involves different receptors and/or different stimulation energy

Anterior canal

sensitive to spinning around y-axis (saggital plane) moving your head to look at your belly and then back and up to look at the ceiling, nodding your head for "yes"

horizontal canal

sensitive to spinning left/right (piroutte) around z-axis (transverse plane), shaking your head to indicate "no"

posterior canal

sensitive to spinning on x axis (coronal plane) moving your head in direction of either shoulder

Mechanoreceptors

sensory receptors that are responsive to mechanical stimulation (pressure, vibration, movement) -like the hair cells in hearing, hair cells act as the mechanoreceptors in each of the five vestibular organs -head motion causes hair cell stereocilia to deflect, causing a change in hair cell voltage and altering neurotransmitter release

Vestibular system (aka vestibular labyrinth)

set of 5 organs (three simicircular canals and two otolith organs) located in each inner ear that sense head motion and head orientation with respect to gravity -"sixth sense" that is often overlooked -evolutionarily very old and hasn't changed much over time

Hair cells

support the stereocilia that transduce mechanical movement in the vestibular labyrinth into neural activity sent to the brain stem

Otolith Organs

the mechanical structures in the vestibular system that sense both linear acceleration and gravity (2 in each ear)

Angular VOR

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

Objective vertigo

the perception of movement in surrounding objects

Subjective vertigo

the sensation of movement

Cristae

the specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla -each crista has about 7000 hair cells and associated supporting cells and nerve fibers -cilia of hair cells project into a jellylike cupula that forms an elastic dam extending to the opposite wall of the ampulla, with endolymph on both sides of the dam

Semicircular Canals

the three toroidal (doughnut shape) tubes in the vestibular system that sense angular motion (3 in each ear)

velocity storage

the time course of the perceptual decay is more gradual than that of the semicircular-canal signal sent to the brain -its called velocity storage because the perception of rotation persists after the afferent signal from the semicircular canals has dissipated

Visual system works with...

the vestibular system in the MIDBRAIN (pons)

why don't people feel as if they are turning upside down when looking at a rotating display

the vestibular system's sense of gravity stops the illusion -astronauts without gravity feel as if they are tumbling under these circumstances

Multisensory parieto-insular cortex

this area receives input from both the semicircular canals and the otolith organs. Lesions in this area has patients reporting illusions of translation and illusory tilts. Some rare problems are rotational vertigo (illusory sense of spinning).

vestibular information is combined with visual information b/c

to yield a "consensus" about our sense of spatial orientation

Where is the vestibular cortex

we don't have one! -projections gather at the vestibular nuclei in the medulla -and nerve projections from the five vestibular organs spread out to other major systems (visual, autonomic, somatosensory) providing them with critical information about movement -much of the vestibular information relays through the thalamus which is a key relay station for sensory information

without vestibulo-spinal responses...

we would be unable to stand up in the dark -patients with vestibular loss actually over-compensate for body sway. have you ever seen someone with Parkinson's lean too far in the opposite direction their body sways or moves?

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 -VORs are accomplished by 6 oculomotor muscles that rotate the eyeball