Chapter 12-Spatial Orientation and the Vestibular System

rotation perception

-at first constant rotation is perceived accurately -soon subjects feel like they're slowing down and feel stopped at 30 seconds -when rotation stops, subject feel as if they are rotating in the opposite direction

head rotation and neurons

-canal afferent neurons sensitive to back and forth rotations of head too -greatest sensitivity to rotations at 1 Hz or less (faster rotations would be dangerous) -overall normalized amplitude of the canal neuron response scales with head rotation frequency

acceleration

-change in velocity -derivative of velocity, so linear=change in linear velocity, angular=change in angular velocity

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 -get a balance in info that comes through

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 spikes/s (relatively high, allows for amplitude coding by increase and decrease in firing rates) -changes in firing rate are proportional to angular velocity of the head aligned with the canal the neuron is in

hair cell responses

-in the absence of stimulation, hair cells release neurotransmitters at a constant/base rate -when hair cell bundles bed, 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

coding of amplitude in the otolith organs

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

linear motion

-movements represented in terms of changes in the x, y, and z-axes -sensed when the head accelerates or decelerates in a line -translations

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

spatial orientation cortex

-no real vestibular cortex because of auditory and visual cortices; areas of cortex that respond to vestibular info would not respond to visual input too -vestibular information reaches cortex via thalamo-cortical pathways -areas of cortex that receive projections from the vestibular system also project back to the vestibular nuclei

vestibular senses

-often overlooked -"sixth sense" -evolutionarily very old

saccule

-otolith organ -contains about 16,000 hair cells

utricle

-otolith organ -contains about 30,000 hair cells

patients vs. normal vestibular function

-people with normal vestibular function had response gains near or below 1 (close to tilt of platform) -patients with bilateral vestibular loss often had response gains greater than 1 (body tilt exceeded the platform tilt)

vertigo

-sensation of rotation or spinning -identified by Aristotle, forgotten when the senses were listed elsewhere

vestibular organs

-sense head motion and head orientation with respect to gravity -set of five; 3 semicircular canals, 2 otolith organs -also called vestibular labyrinth/system

coding of direction in the semicircular canals

-three semicircular canals in each ear -each canal oriented in different plane -each canal maximally sensitive to rotations perpendicular to the canal plane

spatial orientation

a sense consisting of three interacting modalities: perception of linear motion, angular motion and tilt

vection

an illusory sense of self motion when you are not, in fact moving -ex: feeling of flying while watching and IMAX movie, being stopped in care next to a moving semi (feel like you're rolling backwards)

hair cell

any cell that has stereocilia for transducing mechanical movement in the vestibular labyrinth into neural activity sent to the brain stem -like those that involve hearing, act as mechanoreceptors in each of the five vestibular organs -head motion causes these stereocilia to deflect, causing a change in hair cell voltage and altering neurotransmitter release

spatial disorientation

any impairment of spacial orientation

coding of direction in the otolith organs

arises in part from the anatomical orientation of the organs

roll

around x-axis -shoulder to shoulder

pitch

around y-axis -up, down

yaw

around z-axis -left, right

otoconia

calcium carbonate crystals in the gelatinous structure encasing hair cells in otolith organs -provide inertial mass, enabling them to sense gravity and linear acceleration

angular motion

can be sensed when rotating head rotates in place

knowledge and expectations

can influence perception of tilt and motion

problems with the vestibular system

can lead to peculiar sensations -spatial disorientation -dizziness -vertigo -imbalance -blurred vision -illusory self-motion **visual info helps, but not as effective w/o vestibular system

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

all movement

creates optic flow

excitation

depolarization by bending towards kinocilium increases firing

visual info

dominates mechanical info for movement perception -adults less affected by visual movement than infants/babies though

endolymph

fluid filling inner membrane of smaller toroid (0.3 mm in diameter) of semicircular canals

perilymph

fluid filling outer part of semicircular canals

tilt

head orientation with respect to gravity

utricular macula

horizontal plane -sensitive to horizontal linear acceleration and gravity

yaw rotation thresholds

humans 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

inhibition

hyperpolarization by bending away from kinocilium decreases firing

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

cupula

jellylike structures that form an elastic dam extending to the opposite ampulla wall, with endolymph on both sides -cilia of hair cells project into this

angular VOR

most well studied VOR -ex: when head turns to left, eyeballs rotate partially to right to counteract this motion

dizziness

nonspecific spacial disorientation

rotational vection

observers looking at a rotating display report this -subjects have illusion of tilt, but do not feel as if they turn upside-down (sensation stopped by vestibular system's sense of gravity, astronauts without gravity feel as if they are tumbling under these circumstances)

perception of linear velocity

our brains mathematical integration of acceleration registered by the otolith organs

equilibrium

our vestibular sense comprised of spatial orientation perception-encompassing our perception of linear motion, angular motion, and tilt-combined with reflexive vestibular responses like posture, vestibulo-ocular reflexes

autonomic nervous system

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

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 thing they tilted, rotated, or translated

x-axis

points forward and backward -+=forward, -=backward

y-axis

points laterally -+=out left ear

z-axis

points vertically -+ is out of the top of the head

vestibulo-autonomic responses

regulate blood pressure

motion sickness

results when there is a disagreement between motion and orientation signals provided by the semicircular canals, otolith organs and vision -could be evolutionary response to being poisoned

spacial orientation

sense comprised of three interacting sensory modalities: our senses of linear motion, angular motion and tilt **most motion requires a combo of all 3

why differing "modalities"?

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

mechanoreceptors

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

amplitude

size (inc. or dec.) of a head movement (e.g. angular velocity, linear acceleration, tilt)

macula

specialized detector of linear acceleration and gravity -in saccule and utricle -roughly planar and sensitive primarily to shear forces

cristae

specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla -each has about 7000 hair cells, associated supporting cells, and nerve fibers

Vestibulo-Ocular Reflex (VOR)

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

Ménière's sydrome

sudden experience of dizziness, imbalance and spatial disorientation -can cause sudden falling down -can cause repeated vomiting from severe motion sickness -possible treatments: medications, implanted devices, or sometimes removal of vestibular apparatus

Mal de Débarquement Syndrome

swaying, rocking, or tilting perceptions felt after spending time on a boat or in the ocean -usually goes away after a few hours, but some people experience it continuously, causing problems

kinocilium

tallest stereocilium

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 (acceleration and tilt) -source of linear velocity and gravity

sensory integration

the process of combining different sensory signals -typically leads to more accurate information than can be obtained form individual senses alone

semicircular canals

three toroidal (3/4 of a donut shape) tubes, 15mm long & 1.5 mm in diameter in the vestibular system that sense angular acceleration, a change in angular velocity -source of angular motion

velocity storage

time course for perceived velocity is slower than time course of habituation for velocity neurons

saccular macula

vertical plane -sensitive to vertical linear acceleration and gravity

tilt perception

very accurate for tilt angles between 0 (upright) and 90 degrees (lying down)

threshold estimation

what is the minimum motion needed to correctly perceive motion 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 -can also reproduce velocity of passive-motion trajectory-implies brain remembers and replicates the velocity trajectory

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

ampulla

where canals join in the vestibule -cross section of each canal that swells substantially

vestibulo-spinal responses

whole family of responses that work together to keep us from falling over -without these, we would be unable to stand up in the dark -patients with vestibular loss actually over compensate for body sway (ex: elderly)