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)