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• Vestibular organs: the set of five organs –three semi-circular canals and two otolith organs- located in each inner ear that sense head motion and head orientation with respect to gravity
• Spatial orientation: a sense consisting of three interacting sensory modalities: the sense of linear motion, angular motion and tilt -Deficits of the vestibular organs:
  • Dizziness: any form of perceived spatial disorientation, with or without instability o Vertigo: a sensation of rotation or spinning o Spatial disorientation: any impairment of linear motion, angular motion or tilt o Imbalance
  • Blurred vision: normally the vestibular system helps us see clearly by reflexively rotating the eyeballs in the sockets to compensate for our head rotation
  • Illusory self-motion

Modalities and qualities of spatial orientation

• Spatial orientation is based on three sensory modalities, different modalities because they require different types of stimulation energy:
  • Angular motion: rotational motion like the rotation of a spinning top or swinging saloon doors that rotate back and forth → angular acceleration
  • Linear motion: translational motion  → linear acceleration
  • Tilt: attaining a sloping position like that of the leaning tower of Pisa → gravity

Sensing angular motion, linear motion and tilt

• Two types of vestibular sense organs:
  • Semi-circular canals: sense angular acceleration, which is the rate of change in angular velocity
  • Otolith organs: transduce both linear acceleration, which is a change in linear velocity, and gravity
  • Provide a predominant contribution to your sense of head tilt and to your sense of linear motion

Basic qualities of spatial orientation: amplitude and direction

• Each spatial orientation modality contains two qualities:
  • Amplitude: the size of the head movement (angular velocity, linear acceleration, tilt) o Direction: the line along which one faces or moves, with reference to the point region toward which one is facing or moving

Direction

• Three directions define our sense of angular motion:
  1. Roll angular velocity
  2. Pitch angular velocity, as when you nod yes
  3. Yaw angular velocity, as when you nod no -Three directions define our sense of linear motion:
  4. Stepping forward/backward
  5. Sliding from left to right
  6. Translating up and down
• Each orientation has two tilt directions:
  1. Pitch tilt (forward/backward)
  2. Roll tilt (left/right) fig 12.5 p333

The mammalian vestibular system

• Figure 12.6 page 334
• Neither the semi-circular canals, nor the otolith organs respond to constant velocity, rather they respond to changes in velocity: accelerations

Hair cells: mechanical transducers

• Head motion causes hair cell stereocilia to deflect. Stereocilia deflection causes a change in the hair cell voltage, which alters the NT release, which, in turn, evokes action potentials in those vestibular nerve fibres that have one or more synapses on the hair cell, these afferent neurons carry these action potentials to the brain.
• In the absence of stimulation the hair cells have a negative voltage and release NTs at a constant rate, evoking a constant rate of action potentials in the afferent neurons. Changes in hair cell voltage, called receptor potentials, are proportional to the bending of the hair cell bundles and control the rate at which hair cells release NT to the afferent neurons.
  • When a hair cell bends toward the tallest stereocilia, the hair cell voltage becomes less negative → This depolarization increases the release of NT, causing an increase in the action potential rate → excitation.
  • When the hair cell is bent away from the tallest stereocilia, the hair cell voltage becomes more negative → This hyperpolarization decreases the release of NT, causing a decrease in the action potential rate → inhibition

Semi-circular canals

• Each inner ear has three semi-circular canals: horizontal, anterior and posterior, which are roughly orthogonal to one another
• Ampulla: an expansion of each semi-circular canal duct that includes that canal’s cupula, crista and hair cells (figure 12.8 page 336)
• Crista: any of the specialized detectors of angular motion located in each semi-circular canal in a swelling called the ampulla
• All the hair cells of a semi-circular canal are aligned, thus rotations in one direction yield increases in the receptor potential of all hair cells in that semi-circular canal, as well as concomitant increases in the action potential rates for all neurons that innervate that semicircular canal
• Each semi-circular canal is maximally sensitive to rotations around the axis perpendicular to it, and insensitive to rotations about axes that fall in the plane of that canal

How amplitude is coded in the semi-circular canals

• The vestibular afferent neurons have relatively high spontaneous firing rate (100 action potentials/second) to allow these neurons to decrease the firing rate for rotations in one direction and increase the firing rate for rotations in the opposite direction
• The horizontal canals in both ears, lie roughly in the same plane and thus form one of the three functional pairs. The horizontal canal afferent neurons on the right all increase their firing rate for yaw head turns to the right, and those on the left all decrease their firing rate.
• The mirror symmetry of the semi-circular in the left and right ears yields functional pairs that involve different vertical canals; the maximum sensitivity axis of the anterior canal on one side roughly parallels the maximum sensitivity axis of the posterior canal on the opposite side. So the right anterior and left posterior canals form one pair, as do the right posterior and left anterior canals. These canal pairs work in a push-pull manner like the horizontal pair -Figure 12.10 page 338

Semi-circular canal dynamics

• Oscillatory movement: back- and forth movement that has a constant rhythm o The firing rate of afferent neurons increases and decreases as the angular velocity increases or decreases, this change in firing rate has the same frequency as the sinusoidal stimulus
• The canals are not very good transducers of low-frequency rotations

 

 

Otolith organs

• Two otolith organs: utricle and saccule (figure 12.13 page 342)
• Macula: any of the specialized detectors of linear acceleration and gravity found in each otolith organ; each macula is roughly planar and primarily sensitive to shear forces (parallel to the macular plane), perpendicular forces have little influence on neural response
• Otoconia: tiny calcium carbonate stones in the ear that provide inertial mass for the otolith organs, enabling them to sense gravity and linear acceleration.

o The displacement of the otoconia, which is the result of gravity or linear motion, drags the gelatinous layer, thereby moving the hair cell stereocilia, leading to changes in the hair cell receptor potential, which in turn cause changes in the rate of action potentials in the afferent neurons

How amplitude is coded in the otolith organs

•    Both the utricle and the saccule include a central band called the striola. On opposite sides of the striola, hair cells are oriented in opposite directions. Since the neuronal response arises from synapses to the hair cells, tilts in opposite directions cause opposite changes in firing rate. So a tilt or an acceleration that maximally excites a hair cell and afferent neuron on one side of the striola will maximally inhibit a hair cell and afferent neuron on the opposite side. Larger accelerations move the otolith organ’s otoconia more, which, in turn, leads to greater deflection of the hair cell bundles, which causes larger changes in the hair receptor potentials.

How direction is coded in the otolith organs

• The plane of the utricular macula is horizontal, the plane of the saccular macula is vertical, thus the utricle will be sensitive primarily to horizontal linear accelerations and horizontal gravitational forces, while the saccule is sensitive primarily to vertical linear accelerations and vertical gravitational forces
• Different hair cells respond maximally to different movement directions, with the direction of maximum sensitivity varying systematically across the plane of each macula

Spatial orientation perception

• Threshold estimation: what is the minimum motion required for correctly perceiving the direction we are moved
• Magnitude estimation: verbal reports of how much they tilt, rotate or translate using physical units like rotation in number of degrees
• Matching: align a visual line with perceived earth-vertical

Rotation perception

• If you are spun on a barstool in the dark at a nearly constant velocity, you will initially perceive an angular velocity that is roughly the same as the actual rotation. However if this rotation lasts more than a second or two you will perceive that you are slowing down, and if the rotation continues for more than 60 seconds you will perceive that you are no longer rotating
• Velocity storage: the perception of rotation persists after the afferent signal from the semicircular canals has dissipated
• When you are rotating at a constant velocity, there is little or no hair cell deflection, because the endolymph and cupula are moving together. When the rotation is suddenly halted however, the cupula stops moving quickly but the endolymph has momentum and tends to keep moving. The hair cells are therefor deflected, and the direction of the hair cell response is opposite to the one measured when the constant-velocity rotation began

Thresholds

• For frequencies above 1Hz, direction recognition thresholds are roughly constant; your head has to be moving at a speed of just a little below 1 degree per second → we are very sensitive to rotation
• For frequencies below 0.5 Hz, thresholds increase with decreasing frequency

Translation perception

• When subjects are passively translated short distances while seated in a chair in the dark and then asked, while still seated in the chair, to use a joystick actively move the chair to reproduce the distance they had been passively translated, they do so accurately. Furthermore, they also reproduce the velocity of the passive-motion trajectory → while otolith organs sense linear acceleration, our brains turn this info into a perception of linear velocity
• Humans correctly recognize the direction of translation only when the linear velocity exceeds

3cm/s

Tilt perception

• When roll-tilting your head while looking at a vertical streak of light, the vertical line appears to tilt in the direction opposite to the head tilt
• Static tilt is correctly reported from about 1 degree off vertical in the dark, this sensitivity serves our ability to stand upright

Sensory integration

• Sensory integration: the brain combines signals from multiple different sensory systems

Visual-vestibular integration

• Vection: an illusory sense of self-motion, caused by moving visual cues, when you are not actually moving
• Retinal signals converge with the semi-circular canal signals in the vestibular nuclei, which is the first place in the brain that vestibular info reaches
• In a rotational vection experiment subjects typically experience rotational vection and a simultaneous illusory sensation of tilt that gradually builds up to a relatively constant level. These perceptions are contradictory, since we cannot ben rotating relative to gravity while also maintaining a constant orientation with respect to gravity
• Illusory motion is larger when there are no otolith cues to contradict the visual cues, which means that under normal circumstances info from the vestibular system is combined with the visual info to yield consensus about our sense of spatial orientation

Reflexive vestibular responses

• Vestibulo-ocular reflex (VOR): a short-latency reflex that helps stabilize vision by counterrotating the eyes when the vestibular system senses head movement
• Balance system: then sensory system, neural processes and muscles that contribute to postural control

Vestibulo-ocular responses

• Angular VOR: the compensatory eye rotation evoked by the semi-circular canals when they sense head rotation o When the head rotates to the left, the reflex pathways cause the eye to rotate to the right with respect to the head, so as to compensate for the head turn
• The six ocular muscles are organized in three pairs that rotate the eye in each of the three directions. Muscles are paired to pull in opposite directions. For example, moving the left eye horizontally requires a coordination of the lateral rectus that pulls the eye to the left and the medial rectus that pulls the eye to the right. To make an eye movement to the right, the lateral rectus is inhibited, and the medial rectus is excited, causing the eye to move right
• The most direct neural path for the VOR consists of an arc of three neurons that yields reflexive eye responses with a latency of less than 10 ms (figure 12.20 page 352):

o The afferent neurons: transmit info from the vestibular periphery to the vestibular nuclei, here the afferent neurons synapse on efferent oculomotor neurons in the oculomotor nuclei, these oculomotor neurons synapse with the oculomotor muscles to rotate the eyes with respect to the head

Vestibulo-autonomic responses

• Motion sickness typically results when there is a disagreement between the motion and orientation signals provided by the semi-circular canals, otolith organs and vision

Vestibulo-spinal responses

• The vestibular system measures the movement of the head and sends commands to the postural control system that help reduce the amount of body sway

Spatial orientation cortex

• The visual system is responsive to constant-velocity visual motion, while the vestibular system responds primarily to changes in velocity and is relatively insensitive to constantvelocity motion→ areas of the cortex related to the perception of tilt and self-motion demonstrate a convergence of visual and vestibular info, as well as info from other sensory systems that contribute to spatial orientation

 

Vestibular thalamocortical pathways

• Vestibular info reaches the cortex via thalamocortical pathways: the vestibular info reaches the cortex via the thalamus. Neurons from the vestibular nuclei carry vestibular info to the thalamus, where that info is processed and relayed to the cortex
• The temporo-parieto-insular cortex is involved in spatial orientation perception → this area receives input from both the semi-circular canals and the otolith organs o Lesions to this area result in illusory tilt and/or illusory translation
• Direction cells: neurons in the hippocampal formation that respond to vestibular stimuli; neurons in the vestibular pathway that leads to the hippocampus through the vestibular cortex

Cortical influences

• The areas of the cortex that receive projections from the vestibular system also project back to the vestibular nuclei. The existence of these pathways suggests that feedback from these areas of the cortex respond to vestibular stimulation likely modulates low-level vestibular processing in the brain stem.

When the vestibular system goes bad

Mal de debarquement syndrome

• When you have been on a boat for several hours, you may experience aftereffects of adaptation: the perception of swaying, rocking or tilting
• Mal de debarquement syndrome: the inability to readily readapt, the symptoms of spatial disorientation, imbalance and rocking last for a month or even years in severe cases

Ménière’s syndrome

• Ménière’s syndrome: the sudden experience of dizziness, imbalance and spatial disorientation so severe that you either have to lie down, or you fall down. This severe motion sickness ensues and leads to repeated vomiting. Other symptoms include: an illusory ringing sound (tinnitus), hearing loss and feeling pain or fullness in the ear