Chapter 12 - Spatial Orientation and the Vestibular System
What part of the vestibular system regulates blood pressure?
Blood pressure is regulated by VESTIBULO-AUTONOMIC RESPONSES
Where do the stereocilia of the hair cells project to?
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
Discuss translation perception during passive translation in the dark. -- What can participants of such a study report CORRECTLY? What does this imply?
*Can report distance traveled correctly. *Velocity of translation motion correctly. The brain's ability to accurately measure distance and velocity means that the brain REMEMBERS and REPLICATES the velocity trajectory. (relative timing... super ch. n.?) *******Think of this to remember: The brain can detect time and distance, so what else can it detect? Velocity! ---Velocity = Distance/Time
Discuss VOR responses in the dark at varied frequencies.
*Same amplitude of head velocity in all conditions *Same time interval in all conditions IN LOW FREQUENCIES: --Lower amplitude of vestibular-ocular reflex IN INTERMEDIATE AND HIGH (that is, 0.5Hz+) --Higher amplitude in vestibular-ocular reflex. There are insignificant changes in amplitude after 0.5Hz (even at the max detectable, 1.0 Hz)
Why do we use sin waves to measure motion trajectories?
-- Fourier analysis: Complex motion trajectories can be broken down into simple sine wave motions at different frequencies. --If we know the responses to single sine wave frequencies, we know a good deal about complex motion perception.
What are HAIR CELLS?
-- Mechanoreceptors that support the stereocilia that transduce mechanical movement in the vestibular labyrinth into neural activity labyrinth = interconnected system canals located in the inner ear
Are semicircular canal afferents sensitive to back and forth rotations of the head?
--Canal afferent neurons are sensitive to back and forth rotations of the head, as well --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
Ménière's Syndrome
--Sudden experience of dizziness, imbalance, and spatial disorientation --Can cause sudden falling down --Can cause repeated vomiting from severe motion sickness --The unpredictability of the attacks can be terrifying for those who suffer from it --Possible treatments: medications, implanted devices, or sometimes removal of the vestibular apparatus itself
Mal de Debarquement Syndrome
--Swaying, rocking, or tilting perceptions felt after spending time on a boat or in the ocean --Aftereffect of adapting to the rocking motion of the ocean --"Getting your sea legs" --Usually goes away after a few hours, but some people experience it continuously, causing problems
What is the resting firing rate of afferent cells in the semicircular canals? What is the benefit of this resting firing rate?
100 spikes/s High firing rate allows canal neurons to code amplitude by DECREASING their firing rate (not just increasing it)
Discuss the gain of VOR. --What is it? --What is the "ideal" gain? --What is the typical gain of horizontal and vertical VOR? --What is the typical gain of rotational VOR? --How does gain of translational VOR change? Why? --Does VOR change in light/dark conditions?
--The "gain" of the VOR is defined as the change in the eye angle divided by the change in the head angle during the head turn. --Ideally the gain of the rotational VOR is 1.0. --The gain of the horizontal and vertical VOR is usually close to 1.0, but the gain of the torsional VOR (rotation around the line of sight) is generally low. --The perception of motion (by the vestibular system) in dark conditions causes a reduction in gain --The gain of the translational VOR has to be adjusted for distance, because of the geometry of motion parallax. When the head translates, the angular direction of near targets changes faster than the angular direction of far targets.
What are yaw rotations? Are humans relatively insensitive to yaw rotations?
-Rotation about the z-axis -Humans are so sensitive to yaw rotation that we can detect movements of less than 1 degree per second -As yaw rotation frequency decreases, it takes faster movement to be detected
What are the three sensory modalities of the spatial orientation? (i.e. which features of sensory information are detected to achieve spatial orientation)
1. Angular Motion 2. Linear Motion 3. Tilt (head orientation with respect to gravity)
What are the three directions for detecting sense of rotation?
1. Roll = rotation around x-axis 2. Pitch = rotation around y-axis 3. Yaw = rotation around z-axis
What are the three experimental paradigms typically used to investigate spatial orientation perception? Hint: TMEM
1. Threshold estimate ---Participants report motion direction; what minimum motion is needed to correctly perceive motion direction 2. Magnitude estimation --Participants report how much (e.g., how many degrees) they think they tilted, rotated, or translated 3. 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
What are the MACULAE of the otolithic organs? What is their shape and why is this conducive to their function?
A specialized detector of linear acceleration; exhibits an oval thickening at the anterior portions of each organ --Roughly planar; shape causes hair cells to be sensitive to primarily shear forces
Describe the discharge rate of an afferent cell in the utricle to: 1. Linear acceleration (tilt) 2. Constant linear (tilt) velocity 3. Linear deceleration (tilt) *Away from kinocilium
Acceleration: Decreased firing with pronounced/sharp decrease Constant: DOES NOT DIP BACK TO BASELINE; REMAINS SUPPRESSED!! Deceleration: Increased firing with pronounced/sharp increase *Baseline firing rate of cells in the utricle is lower than resting firing rate of semicircular hair cells but still substantial
Describe the discharge rate of an afferent cell in the semicircular-canal to 1. Angular rotational acceleration 2. Constant angular velocity 3. Angular rotational deceleration
Acceleration: Increased firing (from elevated rest firing rate); progressive Constant: Baseline (elevated rest firing rate) Deceleration: Decreased firing (below elevated baseline); progressive *See figure
Describe the discharge rate of an afferent cell in the utricle to: 1. Linear acceleration (tilt) 2. Constant linear (tilt) velocity 3. Linear deceleration (tilt) *Towards kinocilium
Acceleration: Increased firing with pronounced/sharp increase Constant: DOES NOT DIP BACK TO BASELINE; REMAINS ELEVATED!! Deceleration: Decreased firing with pronounced/sharp decrease *Baseline firing rate of cells in the utricle is lower than resting firing rate of semicircular hair cells but still substantial
Do hair cells in the semicircular canals sustain stimuli or adapt to stimuli?
Adapt; Constant rotation leads to decreased responding from the canal neurons after a few seconds
Each semicircular canal is maximally sensitive to what kind of movement?
Each canal is maximally sensitive to rotations perpendicular to the canal plane
What is the inner membrane of the semicircular canals filled with?
Endolymph
Semicircular canals are most responsive to rotations of what Hz?
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
What is the gelatinous structure that encases hair cells of the otolith organs?
Hair cells are encased in a gelatinous structure that contains calcium carbonate crystals called OTOCONIA. They allow for stimulation of the hair cells, allowing perception of linear or tilt acceleration. *"ear stones" in Greek
What is PUSH-PULL SYMMETRY as it relates to the vestibular system?
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
De-mystify "the spins" experienced by a drunk person.
High blood alcohol levels cause the cupula to become lighter than the surrounding endolymph. The cupula's temporary "lighter" state causes the vestibular system to perceive input, such as gravity or rotational acceleration, more dramatically than normal.
Discuss tilt perception. --When can humans accurately perceive tilt? --What is the illusion we discussed in class that is associated with tilt perception?
Humans can accurately perceive tilt at angles of 0 degrees and 90 degrees. 0 degrees = upright 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
What are the consequences of the gain being wrong?
If the gain of the VOR is wrong (different from 1)—for example, if eye muscles are weak, or if a person puts on a new pair of eyeglasses—then head movement results in image motion on the retina, resulting in blurred vision. Under such conditions, motor learning adjusts the gain of the VOR to produce more accurate eye motion. This is what is referred to as VOR adaptation. Ethanol consumption can disrupt the VOR, reducing dynamic visual acuity.[9]
How can the three sensory modalities of spatial orientation change?
In AMPLITUDE ---the size of a head movement In DIRECTION ---the line along which one faces or moves, with reference to the point or region toward which one if acing or moving
Otolith organs
Mechanical structures that sense both linear acceleration and gravity
Provide a common example of a Vestibulo-Autonomic response.
Motion Sickness ---Disagreement between the motion and orientation signals provided by the semicircular canals, otolith organs and vision ---Possibly evolutionary response to being poisoned
Patients with vestibular loss actually _____________ for body sway due to the loss of ___________ responses.
Patients with vestibular loss actually OVER-COMPENSATE for body sway due to the loss of VESTIBULO-SPINAL responses (reflexes).
What is the outer part of the semicircular canals filled with?
Perilymph
What are otoconia? What is their purpose?
Small crystals of calcium carbonate in the saccule and utricle of the ear that under the influence of acceleration in a straight line CAUSE STIMULATION OF THE HAIR CELLS by their movement relative to the gelatinous supporting substrate containing the embedded cilia of the hair cells—called also statoconia. --Larger accelerations move the otolithic organ'c otoconia more, which leads to greater deflection of the hair cell bundles --Change in receptor potential is proportional to magnitude of linear acceleration or gravitational shear
What happens when the stereocilia of a hair cell bend away the kinocilium (the tallest stereocilia)?
The hair cell is hyperpolarized (inhibited)
What is the ampulla of the semicircular canals?
Swelling of semicircular canals where the cupula is located. Contains endolymph (inner membrane) where cupula is, but this endolymph IS NOT located in the cupula .
What happens when the stereocilia of a hair cell bend towards the kinocilium (the tallest stereocilia)?
The hair cell is depolarized (excited)
What are Cristae?
The specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla Wikipedia: The crista ampullaris itself is a cone-shaped structure, covered in receptor cells called "hair cells". Covering the crista ampullaris is a gelatinous mass called the cupula. Upon angular acceleration (rotation), the endolymph within the semicircular duct deflects the cupula against the hair cells of the crista ampullaris. The hair cells respond by stimulating neurons that innervate them
What is the coding direction in each of the otolith organs?
Utricular Macula: HORIZONTAL Saccular Macula: VERTICAL (SaVvy) *In part due to anatomical orientation of each organ
Why don't people feel as if they are turning upside down despite experiencing rotational vection? What sort of integration is this evidence for?
The vestibular's system's sense of gravity stops the illusion. Thus, vestibular information is combined with visual information to yield a "consensus" about our sense of spatial orientation
What are the otolith organs?
There are two: 1. Utricle 2. Saccula They sense LINEAR ACCELERATION and TILT
What are the semicircular canals?
Three tubes that sense angular acceleration (a change in angular velocity).
How many cells are in each of the otolith organs?
Utricle: 30,000 Saccule: 16,000
Visual-vestibular integration: What is vection? Give an example of vection.
Vection is the illusory sense of self motion (when you are not actually in motion) Ex: Being stopped in your car at a light next to a semitruck and when the semitruck begins to roll forward, you press the brake because you feel as if you are rolling backwards.
How does vestibular information reach the cortex?
Vestibular information reaches the cortex via thalamo-cortical pathways.
What are VORs?
Vestibulo-ocular reflexes (VORs): --Counter-rotating the eyes to counteract head movements and maintain fixation on a target --VORs are accomplished by six oculomotor muscles that rotate the eyeball
What are 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
How is head rotation perceived in the semicircular canals?
When the head rotates, the inertia of the endolymph causes it to lag behind, leading to tiny deflections of the hair cells
Provide an example of an angular VOR.
When the head turns to the left (i.e. around an axis going from the SKY to the GROUND), the eyeballs are rotated to the right to partially counteract this motion. *The most "well" studied VOR
Without our vestibulo-spinal responses, what couldn't we do? Discuss the correlation of frequency/gain in both health individuals and individuals with vestibular lesions. What stage degrees do different gains occur at?
Without vestibulo-spinal responses, we would be UNABLE TO STAND IN THE DARK. *NOTE: The Vest-Spinal responses are a whole family of reflexes that work together to keep us from falling over **Refer to figure for other answers
When hair cell bundles bend, change in hair cell voltage is proportional to ___________________.
the amount of deflection
How many hair cells does each crista have?
~7000
