[ST:NB] W06 - Visual Integration

visual integration

the stimulus for the vestibular system is acceleration
- has to be a above a threshold to activate the cupola or the otoconial mass
- to drive a signal from the vestibular apparatus

slow head movements generate a weak signal
- so visual input from the midbrain
  - from an area that responds to “optic flow”
  - is integrated with vestibular information in the vestibular nucleus
“optic flow” is not a perception
- it is a part of the visual processing that happens unconsciously
- it tells the brain about the direction of motion and head movement
the optic flow comes from the mid-brain
- nothing to do with visual cortex
- it has nothing to do with perception
  - we are unaware of it
- info goes to the same neurons in the vestibular nuclei
  - that receive info from the inner ear
so the vestibular nucleus neuron gets info
- from the ear
- input from the midbrain
- the inputs cannot be differentiated consciously
- creates optical illusions sometimes

the vestibular system by itself helps in balance and direction
the other important function is to help steadying the gaze
gaze = (head + eyes) orientation
- majority of a gaze involves the eye
- so gaze can be approximated to the motion of the eye
  - within a limited point-of-view
  - the eyes can look in a different direction from the head gaze
when walking, ones head bops up and down
- but the visual world doesn’t bop around
- this is because of the VOR (Vestibular Ocular Reflex)
- this is an optical stabilization mechanism
  - it is fast
  - because the vestibular reaction is instantaneous
  - information gets right to the vestibular nucleus
  - and immediately to the motor neurons that drive the eye position
if the head itself is moving, the eyes can still focus on a stationary object
- this is because of the VOR
the focus isn’t necessarily held constant when the object is moving
- this is because the visual focussing mechanism is very slow
that is why there is not a Visual Ocular Reflex
- it is a Vestibular Ocular System

the vestibular system is a slave to the motor system
- vestibular signals goes to motor neurons instantaneously
- this is to support the VOR
the input is coming from the ear
- then it comes into an area in the hindbrain, the vestibular nuclei
- from there, those vestibular nuclear neurons project to motor neurons
- motor neurons that control the position of the eye
there are 6 muscles that control the position of the eye
- they are called extra-ocular muscles
for horizontal controls, there are two motor neurons
- motor neuron that pulls the eyeball away from the nose (abducting)
- motor neuron that pulls the eyeball towards the nose (adducting)
there are just two synapses between the vestibular apparatus and the horizontal ocular motor neurons
- 2 synapse reflex (du-synaptic reflex)
- there aren’t that many, this is one of the few

post-rotatory nystagmus
- after rotating for 30-60 s,
  - the eyes continue to beat in the direction of rotation
- this is called a beating nystagmus
the horizontal canal is excited, acting the on the optic motor neurons
- gradual VOR occurs in the direction of the rotation
  - and a fast reset
- after a sufficiently long duration of rotation
  - usually 30-60s
- the nystagmus lasts for about 30s
the world moves without actual head movement
- it is hard to fix a gaze on something
- during the nystagmus
this is a good sign of brain health

VOR is a reflex
- they are automatic
- they are modulatable, i.e. they aren’t fixed for everything
the amount the VOR activates depends on the distance of the object from the eyes
- VOR is lesser for far objects
- VOR is more for near objects
VOR modulation amount is automatic
- depends on the flocculus in the cerebellum
- the gain of modulation is changed on the 2-path synapse
  - by the flocculus
- no conscious intervention is necessary for VOR gain modulation

consider VOR beyond eye movement
focus fixation and switching has to be handling both
- head position and eye position have to be compensated for each other
- to result in a “complete” and composite gaze control
ocular motor field: the peripheral and central nervous system
- makes eye movements work
the state of eye movements say something about the health of many regions of the nervous system
- eye movements are often different in people with and without neuropsychiatric disorder
- eye movements are used to describe a person’s character

saccades are reflex eye movements
- example:
  - when reading, like a typewriter
    - the eye reverts back to the beginning of the line at the left
- they can have a velocity of 300-400º/s
  - if the saccade lasted for a second
  - the eye would rotate around a whole circle
target appearing to saccade onset:
- 200 ms (1\5 s)
- pulse sets in to get to step
saccade is ballistic:
- once a target is set, there is no going back
- healthy people when looking at other faces gather information from the eyes and mouth
  - making a triangle
  - occasionally going around the circumference of the face
- under certain disorders, people don’t look at faces this same
for big shift, saccades can be around 5º-10º
micro-saccades are shorter, i.e. < 5º
- critical to making sure that the visual world does not fade
- from how the eye works, if the eye doesn’t move in time continuously,
  - edge detection fails
  - vision fades into nothing
- micro-saccades make sure that eyes move constantly so vision doesn’t fade into nothingness
  - the vision perception system ensures smooth images are perceived
- the VOR with the micro-saccades works continuously help enable steady gaze

saccades are organized in the brain stem
horizontal gaze shift is the simplest saccade
- look to the right
- look to the left
the pons has the horizontal gaze center
- this is a bunch of neurons
- can produce a pattern input to neurons
- that results in a horizontal gaze shift
a neuron that is going to send an excitatory input
- directly to the motor neuron
  - that excites the abducting muscle on the same side
- of both eyes
  - so there is branching of the signal
example: when looking to the right
- the right lateral rectus (muscle on the right of the eye) contracts
- a heavily myelinated connection from the pons to the midbrain carries the message the results in the left medial rectus contracting

internuclear-ophtalmoplegia

a part of the saccade circuit doesn’t work as nature intended
- with the intent to shift one’s gaze towards one side
  - only one eye actually shift gaze
  - the other stays in its old position

there are two major places in the brain stem
- where information leaves the system to go to the eyes
- they are:
  - horizontal gaze center (pons)
  - vertical gaze center (midbrain)
vergence (midbrain)
- the motion of moving the eye pair to midline
  - typically close field
- this does not depends on the circuit between the midbrain and pons
so different dissociations stem from different types of movements
- becomes obvious in different types of eye control disorders
frontal eye fields are part of the forebrain
- is critical for volitional eye movements
superior colliculus is part of midbrain
- for following fast moving objects
- for orienting to sounds
- like a frog jumping to catch a fly
  - visual input to movement in space translation