Transcript Sauve CVE 2015 - Calgary Vision Event

Calgary Vision Event 2015
Review of visual neurology
Intense, practical, in-depth review of clinical visual neurology. Emphasis
on CNS control of extra-ocular muscles (EOM) in vestibular-ocular and
optokinetic reflexes.
Saturday September 19th 2015
Yves Sauvé, PhD, Associate Professor of Ophthalmology and Visual Sciences
University of Alberta
The only visually-dependent event in vision is?
Retina circuits
Zele AJ, Cao D. Vision under mesopic and scotopic illumination. Front Psychol 2015 Jan 22;5:1594.
Center surround antagonistic visual receptive fields
Visual receptive field size and visual acuity
Professor David Heeger, NYU
Visual input to the cortex first arrives in V1 and comes from the LGN
Two streams of visual processing
Dorsal (parietal) stream: magnocellular system (magno=large). “Where, when, how” motion,
form, stereopsis.
Pathway: large M-type retinal ganglion cells to magnocellular layers of LGN & visual cortex,
then to middle temporal part of posterior parietal cortex
Ventral (temporal) stream: parvocellular system (parvo = small): “What” (colour, object
recognition).
Pathway: small P-type as well as large M-type retinal ganglion cells to parvocellular layers of
LGN & visual cortex (blobs and inter-blobs), then to V4 in temporal cortex.
We do not see the visual world as it is
Hermann Grid Illusion #1
The intersections of the white "streets" in A are surrounded by more white than
in B. This results in more inhibition from the surround in on-center, off-surround
receptive fields in A than B: therefore A appears more white than B.
A
B
Peter Keyser: The Joy of Visual Illusions (http://www.yorku.ca/eye/toc.htm)
Our conscious representation of the visual world is
ultimately a subjective creation of our brain
Neuron. 2001 May;30(2):319-33. The prefrontal cortex--an update: time is of the essence. Fuster JM
There are well-defined brain pathways underlying
vision and movement without any awareness
Spering M, Carrasco M. Acting without seeing: eye movements reveal visual processing without awareness. Trends Neurosci. 2015
Apr;38(4):247-58. doi:10.1016/j.tins.2015.02.002. Epub 2015 Mar 10. Review.
Retinal projections to primary visual centers
On the menu
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Saccades
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Smooth pursuit movements
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Optokinetic reflex
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Vestibulo-ocular Reflex (VOR)
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Vergence and accommodation
Control of eye movements
Fundamentals
The eyeball must be moved so that the image of the
target falls on the fovea (the part of the retina with the
highest density of photoreceptors, the largest projection
to the visual cortex and therefore the highest visual
acuity). E.g. written characters are only recognized if
foveated.
The extra-ocular muscles (four recti, two obliques) must
be co-activated in specific combinations to move the eye
up/down & left/right.
The eyeball has a low mass and little resistance to rotation
within the eye socket.
Unlike the limbs, the eyeball doesn’t have to resist or
move external loads.
Extra-ocular motor units are the smallest and fastest in the
human body (10-20 muscle fibres per motor axon).
Five separate anatomical movement systems have been identified
and each is characterized by a particular type of eye movement.
1) Saccades: "jumps of 0.2 sec. duration, then hold at new position, with mini-saccades
2) Smooth pursuit movements: slow tracking of visual targets. Cannot be made in
absence of target (e.g. it's impossible to move eyes smoothly around a static scene).
3) Optokinetic reflex: sensory input is visual, eyes fixate a sequence of objects
moving slowly with respect to head, e.g. landscape viewed from car window.
Slow phase (like smooth pursuit) maintains fixation on an object. Fast phase
(saccade) in direction of head motion relative to scene. Slow pursuit
movement in one direction, saccade back is called “nystagmus”. By definition,
direction of nystagmus is that of fast (saccadic) phase.
4) Vestibulo-ocular reflex: response to head rotation mediated by vestibular
apparatus. Fast phase in direction of head movement, slow phase opposite.
5) Vergence: eyes move towards each other to foveate near objects. This is
linked to accommodation (i.e. focussing of the lens) for near and far
vision. Mediated by superior colliculus and cortical eye fields.
Functional neuroanatomy of the 5 eye movement systems
1)
Saccadic system: Visual (striate) cortex
& posterior parietal cortex perceive
target. Supplementary and frontal eye
fields activate saccade generators in
brainstem and superior colliculi (though
the ability to generate saccades
gradually recovers after a complete
lesion of the colliculi). Cerebellum
probably adjusts gain of transmission in
these pathways.
2 & 3) Smooth pursuit & optokinetic
reflex
systems: Visual cortex,
frontal eye fields, pons, cerebellar
floccular lobe, pontine gaze center,
oculomotor neurons.
4. Vestibulo-ocular reflex (VOR) system:
Vestibular apparatus, vestibular
nuclei, abducens nuclei, oculomotor
nuclei.
5. Vergence system: Midbrain area near
oculomotor nucleus
Saccades
Generation of “reflex” saccades
1. target changes
PPH
4. activity in nucleus prepositus
hypoglossi
PPH terminates the phasic part
of the saccade and holds the
new position
3. The pause in firing of the
omnipause neurons removes their
inhibition of neurons in the
paramedian pontine reticular
formation PPRF. These elicit a burst
of activity in the extra-ocular
muscles
Omnidirectional pause
neurons are so called
because they pause during
saccades in a given
direction
2. hill of activity in SC shifts
Smooth pursuit movements
Fukushima K, Fukushima J, Warabi T, Barnes GR. Cognitive processes involved in smooth pursuit eye movements:
behavioral evidence, neural substrate and clinical correlation. Front Syst Neurosci. 2013 Mar 19;7:4.
Optokinetic reflex
http://www.optometry.co.uk/uploads/articles/cet-2013/april-5-2013-cet-2.pdf
Optokinetic reflex
Vestibulo-ocular reflex (VOR)
• Rotation of the head results in
rotation of the eyes at the same
speed, but in the opposite
direction.
• This stabilizes the image on the
retina.
• Mediated by brainstem nuclei
– input from the vestibular system
– output to extra-ocular muscles
Vestibulo-ocular reflex (VOR)
Vestibulo-ocular reflex (VOR)
Vestibular apparatus: the part of the inner ear labyrinth
concerned with detection of head orientation and
movement.
1) semicircular canals (rotational acceleration sensors),
Arranged in 3 mutually perpendicular planes.
Angular acceleration causes endolymph in the
semicircular canals to move, deflecting hair cells in the
ampulla
http://en.wikipedia.org/wiki/Vestibular_system
http://www.tutis.ca/Senses/L10Balance/L10Balance.swf
Vestibulo-ocular reflex (VOR)
During head movements sensory signals from left and right
horizontal semicircular canals are reciprocal. Interneurons
in the brainstem vestibular nuclei take this reciprocity into
account.
Vestibulo-ocular reflex (VOR)
head velocity
•The vestibulo-ocular reflex (VOR) is mediated by
brainstem nuclei which also receive inputs from
cerebellum and visual centers.
firing rate
•In humans, neurons in the vestibular nuclei
respond to a sudden and maintained change in
rotational velocity of the head, with a time
constant of adaptation of 15 s (i.e. the firing rate
initially rapidly changes, then exponentially
returns toward the rest state, reducing the
change by 63% in 15 seconds).
change
63% drop
15 sec
Vestibulo-ocular reflex (VOR): nystagmus in response to head acceleration,
automatically maintains eye fixation, even with eyes closed. Combines with
optokinetic reflex (e.g. subject motionless, visual field moves.... nystagmus; e.g.
watching scenery from car. Optokinetic reflex dominates eye stabilization in slow
head movements (e.g. up to 1 Hz); VOR dominates as head acceleration becomes
more rapid ( >1 Hz).
post-rotatory nystagmus: vestibular nystagmus occurs during acceleration to
constant velocity, then declines over next 15 sec. During deceleration, endolymph
deflects cupula. When rotation ceases, endolymph is stationary in canal, but
cupula now takes another 15 sec. to return to rest position; nystagmus (and
illusions of motion) persist for this time.
VOR can be elicited by caloric stimulation: warm or cold water (37 + 5oC) is
infiltrated into external auditory canal. This causes convection currents in
endolymph, deflecting the vestibular hair cells, leading to vertigo and
nystagmus. The temperature change may also directly activate the vestibular
nerve endings. Used clinically to assess vestibular function, and also, in
extreme form (iced water) as test for brain-death for organ transplant
approval.
Adaptation over 2 - 3 weeks. The sensitivity of the VOR is state-dependent
and tends to decline over days and weeks if the vestibular apparatus is
constantly over-stimulated. Mechanism: presynaptic inhibition of
transmission interneurons. Occurs in Labyrinthitis, Meniere's disease
(gradual destruction of vestibular nerves), after labyrinthectomy, and in
occupational groups such as aircraft & ship personnel, ballerinas, ice skaters
etc. Note that in rapid spins, skaters minimise head motion by fixating for
most of spin, then rapid and equal accel. + decel. (avoiding accumulated
post-rotatory nystagmus).
Reversing prisms experiment: VOR suppressed after 2 weeks, and reversed
after 3-4 weeks. Shows that pathway includes interneurons whose
transmission is adjustable ("plasticity").
Adaptation of VOR: hypothesized mechanisms.
Ito hypothesis
There is an indirect reflex loop from vestibular
nuclei to motoneurons via cerebellar cortex. A
mismatch between head and eye velocity in the
slow phase of nystagmus is signaled by climbing
fibre input which modifies the gain of
transmission through this indirect loop.
Miles-Lisberger hypothesis:
During slow phase of nystagmus, PCs receive
a) a motor (efference) copy of eye velocity signal
b) vestibular input signalling head velocity.
If there is a difference (mismatch), the PCs are
activated. This activity is a “teaching signal” that
alters the gain of transmission of vestibular input
through the vestibular nuclei to the
motoneurons.
Are you still focusing?
Vergence reflex
Accommodation reflex
Reflex versus voluntary (learned)
“reflex” saccades
LGN: lateral geniculate nucleus,
SC: superior colliculus
PPRF: paramedian pontine reticular formation,
riMLF: rostral interstitial nucleus of medial longitudinal fasciculus
“voluntary” saccades
FEF: frontal eye field
PEF: Posterior eye field
Eye movements, an overview
http://www.tutis.ca/Senses/L11EyeMovemen
ts/L11EyeMovements.swf
Tutis Vilis; University of Western Ontario
How are you looking?
• Of course you are all looking great
• But HOW are you looking?
Dr. Arthur Prochazka
University of Alberta
Dr. Charles Boulet
Black Diamond
http://www.ualberta.ca/~aprochaz/
http://dvvc.ca
Tutis Vilis
University of Western Ontario
http://www.tutis.ca/Senses/index.htm
file://localhost/Users/yvessauve/Documents/talks and slides/2015 Calgary Sept/L11EyeMovements.swf
Derek Bok
Panchantra