Intense, practical, in-depth review of clinical visual neurology. Emphasis on CNS control of extra-ocular muscles (EOM) in vestibular-ocular and optokinetic

Intense, practical, in-depth review of clinical visual neurology. Emphasis on CNS control of extra-ocular muscles (EOM) in vestibular-ocular and optokinetic reflexes.

Yves Sauvé, PhD, Associate Professor of Ophthalmology and Visual SciencesUniversity of Alberta

Calgary Vision Event 2015

Review of visual neurology

Saturday September 19th 2015

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

Peter Keyser: The Joy of Visual Illusions (http://www.yorku.ca/eye/toc.htm)

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

Neuron. 2001 May;30(2):319-33. The prefrontal cortex--an update: time is of the essence. Fuster JM

Our conscious representation of the visual world is ultimately a subjective creation of our brain

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.

There are well-defined brain pathways underlying vision and movement without any awareness

Retinal projections to primary visual centers

- Saccades

- Smooth pursuit movements

- Optokinetic reflex

- Vestibulo-ocular Reflex (VOR)

- Vergence and accommodation

On the menu

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).

Control of eye movements

Five separate anatomical movement systems have been identifiedand each is characterized by a particular type of eye movement.

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).

1) Saccades: "jumps of 0.2 sec. duration, then hold at new position, with mini-saccades

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

2. hill of activity in SC shifts

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

4. activity in nucleus prepositus hypoglossiPPH terminates the phasic part of the saccade and holds the new position

PPH

1. target changes

Generation of “reflex” saccades

Omnidirectional pause neurons are so called because they pause during saccades in a given direction

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.

http://www.optometry.co.uk/uploads/articles/cet-2013/april-5-2013-cet-2.pdf

Optokinetic reflex

Optokinetic reflex

• 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)


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_systemhttp://www.tutis.ca/Senses/L10Balance/L10Balance.swf


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.


•The vestibulo-ocular reflex (VOR) is mediated by brainstem nuclei which also receive inputs from cerebellum and visual centers.

•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).

change63% drop

15 sec

head velocity

firing rate


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 hypothesisThere 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.

Vergence reflexAre you still focusing?

Accommodation reflex

“reflex” saccades

“voluntary” saccades

riMLF: rostral interstitial nucleus of medial longitudinal fasciculus

Reflex versus voluntary (learned)

PPRF: paramedian pontine reticular formation,

PEF: Posterior eye field

FEF: frontal eye field

LGN: lateral geniculate nucleus,

SC: superior colliculus

Eye movements, an overviewhttp://www.tutis.ca/Senses/L11EyeMovements/L11EyeMovements.swf

Tutis Vilis; University of Western Ontario

How are you looking?

file://localhost/Users/yvessauve/Documents/talks and slides/2015 Calgary Sept/L11EyeMovements.swf

• Of course you are all looking great• But HOW are you looking?

Dr. Arthur Prochazka University of Alberta

Dr. Charles BouletBlack Diamond

Tutis VilisUniversity of Western Ontario

http://www.tutis.ca/Senses/index.htmhttp://dvvc.cahttp://www.ualberta.ca/~aprochaz/

Derek Bok

Panchantra