Visual System
Visual pathways and eye movements
Phase 1b lecture
Dr Simon Hickman
Consultant Neurologist
Royal Hallamshire Hospital
Sheffield
Optic nerves and visual pathways
• Basic anatomy of the visual pathways
• Functional approach to visual perception
– Cell specialisation in the optic nerves
– Crossing in the optic chiasm
– Primary visual cortex
– “What” and “where” pathways
• Eye movements
– Types of eye movement
– Horizontal and vertical gaze
Retina
www.arthursclipart.com
Retinal encoding
• 150 million photoreceptors and 1 million nerve
fibres in the optic nerve
• Individual retinal ganglion cells (RGCs) are never
electrically silent
–Action of the horizontal and amacrine cells to allow
inhibition of adjacent areas
• Inputs to ganglion cells originate from
photoreceptors in circumscribed areas of the
retina
–Receptive field - varies in size across the retina
Functional encoding in the optic nerve
• Chromatic sensitivity and spatial summation:
–Parvocellular RGCs
• Low-contrast
• High linear spatial resolution
–Koniocellular RGCs
• Blue-yellow colour opponency
–Magnocellular RGCs
• High-contrast
• Low-resolution
• Motion detection
• Colour blind
Girkin CA, Miller NR. Surv Ophthalmol 2001;45:379-405.
Island of vision
• “…the most helpful mental picture of the visual
field is obtained when we regard it from the
standpoint of visual acuity. We may imagine
the field as an island* of vision surrounded by
a sea of blindness”
• Traquair HM. An Introduction to Clinical Perimetry. London, Henry
Kimpton 1949.
* The idea of the field of vision as an island in a sea of blindness derives
originally from Euclid and from Heliodorus, who thought the field was
cone-shaped with a circular base, outside of which nothing could be seen
Island of vision
• “…The coast-line is somewhat ovoid in shape , and rises steeply so that the
island is surrounded by cliffs vertical at one side, sharply sloping at the
other. Above the cliffs is a sloping plateau which rises more rapidly again
towards the somewhat eccentrically situated summit. This is crowned by a
sharp pinnacle whose sides curve steeply upwards from a narrow base.”
• Fovea occupies 0.01% of the visual field (<2° visual angle), yet 10% of the
retinal ganglion cells (optic nerve fibres) subserve foveal vision
Normal visual field
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cm&part=A3543
Normal visual field is an island of vision
90 degrees temporally to central fixation, 50 degrees
superiorly and nasally, and 60 degrees inferiorly
Visual pathway
Left
Right
Optic nerve head
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Primary visual cortex
Hickman SJ. Neurological visual field defects. Neuro-Ophthalmology 2011;35:242-250.
Optic chiasm
• Functionally (and teleologically) two eyes are better
than one for the exploration of the environment and for
panoramic defence.
• A mechanism for the purpose of fusing the images of
the two eyes and preventing double vision
• Deduced by Sir Isaac
Newton from 1st principles
– Query 15 in:
– Opticks: Or, a Treatise of the
Reflexions, Refractions, Inflexions and
Colours of Light. 1706
Lateral geniculate nucleus
• Synapse of first order
retinal ganglion cell
neurons to 2nd order
neurons
• Magnocellular layers
–1 and 2
• Parvocellular layers
–3 to 6
• Koniocellular layers
–Interspersed
Retinal image:
-Vertically and
horizontally
inverted
Primary Visual Cortex - fMRI
Retinotopic map of the visual world
• Approx. 90% or nerve fibres subserve central 30° of vision
Visual processing
• “We do not see objects as such; we see
shapes, surfaces, contours, and boundaries,
presenting themselves in different
illuminations or contexts, changing
perspective with their movement or ours.
From this complex, shifting visual chaos, we
have to extract invariants that allow us to
infer or hypothesize objecthood.”
• Oliver Sacks. The mind’s eye.
Functional specialisation
• Different attributes of the visual scene are
processed simultaneously, in parallel, but in
anatomically separate parts of the visual
cortex
• Firstly - vision from eye - everything is back
to front and upside down - corrected in brain
Dorsal and Ventral Streams
Dorsal Stream
“Where”
Pathway
Ventral Stream
“What” Pathway
www.arthursclipart.com
• V5/MT (Dorsal occipito-parietal)
– Motion detection
Depth perception
• Brain uses cues to perceive relative size and position of objects
–Familiar size
• Judge distance if know the size of an object
–Occlusion
• If an object is partially occluding another we presume it is in front
–Linear perspective
• Parallel lines converge with distance - convergence implies depth
–Size perspective
• The smaller object will be assumed to be more distant
–Distribution of shadows and illumination
• Patterns of light and dark can give the impression of depth
–Motion parallax
• As we move, objects closer than the object we are looking at seem to move
quickly and in the direction opposite to our own movement, whereas more
distant objects move more slowly and in the same direction as our movement
Attention
• V4
– Colour detection
Colour constancy
• The coloured regions appear rather different, roughly orange and
brown.
–They are the same colour, and in identical immediate surrounds, but the brain
changes its assumption about colour due to the global interpretation of the
surrounding image. Also, the white tiles that are shadowed are the same colour
as the grey tiles outside the shadow.
• Colour constancy and brightness constancy are responsible for the fact
that a familiar object will appear the same colour regardless of the
amount of light or colour of light reflecting from it. An illusion of colour
or contrast difference can be created when the luminosity or colour of
the area surrounding an unfamiliar object is changed.
–The contrast of the object will appear darker against a black field that reflects
less light compared to a white field even though the object itself did not change
in colour. Similarly, the eye will compensate for colour contrast depending on the
colour cast of the surrounding area
http://en.wikipedia.org/wiki/Optical_illusion
Face-selective regions
FFA
FFA
STS
FFA: fusiform face area
OFA: occipital face area
STS: superior temporal sulcus
OFA
FFA
Face recognition processing
Fox CJ, Giuseppe Iaria, Barton JJS. Disconnection in prosopagnosia
and face processing. Cortex 2008;44:996-1009.
How have we evolved to be
able to read this?
• Inferotemporal neurons have evolved for general
visual recognition
• All writing systems share basic topological
similarities
• Shapes of letters “have been selected to resemble
the conglomerations of contours found in natural
scenes, thereby tapping into our already-existing
object recognition mechanisms”
• “The brain constrains the design of an efficient
writing system”
• Oliver Sacks. The mind’s eye.
Conscious perception
•
The Binding Problem
Why do we move our eyes?
• Clear vision of an object requires that its
image be held steadily on the retina
–Best vision is possible when the image lies on
the fovea
• Eye movements evolved to aid vision
• Gaze holding
• Gaze shifting
Types of eye movements
Class of Eye Movement
Vestibular
Visual Fixation
Optokinetic
Smooth Pursuit
Nystagmus quick phases
Main Function
Holds images of the seen world steady on the
retina during brief head rotations or translations
Holds the image of a stationary object on the
fovea by minimizing ocular drifts
Holds images of the seen world steady on the
retina during sustained head rotation
Holds the image of a small moving target on the
fovea; or holds the image of a small near target on
the retina during linear self- motion; with
optokinetic responses, aids gaze stabilization
during sustained head rotation
Reset the eyes during prolonged rotation and
direct gaze towards the oncoming visual scene
Saccades
Bring images of objects of interest onto the fovea
Vergence
Moves the eyes in opposite directions so that
images of a single object are placed or held
simultaneously on the fovea of each eye
Leigh RJ. Proc NANOS 2012
Cortical areas for eye movements
• Primary visual cortex (V1) is the gateway for eye
movements:
–Without it, humans cannot make accurate
visually guided eye movements
• Patients with lesions affecting V5/MT report
akinetopsia (loss of motion vision) and show
impaired saccades and pursuit to targets
moving in the contralateral visual hemifield
Leigh RJ. Proc NANOS 2012
Human cortical areas important
for eye movement control
Liu, Volpe & Galetta. Neuro-Ophthalmology: Diagnosis and
management. Philadelphia: WB Saunders Company 2001
Saccades - several rôles
• Spontaneous saccade
–∼20/min
–Visual search of the environment
• Reflexive or non-volitional saccades - short reaction times:
–New visual, auditory, or tactile cues
• Voluntary (intentional or volitional) saccades carry the eyes to a
predetermined location corresponding to the position of a visual target
or sound, ie they direct the fovea at a goal.
–Can also be made in a predictive fashion when the target is moving in a regular
pattern: the eye movement anticipates the change in target position.
–Voluntary saccades can also be made toward imagined or remembered target
locations, or in response to commands (e.g., “Look right”)
• Saccades can be voluntarily suppressed to maintain steady foveal
fixation
Frontal Eye Field - FEF
• Generating voluntary saccades
• Suppression of saccades during steady fixation
• Subregions of the FEF concerned with vergence and
smooth pursuit
• FEF is involved in triggering of the memory-guided
saccade, possibly with a contribution from the parietal
lobe (PEF)
Leigh RJ. Proc NANOS 2012
Supplementary eye fields - SEF
• Guide saccades during complex tasks:
–Sequences of movements and responses
when the instructional set changes
• Also contribute to predictive smooth pursuit
• The SEF seems to play an important role in shifting
from a more automatic to volitional behaviour
Leigh RJ. Proc NANOS 2012
Hypothetical
scheme of the major
structures that
project to the
brainstem saccade
generator - premotor
burst neurons in
PPRF
• LR - Lateral rectus
• MR - Medial rectus
• PPRF - Pontine paramedian reticular
formation
Liu, Volpe & Galetta. Neuro-Ophthalmology: Diagnosis and
management. Philadelphia: WB Saunders Company 2001
The pursuit system
• Parietal cortex enhances attention on the moving target
• Frontal Eye Field contributes to initiation and maintenance of
smooth pursuit
• Projections descend to the brainstem and subsequently to the
flocculus, paraflocculus, and vermis of the cerebellum
• The flocculus projects to the ipsilateral medial vestibular nuclus,
which, in turn, project to the abducens nucleus
• Upward vertical smooth pursuit mediated by the y-group - a small
cell group that caps the inferior cerebellar peduncle and downward
via the superior vestibular nucleus
Hypothetical
anatomical scheme
for smooth pursuit
eye movements
• MVN - Medial vestibular nuclei
Liu, Volpe & Galetta. Neuro-Ophthalmology: Diagnosis and
management. Philadelphia: WB Saunders Company 2001
Eye movements
Wray SH. Eye movement disorders in clinical practice, 2014.
Actions of extra-ocular muscles
Muscle
Primary action
Secondary action
Tertiary action
Medial rectus
Adduction
—
—
Lateral rectus
Abduction
—
—
Superior rectus
Elevation
Intorsion
Adduction
Inferior rectus
Depression
Extorsion
Adduction
Superior oblique
Intorsion
Depression
Abduction
Inferior oblique
Extorsion
Elevation
Abduction
Wray SH. Eye
movement disorders
in clinical practice,
2014.
Conjugate gaze
Wray SH. Eye movement disorders in clinical practice, 2014.
Vestibular-ocular system
• Holds images of the seen world steady on the
retina during brief head rotations or translations
• MVN - Medial vestibular nucleus
http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v7/ch038/002f.html