Eye Movements

1
Eye Movements
gaze direction is determined by 2 things
position of the head in space and orientation
of the eyes within the head gaze stabilization
is the mechanism used by the brain to compensate
for movement of the head and body gaze
shifting is the movement of the eyes to scan
within the visual space or to follow the
movement of something within sight crossing the
eyes is just plain wrong! )
2
Gaze Stabilization
the vestibulo-ocular reflex (VOR) using signals
from the inner ear to rotate the eyes opposite
that of the head to maintain a stable image
rapid (10ms), 3 neuron pathway rotational VOR
deals with rotations of the head using the
vestibular canals-- does not do well over
constant velocity translational VOR uses
otoliths, but can only partly compensate using
eye movements- most common in animals with
foveas optokinetic response (OKR) monitors
en masse movement across the retina, slower
response, but compensates for movements VOR
misses
3
Gaze Shifting
saccadic eye movements quickly shift things from
peripheral to central vision where visual
accuity is highest have stereotyped patterns
depending upon the degree of shift needed smooth
pursuit eye movements track objects in the visual
field slower than saccades, and require visual
feedback vergence eye movements move the 2 eyes
independently- used to focus on different
distances based on binocular disparity accomodati
on focuses the images on the fovea (related to
vergence) by bending the lens using ciliary
muscles fixation is the process by which eyes
are kept focused at one place eyes may not be
stationary, but the visual field is
4
Moving the Eye
3 opposed pairs of muscles determine the angle of
each eye lateral and medial rectus- horizontal
position superior and inferior rectus-
generally up and down movements when
horizontally deviated, also ends up rotating the
eye superior and inferior obliques- rotation
also dependent upon other 2 pairs for exact
action on the eye all 3 muscles cooperate to
maintain gaze because it can rotate in 3
directions, order of movement is important ie
turn-then- spin is different from
spin-then-turn several sets of rules determine
the order of particular eye movements that
can differ in gaze shifting and stabilization
5
CNS Control of Eye Muscles
3 different cranial nerves ennervate each eye
(III, IV, and VI) all brainstem centers
connected via the medial longitudinal
fasiculus balance of eye forces controls gaze
location
6
Gaze Stabilization
takes inputs from the semicircular canal
lateralis (for rotation in 1 dimension) and
controls the medial and lateral rectus
muscles head rotations occur more smoothly than
eye movements nystagmus is the pattern of
quick eye movements used to track an
object works best for faster head movements
because the vestibular canals work best
then signals from canals act on ipsilateral
vesti- bular neurons (1) to contralateral
abducens motor and interneurons (2) then
motor neurons cause opposite rectus muscles to
contract (3) interneurons block the contrary
motor neurons
7
Gaze Stabilization
uses vestibular nuclei and 2 others (nucleus
prepositus hypoglossi- horizontal integration)
and interstitial nucleus of Cajal-
vertical) both work with the vestibulocerebellum
to feed into motor neuron instructions for VOR
(and OKR, saccades, and pursuit) one
integrator pathway modifies all eye movement
controls OKR uses visual information to fix the
gaze- works with low frequency motions OKR in
lower vertebrates (ie. rabbits, etc) have an
accessory optic organ called the pretectum
whic performs the functions, signaling
vestibular nuclei preoptic nuclei are
sensitive to visual movement in only 1 of 3
directions linked to the different movement
types
8
Brainstem Commands Gaze Shifting
cortical visual processing identifies how the
eyes must be moved brainstem and cerebellar
nuclei receive inputs from the cortex saccades
(ie. NOT visually determined) comes from the
reticular formation in the brainstem saccades
are generated from a combined output of a
specific neural circuit with several
neurons burst neurons determine the amplitude of
the saccade using the number of
spikes omnipause neurons (OPNs) fire at a high
rate EXCEPT during saccades-- silences burst
neurons during fixed gaze oculomotor cerebellum
actually ends saccades
9
Brainstem Commands Gaze Shifting
for pursuit, the cortex has to identify the
object to be followed velocity for eye
movement must be calculated based on the cortex
then sent to th neural integrator (ie. block
opposite movement) and the positional
information of the command
cortical information is sent to the pontine
nucleus, then to cerebellum cerebellum projects
down to the vestibular nuclei and neural
integrator pursuit is also regulated by the
pretectum which projects to vestibular nuclei
and nucleus prepositus hypoglossi as well
as the cerebellum stimulates some of the same
neurons used in saccades
10
Superior Colliculus Regulates Gaze Shifts
superior colliculus receives superficial inputs
from the retina deeper inputs come from most
cortical regions and basal ganglia retinotopic
map in the superficial layers is maintained in
the movement layers-- links visual space to
muscules via a brainstem projection controls
direction of all eye movements, pursuit as well
as saccades burst neurons in the superior
colliculus fire before vestibular burst
neurons, and is involved in movement preparation
(and attention?)
11
Frontal Eye Field
the frontal eye field of the cortex projects to
the superior colliculus, the cerebellum, the
basal ganglia, and the reticular formation,
making it the major cortical region directing
eye movements frontal eye field regulates
saccades, pursuit, vergence has movement neurons
that project to the superior colliculus some
neurons predict patterns of saccades to pattern a
movement other fixation neurons inhibit
saccades in all directions primarily visually
driven neurons are related to attention
within a visual field
12
Parietal Cortex Contributes to Attention
lateral interparietal area (LIP) activity
preceeds that of the FEF and brainstem
regions-- inhibiting activity here causes visual
neglect also becomes activated when distractor
elements are active
both FEF and lateral parietal areas increase
activity when preparing to act (yes,
IPS LIP, don't ask why)
13
Motion Processing Cortex Essential for Pursuit
the middle temporal (MT) and medial superficial
temporal (MST) regions are visual processing
areas activated primarily by motion MT and MST
are key areas for smooth pursuit and tracking
objects both project to the frontal eye field
and vestibulocerebellum (via the pontine
nucleus) as well as the pretectum MT and MST
ectopic activation can redirect saccades and
pursuit but cannot initiate them only
significant if movements are ongoing
14
Basal Ganglia Inhibit the Superior Colliculus
substantia nigra inhibits gaze shifts in the
superior colliculus using GABAergic
inhibition when inhibition stops, gaze shifts
can occur caudate nucleus inhibits the
substantia nigra seems to regulate gaze
shifts regarding reward
15
Gaze Shifts and Visual Attention
gaze shifts are guided by what observer finds
interesting 'interesting' meaning what
information the observer is trying to gain
attention shift always preceeds gaze shift
perceptions of pursuit objects are more accurate
than distracting ones even though saccades
occur, we do not perceive motion from
them other motion during gaze shifts are also
suppressed from notice