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Lecture

PSYB51H3 Lecture Notes - Retina, Stereoscope


Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier

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PSYB51 ± Lecture 06
Space Perception & Binocular Vision
- Why talk about eye movements in a course on perception?
o Spatial Constancy Æ eye movements themselves are not perception but they are
very significant for vision
as we move our fovea it influence our vision &inhibits vision for some time
causing problem for vision such as spatial constancy
Eye Movements
- How to describe a location of an object in space given that space has three dimension Æ
take profs body as the object in space Æ so the orientation of the body could be
explained in 3 possible axis (3 possible axis in which the body can rotate)
o So can explain the orientation of object in space w/ 3 NUMBERS along the rotation
axis Æ the object can be described in space using 6 DIMENSIONS (3 for location
& 3 for orientation)
- /HW¶VDVVXPHWKHKHDGLVVWDEOHÆ eyes could also rotate around 3 axis (3 DIMENSION)
Æ so how many muscles are needed to rotate around 1 AXIS given that muscles can only
contract or relax Æ need 2 MUSCLES to rotate
o THUS, NEED 6 MUSCLES IN TOTAL FOR EYE MOVEMENT
- Six muscles are attached each eye & are arranged in three pairs Æ this the minimum
number of muscles needed to control eye movements
o Inferior/ Superior Rectus & Lateral/ Medical Rectus - around the side, top,
below on the eyes Æ so for horizontal & vertical axis (horizontal & vertical eye
movements)
o Inferior/ Superior Oblique Æ torsional eye movement (movement around the line
of sight) Æ difficult to notice & not voluntary Æ if made voluntarily there must
be something wrong w/ your brainstem
Can fixate on an point in space & rotate your head Æ vestibular system will tell
the eyes to COUNTER-ROTATE to the head Æ only for about 5 DEGREES
x after this the eye will make quick saccadic eye movement (jerk-like
movement) in the torsional space (snap into new position) Æ at this point
± you are perceiving the object that you were fixating on as NOT STABLE
Can make voluntarily torsional movement w/ your head
o Superior Oblique Muscles (weird structure) Æ originate from beside the nose & it
runs through a bone that is like a hook Æ so when muscle pulls backward Æ it
pulls the eye around torsional axis
o Inferior Oblique Muscle --. Does something similar to Superior Oblique Muscles
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- Muscles are Controlled (control on the low level) by 3 Cranial Nerves (indirectly by the
brain) Æ sitting on the brain stem Æ they innovates the muscles
o cranial nerve III : Oculomotor nucleus Æ controls the Medial, Superior, Inferior
Rectus Muscles, as well as Inferior Oblique Muscles
o cranial nerve IV : Trochlear nucleus Æ controls the Superior Oblique Muscles
o cranial nerve VI : Abducens nucleus Æ controls the Lateral Rectus Muscles
Out of these nuclei Æ have the soma of the neurons Æ axons radiate out &
form the CRANIAL NERVES III, IV, VI Æ these nerves are name after the
nuclei from which the axons grow
x E.g. Oculomotor nucleus Æ Oculomotor Nerve ( Cranial Nerve III)
- Levels of Control where one control system connected to another system Æ Complicated
o Eye movements is controlled by extensive network of structures in the brain Æ e.g.
Frontal eye field Æ some structures in the Cerebral Cortex will innovate Superior
Collciulus (a set of nucleus)
- Superior Collciulus Æ a structure in the Midbrain ( brainstem) that plays an important
role in initiating & guiding eye movements Æ similar to the LGN - it has more than one
map & is organized in a Topographic way
o There is layers involved in motor control, there is also other layer that is involved in
visual perception lumbar (not just visual ± is multisensory Æ BUT MAINLY
VISUAL)
- When you have eye movements in response to some stimulus in quick manner Æ a
reaction time about 8 milliseconds or less Æ it is most likely an eye movement controlled
by the Superior Collciulus rather than Cerebral Cortex
o Basically eye movements can be controlled by short cuts Æ have visual input that
goes not only through the Primary Pathway but directly to the Superior Collciulus as
well Æ from Superior Collciulus directly into Motor control regions w/in the Superior
Collciulus & down to muscles
Research w/ Cats shows this Æ When stimulated w/ electrical signals, eye
movements can be observed
Different Type of Eye Movements
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- Smooth Pursuit ± eyes moves smoothly to follow moving object Æ e.g. when prof
moves his finger smoothly from left to right , our eyes follow that movement
o This is used in quick neuropsychological test by asking them to follow the finger w/
their eyes Æ cannot do that --. Indicate some kind of damage -Æ eyes will have this
jerks like eye movement (e.g. saccade)
- Saccade ± rapid movements of eyes that change fixation from one object or location to
another Æ e.g. when reading - have this rapid eye movements disperse w/ fixational
eye movements
o Most of the time when you have saccade movements ± you have fixational eye
movements
o Fixational Eye Movements - Eyes move very quick & then stay stable Æ when you
try to NHHS\RXUH\HVWDEOHEXWWKDWGRHVQ¶WPHDQWKHUHLVDQDEVHQFHRIRFXORPRWRU
activity
Often not possible to keep your eyes stable on one spot --. Will have
Microsaccade ± your eyes shifting all time even if you try to keep your eyes
stable
- Vergence Eye Movements ± Type of eye movement in which two eyes move in opposite
directions, done deliberately --.to diverge or converge Æ normally cannot move one eye
independently --. Movements such these are performed w/ both eyes
o If can move one eye independently w/out the other one --. Indicate there might be
some damage in the brainstem (which is dangerous)
Unless ± you know a trick
Smooth Pursuit
- Demonstration Æ prof moving the finger & fixate on fingerÆ even though there is no
movement relative to finger and retina (finger is fixated in fovea) Æ you still see
movement of the finger
- Can carry out conditions where you have misperception of movements
o sometimes have movement on the retina & identify that as movements Æ but
sometime you just dismiss it
- Function of this eye movement is to keep object of interest stable & on the Fovea
o Advantageous - everything stable on retina is better to be perceived Æ because
photoreceptors on the retina is pretty slow that takes some time until enough light is
there to stimulate them Æ it is advantageous to keep the eyes for some time to catch
enough to photons
o PROBLEM is when you move your eyes & things move on the retina Æ these
movements are caused by the eye movements ± is not really movements that are
occurring outside the world
Have perceptual problem to identify to what kind of movement we are facing Æ
whether it is movement outside the world OR movements caused by eye
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