2. Visual Direction Flashcards
what is visual direction
how the brain organizes images on each eye (from the retina)
what is oculocentric direction
visual direction relative to where an eye is looking
what are visual lines
a geometrical way to represent visual directions
oculocentric direction is equivalent to…
the angle between the principal visual direction (PVD) and a secondary visual direction
what is the principal visual direction
what is secondary visual direction
to the fixation point
to the other object in space
images fixed on the retina have constant _____, even if the eye moves
oculocentric directions
what is a local sign
an oculocentric direction associated w/ a retinal point
what demonstrate that local signs are fundamentally neural
mechanical visual phosphenes
what is uniqueness of local sign organization
no retinal point has the same visual direction as any other retinal point
what is order in local sign organization
visual direction is related directly to retinal position
what is the basis of directional discriminiation
local sign size
what is local sign size mostly determined by
the brain, NOT the retina
how are foveal local signs and foveal cones related in size?
how are peripheral local signs and periph cones related?
foveal local signs are 1/7th the size of foveal cones
peripheral local signs are much larger than peripheral cones
what kind of mapping is local sign order related to
retinotopic mapping
why does kundt partition asymmetry happen
there is a difference in optical mag on either side of the fovea
- less nasal retina stimulated than temporal so the img on the nasal retina is shorter so pt will see a shorter temporal portion of the line and will draw closer to nasal portion of the line
- visual axis is 5 degrees off of the optical axis
what can the kundt partition asymmetry test for
to quantify oculocentric vision problems in amblyopic patients
what do abnormally large local signs cause
spatial uncertainty (inability to discriminate differences of visual direction) and reduce VA
local sign disorder causes…
distorted vision
what are the 4 types of visual acuity
- detection (can you see the object)
- resolution (can you see 2 obj as separate)
- localization (do you see 2 obj in diff directions)
- recognition (is an obj recognizable)
when local sign gets smaller, how does it afftect VA
the smaller the local sign, the better the acuity
are detection and resolution functions determined by local sign?
no
how can resolution be tested
w/ grating acuity
- are the black and white vertial lines seen
- doesnt have to tell you about localization of lines
- grating patterns are related closely to foveal cone size (resolve 15 sec of arc)
how can localization be tested
vernier acuity
- 2 lines vertically separated and horiz displaced
- compare the horiz positions of the 2 lines to see if they can accurately locatlize them
what are some examples of visual defects where poor resolution causes low VA (resoltuion and localization deficits are proportional)
what is the exception
refractive blur
media opacities
retinal disease
strabismic amblyopia: good resolution, bad localization
what are the 2 main types of amblyopia
anisometropic amblyopia: amblyopia caused by uncorrected aniso
strabismic amb: amb caused by early onset and consant unilateral strab
-pt can have both
how is the localization threshold elevation and the resolution threshold elevation related in anisomtrpic amblyopes and strabismic amblyopes
anisometropic: localization threshold elevation is about the same as the resolution threashold elevation
- due to neurological blur, low contrast sensitivity
strabismic: localization threshold elevation much higher than the resolution threshold elevation
- bc of spatial uncertainty (snellen letter directions are confused and overlapping)
how can acuity be improved w/ vision training for strab amblyopes
patching, exercies
improvement occurs due to a reduction in the spatial uncertainty and improved resolution
what is metamorphopsia and what is disorderly here
a perception of object shape that differs from the objects true shape
-local sign as being disorderly
what is metamophopsia cause by
- optical problems
- keratoconic corneal distortion - neuroanatomical
- retinal deformation in disease
- retino-cortical connection problems in amby
what does amsler’s grid test
-nonquantitative clinical method
metamorphopsia
-retina stretched or something => signal to local signs are scrabmbled=> localization of obj/img is incorrect
why is distortion not eveident in peripheral vision
bc normal peripheral spatial uncertainty masks distortion
what is distorted in strabismic amblyopia
foveal local sign order
-disotortios not so evidnet bc of spatial uncertainty
how can amblyopic distortion be measured by
the partitioning task
-called the bisection hyperacuity task
can distortion be changed through vision training?
no
distorted macular vision is a serious impediment to ____, even if resolution and directional discrimination are good
yes, it is a serious impediment to binocularity
what is the principal visual direction (PVD)
the sense of looking at something (in the middle of your visual field)
- oculocentric sense (straight ahead)
- PVD=oculucentric zero
the PVD tells you if you are using the _____
what happens when the PVD is stimulated and when it is not stimulated
PVD tells you if you are using the fovea
- PVD stimulated is the image on fovea
- PVD not stimulated is off foveal-image; stimulus for eye movement
is VA testing a good test to see if the patient is using the fovea
no, bc could be using a diff retinal point to read the chart
we need PVD in order to scan the world with ____
high resolution spatial scanning
what is the solution for using too many brain neurons for high acuity across the entire retina
move a small high-resolution retinal area to scan diff parts of the retinal image sequentially
-need PVD to guide these movements
what is eccentric fixation
an extrafoveal point (peripheral retina) used to fixate objects and can be used as the zero reference (PVD) for visual direction (instead of the fovea)
who uses eccentric fixation
- strabismic amblyopes
- patients with years of bilateral macular disease
what is eccentric viewing
fovea is stil the zero reference but it is damanged so they use a peripheral retinal point to view an object but it appears to be off to the side
who uses eccentric viewing (EV)
- normal scotopic vision
- short duration macular disease
patients consciously use peripheral vision to see
what is the difference between EF and EV
EF=functional problem (strab amby) and EV =disease (AMD, foveal damage)
-EV pt aware of peripheral looking, EF is not
how are EF and EV similar
peripheral vision is used to see images
how do you use visuoscopy to test for EF
have pt look at center of grid target
- if grid target right on the fovea then pt using fovea
- if displaced from the fovea, then patient is using EF
- if grid is temporal to foveal reflex pt is a temporal EF
will you see a foveal reflex if a pt has a macular disease
no
what is ocular dominance
the favoring of one eye for visual input
what does ocular donimance testing assume
refractive error is corrected
what are the 2 types of dominance
- acuity dominance: one eye is favored for seeing fine detail, ipmortant for prescribing monovision cl for presby, dominant eye for dist
- sighting dominance: one eye is favored for controlling fixation (favored PVD)
how can you assess acuity dominance
by noting
- sight difference of VA
- sensitivity of each eye to small spherical lens changes
sighting and acuity dominance usually agree, what are the exceptions
- pt w/ mild left anisometric amblyopia
- macular degeneration in the right eye
- patient selects the right eye via hole in hands, despite poor acuity
what is egocentric direction
the direction of an image with respect to one’s self
where are things located relative to myself
what are the 2 processes that interpret oculocentric directions and convert them to egocentric directions
- gaze registration aka corollary dischage: brain registers eye movements and can re-evalulate the egocentric direction
- visual capture
- charactistics and details in a retinal image may reveal the egocentric direction of the object
egocentric direction = ____ + ______
oculocentric direction + gaze registration
egocentric direction is always relative to the….
orientation of the head
what is the neural pathway of gaze registration
- visual cortex sends info to space perception area
- space perc area tells the motor area where things are located w/ respect to the body
- motor command tells the oculomotor neuron to tell muscles to take eyes to target
- copy of this action sent back to the space perception
=>corollary discharge (anytime a nucleus sends primary info one way, secondary another) aka gaze registration
what is visual capture
can deduce egocentric direction w/o gaze direction info
-looks at spatial order of lines and can deduce wehre something is relative to self
what is visual capture based on
perspective textures (cues) in the retinal image
what is visual capture best for? what is gaze registration best for?
visual capture is best for large objects in static (slow) photopic vision
gaze registration is best for small objects, viewing in dark, and movement
-quicker than visual capture
when can egocentric direction be mistaken
if oculocentric direction, gaze registration, or visual capture are wrong
is visual capture known to have unique disorders
no
when gaze registration is wrong, it causes…
past pointing
what is past pointing
- used as a test of erroneous gaze registration caused by muscle paresis
- when oculocentric direction added to gaze registration, the egocentric direction will be very large and the pt will perceive the object to be very far to the right
why do past pointing patients no verbally report an illusory target direction?
gaze registration and visual capture are both running parallel during the test, when the hand is unseen, the movement of it is more strongly influenced by gaze registration
how is past pointing used clnically
not usually used to diagnose the type and severity of paralysis bc there are more accurate ways
- oculomotor adaptation to paresis reduces past pointing over many days of time
- past pointing is useful bed-side test for acute paralysis
