Visual Direction (mono vs bino) Flashcards
Visual direction
Perceived location of an object in a 2D plane (horizontal and vertical) derived from visual cues.
Specification of visual direction requires what?
A reference point
What is the reference point in object space?
The object being fixated. Can then describe location of surrounding objects in comparison to the reference point.
Reference point in the eye
Fovea
Monocular (oculocentric) visual direction
Involves associating a visual receptor (or receptor field) in the retina with an external position of the object imaged on that receptor. Fixating.
When we measure visual function, we must always keep in mind that the measurement is
Situation dependent. Therefore, document the testing conditions. Ex: taking VAs in dark or light room, at 30cm or at 50cm.
The outcome of perceptual process occurs via which 3 processes?
How can poor ocular motor control affect each?
- Perceptual selection. What do I want to pay attention to?
- Perceptual organization. Which one of these belongs with the other/where are things in space?
- Perceptual Interpretation. Leads to decision, action, motion.
*poor ocular motor control adversely effect selection and organization, leading to false interpretation.
Visual line of sight joins which 3 things
Point in object space, the entrance pupil, the exit pupil, and the corresponding point in the retinal image plane (doesn’t have to be fovea)
Principle visual line
Connects the fovea to a point in object space. Not the same thing as visual axis.
Alpha angle is the angle between __ and __
The optical axis and visual axis. Monocular.
What refractive errors cause the angle alpha to be smaller or larger?
Smaller in myopia
Larger in hyperopia
Positive angle alpha
negative angle alpha
Positive: Visual axis is nasal to optical axis.
Negative: Visual axis is temporal to optical axis.
Angle kappa
Angle between the visual axis and pupillary axis.
Is angle alpha and angle kappa mono or bino
Both mono.
Angle kappa procedure
Ask patient to monocularly fixate on the transiluminator. When you view the pt’s eye, you will see the corneal reflex/first purkinje image. It will appear displaced from the pupillary center, demonstrating angle kappa.
Angle kappa may play what role in cataract surgery?
Multifocal IOL.
In eyes with a large angle kappa, a fovea centric ray may hit the edge of the ring, causing edge glare and poor vision quality.
Want to Center the multifocal IOL on the visual axis, not the pupillary axis if the pt has a large angle kappa.
Line of sight goes through what
Fixation point, entrance pupil and fovea
Visual axis goes through what
Fixation point, nodal point, and fovea.
Optical axis goes through what
?
Pupillary axis goes through what
?
Normal angle kappa/lambda
The corneal reflex will appear to be displaced nasally from the center of the pupil by about 0.5mm. This indicates that the line of sight is nasal to the pupillary axis.
Positive angle kappa
Negative angle kappa
Corneal reflex is displaced temporally.
Hirschberg Test mono or bino
Bino. Checks for strab by looking at the location of the first purkinje image OU.
Normal Hirschberg Test
Symmetrical and slightly nasal in each eye.
When to do hirschberg test
Do it for all patients with EOM testing BEFORE cover test
Hirschberg before or after CT?
Before.
Hirschberg test where is reflex on eso and exo pts
Eso: temporal
Exo: Nasal. Normal.
Do you check hirschberg before or after lamb kappa?
Hirschberg before to avoid an initial disruption of fusion in pts with fragile binocularity. Way to determine if pt is an intermittent trope. basically like a cover test.
What do you do if it looks like pt has a large angle kappa binocularly?
Do unilateral cover test. Cover 1 eye. Does the eye move to pick up fixation and cause the light reflex to move? If so, the pt has strabismus. If the light reflex stays in the same position as before, it means the pt just has a large angle kappa.
What kind of angle kappa will a high hyperope have?
Will have a large positive angle kappa, making the pt appear exo. (pseudo exo). No movement will be observed when doing unilateral CT.
What kind of angle kappa will a high myope have?
The fovea may lie nasal to the optical axis, resulting in pseudo esotropia with a negative angle kappaa. No movement will be observed on a unilateral CT.
A large positive angle kappa can make a true ___ look worse or mask a true ____
exotropia look worse or mask a true eso
A negative angle kappa can make a true ____ look worse or can mask a true ___
Make esophoria look worse or mask a true exo.
1 mm of displacement of purkinje corresponds to how ___ degree ocular misalignment
7-8 degree ocular misalignment
Peripheral vision stimulus gives information about which 4 things
- Location in space (X, Y, Z) Absolute and relative
- Motion
- Orientation
- Depth
Angles of retinal eccentricity and oculocentric direction are measured from the
Principle visual direction
If pt is fixating at a point with fovea and then moves eye to right, what angle is that? + or -
+ Clock wise/right
If pt is fixating at a point with fovea and then moves eye to left, what angle is that? + or -
- CCW, left
Object space
Image space
Neural space
Perceptual space
Object space is out in the world.
Image space is on retina
Neural space is between retina and brain
Perceptual space is what we see- not necessarily consciously
Local sign
Each receptive field has an associated single, unique visual direction. Each receptive field has one local sign. No two receptive fields of one eye share the same local sign.
Humans are good at differentiating visual directions based on which receptive field is activated.
** Same can be said about any single neuron (Ganglion/LGN) within a single retinotopic map along the visual pathway.
What is the principle and secondary visual directions?
Principle is the visual axis. Fixation to fovea.
Secondary is anything else- periphery.
Law of oculocentric visual direction
All objects on the same visual line will be perceived to have the same oculocentric visual direction. Regardless of whether they are at the same distance, if they are on the same visual line, they will be perceived as superimposed or one occluding the other.
Is occlusion a monocular or binocular cue
Monocular cue
Oculocentric visual direction is determined by
Local sign relative to the visual axis. AS the eye changes fixation, the secondary visual directions change accordingly to maintain a constant eye-centered (oculocentric) relationship to the fovea and it’s primary direction of gaze
What causes metamorphopsia
Normal retinal local sign being distorted by stretching or other physical distortions to the retina. Local signs in the damaged retinal region change because the relationship of points relative to the fovea changes.
With mild central distortion, pt still may be able to read 20/20. But 20/20 does not automatically signify normal vision! Listen to pts complaints carefully.
What medications can cause cystoid macular edema
latanoprost
Epi
Niacin
Spatial sampling and discrimination of visual direction
How good the visual system is at discriminating 1 visual direction from another. Limited by spatial sampling of photoreceptors.
A hyperacuity- we are better at it than we would think.
Visual direction discrimination is best at the fovea (where cone sampling density is the highest). Much poorer in the periphery because receptor fields are much larger- larger cones and more than 1 cone feeds information to a single ganglion cell.
Eccentric viewing
Two adults on vacation consciously stopping to take in the view.
GOAL: we WANT the patient to use an eccentric point so they can see better (because foveal area is damaged). Not easy to teach!
Consciously directing fixation with a retinal locus OTHER than the fovea. Usually a learned adaption due to foveal injury or disease! Like macular degeneration.
Eccentric Fiction
Unconciously (starts when pt is very young) fixating with a retinal locus other than the fovea. Involves remapping ones perception of visual space in the affected eye by assign the eye’s primary visual direction to a non-foveal locus. Ex: Primary visual axis does not include fovea!!!!!
Typically develops in early childhood together with amblyopia as an adaptation to strab.
Monocular.
Is eccentric fixation mono or bino?
Mono.
How to evaluate fixation using scanning laser ophthalmoscope (SLO)?
Shows on the retina where the patient is fixating. High tech. Ask pt to fixate on cross/target. Wherever the cross lands on their retina is their eccentric fixation point
Is there indication of retinal dysfunction at the level of the cone photoreceptors in amblyopic eyes?
No.
Is eccentric fixation dynamic or static
Dynamic. With a large target, pt may use their eccentric fixation point. With a small target, they may use their fovea.
Location of eccentric point is defined relative to the
Fovea.
Visuoscopy
Evaluating eccentric fixation using fixation target on direct. Shine target in their eye and ask patient to look at center of fixation target. Should fall on their fovea if they have central fixation. If not, could estimate their eccentric fixation.
Brock Givner Test requirements
One eye has normal central fixation!
Requires cortical interocular transfer for the image tagged on the normal retina to be visible when viewing with the strabismic eye.
Brock givner test steps
- Tag the fovea of the patient’s normal eye using a very bright vertical line to produce a strong after image on their fovea.
- Have subject view a horizontal scale using strabismic eye. Cover eye that was tagged and have them focus on the black dot! The after image should transfer to the anatomical fovea of the strabismic eye.
So pt focuses on black dot with their eccentric position. The line will show up wherever the anatomical fovea is in relationship to the eccentric position.
Nasal eccentric fixation in OD or OS will signify which strab
Eso
Temporal eccentric fixation in OD or OS will signify which strab
Exo.
when to conduct the Past Pointing test?
If the patient recently acquired a strabismus or if it is congenital. Only will work if the patient has a recent paralysis of EOM. If it is long standing, brain will compensate and adapt, causing the patient to become more accurate over time.
How to set up the past pointing test?
First, carry out test with non-strap eye to check for normal response. Occlude amblyopic eye and hold pen 25cm in front. Ask pt to touch pen with their finger.
Now repeat with non-strab eye occluded.Object placed in the field of gaze of a paralyzed EOM, the patient may initially point past the target (over reach) because of the additional innervation needed to direct the eye into that fixation position.
Past pointing test is related to ____ localization. Is this mono or bino?
Oculocentric
Mono! Patch other/good eye.
Pt points a few cm to the side of the pen during the Past Pointing tees. What does this indicate?
That fixation does not coincide with the center of monocular localization (fovea)
Brain thinks EOM is working so the brain will interpret that image is further than it actually is.
Eccentric fixation and the haidinger brush/propeller phenom
Ask patient to look through a blue filter and rotate the light through a polarizer. The patient will detect the propeller like pattern.
The black propeller portion is perceived because blue and yellow are opponent colors (cancel each other out). Because they are looking through a blue filter and the pigment distributed in the retina is yellow!
Still controversial.
Why would you use the Haidinger Brush test
Can be used to demonstrate where the fovea projects into space.
Tell patient to fixate on the dot (eccentric fixation point). Ask patient where the spinning wheel is- this indicates where their fovea is!
What does oculocentric direction tell you
Tells us about where something in space is located using the eye’s fovea as the reference point. Monocular!
Does not account for head movement and for the fact that we have two spatially separated eyes.
Headcentric localization
Localization that takes into account head and body movements. Requires additional information from the EOMs, vestibular and musculoskeletal systems.
Egocenter
The point in the HEAD that serves as the single reference point for perceived visual direction.
Egocentric localization
Combines the oculocentric local signs of the two eyes. Brain creates local sign for an apparent single eye, the cyclopean eye, located midway between the two physical eyes.
Binocular.
Egocentric neglect
Inattention to space to ONE SIDE of the observers VISUAL SPACE. Ex: only marking apples on right side of paper. This indicates a right hemisphere lesion.
Allocentric neglect
Will ignore left side of objects in space regardless of whether the object is on his left or right side.
Ignores left side of paper and ignores left side of objets on right side of paper.
Concept that two eyes do not always have equal influence upon cyclopean perception
Ocular dominance. Sighting preference is usually consistent with ocular dominance.
May vary at different distance and under different viewing conditions.
Corresponding points
Points (one in each eye) having the same local sign and therefore, associated with the same oculocentric visual direction.
Common subjective principal visual direction
Images from each of the two eyes are combined according to their local signs as they are seen from a single cyclopean eye. Images from the fovea of the two eyes (from objects in each eyes principal direction from fovea) are mapped to the common subjective principal direction that originates at the cyclopean eyes fovea
Where does the Common subjective principal visual direction originate?
The cyclopean eye fovea
Why isn’t all monocular localization oculocentric?
Some egocentric information is available monocularly- ex: still getting info from head and auditory. People who lose vision in one eye still tend to localize objects relative to the ego center.
Why isn’t all binocular localization purely egocentric?
It may be biased towards the dominant eye’s principal visual axis- not always at midline.
Ex: In strabismus, egocentric direction and the ego center may shift towards the normal eye, even when both eyes are open.
Another name for Vieth-Muller circle
Horopter
Horopter
A circular area of points in space that represent fixation point and also the nodal points of the two eyes (nodal point is where the line of sight and visual axis cross)
Shows where things in space should be single or double when looking with both eyes open. All relative to the horopter circle.
alpha 1 represents which eye
alpha 2?
Alpha 1 is OS
Alpha 2 is OD
Assumptions of the horopter
- The eyes are circular
- The distribution of receptors are even across the retina.
- No influence from higher areas of the brain occur to change it
All false
Where in space you can place objets such that all of their images fall on the corresponding points on the retina of both eyes
Horopter
Images of objects not on the horopter fall on ____ points on both retina.
Non-corresponding. An object not on the horopter will create different angles in each eye, resulting in an angle of disparity.
Uncrossed disparity
Things farther from the viewer than the horopter. The secondary line of sight does not cross until after the horopter.
Crossed disparty
Things closer to the viewer than the horopter. The secondary line of sight crosses before the horopter.
Uncrossed disparity: The right eye projects its image to the ____ of fixation and the left eye projects its image to the___ of fixation
Right
Left
crossed disparity: The right eye projects its image to the ____ of fixation and the left eye projects its image to the___ of fixation
Left
Right
How to measure the angle
From the visual axis (Fovea) to the secondary line of sight.
Clockwise angles
Visual axis. To the right is the secondary line of sight.
Positive angle.
Counter clockwise angles
Visual axis. to the left is the secondary line of sight. Negative angle.
How to calculate binocular disparity (n)
n= alpha left - alpha right
Any objects on the horopter have ___ binocular disparity
Zero.
Panums fusional area
The area upon the cyclopean retina within which similar images can be fused
Basis of stereopsis
Fusion of images with small amounts of binocular disparity.
Real depth
One real object in space produces two retinal images- one on each eye.
What do you perceive if two images do not fall on corresponding retinal points (not on the horopter) but do fall within panums area?
Single object perceived with stereo depth
What do you perceive if two images do not fall on corresponding retinal points and do not fall within panums area?
Double
Dichoptic presentation
We can present TWO objects to the two eyes to produce two retinal images.
If the images are similar, they can undergo sensory fusion. Brain is tricked into perceiving one object.
