Disparity

Exotropia has crossed or uncrossed disparity

Crossed Fixating on one object, other eye is turned *outward* and will show right objects on the left - crossed

What type of disparity would an object inside the Veith Muller circle exhibit

Crossed disparity Positive disparity stimulus for convergence

Why do we see three single circles on the Randot local stereo test, with one popping out at us

*Panum's area* allows fusion of small disparities, single vision with small amount of difference Panum's is an area in retina of one eye that, when stimulated simultaneously with a point of other eye, provides single vision Not correspondence, but larger area

How long could you let your patient look at a target before they should know what target has depth

1 millisecond minimum can tell stereopsis But after 1000 milliseconds, or one second, stereopsis levels off ! Maximum depth perception at 1 second

Why do people with acquired nystagmus perceive oscillopsia

Alpha is not sensed to be moving during the slow phase of nystagmus - thus world perceived to move in opposite direction Fast phase has suppression. So perceive egocentric to be moving with changing oculocentric direction velocity

What are the Levels of Fusion target names by Worth

1st degree - non-identical targets, non fusable 2nd degree - identical targets, fuse 3rd degree - have stereo disparity Not a hierarchy! No fusion for stereo

How many seconds of arc are in one radian

206,000 seconds of arc = 1 radian

What is stereothreshold

Aka stereo acuity Minimum relative disparity required for stereopsis PD (delta d) / d^2

Where is physiological diplopia present

All ranges of depth, basically No matter how far apart objects are, always able to see physiological diplopia..

Stereothreshold is a sensitive screener for what clinical diseases

Amblyopia Refractive error Anisometropia Because monocular blur is the worst results for stereothreshold! But binocular blur does also decrease as well

In real life, is it easier to tell objects are closer/farther apart if your PD is bigger or smaller

Bigger PD Relative disparity = PD (delt d) / d^2 If you increase PD, delta d *decreases* thus easier to tell apart Smaller distance needed to tell what object is closer

Your patient with MS has optic neuritis in the right eye. How will they see the target moving to the right

Closer to them Crossed disparity, because left eye can track but right eye lags making a cross

In real life, is it easier to tell objects are closer or farther from each other when they are closer or farther from you

Closer to you 0.02 cm apart to be able to tell what is closer at near, compared to 4.5 cm apart far away Less space between objects required to tell what is closer. *Increases rapidly as distance gets large*

How do you calculate the convergence amount from a Brewster stereoscope

Convergence = optical separation - target separation / target distance *from lens* Optical separation from prisms BO of plus lenses Target separation from right and left eyes

What is the physiological diplopia threshold

Critical retinal disparity for double vision Small disparities don't cause diplopia, normally eliminated and just fuse

What is relative disparity

Difference in absolute retinal disparity between two objects Sign relative to the tree, crossed or uncrossed disparity Total oculocentric difference of left eye - total oculocentric difference of right eye (between two objects)

Compare dioptic and dichoptic stimulation of image presentations

Dioptic - two eyes see the stimulus simultaneously Dichoptic - separate target for each eye

How does a Brewster stereoscope change the effect with plus lenses used to view the target

Eyes are looking through the lenses more inward - both eyes have *base out prisms* Changes accommodative demand, increased

What are the cortical disparity detectors

Far Near Tuned zero Tuned inhibitory Tuned far (uncrossed disparity) Tuned near (crossed disparity) Different brain cells respond to different disparity types

Your patient with MS has optic neuritis in the right eye. What direction will they see a target moving to the left

Farther away Uncrossed disparity, because right eye is not lagging but left eye has to move back

What are our regions of depth

Fine and coarse depth Little to a lot of disparity present

Compare fine and coarse stereopsis as far as the response of cortical cells

Fine stereo uses parvocellular units, tuned to small select range, accurate responses Coarse stereo uses magnocellular units, far/near detectors are not selective so there's a large range of response

You observe a picture of a dot and a line. The stimulus is dichoptic, left eye sees the dot closer to the line. Where would you look to create equal disparity between the line and dot

Fixating at a point midway between distance of dot and line Dot is always closer, line is always further away - relative disparity never changes. Just amount. And can say it different ways :|

You observe a picture of a dot and a line. The stimulus is dichoptic, left eye sees the dot closer to the line. Is the line relative to the dot crossed or uncrossed disparity

Fixating on the dot, uncrossed disparity Left line is close, right line is further out right

You observe a picture of a dot and a line. The stimulus is dichoptic, left eye sees the dot closer to the line. Is the dot relative to the line crossed or uncrossed disparity

Fixating on the line, crossed disparity Left eye sees dot closer, right eye's dot is much further left

Why would stereopsis be able to detect micro-strabismus that you did not see on cover test

Fixation for micro-strabismus is in front of or behind the target, so stereothreshold is raised - worse acuity *Vergence angle errors* This is the same effect for retinal eccentricity - central visual field has best stereothreshold - lower

What defines the Veith Muller circle

Fixation point Not any object of interest in the background!!

How can you get better at discerning if something is closer than the other

Get closer to the target (decrease d^2) Bigger PD via telestereoscope Equation of *relative disparity = PD(delt D) / d^2* rearranged for distance between objects as primary concern

Relative disparities are processed interactively over a large region of binocular field for what stereopsis

Global stereopsis Random dot test, Leaf room

What types of instruments in clinic are set up that the red filter eye sees the red target

Hess Lancaster screen - patient matches their laser green light where the doctor shines the laser red light Home therapy system with R/G anaglyph (dark background)

Why do people who feel vertigo from acquired nystagmus

If damaged vestibular nerve on one side, hyperstimulation from other side Eyes drift toward the damaged side, then saccade to refixate The brain senses no physical head movement, no body movement, but objects appear to move in direction of fast phase

How do you calculate relative disparity for objects at infinity

Large distances, must use *difference in angles of binocular parallax* or difference in vergence demand Vergence demand of distant target is *zero* (Do not use PD(delt d)/d^2)

What stimulus characteristics have an effect on stereothreshold

Length of vertical lines Nature of target Luminance of target Exposure time to target Blur effect Also Fixation position, fixation disparity (trope) Retinal eccentricity

Should stereopsis be tested in the dark or light

Lighted room Lower retinal luminance creates higher stereothreshold (worse)

Relative disparity is processed independently for each local target pair in what type of stereopsis

Local stereopsis Contour stereopsis Simple line targets

If a patient cannot see the central dots on the orthoscope suppression target, what do these results indicate

Lost central fusion = central suppression May have peripheral fusion still!! Central suppression is most common first sign of suppression, slowly lose it until chronic like strabismus

Why does fixation matter for stereothreshold

Lowest stereothreshold - best value - when fixation is accurate If fixation is left/right, in front/behind the target, stereothreshold is worse because error in *vergence angle*

What is Brewster stereoscope

Measuring dichoptic stereo, relative disparity in one letter per row Can test distance and near Use Xl - Xr / d because separate target for each eye

What is the Howard Dolman apparatus

Measuring distance stereopsis in real world, dioptic presentation Aligning two rods side by side, in your perception Calculate standard deviation of how off the person was from equally side by side rods Use PD (st dev) / distance^2 for total stereo

What is the Verhoeff stereopter

Measuring real world stereo, dioptic presentation Three rods, which is further or closer to you Pass/fail (4/4) at 1 meter, or different distances for threshold - calculate with fixed delta d 2.5mm

When two images are shown with disparity, what are the options your eyes have

Physiological diplopia - see double Motor fusion - use fusional vergence to get to appropriate vergence angle, eliminate disparity Sensory fusion - merging image of right eye to left eye, single perception whole image via cortex

What is the most effective at increasing stereothreshold: monocular or binocular blur

Monocular blur has more effect on stereothreshold, worst VA's Worse than blurring both eyes together! Stereopsis requires both eyes....relisten

What is the limit of sensory fusion

Monocular sensations (each eye's image) must be suitably related to each other otherwise will not allow sensory fusion Rivalry instead

What is exhibited through the Helmholtz target

Motor fusion can happen without sensory fusion Still see stereopsis depth without sensory fusion! Two rocks show rivalry with different angle of outer lines; luster with black on white lines

Do infants with nystagmus experience oscillopsia

Never!! Nystagmus registers alpha, where eye is pointed. Lines up with beta, cancel each other out Egocentric direction is always zero with nystagmus, retinal image is moving Blurry image, but they know their eyes are moving. So no perception of the world moving

Do you need to have sensory fusion to have stereopsis

No, brain does not have to be able to fuse images together to be able to see depth Motor fusion drives the response of *Helmholtz target* mirror diamond gems But you will see rivalry and luster

Where is disparity zero

Oculocentric direction for left and right eyes are equal Anywhere on Veith Muller circle, circle including fixation point and two entrance pupils - single object

What is the vergence demand of a single object in space

PD/d To be in prism diopters, cm/m

If a patient cannot see the numbers in the side of the orthoscope suppression test, what do these results indicate

Peripheral suppression Automatically know central suppression - cannot have solely peripheral suppression

Compare relative disparity as real world vs stereogram

Real world: *Relative disparity (radians) = PD(dela d) / distance^2* Stereo: Xl - Xr / d

What is the disparity range that permits stereopsis

Region of depth Depth y axis, disparity x axis - bellcurve

How can we classify stereopsis

Region of depth - fine/coarse Characteristic of stimulus - local/global

Does a patient with a larger PD have better relative disparity in real world scenarios or on a stereopsis test

Relative disparity is better in real world, when using both eyes together PD (delta d) / d is real world disparity, takes PD into account

In a red-green anaglyph, you are observing dichoptic targets on a white background. The object on the left is a green dot and line; on the right, a red dot and line. You are wearing glasses with green over OD, red OS. What do you see as the image?

Right eye sees right object as black (originally red) Left eye sees left object as black (originally green) Uncrossed images

A tree is farther away than the parked car. You note when you look at the car, there is uncrossed disparity. If you fixate on the tree, how does the disparity change relative to the car

Same amount of disparity, but opposite sign indicating crossed disparity Fixation is not important for relative disparity!! Will change absolute disparity, but not the objects distance compared to each other

What types of instruments in clinic are set up that the red filter eye sees the green target, and vice versa

Sherman cards R/G tranaglyphs for VT

How does the length of a vertical line affect stereopsis

Shorter lines, higher or worse stereo (10 min arc line, 4 arc seconds stereo) Longer lines, lower or better stereo 30 minutes of arc line is maximum length we are best at, same for even longer lines

Why do patients with visual neglect experience straight ahead as moved rightward

Slant of retinal image provides information on alpha, ocular direction They ignore everything in left hemifield, changes perception to shift toward the right Minification of left side, perceived farther away, outside VM?

What is stereopsis

Specific sensation of depth from *retinal disparity* Provides *relative* depth information, not absolute depth or how far actual object is. Only how far it is from another object Stimulus is *lateral disparity* only because eyes are displaced laterally

What is the deficiency of stereopsis that is limited to crossed or uncrossed disparity

Stereoanomaly A third of population cannot see one type of disparity, but stereothreshold is normal - *brief exposure, three choices*

What is the specific sensation of depth from retinal disparity

Stereopsis *Relative depth* information Stimulus is *Lateral Disparity* No sensory fusion needed! Can see double and have stereopsis

What is unique about how a random dot stereogram creates disparity

Stereopsis does not require monocularly identifiable contours Random elements interpreted over visual field, filling in the spots Unambiguous response, shows if patient has stereopsis or not - microtropia. Can still cause amblyopia, decreased VA's

What does Hess Lancaster measure

Subjective measurement of deviation of the eyes in multiple gaze positions Red light seen with red filter because it is a laser, only red wavelengths of light will travel through it

How do we see 3D movies

Synchronous shutter stereoscope Alternates between right and left eye (*dichoptic*) at a temporal frequency 60 Hz or higher, so the brain fuses each image with minimal disparity to see depth Same system for vision therapy screens

How could you increase PD extensively

Telescope on a boat Much easier to see stereopsis at distance with larger PD, require smaller separation between objects to tell which is closer

Do we have better stereopsis with vertical lines or Snellen letters

Thin vertical lines Bulkier targets will create a higher stereothreshold Thus cannot compare different stereopsis tests!!! Results are different if same person

Describe a wheatstone mirror stereoscope

Two mirrors creating separate images for each eye, dichoptic stereopsis Calculate relative disparity = *- (* X left - X right*)*/ target distance from eyes Opposite values because mirrors involved

Esotropia has crossed or uncrossed disparity

Uncrossed disparity Fixation at VM circle represents zero disparity, background object is on one eye only and the other is turned *inward* Eye turned inward will always have objects on the right appear on the right - uncrossed

What type of disparity would an object outside the Veith Muller circle have

Uncrossed disparity Negative disparity stimulus for divergence

How does increased disparity affect depth

Up to a point, we are really good at detecting how far away an object is, fine stereopsis - linear region After this point, various stereopsis disparities are seen as the same distance to us, coarse stereo - non-monotonic region

Why do we see false fusion on a wallpaper with repeating pattern

Various vergence angles possible, changes perception of distance of the wall or floor Disparity from false fusion, because not one spot to fuse

What are we better at, Vernier acuity or stereothreshold

Vernier acuity, relative disparity on a simple target Stereothreshold depends on many characteristics of the stimulus

How do you calculate the relative disparity of a Brewster stereoscope

X left - X right / target distance *from lenses*

Is stereopsis possible with double vision

Yes! No sensory fusion is needed for determining depth Diplopia threshold is within fine stereopsis region, still able to accurately determine which object is closer and by how much

What is the vergence demand of a Brewster stereoscope

Zero, because dichoptic stereopsis, PD does not matter Each eye sees a separate image with septum in the middle