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