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when visual space does not match physical space
Define Local Sign.
unique oculocentric direction associated with a specific retinal point.
It's determined by retinoptic mapping of retinal locus in LGN and cortex. Mechanical visual phosphenes show the neural basis of local signs.
Foveal local signs size to foveal cone size is?
Peripheral local signs to peripheral cones are?
Larger or smaller?
1/7; basis for hyperacuity.
Larger than peripheral cones.
Causes of metamorphopsia.
2 main causes.
Neuroanatomical - retinal disease (most common and evident) and Strabismus/amblyopia (subtle, asymptomatic)
Optical - extreme K distortion such as keratoconus
Define PVD, Principle Visual Direction
is the oculocentric sense of "straight ahead" signaled by the fovea, Zero Direction.
Define Eccentric Fixation, EF.
Found in Pts with what? 2
is a PVD associated with the local sign of peripheral retina. Pt NOT AWARE its off center.
Found in strabsimic amblyopia or Long term macular Dz
Define eccentric Viewing, EV.
Found in who?
PVD assciated w/ fovea but Pt is AWARE of using peripheral vision to see.
Found in short term macular dz or low vision rehab.
A reference point by the visual system located right b/w the eyes. Cyclopean eye if you will.
Egocentric direction is determined by 2 things. Name these 2.
- 1) Retinal location of the object's image in each eye.
- 2) Eye orientation. Possibly determined by EOM proprioception or neurons that drive the mm.
Herin's 2nd Law aka?
Law of identical visual direction.
For every retinal point in one eye there is a corresponding point in the other eye w/ an identical sense of visual direction.
Hering's 3rd Law.
Objects on the visual axis of either eye in symmetrical convergence appear to be on the median plane of the head.
i.e. images on corresponding points are seen from cyclopean eye
Binocular substance angle is?
- alphaL = alphaR. Meaning the angle from each eye's PVD is the same which also mean the image lays on corresponding points.
Explain Vieth-Muller Circle VMO ie theoretical "horopter".
- Eyes are fixed on a target. There is a circle/sphere where the angles are = in L&R eyes that stimulate corresponding points.
Explain Panum's fusional area
Tolerance of binocular vision when images deviate from corresponding points.
longer version: An area on the retina of one eye where an image can be placed and still can be fused with an image on a single location in the fellow eye.
What is Panum's Space?
Zone on either side of Horopter's line where the brain can still accomplish sensory fusion; refers to physical space.
3 Factors that affects Panum's Area
1) Size of PA increases with retinal eccentricity. @ fovea = 5-20' of arc. As one goes to the periphery the area will increase.
2)Target spatial freq strongly influenced PA size. Higher spatial freq = smaller PA. (ex. small snellen letters make Pts more sensitive to diplopia.
3) Both target motion and Pt motion increase PA.
Describe the shapes of Panum's Area due to breif and stationary stimuli.
- Brief stimulus = round shape
- Stationary = larger size, elliptical, larger horizontally. Allows fusional eye movement. Therefore Pts sensitive to vertical diplopia.
Name assumptions of VMO.
- Both retina are spherical.
- Both retina have symmetrical local signs nasal and temporal.
- Right and Left retina are the same size w/ local sign geometry.
Empirical Longitudinal Horopter (ELH)
Fusion Line that is less concave than VMO when determined experimentally.
Methods to measure Empirical Longitudinal Horopter ELH
- 1) ID visual direction
- 2) equidistance (stereoscopic depth matching)
- 3)Singleness (haplopia)
- 4)minimum stereosopic threshold
- 5) zero vergence
Identical Visual Direction Horopter
Most accurate horopter b/c it directly measures the visual direction.
Most popular way to measure horopter. Tells Pt to place rods in the same plane while looking at the center rod.