Lecture 3 Flashcards
what do all points along the circumference of the VMO stimulate?
corresponding points and zero binocular disparity
what shape is the theoretical horopter in near vision?
bow-shaped
what are the 3 assumptions for the theoretical VMO horopter?
both retinas are spherical, both retinas have symmetric distribution of local signs across nasal and temporal retinas, and right and left retina are the same size with the same local sign geometry
what is the empirical longitudinal horopter (ELH)?
the actual horopter from experiments on humans - less concave than VMO (flatter)
what is the deviation between the VMO and ELH horopters called?
Hering-hillerbrand deviation
what are the 2 more commonly used methods of measuring ELH?
identical visual direction and equidistance (stereoscopic depth matching)
what are 5 ways to measure ELH horopters?
identical visual direction, equidistance (stereoscopic depth matching), singleness (haplopia), minimum stereoscopic threshold and zero vergence
which horopter method using this: when 2 targets, each presents to one eye, are perceived as lying in a single visual direction, the images of those targets must be formed on corresponding points?
identical visual direction horopter
which method of measuring a horopter is the only true horopter and most accurate? why?
identical visual direction = only one that directly measures the visual direction
when using the AFPP method (equidistance) - what is the percept of the horopter when the rods are physically lined up in a row?
they are perceived as curving away from the observer
when using the AFPP method (equidistance) - what is the percept of the horopter when the rods are physically curved inwards?
they are perceived as being in a straight horizontal line/row
what is the advantage of the equidistance horopter?
examiner can actually see the shape of the horopter directly
what is the disadvantage of the equidistance horopter?
AFPP method doesn’t reflect the effects of fixation disparity, unlike the identical visual direction
which the the most popular way to measure a horopter?
the Howard-Dolman apparatus - using the equidistance or AFPP method
what does the singleness (haplopia) horopter measure?
the extent of Panum’s area at the fovea and eccentric locations
what does the width of the zone of singleness reflect in the singleness (haplopia) horopter?
Panum’s area
what are 2 variables that affect the shape of the ELH?
skew (relative magnification, R) and curvature (hering-hillerbrand deviation, H)
what is the relative magnification (R) when the target is on the horopter?
zero
a1 = a2, R = 1
what does it mean when R = 1 (relative magnification or skew)?
objective space matches the perceived space = no relative magnification
what happens to an image if a size lens in placed in front of an eye?
the image is magnified and puts the image more nasally = creating uncrossed disparity (larger internal angle)
what happens to the horopter when an eye is magnified?
the new horopter is rotated towards the magnified eye
what does it mean if R > 1?
OD image is magnified
what does it mean if R < 1?
OS image is magnified
what is the typical value of H (hering-hillerbrand deviation)?
+0.1 to +0.2 = indicating the horopter is flatter than VMO
why is the ELH flatter than the VMO?
the original assumption for VMO is not true - nasal packing phenomenon: local sign in nasal retina are spaced closer together and the temporal visual space is overestimated
what happens to the ELH horopter with distance?
with increasing distance the curvature of the horopter changes so that it goes from convex = flat = concave (mirror changes in VMO curvature)
what is abathic distance?
the viewing distance at which the apparent and objective frontal plane coincide in the ELH (when it is truly flat) = about 6m
what is fixation disparity?
the vergence angle of the eye and the angle between the visual axis is slightly inaccurate - the fixation point doesn’t coincide with the actual target
what is exo-fixation disparity?
two eyes slightly under-converge - fixation point behind the target
what is eso-fixation disparity?
two eyes slightly over-converge - fixation point is in front of the target
how big is a normal fixation disparity?
small - usually a few minutes of arc (larger than normal may indicate potential binocular problems)
why is fixation disparity purposeful?
normal FD is beneficial - it serves as a stimulus for the vergence system to maintain its innervation level (doesn’t disrupt sensory fusion)
how does strabismus affect the offset of the ELH?
horopter shifts towards the intersection of the visual axis and sensory fusion is disrupted
what happens in the geometric effect and the induced effect to the horopter?
geometric = magnification along the horizontal meridian induced = magnification along the vertical meridian
what is a size lens? what is a meridional size lens?
size lens = special lens that only changes the image size with no optical effect
meridional = only changes the size along one meridian
what happens when you use an axis 090 meridional size lens on the OD?
the image is magnified in the 180 (horizontal) meridian, makes the internal angles larger = uncrossed disparity
what is “induced” when you use a 180 meridional size lens to one eye and the image is magnified vertically?
the effect is identical to the effect produced by an axis 090 meridional size lens in front of the fellow eye
which horopter method can you use to measure the induced effect?
AFPP (Howard-Dolman) not the Nonius (it won’t rotate because no physical horizontal binocular disparity is being created)
what type of distortion do prisms cause?
non-uniform magnification = more magnification at the apex than the base
what happens when you place BI prisms in front of both eyes?
temporal sides are magnified, horopter bends towards observer at temporal sides and the perceived world bends away from the observer
what are 2 clinical situations where the EHL is more curved than the VMO (H < 0)?
divergence excess and intermittent esotropia
what is an irregular horopter?
a distortion of the horopter from its normal smoothly arced shape - it deviates closer to the observer in a region near the crossing of their visual axes
what are 3 situations that can have an irregular horopter?
early onset constant esotropia, metamorphopsia, and keratoconus
what is the flom notch?
in an inrregular horopter = retional spatial distortion - may be the result of anomalous retina correspondence
