Part 13
When non-corresponding points are stimulated, the eyes may undergo a
disparity-driven vergence movement in an attempt to bring the two retinal
images back onto corresponding points
Moves the eyes in opposite directions so that the principal visual axes
intersect at the object of regard to optimize resolution and minimize
diplopia
Fusion means that a point that stimulates non-corresponding points is fused
on a single cyclopean visual direction
Visual direction is the average of the directions of the diplopic images of
the point
The angular extent of the retinal disparity over which this fusion is
maintained is called Panum’s fusional area
The basic measure of Panum’s fusional area is the diplopia threshold
Panum’s fusional area gets larger as the peripheral angle increases
Consistent with the increase in size of the neural receptive fields as the
distance from the fovea increases
Panum’s fusional area is larger for lower spatial frequencies
Panum’s fusional area is larger for blurred stimuli
Fusion mechanism is insensitive to contrast
Fusion mechanism is sensitive to the spatial frequency content of the
object features to be fused
The fusion limit depends on the highest frequency component
Panum’s fusional area is larger for lower temporal frequencies
Horizontal thresholds range from ~20 at 0.5 Hz to ~5 at 5Hz.
Vertical thresholds are lower and less sensitive to temporal frequency
Fixation Disparity
If the vergence angle of the subject increases esophoria
If the vergence angle is increased this means that the subject was
actively generating a disparity-driven divergence
Only wave to have disparity-driven divergence response is to have an
uncrossed disparity
Amount of static overconvergence during fixation is called eso fixation
disparity
If the vergence angle of the subject decreases exophoria
If the vergence angle is decreased this means that the subject was
actively generating a disparity-driven convergence
Only wave to have disparity-driven convergence response is to have an
crossed disparity
Amount of static overdivergence during fixation is called exo fixation
disparity
If there is fixation disparity
The binoculary-seen element will still be seen as a single element,
being fused
The monoculary-seen nonius lines will not fuse, showing the amount of
fixation disparity
The value of the prism that eliminates the fixation disparity is called
associated phoria
The point of fixation disparity versus prism values is called forced
vergence fixation disparity curve
The subject is tested at both a near and at a far position
Esophoria is when the subject overconverges
To generate a disparity-driven divergence signal, the subject had to
slightly overconverge: eso fixation disparity
Exophoria is when the subject underconverges
To generate a disparity-driven convergence signal, the subject had to
slightly underconverge: exo fixation disparity
The locus of points in space that lie on the intersections of the corresponding
visual lines that project from corresponding retinal points is called the
horopter
The Vieth-Muller circle is the theoretical geometric horizontal horopter
Assumes perfect symmetry in the locations of corresponding retinal
points in the two retinas
Disparity is defined as the difference in the angles subtended as the two eyes
by two points separated in space
Any point not on the Vieth-Muller circle will induce some geometric
disparity
Nonius horopter is a true measure of corresponding retinal points since it is
the only one that allows the observer to make a direct comparison of the
visual directions seen by each eye
Idea is to translate superimposition of monocularly-seen elements, which
defines corresponding points with the vertical alignment of bars
The Apparent Fronto-Parallel Plane horopter is produced by asking the
subject to arrange the rods such that they form an apparent flat plane
perpendicular to the midline
Measurements are much easier to make
The maximum stereoacuity horopter is determined by measuring stereoscopic
thresholds
The zero-vergence horopter requires highly sensitive eye movement
monitoring devices
The Noinus horopter apparatus indicate that the empirical horopter does not
conform to the Vieth-Muller circle because it has a different curvature and
may be displaced from it
Fixation disparity shifts the horopter
An horopter has to pass through the nodal points of the two eyes and the
fixation point
H (hering-Hillebrand deviation) is a measure of its ellipticity
Measure of the curvature of the empirical horopter at the fixation point
and hence its deviation from the Vieth-Muller circle
R0 is a measure of the tilt of the major axis of the ellipse
Measure of the skewness of the empirical horopter with respect to the
objective fronto-parallel plane
Dependent of the relative magnification of the two retinal images
The y-intercept of the analytical plot is equal to R0 and the slope is equal
to H
When R0=1 and H=0, the curve reduces to the Vieth-Muller circle
The alignment of afterimages created to simulate Nonius line sdid not change
significantly as a function of fixation distance
Afterimages are stabilized retinal images that arise from PR mechanisms
Always represent the local sign of the retinal points that were originally
stimulated
The positive value of H may be attributed to the layout of corresponding
points that creates a relative compression of the local signs in the temporal
hemiretina relative to the nasal hemiretina
Nasal packing is reported to be tighter than temporal packing
Magnification of the retinal image from aniseikoina or an afocal magnifier
distorts the perception of space in ways that are predictable and measurable
using the AFPP
A meridional magnifier in front of the right eye with the magnification
horizontal appears to rotate the FPP around the fixation point such that the
right side is farther away and the left side is closer---geometric effect
A meridional magnifier in front of the right eye with the magnification vertical
appears to rotate the objective FPP around the fixation point such that the
left side is farther away and the right side is closer---induced effect
A prism is characterized by increasing angular magnification toward the apex,
curvature of lines oriented perpendicular to the base-apex axis and a slanting
of lines parallel to the axis
Base out prisms cause the fronto-parallel plane to appear to the observer
as though the center of the field is closer to them while base in prisms
have the opposite effect
The vertical horopter in symmetric convergence is usually a straight line
through the fixation point at the midline
In asymmetric vergence it is a complex spiral shape
The object is going to have a smaller retinal image in the eye that is
farther from the object
The midline vertical horopter is tilted away from the observer at the top at
such an angle that the bottom of the horopter intersects the ground at his or
her feet
Vertical meridians are tilted outward at the top with respect to each other
even while the horizontal meridians overlap
Objects are perceived as single if they are above the fixation point and
farther away or if they are below the fixation point and closer
If they were in the same plane as the fixation point they would
have uncrossed and crossed disparity, respectively