Lecture 2 Flashcards
what is Hering’s second law (law of identical visual direction)?
for every retinal point in one eye there is a corresponding retinal point in the fellow eye with an identical sense of visual direction
what are corresponding points?
they unify the visual direction of the two eyes - objects that fall on corresponding points will be seen as lying in a single visual direction under binocular viewing conditions
in the hering window experiment - why do you perceive the house and the tree as overlapping images?
they both have oculocentric directions of 0 (they fall on the fovea) and are on corresponding points
what is Hering’s third law?
objects on the visual axis of either eye in symmetrical convergence appear to be on the median plane of the head - images on corresponding points are seen from the cyclopean eye
what are 4 characteristics of corresponding points?
have identical oculocentric direction in normals, each pair of corresponding oculocentric directions form a single cyclopean eye, all retinal points have a corresponding partner except ONH and monocular crescents of the VF, and normal correspondence is highly stable, not altered by image luminance, contrast, color, size or orientation
what is a binocular substance angle?
when an image stimulates corresponding points they have the same oculocentric direction under binocular conditions
what is the Vieth-Muller circle (VMO)?
or the geometric or theoretical horopter = all points along the circumference of the VMO subtend equal angles in the R and L eyes, they stimulate corresponding points, it passes through the fixation point and the 2 nodal points
what is binocular disparity?
when an image stimulates non-corresponding points on the retina - they create binocular disparity
what is disparity?
it is defined as the difference between the binocular substance angle (n = aL - aR)
what does horizontal binocular disparity allow?
stereoscopic depth perception (crossed or uncrossed disparity)
what might crossed disparity stimulate?
stereoscopic “nearness”, crossed diplopia, and ocular convergence
what might uncrossed disparity stimulate?
stereoscopic “farness”, uncrossed diplopia and ocular divergence
what is the binocular disparity angles in crossed and uncrossed disparity?
crossed = n is positive uncrossed = n is negative
what are the 3 degrees of binocular sensory fusion?
- simultaneous perception (keystone)
- flat fusion (worth 4 dot)
- fusion with stereopsis (stereo acuity testing)
what is motor fusion?
bifoveal fixate on a target by vergence eye movement and aligns corresponding points in the two eyes
how are motor and sensory fusion related?
motor fusion is a prerequisite for sensory fusion
what is an exception for motor fusion being a prerequisite for sensory fusion?
micro-strabismus or mono-fixation syndrome = small angle (usually esotropia) they will have flat fusion even with slightly misaligned eyes they have adapted some sensory fusion using peripheral retina to fuse
what is sensory fusion?
images near corresponding points are fused by visual cortex to create a single perception - requires similarity between 2 monocular images
what is Panum’s fusional area?
an area on the retina of one eye where an images can be placed and still can be fused with an image on a single location in the fellow eye (the zone on either side of the horopter within which it is still possible to maintain sensory fusion)
what type of space does Panum’s area refer to?
physical space
what happens to Panum’s area with retinal eccentricity?
it increases with retinal eccentricity = the further away from fovea the larger the size
what is the size of Panum’s area at the fovea?
about 5-20’ of arc
is diplopia easier to observe in central or peripheral vision?
easier to observe in central vision than peripheral
what happens to Panum’s area with spatial frequency?
the higher the spatial frequency the smaller the PA size (during vergence test small Snellen letters makes the patient more sensitive to the onset of diplopia)
what happens to Panum’s area with target motion and patient motion?
both increase Panum’s area
what is the shape of Panum’s area with a brief stimulus?
round shape
what is the shape of Panum’s area with a stationary stimulus?
elliptical shape, larger horizontally (larger than brief stimulus) - allows fusional eye movements = patient is more sensitive to vertical diplopia
