Lecture 7/8 Flashcards
what are the 6 pictorial monocular depth cues?
relative size, linear perspective, texture gradient, aerial perspective, lighting and shading, and interposition
what are the 3 non-pictorial monocular depth cues?
accommodation, motion parallax and kinetic depth effect
what is relative size?
retinal image size is inversely proportional to distance - requires some knowledge about the object
what are 2 things that can cause a decrease in retinal image size?
object size remains constant but distance increases or distance remains unchanged but the object shrinks
what is size constancy?
the perceived size of an object remains relatively constant even if it’s retinal image size changes
what is shape constancy (form constancy)?
the perceived shape of an object remains relatively constant even if the actual retinal image shape changes, due to changes in viewing angle
what is Emmert’s law?
a projected after-image or eidetic image is altered in size proportion to the distance of the surface on which it is projected
what is the moon illusion?
the moon appears smaller in the zenith (sky) than at the horizon even though the physical distance is constant
what is linear perspective?
perception of parallel lines or edges converging towards a distant vanishing point at the horizon (based on size constancy principle)
what is texture gradient?
distant objects are seen as more densely packed (relates to size principle)
what is aerial perspective?
distant objects appear less sharp than near objects due to scattering of light
how is lighting and shading a monocular depth cue?
the shadow of an object is interpreted as falling behind an object
what is interposition?
the obstruction of the view of a distant object by a closer object
how is accommodation a monocular depth cue?
more accommodation innervation is needed for viewing a closer object (weak depth cue)
what is motion parallax?
viewing a scene while moving, the relative position of objects change depending on their locations relative to the fixation point:
objects closer to fixation = against movement
objects further = with movement
what is the kinetic depth effect?
the flat image of a rotating object acquires a strong perception of 3D depth because of the motion in the image (related to motion parallax)
what are the 2 binocular depth cues?
convergence and stereopsis
what happens to the perceived image with increased convergence by BO prisms?
the perceived image appears to get smaller and closer
what happens to the perceived image with increased divergence by BI prisms?
the perceived image appears to get larger and farther away
why does the converging image that is closer in the vectogram appear smaller?
in order to maintain size constancy it must be perceived as smaller than the original
what is convergence micropsia?
SILO = small in and large out
what is SOLI?
small out and large in = small portion of observers report this instead of SILO (related to patient’s expectations)
what is stereopsis?
a perception based mainly on horizontal relative binocular disparity
what is absolute depth perception?
tells the observer how far objects are from themselves or other objects (distance from object to egocenter in meter, feet, etc.)
what is relative depth perception?
tells the observer about the comparison of depth (judged in % difference or by depth ratio)
what is absolute disparity?
the difference in the external longitudinal angles subtended by an object at each eye
what is absolute depth determined by?
absolute disparity
how do you calculate (n) the retinal disparity angle?
n = angle a - angle b or n= PD(delta D)/D^2
how do you convert geometric disparity (retinal disparity) to seconds of arc?
use equation: n = PD(delta D)/D^2 x 206,256
how do you clinically measure stereoacuity?
using the Howard-Dolman apparatus to find the minimum displacement needed to see the difference in depth
what is the normal stereoacuity threshold in humans?
2-6 seconds of arc
what happens to stereoacuity when there is a larger PD?
the larger the PD - the larger the disparity seen
what happens to stereoacuity when the object is closer?
the closer the object - the larger the disparity seen
what do each part of the equation: n = PD(delta D)/D^2 represent?
n = angular disparity
PD = interpupillary distance
D = fixation distance
delta D = depth interval
which is more accurate for disparity - closer or further away?
more accurate at near than distance
what is the geometric limit for disparity for a 10” arc stereoacuity threshold? (upper limit)
1320 meters or 0.8 miles (beyond this limit you don’t get depth perception from stereopsis)
what is the stereoacuity if a patient can see 9/9 targets in the Titmus stereo fly?
40” arc
what 5 things can decrease stereopsis?
viewing in periphery, low contrast, dim light, blue colors and short exposure time
what is the lower limit of stereopsis?
stereoacuity - the smallest disparity we can detect
what is the upper limit of stereopsis?
what is the largest disparity that can still give us depth sensation
what is patent or quantitative stereopsis?
high quality stereopsis - the perceived magnitude of object depth is proportional to the disparity (extends slightly past Panum’s area = diplopia but patent)
what is latent/qualitative stereopsis?
objects can be perceived as nearer or farther than fixation - but magnitude of depth cannot be perceived (way beyond Panum’s area = diplopia and violates Worth fusional hierarchy)
what are 4 ways to display stereoscopic images?
stereoscope, anaglyph and vectogram, liquid crystal shutter stereogoggles and free fusion
how does a mirror stereoscope work?
two mirrors present distinct images to each eye - the images may be dissimilar producing binocular rivalry for stereopsis (angle of mirrors can vary)
how does a brewster refracting stereoscope work?
uses decentered +5 lenses to create prismatic effect (ex: keystone)
how do anaglyph and vectogram’s produce stereopsis?
anaglyph = red/green or red/blue glasses to separate eyes views vectogram = polarized glasses
how do liquid crystal shutter stereogoggles produce stereopsis?
alternating occlusion of one eye at high speed (15 Hz and higher) - displayed image is synchronized with eye that isn’t occluded
how does free form produce stereopsis?
eyes need to converge/cross
what are the 2 parallel neuronal pathways from retina to visual cortex for stereopsis?
parvocellular = fine stereopsis magnocellular = coarse stereopsis
what is the disparity range for fine and coarse stereopsis?
fine = 2" - 20' coarse = 6' - 10 degrees
what is the color response for fine and coarse stereopsis?
fine = chromatic coarse = black and white
what is the retinal location for fine and coarse stereopsis?
fine = mostly fovea/macular coarse = pan retina/mostly peripheral
what is the motion response for fine and coarse stereopsis?
fine = prefer static images coarse = prefer motion
what is the spatial frequency for fine and coarse stereopsis?
fine = prefer higher spatial frequency coarse = prefer lower spatial frequency
which type of stereopsis, fine or coarse is more susceptible to binocular anomalies?
fine stereopsis (global)(coarse can still function with small/moderate strabismus - can still take advantage of monocular cues)
what is local stereopsis?
stereoscopic depth based on images that can be recognized monocularly as well as binocularly
what is global stereopsis?
brain sees objects through binocular vision that it doesn’t see monocularly
which mechanisms, fine or coarse, are performed in local stereopsis?
both fine and coarse
which mechanisms, fine or coarse, are performed in global stereopsis?
only based on fine stereopsis
what are 2 examples of local stereopsis testing?
line stereogram and eccentric circles (also worth circles in randot book and lifesaver card)
what are 2 examples of global stereopsis testing?
random dot stereogram and autostereogram (“magic-eye photos”)
which test picks up reduced stereopsis better in the clinic?
random dot stereogram works better - global stereopsis is more sensitive to binocular anomalies
what is the Pulfrich phenomenon/effect?
temporal disparity (time delay) creates stereopsis = ND filter reduced retinal illumination and delays the neural signal
which direction is the movement perceived if the ND filter is over the OS? what about over OD?
OS = clockwise OD = counterclockwise
what are 2 potential causes of the Pulfrich effect?
- ay situation that has a significant difference in retinal illumination (anisocoria)
- optic neuropathy/neuritis affecting one eye more (glaucoma too)
what is chromostereopsis? what 2 things cause it?
color induced stereopsis = caused by transverse chromatic aberrations and visual and optic axes of the eye slightly different
what happens to blue and red colors in chromostereopsis?
blue light falls more nasally on retina than red and blue light creates relative uncrossed disparity(perceived as farther away)
