Transcript Stereopsis and Refra..

Stereopsis
• True depth perception. The ability to see in 3D.
• Does not include monocular cues or binocular
kinetic cues.
Interposition
Relative Size
Linear Perspective
Stereopsis
• Occurs due to the
differing positions of the
eyes.
• Retinal disparity: An
image will fall on
noncorresponding retinal
areas.
Stereopsis
• Horopter: an imaginary
arc upon which objects
produce no disparity.
• Objects along the
horopter do not produce
disparity.
• Those in front produce
crossed disparity.
• Those in back produce
uncrossed disparity.
Stereopsis
• Panum’s Fusional
Area: Images from an
object within the area
can be fused.
• Those outside the area
produce diplopia.
Stereopsis
• Fusion is accomplished by disparity selective
cells.
• They respond to specific amounts of disparity
and produce the sensation of depth.
Measurement of Stereopsis
• Stereoacuity: the minimum amount of disparity
one can use to detect depth.
• Measured in seconds of arc (arc sec).
– Adult stereoacuity is less than 40 arc sec.
• Measured with random dot stereograms. Arrays
of dots that appear to have a patternless texture.
Measurement of Stereopsis
Measurement of Stereopsis
• A portion of the stereogram contains two
patterns that are displaced laterally.
• When viewed with polarized glasses, the lateral
displacement creates artificial retinal disparity.
• i.e., a slightly different image is seen by each
eye.
• Creates the sensation of depth.
Measurement in Young Children
• Measurement in preschoolers can be
conducted using the Randot Preschool
Stereoacuity Test.
• Consists of three books that contain
random dot stereograms.
• Each stereogram is in the form of a
shape that is familiar to children.
– Hands, hearts, ducks, elephants, etc.
• Shapes cover a broad range of retinal
disparities
– 800 to 40 arc sec
Measurement in Young Children
• Infants and toddlers can be tested with Randot
Stereo Smile Cards.
– Modeled on the Teller Acuity Cards
• The cards are completely covered with a
random dot array.
• When viewed through polarized glasses, one
side of the card possesses a happy face of
crossed disparity.
– 480 to 120 arc sec
Measurement in Young Children
• The cards are presented following FPL.
• Very difficult to measure stereoacuity in infants
and toddlers.
• The target is not very salient.
Development
• Stereopsis emerges at 3.5 to 6 months of age
and shows rapid improvement.
• Then rises slowly to adult levels.
• Measures 100 arc sec at 3 years, 50 arc sec at 5
years, and 40 arc sec at 7 years.
Development
• May be correlated with the segregation of ocular
dominance columns.
• At birth, neurons from both eyes converge onto
single neurons in layer IV in the visual cortex.
• Later, there is a segregation of these
connections, and connections separate into left
eye and right eye columns.
– i.e., ocular dominance columns.
Development
• Convergence of connections occurs at the next
level.
• Animal studies show that stereopsis occurs at
the same time this segregation into ocular
dominance columns occurs.
Refractive Error
• The degree of myopia, hyperopia, or
astigmatism.
• Measured in diopters.
• The refractive power of a lens.
• The reciprocal of focal length.
• The focal length of the eye is approximately 17
mm.
Refractive Error
• The required refractive power of the eye is
60 D.
Refractive Error
• Emmetropia: No refractive error.
• Ametropia: Presence of refractive error.
Spherical Refractive Error: Due to a
mismatch between the focusing power of
the eye and the length of the eye.
• Reported relative to the 60 D norm.
Refractive Power
• Myopia: the eye is
too long to match its
focal power. The
image is focused in
front of the retina.
• Corrected with a
concave lens.
Refractive Error
• Focal length is longer than 17 mm
• Required refractive power will be less than 60
D.
– E.g. 57 D.
• Corrected with a concave lens with – 3D
power.
Refractive Power
• Hyperyopia: the eye
is too short to match
its focal power. The
image is focused
behind the retina.
• Corrected with a
convex lens.
Refractive Error
• Focal length is shorter than 17 mm
• Required refractive power will more than 60 D.
– E.g. 63 D.
• Corrected with a convex lens with +3 D power.
Refractive Error
• Cylindrical Refractive Error
• Astigmatism: a distortion in the shape of
the cornea.
• The cornea is curved more sharply along
one axis than along the others.
• As a result, the image is distorted.
• Can be corrected by using a lens that
counteracts the distortion
Measuring Refractive Error
• Cycloplegic Retinoscopy: Beam of
light is shone through the subject’s
optical system.
• Ophthalmologist looks through the site
hole and observes the reflected light
and a shadow.
• A mirror inside the retinoscope is
moved in various direction.
• Movement of the shadow is observed
Measurement in Infants and Toddlers
• Usually conducted using cycloplegia which
prevents accommodation.
• Can be difficult in infants and toddlers.
• Can be conducted without cycloplegia, but
requires expertise.
• They can be assessed using a photoscreener
(photorefractor).
Measurement in Infants and Toddlers
• Camera and a flash
source that take a
photograph of the flashed
light as it returns from its
passage through the
optical system.
• Based on the position
and amount of crescentshaped light reflected
from the subject’s pupil,
refractive error can be
determined.
Measurement in Infants and Toddlers
• Can also be measured
using an automatic
refractor or autorefractor.
• An infrared beam is
shone into the eye.
• The reflected beams
return to the autorefractor
and determines the
extent to which the beam
is out of focus.
Measurement in Infants and Toddlers
• Both techniques are fast, objective, and
require little expertise.
• Neither is as accurate as cycloplegic
retinoscopy.
Development
• We are born farsighted (hyperopic).
• This error is reduced through
emmetropization as the eye grows.
– 1 month = 2.2 D (Mayer et al., 2001)
– 1 year = 1.57 D
– 2 years = 1.19 D
– 3 years = 1.00 D
– 4 years = 1.13 D
Development
• The length of the eye increases rapidly at first
during the "infantile" high-growth period and
then more slowly during the "juvenile" slowelongation period.
• This moves the retina away from the cornea so
that, eventually, the length matches the focal
power, producing emmetropia.
• The growth of the eye appears to be controlled
by the amount of blur of the image it receives.