Psychology 4051

Visual

Functions

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 object 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 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 appears to have a patternless texture.

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

• The cards are presented following FPL.• Very difficult to measure stereoacuity in infants

and toddlers.• The target is not very salient.

Measurement in Young Children

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 eye 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 other.• 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 return from its passage through the optical system.

• Based on the position and amount of crescent shaped 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" slow-elongation 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.