BV Flashcards
What is BV?
- Getting the eyes to work together
- Must be able to converge or diverge
- Two images of an object must fall on corresponding parts of the two retinas
– Requires good acuity in both eyes
– Good control of eye movements
– Ability to tell whether an objects is closer or farther than the point of fixation
– Good connections between sensory and motor systems
Response of infants to different cues to depth
– Visual cliff experiment
– Impending collision
– Size and shape constancy
– Reaching
Monocular Cues
- Perspective
- Relative Size
- Relative Height
- Distance Fog
- Texture Gradient
- Depth from focus
- Occlusion
- Color Vision
- Shadow
- Motion parallax
binocular cues
-convergence
-stereopsis
Binocular Depth Cue: Convergence
- Convergence measures depth by measuring the direction your eyes are pointing.
- The angle of convergence is larger when the eye is fixating on near objects.
- As an object gets further away, your eyes almost become parallel
- Brain keeps track of this convergence to determine your distance from the object on which you are focusing
Stereopsis
- Most precise cue to depth depends on retinal disparity
- Two images fall on non-corresponding parts of the retina
– Crossed disparity: lines of sight cross each other between the fixation point and the eyes
– Uncrossed disparity for objects further than the fixation point
– Cells in adult visual cortex respond to crossed disparity (near cells); and respond to uncrossed disparity (far cells)
Preferential Looking
Measures of Stereopsis
- PL for a pair of line displays
– Some lines appear to stand out in front of the others
– Other display is flat - Display with apparent depth suddenly becomes more interesting to infants around 16 weeks of age
- Stereopsis increases from >60 minarc to <1 minarc in a few weeks
- Crossed disparity develops sooner than
uncrossed
Other Stimuli for Stereopsis
- FPL and VEP studies with random dot stereograms (RDS)
– Pattern of dots around the edge identical for right and left eyes
– Pattern of dots in center that are the same but with a crossed or uncrossed displacement - Move the central display around; i.e. a moving RDS stimulus
Development of Stereopsis
Stereopsis emerges at 3.5 to 6 months across studies
Stereopsis and Orthotropia
- Stereopsis does not require the attainment of accurate vergence control
- 88% show brief periods of misalignment up to 6 mos
- Diplopia unlikely due to:
– Panum’s fusional area
» 15 minarc near fixation in adults
– Fovea immature, tolerates small degree of eye misalignment - Stereopsis for crossed disparity precedes uncrossed disparity
Stereopsis not limited by retinal maturation alone
- Studies on stereopsis use stimuli within visual acuity limit of infant
- No abrupt change in visual acuity accompanies the onset of stereopsis
Correlation of onset of stereopsis and segregation of ocular dominance columns
- Segregation of ocular dominance columns occurs at the input layer of the visual cortex (layer IV)
- At birth, afferents from the left and right eyes overlap and synapse onto the same neurons in IV
- Sometime after birth segregate into separate right/left columns and convergence occurs at next cortical layer
– Cats- segregation of afferents at 5 -6 weeks coincides with development of disparity detection
– Monkeys: stereopsis appears at 4 weeks and improves between 4-8 weeks while the left and right eye afferents are segregating
Pre-and Post-Stereoptic Periods
Two aspects of binocular summation differ in the period before and after the development of stereopsis
1. Visual acuity is better by square root of 2 when both eyes are viewing
2. Size of the pupil
Pre- and Post-Stereoptic Periods of fusion vs rivalry
- Using FPL, infants prefer the left screen in the pre-stereoptic period, presumably because they see the fused percept (B2)
- In the post-stereoptic period, infants prefer the right screen, presumably because the left screen has become rivalrous and is unpleasant (B1)
- Avoidance of rivalrous stimuli coincides with emergence of stereopsis
BV Clinical Impact
- Binocular fusion at mean age 12.8 weeks
- Disparity detection by 3-4 months
- Stereopsis
At 16 weeks with PL; 5-6 mo achieve <1 minarc
At 3-4 mo with VEP; youngest age 7-8 weeks
Clinical Impact
* Lack of stereopsis may indicate amblyopia and/or strabismus
