1) The visual system develops post-natally and requires exposure to normal environmental stimuli during critical periods for proper maturation.
2) Monocular deprivation during the critical period can lead to ocular dominance and amblyopia due to abnormal cortical development and lack of binocular interaction.
3) Visual acuity develops rapidly in the first year of life, reaching adult levels of 20/20 by ages 3-5 years for spatial vision functions like grating acuity.
3. Ocular dominance
Most binocular cortical neurons do not receive inputs from two eyes
Closure of an eye of an immature animal results in a lack of cortical cells driven by the deprived
eye
Critical/ sensitive period
•Period in which visual system is influenced by environmental manipulation
•During critical period cortical cells of eye will compete
No binocular action- No stereopsis
8. Synopsis of critical period- Hubel’s and
weisel’s experiment
Synaptic connectivity in the in the cortex is strengthened by the neural activity and lack in the
neural activity result in weakening of this connections
9. Amblyopia
•Reduction in vision secondary to monocular deprivation during the critical period
•Amblyopia results from the abnormal cortical development, not an abnormality of eye
•It can occur secondary to
• Anisometropia
• Strabismus
10. AMBLYOPIA
Occlusion
amblyopia
-One eye occluded in
critical period
-Monocular congenital
cataract
-Lid ptosis
Anisometropic
amblyopia
-Unequal refractive
errors
Strabismic
amblyopia
-Constant unilateral
strabismus present
during critical period
-Diplopia
-Suppression
Meridional
amblyopia
-Cortical neurons are
orientation selective
-Meridional astigmatism
13. Age- months Average visual acuity
1 20/638
1.5 20/540
2.5 20/278
4 20/224
6 20/106
9 20/88
12 20/93
18 20/70
24 20/63
30 20/52
36 20/28
48 20/24
VISUAL ACUITY IN EARLY LIFE
14. Post natal vision
NEONATE
• 8 to 14 inches
• 20/200 - 20/600
• Mothers arm to
her eyes
1-3 MONTH
• Start gazing at
objects
• 2 months- eye’s
coordination
develop
• Hand-eye
coordination
• Color perception
4-7 MONTH
• Enjoys more
complex designs
• Till 1 meter
• Fine movement
of objects
8-12 MONTH
• Focuses on a toy,
crawl pull it and
picks it
• Grasp and throw
objects
• Can perceive
depth
1 YEAR
• Depth
perception
continues
20. Temporal Vision: Critical Flicker Fusion
Frequency
•40 Hz at 1 month
•55 Hz by 3 months- adult level
The retinal and cortical immaturities that slow the
development of grating and Vernier acuity apparently have
little effect on the maturation of temporal resolution
21. Scotopic Sensitivity
•Adult-like at 1 month of age
•Shape of the function- Characteristics of rhodopsin
•Does not depend on postreceptoral processing
•Scotopic sensitivity- 507 nm
•Reaches adult levels by approximately 6 months
22. Colour Vision
•Red–green discrimination arises during the second month of life
•Blue–yellow discrimination
•Adult like on the 1 year of life
•Photopic spectral sensitivity function is adult-like in young infants
Recordings from individual neurons in one hemisphere of striate cortex
1 and 7 are monocular
1 receives from contralateral and 7 receives from ipsilateral
For cat critical period ends in 3 months
Humans- 2 years
Critical period – 7 to 8 years
The peak of the CSF is at the adult location (4 cycles/degree) at
approximately 4 years, and the overall function is adult-like by 9 years