Development of BV

Although all the visual apparatus is intact soon after birth BV is not inborn but must be acquired gradually during the first few years of life. Several factors affect the development of BV

Motor Mechanisms favouring development of BV are concerned w/maintenance of the two eyes in the correct position at rest and during movement

Anatomical factors: structure of bony orbit & contents, structure of eye and posn in orbit (eyes should be visually aligned correctly at rest - slightly divergent. This is only apparent when px is dead)

Physiological factors

• Postural reflexes - these show how the head and eyes work together, independent of visual stimuli. The eyes are maintained in their correct relative posn in the orbit so that the visual axes are correctly aligned despite changes in the head rel. to the body etc. Dolls head phenom. UNCONDITIONED REFLEXES THAT DEVELOP @ BIRTH
• Fixational reflexes - maintenance of two eyes in the correct posn in the orbit. Vis axes aligned as a result of visual stimuli reaching visual cortex. GRADUALLY DEVELOP OVER FIRST FEW YRS OF LIFE

Those fixational reflexes in mo' detail

1. Orientational fixn reflex - ability of each eye INDEPENDENTLY to fix a definite object. Depends on each retinal receptor having visual spatial sense (in turn need functioning retina & adequate FOV). NO BINOCULARITY INVOLVED. Is present @ birth feebly DEVELOPS AS MYELINATION OF NF DEVELOPS & IS COMPLETED W/IN 2-3 WKS. NB Vision at birth is less than 6/60 - the fovea is not developed @ birth - fixation reflexes are present but the VA is too poor for them to be brought into use.
2. Re-fixation reflex (saccades and pursuits) - develops shortly after fixn reflex - the ability of the eye to retain fixn of a moving object (passive) or change fixn from one object to the other (active). STILL NO BINOCULARITY. CAN DO THIS BY 10-12 WKS AT MOST
3. Conjugate fixn reflex - fixn reflex applied to both eyes at the same time - both eyes retain fixn during versional movement. USUALLY PRESENT WITHIN 5-6WKS OF BIRTH AND WELL ESTABLISHED BY 6 MONTHS. Should be able to do motility test then
4. Disjunctive fixn reflex - both eyes retaining fixn at same turn during vergence movement. Develops later than (3), WELL ESTAB BY 6 MTHS.
5. Corrective fusion reflex - elaboration of (3) & (4) - permits eyes to function binocularly even under conditions of stress. FUNCTIONS @ 1YR BUT ONLY FULLY AT 5 YRS. Should be able to do 20 base out prism test & see the fusional movement.
   

***Critical period for establishing cent. fixn = first 3 mths of life. Can easily be lost during this time. Plastic period is up to about 5 yrs.
***Critical period for BV is the first year, but can still go wrong in plastic period

Sensory Mechanisms (visual apparatus, extrinsic oc. muscles)

1. VA - adequate degree of central vision dependent on reasonable integrity of fovea & macular elements, refracting media of the eye, degree of refractive compatibility between the two eyes & adequate perip. vision
2. NORMAL CORRESPONDENCE betw retinal recep of the two eyes - stimulus of corresponding visual points despite existence of 2 separate patterns of stimulation
3. HEMI-DECUSSATION OF ON FIBRES @ CHIASMA enables nerve fibres from corres. areas of the two eyes to become associated with one another ultimately in visual area of occipital cortex. Become closely assoc in optic radiations & near termination of fibres in the visual cortex
4. PROPRIOCEPTIVE IMPULSES OF THE EXTRINSIC OC. MUSC. This provides brain w/sensory info. Not known if plays role in BV development

Central Mechanisms - act of fusion & cortical control of sensory movement

1. Fusion - single picture of obj built up by activity of the striate areas on both sides of the vis.cort. and the final analysis is implemented by the higher visual centres - the relatively crude visual image of the striate area is given meaning & is integrated w/other sensory inputs/past experience. ANATOMICAL/PHYSIOLOGICAL POTENTIAL PRESENT @ BIRTH, ESTAB. GRADUALLY DURING FIRST YEARS OF LIFE. All the different fixation reflexes need to be working properly for it to be maintained.
2. Cortical Motor Control - the integrity of parts of the cerebral hemisphere controlling cranial nuclei concerned in final efferent impulses to extrinsic ocular nuclei

Summary

All of the the above mechanisms develop side by side and each require the others to be functioning correctly - one visual system. They are all conditioned reflexes w/the exception of the postural reflex and depend on visual stimuli. If px has congenital cataracts you have to get rid of em fast!

Critical period(s)

• 2-3 months for fixation
• 2-3 yrs for VA

VA won't develop if the above factors aren't favourable and during the plastic period (up to about 5 yrs) any disturbance may break it down. After the age of 5 the reflexes start to acquire the fixity comparable to an unconditioned reflex. They will be maintained throughout life unless there's some serious kind of pathology

If BV isn't allowed to develop the neurons and cortical cells develop differently and the system is abnormally conditioned ie ARC & SUPPRESSION. These will become fixed at around 5 yrs also. EARLY DETECTION IS IMPORTANT FOR TREAMENT WHICH MUST AT LEAST TAKE PLACE DURING THE PLASTIC PERIOD TO BE SUCCESSFUL.