Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
Chiasm
ON
LGN
ON
M
LGN
P Striate cortex
M
P
M Pathway subserves:
motion & pursuit
direction
speed judgement
coarse stereopsis
Parieto-occipital
Temporo-occipital
Extrastriate cortex
Parieto-occipital M
Temporo-occipital P
P Pathway subserves:
fine acuity
shape
colour
fine stereopsis
Figure 2.2 Simplified illustration of the sensory
visual path demonstrating parallel pathways and
increasingly complex visual processing from each eye
through optic nerves (ON), lateral geniculate nucleus
(LGN), striate and extrastriate cortex Figure 2.3 A baby’s vision is tested using Teller
acuity cards, each with varying spatial frequency.
Note an example of the card on the table
neural layers of the retina, reaching the magnocellular
and parvocellular ganglion cell layers in
the inner retina. M and P parallel visual paths
project from the M and P ganglion cells in the
inner retina and travel in the optic nerve to the
lateral geniculate nucleus (LGN).
acuity and continues to improve to 6/6 by age
2 years (Figure 2.3). 2 Similarly, contrast sensitivity
for detecting movement and lower spatial
frequencies is thought to develop from about
3 months and be mature by 6 months. 3
Lateral geniculate nucleus
The M and P ganglion cells reach separate M
and P laminae in the LGN.
Striate cortex (V1)
The M and P paths project from separate
LGN laminae to the striate cortex. M neurons
project to lamina IVB and P neurons project to
laminae IVA, II and III.
Extrastriate cortex
M neurons project to parieto-occipital regions
of the extrastriate cortex and P neurons project
to temporo-occipital regions. Information then
combines into an integrated visual perception,
including motion, colour and fine detail. 1
The clinical significance of this is reflected in
the illustration that different visual functions
develop at different ages. Studies have shown
that at 6 months visual acuity is 6/36 on Teller
Integration components of normal
binocular vision
When an individual with normal binocular
vision is looking straight ahead at an object in
the primary position of gaze, equal visual
information is falling on corresponding points of
each retina and in particular the macula of each
eye. The information is being integrated within
the visual centre, the extrastriate cortex, and the
information is passed to the motor nuclei of the
extraocular muscle via the motor fusion centre
in the brainstem.
Sensory integration – extrastriate
visual cortex
Hubel and Weisel suggested that the
difference between uniocular deprivation and
binocular deprivation of vision depended on
interaction of information carried in two distinct
visual pathways from each eye to the visual
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