Nem’s Notes…
Phase 2 Year 3
NEUROSCIENCE 9 (page 1 of 1)
Visual Pathways & Function
Physiological
Optics
During normal function the eye focuses images sharply on the retina and fovea. If the
refractive mechanisms focus images behind the retina then this is termed
hypermetropia (long sight) and can be corrected with convex lenses, whereas if the
image is focused in front of the retina then this is termed myopia (short sight) and can
be corrected with concave lenses.
Astigmatism is caused when the cornea or lens of the eye does not have the same
curvature vertically and horizontally causing images to appear distorted in one plane.
This can be corrected using appropriately constructed lenses. It may occur with or
without hypermetropia or myopia.
Visual Pathway The optic nerves coming out of the eyes come together at the optic chiasm and then
continue in the optic tract to the visual cortex. At the optic chiasm the optic nerves
from the nasal retinas cross over in a partial decussation to join the opposing optic
tract. The temporal fibres do not cross. This is significant for understanding of visual
field defects.
Left Field
Nasal retina of left eye
Temporal retina of right eye
Right Field
Nasal retina of right eye
Temporal retina of left eye
Visual processing continues beyond the primary (striate) visual cortex (V1) into
extrastriate regions such as V2, V3, V4 and V5. These are responsible for functions
such as spatial vision and object recognition. Lesions to extrastriate areas can give
rise to perception difficulties.
Field Defects
Lesions to the optic nerves, chiasm or tracts can lead to visual field defects.
Site of Lesion
Transection of Optic Nerve
Transection of Chiasm
Transection of Optic Tract
Transection of Optic Radiation
Transection within Cortex
Pupillary
Reflex
Effect
Loss of vision in that eye
Bitemporal hemianopia
Homonamous hemianopia on opposing side
Superior Quadrantanopia on opposing side
Hom. Hemianopia with macular sparing (opp)
The pupillary reflex occurs when a light is shone into the eye. This causes the pupil to
constrict. At the same type the opposing eye will also constrict via a consensual
response. The afferent pathway involves the optic nerve, whilst the efferent pathway
involves the oculomotor nerve.
Reflex Response
Near
Reflex
Defect
No direct response
Consensual response normal
Efferent defect in illuminated
eye
No direct response in affected eye
Direct and Consensual normal in unaffected eye
Afferent defect in affected eye
The ‘Near Reflex’ is not strictly a reflex as it involves a triad of responses:
(a) Pupil constriction (sphincter pupillae)
(b) Convergence (medial rectus)
(c) Accommodation (ciliary muscle)
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