Neuro Visual Tracts Flashcards
What is accommodation
Adjustment of the eye for seeing objects at various distances
This is accomplished by the ciliary muscle, which flattens or thickens the lens of the eye for distant or near vision
Presbyopia
Loss of lens elasticity with age
Optic disc
Blind spot in retina where optic nerves from ganglion layer exit the eye
Located medially to the macula
Visual fields
Light passes from objects in the visual field, through the pupil to subtend an image upon the retina, creates a retinal field
Image formed on retina is inverted in both lateral and vertical dimensions
Object of attention is focused and centered at the location of the fovea centralis and macula lutea
Retinal fields
Location on the retina that an object in visual field is projected
Each visual field is divided into retinal hemifields and each hemifield is divided into quadrants
L/R visual field in comparison to retina
The left half of the visual field forms an image upon the nasal half of the left retina and the temporal half of the right retina, visa versa
Decussation of CN II
The optic nerves partially decussate in the optic chiasm
Nasal half of each retina = contralateral optic tract
Temporal half of each retinal field = ipsilateral optic tract
This brings together comparable areas of both retinas creating depth perception
Optic tract and LGN
Optic tract curves posteriorly around the cerebral peduncle to terminate in the LGN
Fibers terminate in precise retinotopic pattern
LGN organization
Ventral base of the nucleus is formed by the incoming optic tract fibers
Dorsal and lateral borders are formed by the outgoing optic radiations
LGN magnocellular layers
Magnocellular layers: contain large cells
Consist of ventral layers 1 and 2 of LGN, receive ganglion cell inputs relaying from rods
Larger receptive fields and thick, rapidly conducting axons, sensitive to moving stimuli
LGN parvocellular layers
Dorsal layers 3-6
Contain small cells
Receive ganglion cell inputs relaying from cones
Small receptive fields, slower conducting axons, tonically responsive to stationary stimuli, high-acuity color vision
Ganglion cell axons arising in temporal retina in relation to layers
Fibers remain uncrossed and terminate in layers 2, 3, 5 of the ipsilateral LGN
Ganglion cell axons arising in nasal retina in relation to layers
Fibers cross and terminate in layers 1, 4 and 6 of the contralateral LGN
List the layers and side of LGN that the nasal and temporal regions of the right visual field would terminate in
Nasal portion of right eye- layers 1, 4, 6 of left LGN
Temporal portion of left eye- layers 2, 3 and 5 of left LGN
Try the same with left visual field
Optic radiations
Secondary neurons from LGN give off a bundle of fibers- the optic radiation
These relay to the primary visual cortex, located on the upper and lower banks of the calcarine sulcus
Also referred to as the geniculostriate or geniculocalcarine pathway
Fibers from lower quadrant of the contralateral hemifields
Originate from dorsomedial portion of LGN
Arch caudally to pass through the retrolenticular limb of the internal capsule
Target superior bank of the calcarine sulcus, on the cuneus
Fibers from upper quadrant of the contralateral hemifields
Originate from ventrolateral portion of LGN
Arch rostrally, passing into the white matter of the temporal lobe to forma a broad U-turn, the Meyer loop
Target inferior bank of the calcarine sulcus, on the lingual gyrus
Fibers conveying info from the macula and fovea
Originate from central regions of the LGN and pass to caudal portions of the visual cortex
Temporal lobe damage and visual field defects
Can produce a superior visual field deficit because of Meyers loop
Macula representation in cortex
Macula is represented by disproportionately large volumes (relative to size) of the LGN and visual cortex
It is represented most posteriorly, in the region of the occipital pole
Projection of visual field in cortex
The upper left quadrant of the visual field is represented in the lower right quadrant of the visual cortex and so on
Inferior visual fields project above the calcarine sulcus, superior visual fields below
Macula is represented more posteriorly, and peripheral fields more anteriorly
Calcarine sulcus - broadmanns area
Bordmanns area 17 - calcarine sulcus/primary visual cortex
Also known as striate cortex, because you see numerous myelinated fibers within a discrete layer, giving a striped appearance
Visual association cortex
Area 17 is surrounded by areas 18 (immediately superior and inferior to 17) and 19 (immediately superior and inferior to 18)
Includes parieto-occipito-temporal area:
Heavily interconnected with 17, receives targets from LGN and helps interpret location, motion, form and color
Superior colliculus
Spatially directs head movements and visual reflexes
Receives input from the retina/optic tract as well as area 17
Important for directing eye movements
Also receives spinotectal and auditory inputs
Pretectal/pretectum area
Located near midbrain/forebrain junction
Receives input from optic tract, LGN and suprachiasmatic nucleus
Important in the pupillary light reflex
Pupillary reflex arc
Retinal axons terminate in the olivary pretectal nucleus
Bilateral projections to Edinger-westphal nucleus
Parasympathetics exit with CN III, synapse in ipsilateral ciliary ganglion
Postganglionic fibers (short ciliary nerves) excite pupillary constrictor muscle
Decussation of retinal fibers permits consensual response in opposite eye
Optic nerve damage and pupillary light reflex
Shining a light into that eye will produce diminished direct and consensual responses, but both will be present when the undamaged side is illuminated
Total lesion of optic nerve effect on light reflex
Blindness in that eye
Neither a direct nor a consensual response would be elicited when the eye is illuminated
Lesion in optic tract or pretectum effect on light reflex
Neither response is lost, although reflexes may be weaker
Large lesion in posterior/dorsal midbrain effect on light reflex
Would weaken pupillary responses bilaterally
Lesions in oculomotor nucleus or nerve effect on light reflex
Both direct and consensual responses will be lost ipsilaterally, but will be present contralaterally
Macular sparing
Visual field loss that preserves vision in the center of the visual field
Congruous visual field deficits
When the visual field loss of one eye can be superimposed on that of the other eye
If lesion is closer to the visual cortex, the more congruous it is likely to be
The more anterior a lesion is in the optic tract or radiations, the more likely it will be incongruous
Overarching concept
Damage anterior to chiasm affects only ipsilateral eye
Damage at chiasm causes heteronymous deficits
Damage behind chiasm causes homonymous deficits
Associative visual agnosia
Infarction of the left occipital lobe and posterior corpus collosum
Typically due secondary to occlusion of posterior cerebral artery
Disconnects the language area from the visual cortex
Patient cannot name or describe an object in the visual field, but he can recognize and demonstrate its use
-Visual perception is intact
Patients may also be alexic (unable to read) & writing ability my be affected (agraphia)
