Neurology of Vision Natesan Green
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Transcript of Neurology of Vision Natesan Green
8/6/2019 Neurology of Vision Natesan Green
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NEUROLOGY OF VISION
NATESAN.D
SUBHRANSU
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VISION¶the sense of sight, which¶the sense of sight, which
perceives the form, color, size,perceives the form, color, size,
movement, and distance of movement, and distance of
objects·objects·
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THE VISUAL PATHWAY
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OPTIC NERVE
� The axons of the ganglion cells
� Extends from the retina to optic chiasm.
� It is 5 cm long.
-Intraocular( 1 mm)
-Intraorbital( 25 mm)
-Intracanalicular( 9 mm)
- Intracranial(15 mm)
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OPTIC CHIASM
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WILBRAND·S KNEE
Wilbrand¶s knee
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OPTIC TRACT
� 55% fibres are from contralateral nasal retina
� 45% fibres are from same side temporal retina
� The retinotopic organisation is well maintained, but with
change in orientation of the fibres.
� Incongruity of the visual field defect.
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LATERAL GENICULATE BODY´the gateway to cortexµ
� The Subcortical thalamic relay nucleus of vision,starting the process of co-ordinating vision fromthe two eyes
� C-shaped 6 defined layers
� Layers 1, 2 receive from large M ganglion cellsMagnocellular division
� Layers 3,4,5 & 6 from small P ganglion cellsParvocellular division
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LATERAL GENICULATE BODY
� Each layer receives input from one eye only- Layers 1,4,6 from contralateral eye
- Layers 2, 3,5 from ipsilateral eye
� Responses of neurons are similar to retinal GC(on-centre & off-centre organization)
� LGB also receives input from brain stem,reticular formation & feedback from cerebralcortex
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OPTIC RADIATIONS
� Third order neuronal axons.
� Projection of LGB on to striate cortex.
� Two portions:
± Fibers from the inferior retina("Meyer's loop" or ́ Archambault's loopµ).
± Fibers from the superior retina
("Baum's tract")
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PRIMARY VISUAL CORTEX(V1)(AREA 17)
� On medial aspect of each occipital lobe
� Its neurons arranged in the form of columns (1x1x2
mm) forming 6 distinct layers
� The input from the two eyes remain separated (Ocular
dominance columns)
� Fovea has broad presentation
� Its neurons (layers 2,3,4) project into areas 18 & 19
(association or secondary visual areas)
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Visual area V2(prestriate cortex)
� the first region within the visual association area.� receives strong feeds from V1 (direct and via the
pulvinar)
� sends strong signals to V3, V4, and V5.
� also sends strong feedbacks to V1
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OTHER ASSOCIATION AREAS
� V3- DORSAL V3 POSTERIOR PARIETALVENTRAL V3 INFERIOR TEMPORAL
� V4- (Part of ventral stream)
V2 POSTERIOR INFERIOR TEMPORAL
� V5 (MIDDLE TEMPORAL AREA )
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VISUAL PROCESSING
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VISIBLE SPECTRUM
370-730 nm
SCOTOPIC: 507 nm ,low light, rods (cones
unresponsive), 10-6 to
10Cd/m2
MESOPIC: mediumlight, rods and cones,
0.01 and 10Cd/m2
PHOTOPIC: 555 nm,
bright light , cones(rods saturate),
0.01 and 108Cd/m2
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A BASIC CONNECTIVITY PATTERN OF
THE VISUAL PATHWAY
Photoreceptor Bipolar cell Ganglion
cell LBG Striate cortex
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Photoreceptors release glutamate in darkness andexhibit graded hyperpolarizations in response to
different luminance levels
Hyperpolarization of photoreceptors results in
decreased glutamate release
PHOTOTRANSDUCTION
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` Bipolar cells are of two
types:
´ONµ cells
´OFFµ cells
` Ganglion cells are of 3
types :
Parvocellular-X,Magnocellular-Y,
Koniocellular
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BIPOLAR CELLS OFTHE RETINA
MECHANISMS FOR ¶ON· AND ¶OFF· RESPONSES
Bipolar cells exhibit graded
electrical potentials (i.e., NOT
action potentials)
They release neurotransmitter(glutamate) in proportion to the
level of depolarization as opposed
to the rate of action potentials
¶OFF· BIPOLAR
¶ON· BIPOLAR
0 mV
-40 mV
0 mV
-40 mV
LIGHT ON
LIGHT ON
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0 mV
-40 mV
light on
LGN
µOFF¶ bipolar (glutamate depolarizes)
0 mV
-40 mV
light on
photoreceptor
0mV
-40 mV
light on
µON¶ ganglionµOFF¶ ganglion
light onlight on
A LIGHT SPOT IN AN
OTHERWISE DARK
FIELD:
µON¶ bipolar (glutamate hyperpolarizes)
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0 mV
-40 mV
light off
LGN
0 mV
-40 mV
light off
photoreceptor
0mV
-40 mV
light off
µON¶ ganglionµOFF¶ ganglion
light off light off
A DARK SPOT IN AN
THERWISE ILLUMINATED
FIELD:
µOFF¶ bipolar (glutamate depolarizes)
µON¶ bipolar (glutamate hyperpolarizes)
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GANGLION CELL OUTPUT OF THE RETINA
PARVOCELLULAR-
(X)
MAGNOCELLULAR(Y)
KONIOCELLULAR
SURROUND
INHIBITION
YES YES NO
COLOROPPONENCY
YES NO YES
RECEPTIVE FIELDSIZE
SMALL LARGE --------
RESOLUTION HIGH LOW ---------
RESPONSE TOLIGHT
SUSTAINED TRANSIENT ---------
LOW-CONTRAST,
MOVING STIMULI
POOR
RESPONSE
STRONG
RESPONSE
--------
PERCENT OFGANGLION CELL
POPULATION
80% 10% 10%
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PARVOCELLULAR
LAYERS
MAGNOCELLULA
LAYERS
KONIOCELLULAR
LAYERS
THE LGB: LAYERING CORRESPONDS TO TYPE
OF GANGLION CELL
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Neurons in the Cerebral Cortex
Simple CellsNeurons with fixed excitatory & inhibitory zones in theirreceptive fields
Found only in the Primary Visual Cortex (V1)
Complex Cells
Receive input from a combination of Simple Cells
Have receptive fields that respond to particular orientationsof light but cannot be mapped into fixed excitatory &inhibitory zones
Located in V1 and V2End-stopped (Hyper-complex) Cells
Strongly resemble complex cells but have an inhibitory area atone end of its bar-shaped receptive field
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PRIMARY VISUAL CORTEX (AREA 17 / V1)
CORTICAL COLUMNS
OCULAR DOMINANCE
COLUMNORIENTATION
COLUMNS
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ORIENTATION
COLUMNS
OF
THE STRIATE
CORTEX
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COLUMNS AND BLOBS
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COLOUR
Theories of colour vision:` TrichromaticTheory
` Opponent-process Theory
` RetinexeTheory (The Land Effect)
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�Horopter
�Panum's
Fusional
Area
�Retinal
disparity
Stereopsis ² the ability to appreciate depth
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-Damage to IT (TE + TEO) impairs object identification
(but only via visual information)
-Damage to parietal cortex (MT, MST, 7a, VIP, LIP) impairs visuospatial
abilities(e.g., reaching to an object)
¶ WHAT· (TEMPORAL) AND ¶ WHERE·
(PARIETAL) PATHWAYS
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DISORDERS OF OBJECT RECOGNITION
Visual AgnosiaThe inability to recognize objects
despite otherwise normal vision
Prosopagnosia
The inability to recognize faces
without an overall loss of vision
or memory
The Fusiform Gyrus in the
Inferior Temporal Cortex is
specialized for face recognition
Identifying car models, bird
species
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MOTION DETECTION
Medial Temporal Cortex
Middle Temporal Cortex &
Medial Superior Temporal Cortex
important in motion detection
Involved in distinguishing
between moving objects
& head changes
Damage to Medial Temporal
Cortex results in motionblindness
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THANK YOU