NEUROPHYSIOLOGY OF VISION

DR NILESH KATE
MBBS,MD
ASSOCIATE PROF
DEPT. OF PHYSIOLOGY
NEURO
PHYSIOLOGY
OF VISION.

PROCESSING & TRANSMISSION OF VISUAL
IMPULSE IN VISUAL PATHWAY
 Optic nerve
 Optic chiasma
 Optic tracts
 Lateral geniculate bodies
 Optic radiations

OPTIC NERVE
 Axons of the retinal
ganglionic cells.
 Macular fibres – pass
straight into temporal
part of optic disc.
 Temporal fibres – above &
below as arcuate fibres.
 Nasal fibres – to nasal half
of disc as superior &
inferior part radiating
fibres.
Wednesday, June 8, 2016

OPTIC CHIASMA
 Flattened structure above
pituitary fossa.
 Fibres from nasal half of
retinae decussate &
temporal half remains same.
 Visual impulses from
temporal half of visual field
cross while from nasal half
remains same.

OPTIC TRACTS
 From Posterolateral
angle of Chiasma to
lateral geniculate
body.
 Contains temporal
fibres of same side &
nasal fibres of
opposite side.

LATERAL GENICULATE
BODIES
 Retinotopic
projection.
 Optic tract fibres project
detailed spatial
representation of retina
on LGB.
 Lamellar structure of
LGB.
 6 Layers.
 1,4,6 receives from
nasal half.
 2,3,5 from temporal
half.
 In each layer point to
point representation.

LATERAL GENICULATE
BODIES
 Magnocellular &
Parvocellular layers.
 1& 2 are Magnocellular
receives fibres from M
ganglion cells of retina.
 3-6 are Parvocellular
receives from P ganglion
cells of retina
 Functions.
 Relay station.
 Visual perception & to
“Gate” the transmission of
signals. – how much to pass
to cortex.
 10-20% from retina.
 80-90% from visual cortex
& other brain regions.
 Signals from 2 eyes are
kept apart in LGB.

OPTIC RADIATIONS
 Axons of LGB relay
cells to visual cortex
on same side.
 Maintains Retinotopic
organization.

PROCESSING & ANALYSIS OF VISUAL
IMPULSE IN THE VISUAL CORTEX
 Retinotopic organization
 Functional anatomy & organization of visual
cortex
 Visual areas (Classical nomenclature)
 Primary visual cortex
 Peristriate cortex
 Parastriate cortex
 Modified nomenclature of visual areas
 Histological layers of primary visual cortex

RETINOTOPIC ORGANIZATION
 Visual cortex – cortical
retina.
 Its only in Visual
Cortex that impulses
from corresponding
points of 2 retina
meet.

FUNCTIONAL ANATOMY &
ORGANIZATION OF VISUAL CORTEX
 Visual areas.
 Primary visual
cortex.
 Peristriate cortex.
 Parastriate cortex

MODIFIED NOMENCLATURE
OF VISUAL AREAS
 V1 – prim visual
cortex (Brodman 17)
 V2 – greater part of
Brodmann 18
 V3 – narrow strip of
area 18.
 V4 – area 19.
 V5 – Middle temporal
area.

HISTOLOGICAL LAYERS OF
PRIMARY VISUAL CORTEX
 6 layers.
 I-III – thin, contains
pyramidal cells.
 IV – thickest, contains
stellate cells.
 Subdivided into a ,b , cα
& cβ
 V & VI – relatively
thin.

PHYSIOLOGICAL CONSIDERATION OF
VISUAL CORTEX
 Concept of receptive field of striate cortex
 Columnar organization of the striate cortex
 Orientation column
 Ocular dominance column
 The colour blobs

CONCEPT OF RECEPTIVE FIELD
OF STRIATE CORTEX
 Simple cells
 Complex cells
 Hyper complex cells
 Retinal ganglion cells &
LGB respond to both
diffuse & spot stimuli but
cortex respond to straight
line, bar or edge in proper
spatial orientation.
 So depending on receptive
field peculiarities cell types
are..

SIMPLE CELLS
 Mainly in layer IV.
 Form the first relay station.
 Respond to lines, bars & edges
only in particular orientation.
 This orientation most effective
in evoking in response is
“receptive field axis
orientation”
 Thus it detects not only lines &
borders but also orientation.

COMPLEX CELLS
 Above & below layer IV
in area 17,18,19.
 They respond to lateral
movement of linear
stimulus without
change in orientation.
 They receive signals
from both the eyes.

Complex cells
 Thus play an
important role in
detection of lines,
bars, edges especially
when moving.
 So simple & complex
cells together called
“Features Detectors”

Hyper complex cells
 In layer II & III of
17,18 & 19.
 Require line stimulus
of specific length.
 Thus play a role in
detection of specific
length, angle & shapes.
 Types (by Hubel &
Wiesel)
 2 lower
 2 higher.

Columnar organization of the
striate cortex
 Visual cortex organized
into vertically oriented
functional modules –
hypercolumns.
 They respond to lines of
all orientation from
particular region in space.
 Each include 3 types of
columns.
 Orientation column
 Ocular dominance
column
 The colour blobs

Orientation column
 Like sensory cortex visual
cortex divided into vertical
columns from pial surface to
white matter .i. e vertical
grouping of cells with
identical orientation
specificity.
 As one from one column to
next orientation preference
changes 5-10 degree.

Ocular dominance column
 Simple cells in layer IV
receives input from single eye
& complex & hyper complex
cells of layer above & below IV
receives binocular inputs.
 So group of binocular complex
& hyper complex cells in layer
II,III,IV & VI & cells in layer IV
that receives inputs from same
eye is called ocular dominance
column.

The colour blobs
 Among primary visual
columns are special
column like areas called
colour blobs which
responds specially to
colour signals.
 These are primary areas
for deciphering colours.

Concept of serial & parallel
processing of visual information
 Parallel processing pathways
 Magnocellular pathways
 Parvocellular pathways
 Serial processing of visual information
 Serial processing in the retina
 Serial analysis of visual image in the visual cortex

Parallel processing pathways
 2 types of cells found entire pathway from
retina to cortex.
 Large ( Magnocellular)
 Small (Parvocellular)
 Like 2 separate lanes of road.
 So different features of images analysed by
these pathways.

Concept of parallel processing
pathway

Serial processing of visual
information in cortex.
 Sequence from simple to
complex to hyper complex
cells forms a system of
serial analysis with more &
more details being
presented.
 As simple cells are
monocular & complex cells
are binocular , complex
cells are at advance stage.

VISUAL PERCEPTION

VISUAL PERCEPTION
Light sense.
Form sense.
Contrast sense.
Colour sense.

Light sense.
 It’s awareness of light.
 Minimum brightness
required to evoke
sensation of light.
 Human eye can work
normally over wide
range of illumination
by visual adaptation.

Dark adaptation.
 Def – ability of eye to
adapt to decreased
illumination.
 Time taken to adapt is
dark adaptation time.
 Rods used more in
dim light ( scotopic
vision) than cones
( photopic vision)

Dark adaptation curves
 Grapg shows visual
threshold falls
progressively in dark
room for ½ hour until
relatively constant
value is reached.

Dark adaptation curves
 Graph shows increase in
sensitivity of retina in 2
steps –
 First rapid, short & small
due to cone adaptation
 Second slow, long & large
due to rods adaptation.
 Breakpoint is cone-rod
break or alpha point.

Mechanisms of dark
adaptation.
 Visual pigment
mechanism. – reversal of
mechanism of light
adaptation i. e.
regeneration of visual
pigments.
 Change in Pupillary size. –
dilation about 30 times
 Neural mechanism – visual
chain in retina.
 Vit-A deficiency.
 Elevates threshold for
dark adaptation due to
depletion of
photosensitive
pigments.
 Night blindness –
Nyctalopia.

Light adaptation.
 Def – process by
which retina adapt
itself to bright light.
 Very quick, over in 5
min.
 It’s merely
disappearance of dark
adaptation.

Mechanism.
 Neural adjustment –
responsible for transient
effect.
 Visual pigment
mechanism – reduction
of rhodopsin & cone
pigment due to bleaching
in light.
 Pupillary mechanism –
constriction.

The form sense
 Ability to discriminate
between shapes of
objects.
 Cones are imp.
 Snellen’s chart is
measure of form
sense.

visual acuity
 Measurements of
threshold of
discrimination of 2
separate targets is
visual acuity.

Components of visual acuity.
 Minimum visible-
visibility or
detection.
 The ability to
determine whether
object is present in
empty field
 Depend on size, shape
& illumination.

 Resolution –
 Discrimination of 2 separate
points.
 Minimum separation is
minimum resolvable.
 Measurement of threshold of
discrimination is function of
fovea centralis called ordinary
visual acuity.
 Distance measured by angle
subtended by 2 points at nodal
point of eye. (MAR)

 Recognition.
 Component by which we not
only discriminate spatial
characteristics but also
identifies pattern with
previous experience.
 Spatial resolution with
cognitive components.
 E. g- identification of faces.

 Minimum
discriminable or
hyperacuity.
 Spatial distinction.
 Threshold is lower
than ordinary acuity.
 Vernier acuity.

Snellen’s test types
 Visual acuity is same as
minimum resolvable.
 Principle – 2 points seen
separate only if they form
an angle of 1 min at nodal
point.
 Each line form angle of
1min
 Each letter form an angle
of 5 min.

Critical flicker fusion frequency
 When light stimuli given
intermittently produces
flicker sensation.
 If frequency of it increased it
fuses to form continuous
stimulation
 This frequency is Critical
flicker fusion frequency.
 Temporal resolving power
of visual system.

Contrast sensitivity.
 Ability to perceive
slight change in
luminance between 2
regions separated by
borders.
 Loss of contrast is
more important than
loss of acuity.

Encoding of contrast.
 At border between
light & dark.
 In lighted portion
center is illuminated &
on cell ganglion cell
activity increased.
 In darkened portion
surround is illuminated
& on cell ganglion cell
activity decreased.

NEUROPHYSIOLOGY OF VISION

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