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, May 3, 2023

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 PERCEPTIONLight 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.


Components of visual acuity. 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)


Components of visual acuity. 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.


Components of visual acuity. 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.


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NEUROPHYSIOLOGY OF VISION

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