NEUROPHYSIOLOGY OF VISION

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  • DR NILESH KATE MBBS,MDASSOCIATE PROFDEPT. OF PHYSIOLOGYNEURO PHYSIOLOGY OF VISION.

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

  • OPTIC NERVEAxons 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.

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  • OPTIC CHIASMAFlattened 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.

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  • OPTIC TRACTSFrom Posterolateral angle of Chiasma to lateral geniculate body.Contains temporal fibres of same side & nasal fibres of opposite side.

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  • LATERAL GENICULATE BODIESRetinotopic 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.

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  • LATERAL GENICULATE BODIESMagnocellular & 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.

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  • OPTIC RADIATIONSAxons of LGB relay cells to visual cortex on same side.Maintains Retinotopic organization.

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  • PROCESSING & ANALYSIS OF VISUAL IMPULSE IN THE VISUAL CORTEX Retinotopic organization Functional anatomy & organization of visual cortexVisual areas (Classical nomenclature) Primary visual cortexPeristriate cortexParastriate cortex Modified nomenclature of visual areasHistological layers of primary visual cortex

  • RETINOTOPIC ORGANIZATIONVisual cortex cortical retina.Its only in Visual Cortex that impulses from corresponding points of 2 retina meet.

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  • FUNCTIONAL ANATOMY & ORGANIZATION OF VISUAL CORTEXVisual areas.Primary visual cortex.Peristriate cortex. Parastriate cortex

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  • MODIFIED NOMENCLATURE OF VISUAL AREASV1 prim visual cortex (Brodman 17)V2 greater part of Brodmann 18V3 narrow strip of area 18.V4 area 19.V5 Middle temporal area.

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  • HISTOLOGICAL LAYERS OF PRIMARY VISUAL CORTEX6 layers.I-III thin, contains pyramidal cells.IV thickest, contains stellate cells.Subdivided into a ,b , c & cV & VI relatively thin.

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  • PHYSIOLOGICAL CONSIDERATION OF VISUAL CORTEXConcept of receptive field of striate cortexColumnar organization of the striate cortexOrientation columnOcular dominance columnThe colour blobs

  • CONCEPT OF RECEPTIVE FIELD OF STRIATE CORTEXSimple cellsComplex cellsHyper complex cellsRetinal 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..

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  • SIMPLE CELLSMainly 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 orientationThus it detects not only lines & borders but also orientation.

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  • COMPLEX CELLSAbove & 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.

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

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  • Hyper complex cellsIn 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 lower2 higher.

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  • Columnar organization of the striate cortexVisual 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 columnOcular dominance columnThe colour blobs*

  • Orientation columnLike 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.

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  • Ocular dominance columnSimple 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.

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

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  • Concept of serial & parallel processing of visual informationParallel processing pathwaysMagnocellular pathwaysParvocellular pathwaysSerial processing of visual informationSerial processing in the retinaSerial analysis of visual image in the visual cortex

  • Parallel processing pathways2 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.

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  • Concept of parallel processing pathway*

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  • 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.

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  • *VISUAL PERCEPTION

  • VISUAL PERCEPTIONLight sense.Form sense.Contrast sense.Colour sense.

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

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  • 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)

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  • Dark adaptation curvesGrapg shows visual threshold falls progressively in dark room for hour until relatively constant value is reached.

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

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  • 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 timesNeural mechanism visual chain in retina.

    Vit-A deficiency.Elevates threshold for dark adaptation due to depletion of photosensitive pigments.Night blindness Nyctalopia.

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  • Light adaptation.Def process by which retina adapt itself to bright light.Very quick, over in 5 min.Its merely disappearance of dark adaptation.

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

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  • The form sense

    Ability to discriminate between shapes of objects.Cones are imp.Snellens chart is measure of form sense.

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  • visual acuityMeasurements of threshold of discrimination of 2 separate targets is visual acuity.

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

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  • 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)

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  • 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.

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  • Components of visual acuity.Minimum discriminable or hyperacuity.Spatial distinction.Threshold is lower than ordinary acuity.Vernier acuity.

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  • Snellens test typesVisual 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 1minEach letter form an angle of 5 min.

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  • Critical flicker fusion frequencyWhen light stimuli given intermittently produces flicker sensation.If frequency of it increased it fuses to form continuous stimulationThis frequency is Critical flicker fusion frequency.Temporal resolving power of visual system.

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  • Contrast sensitivity.Ability to perceive slight change in l