VISION 3VISION 3
LEARNING OBJECTIVESLEARNING OBJECTIVES
Visual processingVisual processing
Visual pathwaysVisual pathways
Lesions in the pathways & testLesions in the pathways & test
Visual cortexVisual cortex
Visual acuity & testingVisual acuity & testing
miscellaneousmiscellaneous
Learning outcomesLearning outcomes
At the end of this lecture students should beable toAt the end of this lecture students should beable to
25-27.13. Trace the neural pathways thattransmit visual information from the rods and25-27.13. Trace the neural pathways thattransmit visual information from the rods and
cones to the visual cortex ( Visual pathway ). cones to the visual cortex ( Visual pathway ).
25-27.14. Discuss visual pathway; functions ofthe primary visual cortex and visual25-27.14. Discuss visual pathway; functions ofthe primary visual cortex and visual
association areas. association areas.
25-27.15. Describe the effects of lesions in thevisual pathway.25-27.15. Describe the effects of lesions in thevisual pathway.
Visual processingVisual processing
Occurs at diff levels: in the retina, lateral geniculate body, inthe visual cortexOccurs at diff levels: in the retina, lateral geniculate body, inthe visual cortex
In rods & cones- hyperpolarizationIn rods & cones- hyperpolarization
Horizontal cells: lateral inhibition to enhance the visualcontrast.Horizontal cells: lateral inhibition to enhance the visualcontrast.
Amacrine cells also provide further lateral inhibition.Amacrine cells also provide further lateral inhibition.
This is essential to allow high visual accuracy in transmittingcontrast borders in the image. This is essential to allow high visual accuracy in transmittingcontrast borders in the image.
Ganglion cells: only retinal cell capable of firingaction potential (all others graded potentials).Ganglion cells: only retinal cell capable of firingaction potential (all others graded potentials).
Retina presents Spatial representation in lateralgeniculate body.Retina presents Spatial representation in lateralgeniculate body.
Ganglion cells, lateral geniculate body,neurons in visual cortex respond tostimuli in their receptive fields with oncenters & inhibitory surroundsGanglion cells, lateral geniculate body,neurons in visual cortex respond tostimuli in their receptive fields with oncenters & inhibitory surrounds
Or off centers & excitatory surroundsOr off centers & excitatory surrounds
This may due to lateral inhibition bynearby neuronsThis may due to lateral inhibition bynearby neurons
This phenomenon helps to sharpen theedges of a stimulus & improvediscriminationThis phenomenon helps to sharpen theedges of a stimulus & improvediscrimination
Corresponding points: the points onthe retina, on which the image of anobject must fall, if it is to be seenbinocularly as a single object.Corresponding points: the points onthe retina, on which the image of anobject must fall, if it is to be seenbinocularly as a single object.
Diplopia(double vision): images nolonger form in the correspondingpoints of retinaDiplopia(double vision): images nolonger form in the correspondingpoints of retina
Visual Pathways Visual Pathways
The main, consciousgeniculocalcarine pathway, the longpathway from the retina via thelateral geniculate nucleus (LGN) tothe primary visual cortex (V1).The main, consciousgeniculocalcarine pathway, the longpathway from the retina via thelateral geniculate nucleus (LGN) tothe primary visual cortex (V1).
Optic pathways & lesions producing fielddefectsOptic pathways & lesions producing fielddefects
Heteronymoushemianopia(bitemporal)
Homonymoushemianopia
FIELD OF VISION: portion of theexternal world visible out of that eye,when eye is fixed at an objectFIELD OF VISION: portion of theexternal world visible out of that eye,when eye is fixed at an object
BINOCULAR VISION: central parts of2 eyes coincide, therefore anything inthis portion of the field is viewedwith binocular visionBINOCULAR VISION: central parts of2 eyes coincide, therefore anything inthis portion of the field is viewedwith binocular vision
Collaterals of visual pathway Collaterals of visual pathway
The subcortical, tectal pathways helpin the visual associative functions.(eg; sleep wakefulness cycle etc)The subcortical, tectal pathways helpin the visual associative functions.(eg; sleep wakefulness cycle etc)
Also to temporal, parietal, frontallobes- dorsal & ventral visualpathwaysAlso to temporal, parietal, frontallobes- dorsal & ventral visualpathways
PerimetryPerimetry
Peripheral portion ofvisual fields are mappedwith an instrument-perimeterPeripheral portion ofvisual fields are mappedwith an instrument-perimeter
Central visual fields aremapped with tangentscreenCentral visual fields aremapped with tangentscreen
Physiological scotoma-optic disc area (10- 15°isopter, around 86-90°meridian on temporalfield)Physiological scotoma-optic disc area (10- 15°isopter, around 86-90°meridian on temporalfield)
Objective or pathologicalscotoma - blind spot dueto diseaseObjective or pathologicalscotoma - blind spot dueto disease
Monocular and binocular visual fields
The dashed line encloses the visual field of the left eye;
the solid line, that of the right eye.
The common area (heart-shaped clear zone in the center)
is viewed with binocular vision
Meanings:Meanings:
Homonymous, heteronymous, hemi,anopiaHomonymous, heteronymous, hemi,anopia
What is macular sparing ?Loss of peripheral vision with intactmacular vision in occipital lesions.Reason: macular representation isseparate from that of peripheralfields & proportionately large area isdevoted, so extensive lesions onlycan damage macular visionWhat is macular sparing ?Loss of peripheral vision with intactmacular vision in occipital lesions.Reason: macular representation isseparate from that of peripheralfields & proportionately large area isdevoted, so extensive lesions onlycan damage macular vision
Test ?Test ?
Visual areas in occipital cortexVisual areas in occipital cortex
VISUAL CORTEXVISUAL CORTEX
Primary visual cortex or striatecortex or V1.Primary visual cortex or striatecortex or V1.
Located in the calcarine fissure onthe medial aspect of the occipitallobes.Located in the calcarine fissure onthe medial aspect of the occipitallobes.
Disprortionately large area devotedto macular neurons.Disprortionately large area devotedto macular neurons.
Secondary visual areas or associationareas or V2 : lie lateral, anterior,superior & inferior to the V1.Secondary visual areas or associationareas or V2 : lie lateral, anterior,superior & inferior to the V1.
V2 for analysis of visual details liketexture, color, recognizing letters,reading etc.V2 for analysis of visual details liketexture, color, recognizing letters,reading etc.
Other more distant sec visual areasare V3 onwards upto V12.Other more distant sec visual areasare V3 onwards upto V12.
Various different cells like simple,complex, hyper complex, orientationcolumns, binocular cells etc withcomplex functions.Various different cells like simple,complex, hyper complex, orientationcolumns, binocular cells etc withcomplex functions.
Other Visual AreasOther Visual Areas
V8 – color visionV8 – color vision
Middle temporal (MT) – Motion.Middle temporal (MT) – Motion.
Thus there is a parallel processing of visualinformation along multiple paths. All thisinformation is put together to produce aconscious visual experience.Thus there is a parallel processing of visualinformation along multiple paths. All thisinformation is put together to produce aconscious visual experience.
VISUAL ACUITYVISUAL ACUITY
Resolving power=Acuity(degree towhich details & contours of objectsperceived)Resolving power=Acuity(degree towhich details & contours of objectsperceived)
defined as shortest distance by which2 lines can be separated & still beperceived as 2 separate lines.defined as shortest distance by which2 lines can be separated & still beperceived as 2 separate lines.
Test: By Snellen’s chartTest: By Snellen’s chart
Each row on the test is assigned a distance, infeet or meters, at which a person with "normal"vision should be able to read it.Each row on the test is assigned a distance, infeet or meters, at which a person with "normal"vision should be able to read it.
Visual acuity= Distance at which test is made /distance at which the smallest optotypeidentified subtends an angle of 5 minVisual acuity= Distance at which test is made /distance at which the smallest optotypeidentified subtends an angle of 5 min
The results of the test are given in a fraction foreach eye. The top number indicates the distancethat the patient was made to stand from thechart.The results of the test are given in a fraction foreach eye. The top number indicates the distancethat the patient was made to stand from thechart.
The bottom number is the distanceassigned to the last row they were able toreadThe bottom number is the distanceassigned to the last row they were able toread
Normal acuity is thus described as 20/20in the United States and 6/6 in countriesthat use the metric systemNormal acuity is thus described as 20/20in the United States and 6/6 in countriesthat use the metric system
Healthy young person’s vision may beeven better 6/5Healthy young person’s vision may beeven better 6/5
For eg; someone with 6/36 vision meansthis person needs to approach to adistance of 6m to read letters that aperson with normal acuity could read at36m.For eg; someone with 6/36 vision meansthis person needs to approach to adistance of 6m to read letters that aperson with normal acuity could read at36m.
In an even more approximate manner,this person could be said to have "half"the normal acuity.In an even more approximate manner,this person could be said to have "half"the normal acuity.
Factors affecting visual acuityFactors affecting visual acuity
Optical factors (image formingmechanisms) Optical factors (image formingmechanisms)
Retinal factors- state of conesRetinal factors- state of cones
Stimulus factors- illumination,contrast between stimulus &background, length of exposure etc.Stimulus factors- illumination,contrast between stimulus &background, length of exposure etc.
Binocular vision- thecentral portions of visualfields of two eyescoincideBinocular vision- thecentral portions of visualfields of two eyescoincide
Fusion- the impulsesfrom two eyes are fusedat cortical levelFusion- the impulsesfrom two eyes are fusedat cortical level
Diplopia- double visiondue to displaced image ofone eye, which no longerfall on correspondingpoint.Diplopia- double visiondue to displaced image ofone eye, which no longerfall on correspondingpoint.
Strabismus or SquintStrabismus or Squint
Lack of fusion of eyes in one or morevisual coordinatesLack of fusion of eyes in one or morevisual coordinates
Suppression of image in that eye if notcorrected within 6 years of ageSuppression of image in that eye if notcorrected within 6 years of age
Critical Fusion frequencyCritical Fusion frequency
Time resolving ability of eye - therate at which stimuli can bepresented and still be perceived asseparate stimuli.Time resolving ability of eye - therate at which stimuli can bepresented and still be perceived asseparate stimuli.
Motion picture- frames are above CFFMotion picture- frames are above CFF
Movie flicker- when it slows downMovie flicker- when it slows down