VISION 3

LEARNING OBJECTIVES LEARNING OBJECTIVES Visual processing Visual processing Visual pathways Visual pathways Lesions in the pathways & test Lesions in the pathways & test Visual cortex Visual cortex Visual acuity & testing Visual acuity & testing miscellaneous miscellaneous

Learning outcomes Learning outcomes At the end of this lecture students should be able to At the end of this lecture students should be able to Trace the neural pathways that transmit visual information from the rods and Trace the neural pathways that transmit visual information from the rods and cones to the visual cortex ( Visual pathway ). cones to the visual cortex ( Visual pathway ) Discuss visual pathway; functions of the primary visual cortex and visual Discuss visual pathway; functions of the primary visual cortex and visual association areas. association areas Describe the effects of lesions in the visual pathway Describe the effects of lesions in the visual pathway.

Visual processing Occurs at diff levels: in the retina, lateral geniculate body, in the visual cortex Occurs at diff levels: in the retina, lateral geniculate body, in the visual cortex In rods & cones- hyperpolarization In rods & cones- hyperpolarization Horizontal cells: lateral inhibition to enhance the visual contrast. Horizontal cells: lateral inhibition to enhance the visual contrast. Amacrine cells also provide further lateral inhibition. Amacrine cells also provide further lateral inhibition. This is essential to allow high visual accuracy in transmitting contrast borders in the image. This is essential to allow high visual accuracy in transmitting contrast borders in the image. Ganglion cells: only retinal cell capable of firing action potential (all others graded potentials). Ganglion cells: only retinal cell capable of firing action potential (all others graded potentials). Retina presents Spatial representation in lateral geniculate body. Retina presents Spatial representation in lateral geniculate body.

Ganglion cells, lateral geniculate body, neurons in visual cortex respond to stimuli in their receptive fields with on centers & inhibitory surrounds Ganglion cells, lateral geniculate body, neurons in visual cortex respond to stimuli in their receptive fields with on centers & inhibitory surrounds Or off centers & excitatory surrounds Or off centers & excitatory surrounds This may due to lateral inhibition by nearby neurons This may due to lateral inhibition by nearby neurons This phenomenon helps to sharpen the edges of a stimulus & improve discrimination This phenomenon helps to sharpen the edges of a stimulus & improve discrimination

Corresponding points: the points on the retina, on which the image of an object must fall, if it is to be seen binocularly as a single object. Corresponding points: the points on the retina, on which the image of an object must fall, if it is to be seen binocularly as a single object. Diplopia(double vision): images no longer form in the corresponding points of retina Diplopia(double vision): images no longer form in the corresponding points of retina

Visual Pathways Visual Pathways The main, conscious geniculocalcarine pathway, the long pathway from the retina via the lateral geniculate nucleus (LGN) to the primary visual cortex (V1). The main, conscious geniculocalcarine pathway, the long pathway from the retina via the lateral geniculate nucleus (LGN) to the primary visual cortex (V1).

Optic pathways & lesions producing field defects Heteronymous hemianopia (bitemporal) Homonymous hemianopia

FIELD OF VISION: portion of the external world visible out of that eye, when eye is fixed at an object FIELD OF VISION: portion of the external world visible out of that eye, when eye is fixed at an object BINOCULAR VISION: central parts of 2 eyes coincide, therefore anything in this portion of the field is viewed with binocular vision BINOCULAR VISION: central parts of 2 eyes coincide, therefore anything in this portion of the field is viewed with binocular vision

Collaterals of visual pathway Collaterals of visual pathway The subcortical, tectal pathways help in the visual associative functions. (eg; sleep wakefulness cycle etc) The subcortical, tectal pathways help in the visual associative functions. (eg; sleep wakefulness cycle etc) Also to temporal, parietal, frontal lobes- dorsal & ventral visual pathways Also to temporal, parietal, frontal lobes- dorsal & ventral visual pathways

Perimetry Peripheral portion of visual fields are mapped with an instrument- perimeter Peripheral portion of visual fields are mapped with an instrument- perimeter Central visual fields are mapped with tangent screen Central visual fields are mapped with tangent screen Physiological scotoma- optic disc area (10- 15° isopter, around 86-90° meridian on temporal field) Physiological scotoma- optic disc area (10- 15° isopter, around 86-90° meridian on temporal field) Objective or pathological scotoma - blind spot due to disease Objective or pathological scotoma - blind spot due to 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, anopia Homonymous, heteronymous, hemi, anopia What is macular sparing ? Loss of peripheral vision with intact macular vision in occipital lesions. Reason: macular representation is separate from that of peripheral fields & proportionately large area is devoted, so extensive lesions only can damage macular vision What is macular sparing ? Loss of peripheral vision with intact macular vision in occipital lesions. Reason: macular representation is separate from that of peripheral fields & proportionately large area is devoted, so extensive lesions only can damage macular vision Test ? Test ?

Visual areas in occipital cortex

VISUAL CORTEX Primary visual cortex or striate cortex or V1. Primary visual cortex or striate cortex or V1. Located in the calcarine fissure on the medial aspect of the occipital lobes. Located in the calcarine fissure on the medial aspect of the occipital lobes. Disprortionately large area devoted to macular neurons. Disprortionately large area devoted to macular neurons. Secondary visual areas or association areas or V2 : lie lateral, anterior, superior & inferior to the V1. Secondary visual areas or association areas or V2 : lie lateral, anterior, superior & inferior to the V1.

V2 for analysis of visual details like texture, color, recognizing letters, reading etc. V2 for analysis of visual details like texture, color, recognizing letters, reading etc. Other more distant sec visual areas are V3 onwards upto V12. Other more distant sec visual areas are V3 onwards upto V12. Various different cells like simple, complex, hyper complex, orientation columns, binocular cells etc with complex functions. Various different cells like simple, complex, hyper complex, orientation columns, binocular cells etc with complex functions.

Other Visual Areas V8 – color vision Middle temporal (MT) – Motion. Thus there is a parallel processing of visual information along multiple paths. All this information is put together to produce a conscious visual experience.

VISUAL ACUITY Resolving power=Acuity(degree to which details & contours of objects perceived) defined as shortest distance by which 2 lines can be separated & still be perceived as 2 separate lines. Test: By Snellen’s chart

Each row on the test is assigned a distance, in feet or meters, at which a person with "normal" vision should be able to read it. Each row on the test is assigned a distance, in feet 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 optotype identified subtends an angle of 5 min Visual acuity= Distance at which test is made / distance at which the smallest optotype identified subtends an angle of 5 minoptotypesubtendsoptotypesubtends The results of the test are given in a fraction for each eye. The top number indicates the distance that the patient was made to stand from the chart. The results of the test are given in a fraction for each eye. The top number indicates the distance that the patient was made to stand from the chart.

The bottom number is the distance assigned to the last row they were able to read The bottom number is the distance assigned to the last row they were able to read Normal acuity is thus described as 20/20 in the United States and 6/6 in countries that use the metric system Normal acuity is thus described as 20/20 in the United States and 6/6 in countries that use the metric system Healthy young person’s vision may be even better 6/5 Healthy young person’s vision may be even better 6/5

For eg; someone with 6/36 vision means this person needs to approach to a distance of 6m to read letters that a person with normal acuity could read at 36m. For eg; someone with 6/36 vision means this person needs to approach to a distance of 6m to read letters that a person with normal acuity could read at 36m. 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 acuity  Optical factors (image forming mechanisms) Retinal factors- state of cones Retinal 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- the central portions of visual fields of two eyes coincide Binocular vision- the central portions of visual fields of two eyes coincide Fusion- the impulses from two eyes are fused at cortical level Fusion- the impulses from two eyes are fused at cortical level Diplopia- double vision due to displaced image of one eye, which no longer fall on corresponding point. Diplopia- double vision due to displaced image of one eye, which no longer fall on corresponding point.

Strabismus or Squint Strabismus or Squint Lack of fusion of eyes in one or more visual coordinates Lack of fusion of eyes in one or more visual coordinates Suppression of image in that eye if not corrected within 6 years of age Suppression of image in that eye if not corrected within 6 years of age

Critical Fusion frequency Time resolving ability of eye - the rate at which stimuli can be presented and still be perceived as separate stimuli. Time resolving ability of eye - the rate at which stimuli can be presented and still be perceived as separate stimuli. Motion picture- frames are above CFF Motion picture- frames are above CFF Movie flicker- when it slows down Movie flicker- when it slows down