Sense of vision – part 2

Visual pathway

Disorders of the visual field

Pupillary reflex, accommodation reflex

Stereoscopic vision

Colour vision

Successive and simultaneous contrasts

Practical tasks

1. Detection of the central visual acuity

2. Reaction of pupils to light and accommodation stimulus

3. Examination of the colour vision by the use of

pseudoisochromatic charts

4. Successive and simultaneous contrasts

5. Additive mixing of colours by the use of Maxwell´s discs

6. Stereoscopic vision

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Disorders of the visual field

Blind spot

- area in the retina where the optic nerve passes

through it

- the only physiological scotoma

A scotoma - area of absent or diminished vision

(surrounded by an area of normal vision)

Scotomas - may be caused by diseases of:

1. the eye – macular degeneration, detachment of

retina, cataract

2. optic nerve - e.g. demyelinating process

3. visual cortex – e.g. tumours of the brain

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http://www.dwp.gov.uk/img/visual-fields.gif

Blurred central vision -

(in macular degeneration) Tunnel vision (in tumours of hypophysis)

Binasal hemianopsia

Impaired peripheral vision

(in glaucoma)

Disorders of the visual field

http://www.eschenbach-optik.com/en/uploads/RTEmagicC_sehhilfen_makula_lesen_06.jpg.jpg

http://upload.wikimedia.org/wikipedia/commons/thumb/0/0f/Binasalvf.png/230px-Binasalvf.png

http://pituitary.ucla.edu/images/site/Visual3.3.jpg

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tmCTvq0y9aGMaWJtHSUE2PCZNZG

https://en.wikipedia.org/wiki/Macular_degeneration

Glaucoma

• progressive optic neuropathies characterized by

degeneration of retinal ganglion cells and resulting

changes in the optic nerve head.

• if untreated may lead to blindness

• aqueous humour - secreted by the ciliary epithelium

into posterior chamber (behind iris)

• circulates through the pupil into the anterior chamber

• drained by the canal of Schlemm into venous system

• constant pressure 22 mm Hg or less

• if aqueous humour is not absorbed - increase of

intraocular pressure impedes blood flow to the retina

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http://upload.wikimedia.org/wikipedia/commons/3/3f/Flow_of_aqueous_humour_eye_EDA02.JPG

Cataract

normal lens is transparent

cataract

- slight or complete opacity of the lens (cloudy lens)

- causes obstruction for the passage of light – blurred vision

- often related to aging

http://cd.hpathy.com/wp-content/uploads/2011/09/cataract.jpg

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- visual images are inverted as they pass

through the lens (nodal point of the

optical system of the eye is in lens)

- the nasal retina receives rays from the

temporal half of the world (hemifield)

- the temporal retina receives rays from

the nasal half of the world (hemifield)

Visual pathway and its disorders

nasal

temporal temporal

http://www.bioon.com/bioline/neurosci/course/bvis1.gif

- visual information leaves the eye by way of the optic nerve (1)

-partial crossing of axons at the optic

chiasm (2)

- nerve fibres from nasal part of retina cross

over

- temporal fibres remain at the same side

- after the chiasm, the axons are called

the optic tract (3)

- the optic tract terminates in the lateral

geniculate nucleus (LGN) - the axons

synapse here

- LGN axons form optic radiations –

terminate in the primary visual cortex, in

occipital lobes

http://www.bioon.com/bioline/neurosci/course/bvis2.gif

left eye right eye

1

2

3

complete

blindness

bitemporal

hemianopsia

(often due to

tumours of hypo-

physis that com-

press the chiasm)

homonymous

hemianopsia

Disorders of the visual pathway

Hemianopsia

• heteronymous – contralateral (bitemporal, binasal)

• homonymous – homolateral (right, left) http://www.bioon.com/bioline/neurosci/course/bvis2.gif

Task: The reaction of pupils to light and accommodation stimulus

Introduction

• the pupil – an opening located in the centre of the iris

• pupillary diameter (2-8 mm) - regulated by tone of small muscles in iris

M. sphincter pupillae (circular muscle)

– its contraction decreases the diameter of pupil (miosis)

– controlled by the parasympathetic NS

M. dilator pupillae (radial muscle)

– its contraction increases the diameter of pupil (mydriasis)

– controlled by the sympathetic NS

The pupillary light reflex

• reflex that controls the diameter of the pupil, in response to the intensity of light that

falls on the retina of the eye (reflex = involuntary, quick, stereotyped response to stimulus)

• greater intensity light - causes myosis (allowing less light in)

• lower intensity light - causes mydriasis (allowing more light in)

Reflex arc for the pupilary light reflex

- light stimulates receptors in retina

- signal is transmitted from retina by n. opticus (afferent fibres), however 10-15 % of

fibres bypass the LGN (leave the primary visual pathway) and terminate in the

Edinger-Westphal nuclei on both sides of the midbrain (therefore both eyes react

to light at one time)

- nerve fibres (efferent, parasympathetic) from the Edinger-Westphal nuclei form the

oculomotor nerve (IIIrd) - terminates in m. constrictor pupilae

The accommodation reflex

- a vision reflex that enables quick transfer of the

focus between near and distant objects

- comprises coordinated changes in

1. pupil size (miosis when focusing closer)

2. lens shape (accommodation when focusing closer)

3. vergence (convergence of the eyeballs – when

focusing to a close dostance)

far object (parallel) close object (convergence)

Pathway for

accommodation reflex

Receptors: rods and cones

Afferent nerve: optic n.

Centre: oculomotor nucleus

Efferent: oculomotor nerve

Effector:

1. constrictor pupillae

2. ciliary muscles

3. m. medial rectus

Task - Procedure

- look at the size of the pupil in normal room illuminaton

1. illuminate the eye by a torch and observe the reaction of the pupil, estimate

approximately the diameter of the pupil

2. switch the torch off and observe the reaction of the pupil

3. shade the other eye with a hand or a note-book, after illumination of the first eye

observe the consensual reaction of pupils

4. stop the illumination of the first eye and observe the consensual reaction of both

eyes

5. ask the examinee to focus on your finger that is moving towards the examinees

nose – observe size of the pupils and vergence of eyes

Result and conclusion

• describe your observations for 1 – 5 (or make a drawing)

Retina

A. Horizontal cells

– horizontal communication of several rods and cones

B. Amacrine cells

– horizontal communication of several ganglion cells

Sclera (outer layer)

Vascular layer

Pigment layer

1

2

3

A

B

- the light sensitive tissue lining the inner surface of the eye

- cells of retina:

1. Receptors

• rods

• cones

2. Bipolar cells – transmission of AP

from the recptor cells to the ganglion cells

3. Ganglion cells

• their axons form n. opticus that transmits the signal to CNS

light

The receptor cells

Cones (7 millions)

• colour vision - photopic vision

• higher threshold – need daylight conditions

• high visual acuity

• maximum density in fovea centralis

• low density outside the fovea

Rods (125 milions)

• operate in reduced light (scotopic vision)

• low threshold for light detection

• no well defined image

• not present in fovea centralis

• maximum in parafoveal region

• in direction to peripheral parts of retina their count rapidly decreases

Fovea

centralis Blind spot - optic nerve disc - no receptors

Yellow spot – Fovea centralis

• the upper layers of retina are „moved to the side“ -

mainly in the central fovea

• the light can more directly reach the receptors

• better image resolution

• because of the pigment layer behind retina – the yellow

spot appears darker in ophtalmoscopy

Fovea centralis

- best image resolution

(ability to recognize details)

1. highest density of receptors

2. highest count of nerve fibres - no

convergence

Periphery

– lower density of receptors, convergence

light

pigment

Examination of the central visual acuity

Introduction

• visual acuity (visus) – ability to see details sharply

• the ability of retina to distinguish two close points as separate and not fused into

one = minimum separabile

• 2 points can be distinguished if 2 sense receptors in retina are stimulated and

between them one receptor remains unstimulated

• visual acuity depends on

– the density of receptors

– the angle of observation (angle of light rays)

• the object you focus on - is imaged in fovea centralis

• fovea centralis – the highest density of receptors, therefore

the best visual acuity (central visual acuity)

• 2 points that are imaged in fovea centralis can be distinguished as 2 if they are

observed under visual angle of 1 minute

1´

receptors

of retina

the observed

points

Optotype

• serves for examination of visus

• Snellen´s optotype – a glass board with symbols (E, letters, numbers)

• symbols are in lines with decreasing size

• each line – signed with a number from which a healthy eye should be able to

distinguish details and recognize the symbols normally

• symbols in the last line that a healthy eye should recognize correctly are seen

in an angle of 5 minutes and details in 1 minute

E 1´

5´

Procedure

• examine each eye separately (the non-examined eye is covered)

• switch the optotype on

• according to the type of the optotype the patient stands in distance of 5 or 6 m (d)

• the examinee is asked to read lines of symbols starting from the top continuing to

the bottom (largest → smallest)

• record the number of line that was read without a mistake (D)

Result

• write result in form: visus V=d/D

• d – distance of the patient from the optotype

(5 or 6 meters – depending on the optotype)

• D – number of the last line that was read without a mistake

Conclusion

• evaluate if the visus is normal

• V ≥ 5/5 or 6/6 (≥1) normal visus (normal central visual acuity)

• V < 5/5 or 6/6 (<1) impaired visus Landolt´s optotype

circles with opening

Perception of 3 dimensions

1. Determination by stereopsis – binocular vision

• the eyes are approximately by 6 cm apart

• images at 2 retinas are not identical

- different angle of observation

• object that we focus on is imaged at the same points of both retinas (corresponding points)

(„the same“ spot on both retinas)

• corresponding points in retina have the same cortical projection – two images are fused into one

• two eyes - two images in retina - one perception

• horopter – a circle passing through nodal points of both eyes and the point we focus on

• all points of the horopter circle are imaged to corresponding points

corresponding points

• points outside or inside horopter circle are not

imaged at the corresponding point, but to the

disparate points

• they do not have the same cortical projection

• the image is not sharp

• the bigger the distance from horopter (disparity),

the less sharp the image

• these blurred images contribute to the perception

of depth – 3 dimensional perception

disparate points

2. Determination by sizes of retinal

images of known objects

- distant objects – smaller, close objects –

larger in size

3. Determination by movement parallax

(monocular, motion parallax)

- head produces relatively large apparent

displacement of nearby objects and

relatively small displacement of distant

objects

- quickly moving objects – closer

- slowly moving objects – more distant

http://www.adamdalyonline.com/wp-content/uploads/2011/04/Perspective.jpg

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close

distant

Task: Stereoscopic vision

Principle

- by observation of stereoscopic photographs through a prism of a

stereoscope get the perception of a 3 D image

- stereoscopic photographs – 2 photos of the same object taken in an

angle as seen by 2 eyes

B. fix the eyes at an object in distance approx. 70 cm

- with a finger exert slight lateral pressure on the eyeball

- double image (diplopia) should occur, because an image

was formed on disparate points of retina of the shifted eye

A. put both index fingers in front of your eyes – one closer (20

cm), the other one in more distant position (40 cm)

fix the eyes on the closer finger (now is on the horopter)

- the closer finger is sharp, the further finger is double and blurry

- if you close one eye, one of the further objects disappears

fix the eyes on the further finger (now is on the horopter)

- the closer finger is double and blurry, the further finger is sharp

- if you close one eye, one of the further objects disappears

Procedure

C. place stereoscopic photographs into the holder of stereoscope

- by moving the holder forwards and backwards achieve such a convergence of the

eyes when both photographs fuse into a single image that is perceived as 3-D

Result and conclusions: describe your observation

Strabismus (squint, cross-eye)

• condition in which the eyes are not

properly aligned with each other:

• lack of coordination between

the extraocular muscles

• prevents bringing each point of the visual field

to the same point in retina

• prevents proper binocular vision, which may adversely affect depth perception.

• in a young child’s early efforts to fixate the two eyes on the same object, one

of the eyes fixates satisfactorily while the other fails do so

• abnormal fusion of the images on both retinas (to disparate points)

• the affectes eye becomes supressed and is not used for precise vision

Colour vision

• rods – black/white vision

• cones – colour vision Helmholtz trichromatic theory

• explains the colour vision

• photopigment – chemical substance (opsin) in photoreceptors, that is sensitive to light, chemical changes in photopigments give rise to receptor potential

• cones contain 3 different types of photopigment with different absorption maximum (i.e. maximum sensitivity)

• 420 nm – blue (cyanolab)

• 540 nm – green (cholorolab)

• 560 nm – red (erythrolab)

• all the other colours are a mixture of RGB

• elementary colours: red, green, blue

• additive mixing (in retina or higher centres for vision)

– of 3 elementary colours – stimulation of cones by red, blue and green colour (at the same time) - perception of white colour

• opponent colours = 2 colours which by additive mixing in the same proportion results in perception of white, e.g.

– blue-yellow, red-green

– and many others - any couple of the Maxwell´s triangle

Maxwell´s triangle

w

• human eye can detect almost all gradations of colours when only red, green

and blue monochromatic lights are appropriately mixed

• mixing of colours - red/green/blue cones generate potentials with different amplitude

e.g. orange light 580 nm

- red cones - 99% of their peak stimulation

- green cones - 42 % of their stimulation

- blue cones - no stimulation

e.g. blue light 450 nm

- red cones – 0% of their peak stimulation

- green cones 0 % of their stimulation

- blue cones – 97 % of their stimulation

Disorders of colour vision – Colour blindness

• about 8% of men and 1% of women have colour vision impairment

1. anomalia – (colour weakness) resulting from a deficiency of colour-sensitive pigment

2. anopia – absolute blindness – absence of a colour pigment

• Protanomalia/ protanopia – red colour deficiency/ blindness

• Deuteranomalia/ deuteranopia – green colour deficiency/ blindness

• Tritanomalia/ tritanopia – blue colour deficiency/ blindness

• Trichromatic people – normal vision, anomalia

• Dichromatic people – blindnenss for 1 colour (deuteranomalia is the

most frequent type)

• Monochromatic people– blindness for 2 colours

• Achromatic people – do not recognize any colours at all

Task: Detection of colour blindness by the use

of pseudoisochromatic charts

Principle

• Ishiharas´s peudoisochromatic charts – set of 38 charts composed of

irregular mosaic of circles differing in colour

• an eye with normal colour vision can discriminate figures or lines

• a person with disorder of colour vision

– a/ does not recognize any figure or line

– b/ recognizes different figure than normal eye

• normal eye – 5

• red-green blindness - 2

Procedure

• perform the examination in a room with good illumination

• put the charts in normal reading distance

from the examinee´s eyes in a right angle

• charts 1-25 must be read within 3 seconds

• charts 26-38 are for illiterate persons (children) – must be read within 10 s

• compare the examinee´s results with normal results given (see the text in

Ishihara´s book)

Result

• did the examinee read all charts appropriately?

Conclusion

• is the patient´s colour vision normal?

On line tests for colour vision (Ishihara plates)

http://www.color-blindness.com/ishihara-38-plates-cvd-test/#prettyPhoto/1/

Task: Additive mixing of colours by use

of Maxwell´s discs and monochromatic filters

Principle

• by observing the fast rotation of Maxwell´s discs additive mixing of colours in

retina is achieved

• additive mixing at level of higher centres is achieved by placing of different

monochromatic filters in front of each eye

Procedure

• attach a Maxwell´s disc to an engine

• gradually increase the speed of rotation until additive mixing of colours on the

retina is achieved

• observe individually by each eye and then by both eyes

• repeat procedure with different discs

= simultaneous stimulation of retina by different wavelengths (light of different

colours) which are summed up and result in final perception

possible due to integration abilities of retina or brain

• put a green monochromatic filter in front of one eye and a red monochromatic filter in

front of the other eye

• look at a white surface with both eyes until the perception of final colour is achieved

Result

• describe your observation, draw pictures

Successive and simultaneous contrast

The terms "simultaneous contrast" and "successive contrast" refer to

visual effects in which the appearance of a patch of an object /field is

affected by other fields that are nearby in space and time.

lighter

darker

Tasks

1. Look at a black-white picture, then transfer your look

at a white surface (sheet of paper, wall) – describe

your observation - „the afterimage“ (several pictures

available)

2. Look at a colour picture - then transfer your look at a

white surface – describe your observation – „the

afterimage“ (several pictures available)

Successive contrast

• based on partial adaptation of receptors in retina

• exposure of retina to a colour causes adaptation of the exposed

cells to this colour and leads to higher sensitivity to opponent

colour

• when neutral light is restored (white backgound) a temporary

illusion of a light composed of the "missing" wavelengths (the

complementary colour) is seen

• exposure to white (black) – higher sensitivity to black (white)

Simultaneous contrast

• is based on lateral inhibition of visual neurons

Lateral inhibition

• neighbouring visual neurons have influence on each other (inhibitory)

• the "edge" neurons receive inhibition from neighbours on one side

• this causes their higher sensitivity to complementary field (light, dark)

• inhibition greatly increases the visual system's ability to respond to edges of a

surface

less sensitive to dark, more

sensitive to light

Surrounding field -

inhibitory effect

less sensitive to light, more

sensitive to dark

Tasks

Simultaneous contrast

• look steadily at the centre of the left grid

• notice grey clouds at the intersections of the light bars

(except the one you look straight at)

Explanation (based on lateral inhibition)

• the clouds appear because intersections have lateral

inhibitions from white areas on 4 sides,

• the middle of the bars have lateral inhibition from white

areas on only 2 sides the intersections receive (roughly)

twice the lateral inhibition that the bars do

• this makes the neural signal triggered by the intersection

weaker (i.e. white looks much less white and more to

opponent colour = black) than the signal triggered by a

section between intersections (white looks a little less

white – light greyish)

Simultaneous contrast

• attach the Maxwell´s disc to the engine

• start it, increase frequency of rotation

• the black narrow band of the disc will be perceived in the complementary colour

to the basic colour of the disc

- an intense green light strikes the retina – it makes the green receptors „tired“

- because of links by horizontal cells – it makes also the neghbouting receptors in

the narrow strip tired to green

- when the white strip falls to the „tired“ part of retina, we get perception of the

opponent (complementary colour)

Lateral inhibition

• neighbouring visual neurons have influence on each other (inhibitory)

• the "edge" neurons receive inhibition from neighbours on one side

• this causes their higher sensitivity to complementary colour

• inhibition greatly increases the visual system's ability to respond to

edges of a surface

the "edge" neurons – less sensitive

to green, more sensitive to red

(complementary colour)

inhibitory effect