Overview of the Visual System Dr. Patrick Degenaar Lecturer in Neurobionics (Institute of Biomedical Engineering) (Division of Neuroscience, Faculty of

Overview of the Visual System

Dr. Patrick DegenaarLecturer in Neurobionics

(Institute of Biomedical Engineering)(Division of Neuroscience, Faculty of Medicine)

Dr. Patrick Degenaar

Biological Vision

VS

Hawk Oscar

“He/she has eyes like a Hawk!”

Hawk’s eyes are smaller!

Hawks have fewer neurons to perform visual processing!

Why???


What is the purpose of vision?

Scene selection

Light intensity

Colour

Night vision

Image processing

(head and eye movements)

(retina and pupil)

(Retina cones)

(Retina rods)

(post-receptoral pathway)

The Eye is not a camera!!!!!!

-- The purpose of vision is to understand the world around us!


LateralGeniculate

Nucleus(LGN)

Visual Cortex

Optic Nerve

Optic Nerve

The visual system


Glaucoma

Myopia

Optic Neuritis

Optic Neuropathy

Papilledema

Diplopia

Retinitis Pigmentosa

Macular Degeneration

Cancer

Cataphracts

Orbital cellulitis

Diabetic retinopathy

Things that can go wrong……..


The eye: Physiological Optics


The eye as an imaging systemPrincipal refraction occurs at the cornea

Focal power of the cornea is ~ 60 dioptres

The lens provides variable focus (accommodation)

Lens power varies with age ~ 6 dioptres in adults

For small pupil sizes resolution is limited by diffraction

For large pupil sizes aberration limits resolution


Anterior chamberFormation

Secretion / diffusion / ultrafiltration from ciliary body

‘Plasma-like’ Replaced every 100 minutes

Drains through trabecular meshwork

Function Maintenance of IOP (16mm Hg) Contributes to transparency

Metabolic support for lens, cornea & vitreous

Lens

Anterior chamber

Cornea

Trabecularmeshwork

Iris

Schlemm’s canal


Glaucoma

Lens

Anterior chamber

Cornea

Trabecularmeshwork

Iris

Schlemm’s canal


Inner Nucleus Layer

Ganglion Cell Layer

Outer Nucleus Layer

Inner Plexiform Layer

Outer Plexiform Layer

Pigment Epithelium Layer

Photoreceptor Layer

Structure of the Retina

Ramon y Cajal (1900)

Retina sectionCajal’s drawing


Photoreceptor cells

Horizontal Cells

Bipolar Cells

Amacrine Cells

Ganglion cells

Retina FunctionRetinal Epithelium


Photoreceptors

Rods Scotopic Monochromatic

Cones Photopic Colour vision


Why do we need colour?


Colour sensitivity

L - cone

M - coneS - coneRod


Colour blindness Congenital colour deficiencies

- affecting 8% population

Protanopia: (protanomally): missing (abnormal) L cone

Deuteranopia : missing (abnormal) M cone

Tritanopia : missing (abnormal) S cone

Monochromacy


Ishihara tests for colour blindness


Photoreceptors and photopigments


Light closes the cGMP channels in the outer segment

Physiology of phototransduction

In darkness photoreceptors are relatively depolarised.

A constant depolarising current flows through cGMP gated cation channels in the outer segment

Light hyperpolarises photoreceptors


A biochemical cascade links photo-isomerisation of the visual pigment to the closure of the cGMP cation channel

GAIN……..GAIN……..hyperpolarisationhyperpolarisation

Phototransduction


Photoreceptor distribution


Equal visibility with eccentricity

Arises from:

arrangement of the photoreceptor mosiac

post receptoral processing of the image


Fovea – avascular zone

Photoreceptors

FoveaFovea Maximum density of Maximum density of

photoreceptorsphotoreceptors High spatial resolutionHigh spatial resolution


ElectroretinogramElectroretinogram

ERG- retinal activity

ERP: phototransduction A-wave (LRP): photoreceptor (16-25ms) B-wave: ON-bipolar (+HC’s) Ops: amacrine cell circuits C-wave: K+ flux PR-RPE space (400-500ms)


Visual Processing begins in the retina

Red Channel

Green Channel

Red-green edges via subtraction


Colour opponent channels

Only two colour opponent channels are necessaryOnly two colour opponent channels are necessary

Red and Green cone sensitivitiesare very close together


Initiated at in the Initiated at in the outerplexiform layer of outerplexiform layer of the retinathe retina

Bipolar cells receive Bipolar cells receive direct photoreceptor direct photoreceptor input at the receptive input at the receptive field centre together with field centre together with surround input fed via surround input fed via horizontal cellshorizontal cells

Visual Processing begins in the retina


++++ --

++ ---- ++

-- ++

--B on / Y off

R on / G off R off / G on

G on / R off G off / R on

R = L cone R = L cone

G = M cone Y = L + MG = M cone Y = L + M

B = S coneB = S cone

Basic Currency of Parvocellular PathwayBasic Currency of Parvocellular Pathway

Colour opponent primate ganglion cells


Cones:Cones:All hyperpolarizeAll hyperpolarize

Bipolar Cells:Bipolar Cells:hyperpolarize –OFFhyperpolarize –OFF

depolarize – ONdepolarize – ON

Ganglion Cells:Ganglion Cells:

ON and OFFON and OFF

The retinal cone pathway

On - centre Off - centre


ON – centre ganglion cellON – centre ganglion cell OFF – centre ganglion cellOFF – centre ganglion cell

Centre surround physiology


Retina Summary

Layered structureLayered structure

Photoreceptors are the last layer in the light Photoreceptors are the last layer in the light pathpath

More rods than conesMore rods than cones

Non-uniform and different spacing of rods and Non-uniform and different spacing of rods and conescones

Cones mediate high spatial resolution at high Cones mediate high spatial resolution at high light levelslight levels

Rods operate at low light levelsRods operate at low light levels


Retinal physiology - summaryWavelength encoding is achieved by a subtractive Wavelength encoding is achieved by a subtractive

process - different spectral classes of cone are process - different spectral classes of cone are

combined in opponency.combined in opponency.

Signals in the retina are all analogue apart from the Signals in the retina are all analogue apart from the

ganglion cells which produce action potentials for ganglion cells which produce action potentials for

communication with the CNScommunication with the CNS

Ganglion cells are of two broad types; P-type, which are Ganglion cells are of two broad types; P-type, which are

colour opponent and have sustained responses, and M colour opponent and have sustained responses, and M

type, which are achromatic and transient.type, which are achromatic and transient.

Centre-surround opponency provide the first spatial Centre-surround opponency provide the first spatial

processing of visual information. Surround receptive processing of visual information. Surround receptive

fields result from horizontal and amacrine cells.fields result from horizontal and amacrine cells.


Central projections of retinal Ganglion cells

Major Pathway:

Primary Visual Pathway

retina - lateral geniculate nucleus (LGN) - cortex

Other Pathways:Pretectum - pupillary light reflexPretectum - pupillary light reflex

Hypothalamus - circadian axis inputHypothalamus - circadian axis input

Superior colliculus - eye movements orientationSuperior colliculus - eye movements orientation


What happens here ?What happens here ?

Primary visual pathway

Retina

optic nerve

Chiasm

LGN

optic radiation

cortex


AnatomyAnatomy

LGN shows precise LGN shows precise

segregation of Left/Right segregation of Left/Right

eye. Together with eye. Together with

functional segregation of P functional segregation of P

and M retinal inputsand M retinal inputs

PhysiologyPhysiology

The receptive fields are The receptive fields are

generally of the simple generally of the simple

concentric centre-surround concentric centre-surround

of the types. of the types.

LGN – structural segregation


What do we know about the visual cortex?

Single unit

recording

Primate Cortex

Organisation:

Hubel and

Wiesel


V1 – characterised by the emergence of edge detection

Generation of simple receptivefields


What is the functional significance of bringing Left and Right together ?

Ocular dominance stripes in V1


Random Dot Stereograms

All dots are exactly

the same in each

slide

Except the dots in

the square

The dots in Slide one

are moved over one

column in slide two

(shown by the yellow

section)


Random Dot Stereograms

Moving the dots creates disparate points for that square region when seen through the stereoscope.

Even without the aid of

any monocular cues,

you still see a 3D pop-

out square.


Very complex

Understanding of visual

system decreases

rapidly after v1

Visual Cortex Map


Primary visual cortex - summary

Layered structure, with specific cells types in Layered structure, with specific cells types in

different layersdifferent layers

Information from each eye is incorporated to Information from each eye is incorporated to

generate neurones sensitive to depthgenerate neurones sensitive to depth

Cells are sensitive to edges – orientation Cells are sensitive to edges – orientation

columnscolumns

Many cells are motion sensitive.Many cells are motion sensitive.

Colour cells are segregated from non colour Colour cells are segregated from non colour

cellscells


Biological Vision

VS

Hawk Oscar

“He/she has eyes like a Hawk!”

Hawk’s eyes are smaller!

Hawks have fewer neurons to perform visual processing!

Why???


Reading and Learning

Web resource on the retina: http://webvision.umh.es/webvision

Visual Cortex “A Vision of the Brain” – S. Zeki

[Blackwell]


Supplementary information

Ref:

http://webvision.umh.es/webvision


Supplementary informationIntensity

Centre surround response

x

x

Cones

Horizontal cellsBipolar

cell

The original 2D image that hits the retina

If we take a 1D intensity profile…

The centre surround processing will output an image which effectively transmits features which can be interpreted as edges later in the visual cortex

Documents

Overview of the Visual System Dr. Patrick Degenaar Lecturer in Neurobionics (Institute of Biomedical Engineering) (Division of Neuroscience, Faculty of