The Visual System

Dr. Farid Youssef

November 2002

Objectives1 Describe the principles of optics, errors of refraction

and their correction.2 Explain spherical and chromatic aberrations and

mechanisms of accommodation.3 Describe type of eyeball movements and their control

centres.4 Explain the processing of the form, movement and

colour of objects and relate to sequential and parallel processing.

5 Describe the principles of assessment of defects in field and colour vision.

6 Describe Visual Evoked Potentials and their clinical applications.

Basic Principles of Optics

• Speed of light in air - 3x108m/sec.

• Much slower through transparent solids and liquids, e.g. glass 2x108m/sec.

• The refractive index of a substance is the ratio of the speed of light in air to the speed of light in the substance.

• When light travels from one medium to another it changes direction providing it strikes the surface at an angle and not at 90o. This process is called refraction.

• A convex lens allows light rays to converge to a focal point.

• A concave lens allows light to diverge. • A cylindrical lens focuses light to a focal line as

opposed to a focal point. • The focal length of a lens is the distance from the

lens to the focal point when the lens focuses light form a distant source.

• The refractive power of a lens is measured in diopters and is equal to one metre divided by the focal length.

Basic Principles of Optics

The Eye• The main function of the eye is to bring light into

focus upon the visual receptors i.e. the rods and cones of the retina (in this way the eye is remarkably like a camera).

• The Process: an image is formed upon the retina light energy is transduced into electrical signals information needed to create the image is encoded by neurons within the retina and the brain the information is used by the visual cortex to create the visual perception we call ‘seeing’.

How is this Achieved?

• Light must enter the eye - this occurs via an opening in the eye called the pupil.

• The amount of light entering the eye is controlled by the iris (a group of radial and circular muscles that surround the pupil) and is proportional to the square of the diameter, i.e. the area of the pupil.

• Changing the size of the pupil also alters the depth of field of the image and the amount of spherical aberration produced by the lens.

How is this Achieved?

• Light must be focused upon the retina - focusing is achieved by the ability of the components of the eye to refract light.

• The majority of refraction takes place at the interface between the cornea and the air.

• The lens is the other principal agent of refraction.

• Accommodation is the ability of the lens to change its refractive power thus allowing the eye to focus on images at varying distances.

Terms Describing the Eye

• Emmetropia – the normal eye; when the ciliary muscles are completely relaxed the emmetropic eye has the ability to focus light from a distant object onto the retina; it is also able to accommodate to see objects at close range.

• Presbyopia – as a person ages the lens becomes larger, thicker and less elastic. As a result the power of accommodation decreases greatly to about 2D at 50 years of age and 0D at 70.

• Hyperopia – or ‘far sightedness’ is as a result of the lens focusing the image behind the retina. This can be due to an eyeball that is too short or a lens system that is too weak.

• Myopia – or ‘near sightedness’ is a result of the image being focussed in front of the retina. This may be due to an eyeball that is too long or a lens system that is too strong.

• Astigmatism – the image in different planes focuses at different distances from the retina. It is most often due to a ‘wavy’ corneal surface.

Terms Describing the Eye

Other Considerations• The design of the eye is such that light from the

focus of our gaze, i.e. the fixation point, is always brought to focus upon a specific region of the retina called the fovea. This is referred to as the visual axis.

• Normal visual acuity for the human eye is approximately 45 seconds of an arc. The Snellen chart measures visual acuity.

• There is a region upon the retina called the blind spot or optic disk. This region is devoid of photoreceptors.

• Depth Perception is achieved by three means:– If one is aware of the size of the object then the

brain automatically calculates roughly how far away the object is based upon the visual stimulus.

– A phenomenon known as moving parallax. – Due to binocular vision the image formed upon

each retina is slightly different. This is called stereopsis.

Other Considerations

• The shape of the eye is maintained by two fluid filled regions, one in front of the lens and one behind the lens.

• Behind the lens the chamber is filled with a thick jelly-like substance called the vitreous humor.

• The region in front of the lens is divided into a posterior and anterior chamber by the iris and is filled with aqueous humor.

• Glaucoma is a condition in which the intra-ocular pressure is higher than normal.

Other Considerations

The Retina

• The retina is divided into 10 layers:– Pigment layer– Photoreceptor layer– Outer limiting membrane– Outer nuclear layer – Outer plexiform layer– Inner nuclear layer– Inner plexiform layer– Ganglion cell layer– Layer of optic nerve fibres– Inner limiting membrane

• The choroid layer is a pigment layer that surrounds the eye and is continuos with the iris. Functions include:

– It is designed to absorb excess light entering the eye and prevents scattering of light.

– Phagocytosis of the end of the outer segments of the rods that are continually being removed.

– Conversion of the utilized photopigments back to an active form.

– Storage of vitamin A.

The Retina

• This is a multi-step process designed to convert the light energy of photons into electrical impulses in the rods and cones.

• Steps:1 Light energy is ‘captured’ by the visual pigments

called opsins.

2 Rhodopsin is composed of the protein scotopsin + retinal (derived from vitamin A) in its 11-cis form. Only this form can bind with scotopsin to form rhodopsin.

Phototransduction

Phototransduction

3 Light energy is absorbed by rhodopsin and this initiates a series of reactions due to the conversion the 11-cis-retinal to all-trans-retinal.

4 All-trans retinal is converted back to the cis form via an isomerase directly or indirectly via conversion to retinol a form of vitamin A.

• How does this alter the membrane potential of the rods?

Phototransduction

5 Under dark conditions the outer segments of the rods and cones are extremely permeable to sodium ions.

6 The decomposition of rhodopsin decreases the permeability of the outer segment to Na+ resulting in cell hyperpolarization as the inner segment is still pumping sodium out of the cell.

7 Metarhodopsin II transducin phosphodiesterase cGMP. cGMP is responsible for the leaky nature of the outer segment membrane to sodium.

Visual Processing

• Begins within the retina itself, continues in the Lateral Geniculate Nucleus (LGN) and is completed in the visual cortex.

• Ganglion Cells - the output cells of the retina.

• The axons of ganglion cells coalesce to form the optic nerve of each eye.

• Ganglion cells transfer information by firing action potentials. Photoreceptors, horizontal and bipolar cells all respond with graded changes in their membrane potentials.

• Each ganglion cell has a receptive field, that is a particular area upon the retina which when stimulated by light modulates the activity of the ganglion cells.

• The receptive fields of ganglion cells are:– circular – divided into two regions: a central circular zone,

the centre, and the region around it called the surround.

• Ganglion cells process information along two parallel pathways.

Visual Processing

Types of Ganglion Cells• X cells - most numerous in the retina, medium size, 10-15m,

transmit at a rate of 14m/s, small receptive fields. Mainly responsible for the visual image (fine detail) and colour vision.

• Y cells - as large as 35m and transmit at 50m/s or greater, broad dendritic fields and are believed to be responsible for detecting rapid changes in light intensity or rapid movements across the visual fields. They make up only 5% of the total.

• W cells - 10m in size and transmit signals at 8m/s. Receive most input from the rods, via inter-neurons, have wide extensive dendritic trees and are believed to be responsible for detecting directional movement.

Visual Processing

• Information is transferred from the photoreceptors to the ganglion cells via the inter-neurons, i.e. the bipolar, horizontal and amacrine cells.

• Bipolar cells have a receptive field arrangement with centre-surround organization. On centre bipolar cells depolarize in response to light while off-centre bipolar cells hyperpolarize.

• To achieve this glutamate is released from the cones and this in turn activates ionotropic and metabotropic receptors respectively.

The Visual Pathway

• The visual field is the view seen by the two eyes without movement of the head. There are also a corresponding left and right visual fields.

• Ganglion cells converge upon the optic disk where they are myelinated and leave as the optic nerve.

• This sends projections to three sub-cortical regions after crossing at the optic chiasm, the LGN (visual processing), the superior colliculus (eye movements) and the pretectal region (pupillary reflexes).

Lateral Geniculate Nucleus• The vast majority of neurons from the retina terminate

here.• Due to crossing at the optic chiasma the left hemifield

of vision is represented by the right optic tract and ends in the right LGN.

• The LGN has a visuotopic map, i.e. a specific point on the retina always stimulates the same point in the LGN.

• Neurons from the fovea and the regions around it make up approximately half of the neural mass in the LGN.

• The LGN has a very orderly arrangement.• The first two layers are called the magnocellular

cells layers and receive input from the M or Y cells. • The other four are called the parvocellular layers

and receive input from the X or P cells.• A layer receives input from one eye only, with the

contralateral nasal hemiretina ending in layers 1, 4 and 6 and the ipsilateral temporal retina in layers 2,3, and 5.

Lateral Geniculate Nucleus

The Visual Cortex

• From the LGN fibres pass to an area termed the primary visual cortex or visual area 1 (Brodmann’s area 17) or the striate cortex.

• It is located in the region of the calcarine fissure, extending to the occipital pole on the medial aspect of the occipital cortex.

• The secondary visual area surrounds this region, is located in Brodmann’s area 18 and is also called the V-2 region. This area is responsible for the analysis of visual signals.

• The cells from the LGN synapse in layer 4C on spiny stellate cells. This layer responds to the small circular spots of light similar to those that have activated the visual system so far.

• The layers above and below this respond to more complex patterns of activation.

• Their receptive fields are rectangular as opposed to circular. Two types of cells have been identified depending upon their pattern of activation, simple cells and complex cells.

The Visual Cortex

• Simple cells are similar to those found in the LGN, they have on and off zones, but a line of light activates them and their receptive field is larger. In addition the on zone is always in a particular axis of orientation.

• Complex cells have a larger receptive field and the axis of orientation is also important. However they do not have distinct on and off zones and so the precise position of the stimulus in the receptive field is not as important.

The Visual Cortex

Colour Vision

• Colour greatly enhances contrast between and within objects and so improves visual processing.

• Colour vision is achieved by the existence of three different photo-pigments with different but overlapping absorption spectra. This is called trivariancy.

• The first makes a strong contribution to blue light (B or S – 420nm). The second contributes mostly to green (G or M – 531nm) and the third contributes most to red (R or L – 558nm).

• Although colour vision greatly enhances our ability of contrast it is limited in two ways.

• The first of these is due to the fact that if an object only stimulates a single cone then no contrast can be achieved.

• The second is a property of short wavelengths of light termed chromatic aberration. Images produced by such light are inherently blurred.

Colour Vision

• Colour opponent theory –certain colours can never be perceived, e.g. a reddish green or a bluish-yellow colour. This is believed to be due to an opponent system. The antagonistic pairs are red-green; blue-yellow and black-white. Presumably this is because light that excites a green cone when processed inhibits a red cone.

• It is important to appreciate that the brain computes colour perception by not only analyzing the response of the cones that are stimulated by the incident light but by analyzing the response of all the cones to light.

Colour Vision

Role of the Pretectal Area

• Light shone directly into either eye initiates a reflex constriction of the pupil - the direct response. At the same time there is also a reflex constriction of the pupil in the other eye - the indirect response.

• These reflexes are mediated by the ganglion cells that detect overall changes in brightness and project to the pre-tectal area.

• Efferent pathway: Pre-tectal area Edinger Westphal nucleus ciliary ganglion postganglionic fibres smooth muscle of the iris.

Eye Movements and the Superior Colliculus

• Three pairs of extra-ocular or extrinsic muscles control eye movements, namely, the medial and lateral recti, the superior and inferior recti, and the superior and inferior obliques.

• These muscles are reciprocally innervated and connected via the medial longitudinal fasiculus. This allows one muscle of each pair to contract while the other relaxes. Cranial nerve 4 innervates the superior oblique, 6 the lateral rectus and 3 the rest.

• Fixation movements - 2 types voluntary and involuntary.

• A bilateral region in the pre-motor cortex of the frontal lobe controls voluntary movements.

• The secondary visual cortex via the superior colliculus achieves involuntary fixation.

• There are three types of movements involved in involuntary fixation.

Eye Movements and the Superior Colliculus

1 A continuous tremor of 30-80Hz due to successive contractions of the extra-ocular muscles.

2 A slow drift across the fovea.3 A sudden flicking movement that occurs when

the spot of light reaches the edge of the fovea ands is designed to bring it back into focus.

• Saccadic movement is the sudden jumping of the eyes to maintain focus upon a continual changing visual field, e.g.s driving in a car or reading.

Eye Movements and the Superior Colliculus

• Saccades occur extremely rapidly and the visual image is suppressed during the eye movement.

• Another type of movement is smooth pursuit and involves the eyes following the course of movement of an object.

• The superior colliculi have a topographical map of both the retina and the somatic sensations of the body and thus are responsible for turning the eyes and the head towards or away from a particular light source.

Eye Movements and the Superior Colliculus