Common Causes of Visual Loss in Children
Presented on Friday 15 May 2009
The Visual Pathway
Dr J Skillen
How the brain and eye works to create a visual representation of our environment
The Visual Pathway
Notes: The 'visual pathway' is a term used to describe the structures involved in relaying information from the eye to the visual cortex. This pathway provides the brain with basic visual signals: the visual cortex is the first area in the brain responsible for visual processing and can be thought of a sorting office. Its role is to carry out simple visual tasks, and then to filter the information before sending it off to reach areas in the brain responsible for more complicated visual tasks.
The Visual Pathways
To simplify our understanding of the visual pathway we shall divide it into sections
- Creating a visual image on the retina
- Converting light rays into electrical impulses
- Pathways involved in to delivering and sorting the retinal image to visual area in brain
- Higher visual pathways
Notes: To simplify our understanding of the visual pathway we shall divide it into three parts.
1) visual information entering the eye and being focused onto the back of the eye
2) visual information being organised into basic channels
3) Transferring the signals from the eye to the first part of the brain used for interpreting the visual signals.
Structure of the eye
Light rays passing through the eye
Notes: The eye contains a number of structures with the main aim of allowing light (and therefore information about our environment) to be focused onto the back of the eye. All vision is created from light rays which is why nothing can been seen in the dark. From the different levels and distribution of light we are able to detect colours, know what objects are, where they are positioned and know when something is moving.
Notes: The 'whites' of the eyes are the visible part of the sclera, which is a tough outer-coat which protects the structures within the eye from damage. The coloured part of the eye called the iris is found underneath a clear structure called the cornea. The cornea is clear to allow the eye to enter the eyeball but also is shaped in a particular way so that light entering the eye, and therefore information about our environment, can be bent and focused onto the back of the eye. The iris is the coloured part of the eye and the pupil is the black hole in the centre of the iris. The iris contains muscles which tighten when the light levels outside are high, this makes the pupil become small like a pinhole, when it gets dark the iris relaxes and the pupil becomes bigger in size to allow as much light as possible to enter the eye. It is essential to control the amount of light which enters the eye - too much light and it becomes uncomfortable and difficult to see clearly which is why the pupils becomes small in bright sunlight.
Accommodation - lens and ciliary muscle
Notes: Like the cornea the lens inside the eye is convex shape which means that light rays entering it will be bent to form a single focus point - where the image will be sharp and clear. The ciliary muscle can either relax or tighten which will make the lens flatten or become more curved. When the muscle contracts the lens changes shape and becomes more curved making the lens more powerful and able to bend the light more. We would need this to happen when looking at objects close to us. When the muscle contracts this is a process called accommodation. We use accommodation when reading. Accommodation is triggered when a blurry image is detected and is an automatic process - we are not normally aware of blurry images as the process has been activated even before we are aware of our vision being blurry.
A good way to understand how light must be bent to allow us to see clearly and the role the lens plays in focusing objects is to think of what happens to most of us when we reach middle age. This is a time when we will start to become aware of blurry vision.
The lens is made up of many layers which increase in number every year. By the time we reach middle age the extra layers make the lens harder which in turn makes it difficult for the ciliary muscle to bend the lens. So even though a signal of blurry vision is activated - the lens and ciliary muscle are no longer able to compensate and we are then aware of blurry vision. This is most likely to be first noticed when trying to read - especially if the light is poor. The way people try to get around this difficulty is to move the reading book further away, that is, less light bending is now required and the print is no longer blurry. However, this is only a temporary solution and in the end reading glasses are needed to supplement the refraction within the eye.
Notes: Posterior Structure - towards the back of the eye
The vitreous is the clear jelly which forms the bulk of the eye and gives the eye its shape. Its other role is to keep the retina flat onto the back surface of the eye. Without this pressure the retina would peel off the back of the eye and fold over. The retina is the layer at the back of the eye, which is a sensory tissue which can detect different light levels being scattered across the back of the eye. The retina extends all around the back of the eye. This is one of the most important structure of the eye as it’s the only part of the eyes which turns information from light into the type of information that is useful for the brain - ELECTRICAL IMPULSES. Brain cells, are known as neurons and respond to electrical impulses therefore the retina is the most important structure in the visual pathway for translating light rays into a medium that the brain can use.
The photoreceptors are the cells in the retina that turn light into electrical signals of which there are two types, rods and cones – so called because of their shape. The electrical signals are collected and passed onto the ganglion cells (via other retinal layers) which then take the electrical impulses out of the eye to the brain via the optic nerve.
The retinal image
Notes: The optics of the eyes are the reason why an object – such as the green
arrow shown – is seen inverted – or upside down at the back of the eye . So
anything in the real world which are
above us will be deflected towards the bottom of the retina
below us will be deflected towards the top of the retina
The photoreceptors are distributed in a set fashion within the retina
- Rods – peripheral area
- Cones – central area
The photoreceptors then connect to ganglion cells – 2 types are of particular interest
Notes: The retina contains cells called photoreceptors which are responsible for turning light into electrical signals ready for the brain to process. The rods and cones are distributed through the retina in a particular way – with the majority of rods found in the periphery with cones being concentrated in the central area of the retina. The reason for this is due to the different functions of each type of photoreceptor.
Magnocellular (Big cell) and Parvocellular (Little cell) channels
- Different ganglion cells exist and are organised so that they feed into one of two channels entering the optic nerve.
- Parvocellular channel
- dominate central visual area
- Carry information about colour and detail (spatial form) to the brain
- Magnocellular channel
- to be found in the peripheral retina
- Carry information about movement and location to the brain
The two channels are the
Notes: Information gathered at the retina which is in the form of electrical impulses, is carried through to the brain via 2 separate channels – one takes information gathered by one type of ganglion cell into the Parvocelluar channel and the other type of ganglion cells into the Magnocellular channel. These channels stay separate all the way into the visual cortex and carry information, which is different within each channel.
2 channels then go into the 'What' and the 'Where' Stream
- Contributions from the magno and parvo channels then feed into the ventral 'what' stream and the dorsal 'where' stream.
- These are examples of visual streams which carry information to higher brain areas for more complicated processing.
- A mixture of information from the magno and parvo feed in - one stream does not become the other.
- Important point is that information at the retina remains in the separate streams all the way through the visual pathway, that is, from the retina to visual cortex.
Notes: The WHAT and WHERE streams also called the ventral and dorsal processing streams and are channels of visual information which are taken to higher points in the brain for complex processing. The WHAT stream is interested in what the object of interest is – that is, is it big or small, dark or light, made up of colour. The WHERE stream is less interested in the details of the object and more interested in where it is to be found – the position of the object and how fast its moving.
We will not go into detail about this as another talk is in this series explaining the differences between the two but suffice to say that the organisation of visual information at these low levels of the visual pathway have a bearing on the type of processing which occurs in the higher areas of the brain.
The Visual Pathway
Notes: The optic nerves from each eye then take the electrical impulses to a structure called the optic chiasm. This is an important structure as the fibres carrying visual information from each eye then CROSS-OVER at the chiasm. Why is this important? It means that from this point onwards each side of the brain contains information from each eye rather than information from the right eye being processed only by the right side of the brain and vice versa.
Notes: The information then travels through the optic tracts into a structure called the LATERAL GENICULATE NUCLEUS. This site is important as each LGN within each side of the brain contains information which is segregated into A) which eye the information originates from and b) separates whether the signals are carrying information which is from the Parvocellular or Magnocellular stream. The information is separated because this structure has six different layers, 4 of these layers contains parvocellular information and 2 layers contain magnocellular information whilst the split of right and left eye information being 50:50.
The Visual Cortex
Notes: The optic radiations then take the signals to the final area in the visual pathway – the visual cortex. The optic radiations also divide information into what is seen above us and what is seen below us as well as whether its from the right or left hand side.
The information is then taken to the visual cortex.
The visual cortex is a complicated structure containing lots of compartments which splits the types of visual information into lots of different areas. Its sometimes called the striate cortex because when under the microscope it has a stripey appearance as shown in the picture.
Visual Cortex – 6 Layers
Ocular Dominance columns
- channels which contain info from
RIGHT eye only red
LEFT eye only green
Layer 4c alpha – Magno
Layer 4c beta – Parvo
Notes: At the visual cortex information in categorised into whether its parvo or magno and whether information comes from the right or left eye. Although this area of brain has 6 known layers all the input into the cortex is into layer 4c. Outputs then extend to a specific layers depending on the type of information which it carries, for example
Colour information will come into layer 4cbeta then go to layer ...before going on the areas higher up in the brain which is responsible for colour processing.
The visual cortex is capable of making some easy visual judgments such as understanding the position of objects AND coordinating the information from each eye – a process known as binocular vision. Details of patterns are also processed with a high amount of detail. So if you only has the visual cortex working it would still be possible to do some simple visual tasks.
Colour vision analysis and detailed motion processing in general is the result of processing in areas of the brain which are dedicated for this purpose. The visual pathway needs to organise the flow of information so that the correct signal are reaching the right parts of the brain.
- Magno and Parvo are then reorganised and then feed into the WHAT and WHERE pathways
- Dedicated areas of the brain for motion and for colour -
- Different areas for recognising shapes when they are still or when they are moving
Notes: So the flow of information to the back of the eye is the key route to doing simple visual tasks as well as segregating and ordering information. The reason for this is so that information can then be sent to the most appropriate areas of the brain for more detailed analysis. Only colour information is sent to the brain area interested in colour – otherwise a lot of neurons and pathways would be carrying information to areas in the brain which would just switch off and ignore the information.
Low Level Visual channels:Magnocellular and Parvocelluar channels
Notes: This requires a high level system which is so sophisticated and unique that the mechanisms to understand how it all works are not yet fully understood. The fact that the visual system cannot be replicated using even the highest spec computer is testament to the super-advanced processing capabilites which exist swithin the human brain.
So what do we know up to now about how the brain understands our visual world?
A natural visual scene
Notes: The amount of visual processing which takes place in order for us to see what is within our visual field is vast. Particularly when we consider that the brain can keep an up to date picture when our environment is changing. Even looking at a simple uncomplicated visual scene, such as the picture of the deer in its natural environment. Many computations are taking place for us to see the picture in front of us.
Notes: First we need to distinguish what we are seeing – and separate what the object of interest is from the visual signals that make up the background. This requires the process of object recognition and figure-ground segregation.
We also need to make a judgement as to where the object is and its distance from us as viewers. Being able to make this judgement allows us to know when we are likely to be in danger – for example if we were looking at a lion rather than a deer – we may well not want to be as close as this when taking the photograph. We are using our visual localisation and depth perception skills to get this information and basing our judgements on learned behaviour.
Most people viewing this scene will also recognise the range of colours contained within the scene, we can recognise where the sky ends and the land begins and pick out colour differences of trees from the fields
We are also able to see the deer in detail using our central vision, picking out the colour change within its skin and the pattern of light and shadow – these features are all part of form vision or spatial vision. We can pick up all these details and still be simultaneously aware of the deer's surroundings (using our peripheral vision).So not only does the brain have to process all the visual information falling within the visual field but it also has to contend with constant visual updates streaming into the brain at a rate of knots. Especially if we consider that the deer will be moving across the landscape. Its movements will be picked by us in an instant.
High Level Visual Pathways:
Location of What and Where Pathway
Notes: Specialist areas in the brain also exist as well as specialist pathways or channels. These areas are typically beyond area V1, and respond maximally to particular types of visual information. Links from one specialist area to another are thought to exist in a structured way to form the What and where pathways - the basic of vision is to know what things are and where they are. The what pathway is also called the ventral stream and the WHERE pathway is known as the dorsal stream. Because all these areas are above area V1 in the processing stream – they are regarded as higher visual areas. This means that the type of information which can be processed will be more detailed and be more complicated and involve interaction with different areas of the brain that control other parts of our bodies.
BUT – connections from higher visual areas also feedback to lower visual areas rather than all information flowing in one direction.
The diagram shows the WHERE pathway starting at area V1 and extending up to the posterior parietal cortex – shown in GREEN. (arrow pointing upwards). The DORSAL pathway extends from area V1 to the inferior temporal lobe – shown in PURPLE) (downwards arrow).
Specialised Visual Areas –
Occipital Cortex (area V1-V5)
Notes: The reason for having specialised areas is to limit the amount of time taken to sift through the different types of visual information. Instead the brain is geared up to have separate troops arranged to spring into action and fire in response to their preferred type of visual information. All these troops can be active at the same time and are stationed in different locations within the brain.
Within the occipital lobe of the brain there exists 5 distinct zones – responsible for processing particular types of information.
Area V1, already described as a sorting office – separates information used for different tasks as does areas V2 to a finer degree. Segregation of colour, shape or (form) and motion occurs here which will then be passes onto other areas listed below. Most cells in area VI and V2 will respond to the orientation of visual signals and will be spatial frequency selective. This means that they will identify the basic patterns making up visual patterns.
Area V3 signals geometrical shapes whilst V4 is predominately used to process colours. Area V5 also known at area MT responds to motion signals.
Damage to area V4 and V5 can lead to isolated visual problems of colour and motion whilst the other functions of vision remain entirely unaffected – this type of damage can happen after certain types of strokes.
Purpose of DORSAL 'Where' Stream
- Vision for action
- Integrates motion with object locations
- Links visual information with visual guided action - creates visual control over skilled movements
- Guides visual attention to areas requiring attention eg; predator, warning sign of fast approaching car
Notes: What are the purpose of these pathways?
The WHERE or dorsal pathway links particular parts of the visual cortex to the parietal cortex. This area of the brain does many things such as processing body sensations for example from touch, it also gives us reasoning powers and our intelligence. When visual information is fed into the parietal cortex the brain can make sense of motion signals with respect to the body's response to it. This type of responses can be summarised as VISION FOR ACTION.
Purpose of Ventral 'What' Stream
- Vision for perception
- Links visual cortex with visual recognition – takes place in the infero-temporal cortex
- Processing of detailed spatial vision see patterns and distinguish objects and faces
- Stores visual memory
- Makes sense of the significance of objects and faces
Notes: The what pathway or ventral stream connects sections of the visual cortex with the temporal cortex. This area of the brain is predominately used for memory, behaviour and emotions as well as for visual based tasks. When visual information is fed into this cortex it tries to make sense of the object in our environment by the process of learned responses. This type of response can be summarised VISION FOR PERCEPTION.
The Where Pathway
- REAL TIME ACTIONS – VISUALLY GUIDED BEHAVIOUR
- Helps to separate the wood from the trees
- Spatial Awareness - navigation in environment
- Involved in guiding eye movements
- Subconscious and automatic
Notes: Therefore, in terms of visual skills, having a where pathway integrates the visual signals from our environment and takes them up to the control centres for actions to take place. These actions are generally our visually guided behaviours – automatic processes such as reaching to pick something up – or moving safely around our environment. Knowledge of the position of things in our environment, how fast something is moving is linked into the centres which know the position of our arms and legs. We can then easily make appropriate actions to be safe in our environment and make accurate reaching and grabbing movements. The vast majority of these processes are automatic and not part of our conciousness.
All the information fed into the where pathway is generated from area v1 – v2 v3 and v5.
The What Pathway
- STORES VISUAL MEMORY/(VISUAL HARDRIVE OF THE BRAIN
- REQUIRES CONCIOUS AWARENESS
- SLOWER PROCESSING SPEED THAN THE WHERE PATHWAY
Notes: The ventral pathway processes at a slower rate than the dorsal pathway and is associated with concious awareness. The details of objects are recognised allowing for object and people to be identified – making full use of the visual memory which is also located within this brain region. Images and pictures which are familiar are stored onto the hard drive for faster recollection next time the same stimulus appears.
All the information fed into the where pathway is generated from area v1 – v2 v3 and v4.
Signs of Dorsal 'WHERE' Pathway deficits
- Difficulties reaching and grasping
- Clumsy child syndrome
- tripping up and difficulty with stairs
– unable to single out information when in crowded and busy environments,
- difficulty hunting for a toy in a tightly packed box of toys
- Behavioural changes in busy crowded surroundings
Notes: By understanding where in the brain different types of visual information are processed we can then begin to understand the difficulties that may arise when things go wrong. Problems with the 'what' and 'where' pathway gives rise to specific types of visual difficulties which can be tricky to recognise during normal eye tests. In fact the ability to read letters on a chart can be completely normal and yet a person may be profoundly affected by dorsal or ventral based visual difficulties.
Signs of Ventral 'WHAT' Pathway deficits
- Poor facial recognition – may mistake strangers as being someone known to them
- Poor recognition of facial expressions
- Poor visual memory – have difficulty remembering routes
- Difficulty remembering where belongings were left
- Difficulty recognising and remembering familiar people – when based on visual cues only
- Unable to name colours
Notes: Problems with the ventral or WHAT pathways lead to difficulties with facial recognition both in terms of mistaking people to be known when they are strangers and not recognising people who are familiar. The other visual difficulty linked to visual memory is problems remembering routes taken when in unfamiliar place. Children in school may also have problems copying from the blackboard as recalling sentences written on the board proves difficult.
Strategies can be used to help get around these processing difficulties although an understanding of the nature of the difficulty is required first.
Importance of early intervention
- How long does the process of learning to see take?
- What are the developmental stages?