Before considering CVI as a cause of low vision it is important to understand that up-to-date eye tests are essential to ensure that near or long sight, and astigmatism, as well as lack of focussing, are identified and corrected with spectacles or contact lenses. In addition, eye, and eye movement disorders need to be checked for. For those with additional support needs or learning difficulties, sight tests need to be planned carefully. The UK charity SeeAbility has a lot of useful information on their website.
Reduced Visual Acuity
How clearly detail is seen. With typical vision, sometimes called 20/20 vision, you can clearly see, at arms-reach, a single line as narrow as a tenth of a millimetre, that's about the width of a single human hair. So with typical vision, two human hairs next to each other, need a gap at least one tenth of a millimetre between them, to be seen clearly and separately. This measure is called visual acuity, or clarity of vision, and the 20/20 (or 6/6 as used in the UK) is the typical measurement, with bigger numbers meaning that only wider lines and gaps are visible. When things need to be bigger and separated further apart to be seen clearly, this is called Reduced Visual Acuity. Further reading: What Is CVI? Visual Acuity and Visual Acuity The Technical Stuff, Lessons: 3b Visual Acuity and 5b Reduced Visual Acuity. See also our section Visual Acuity - Free Resources, including link to sight-sim, a free programme (shown in image below) where any photo can be altered to show different levels of reduced visual acuity and / or reduced contrast sensitivity.
Reduced Contrast Sensitivity
Look at some wood (image below). Your ability to see the shades of brown in the wood is called your contrast sensitivity. Where the subtle differences between shades and tints of colours can't be seen clearly, or at all, (needing greater contrast), this is called Reduced Contrast Sensitivity. Further reading: What Is CVI? Colour & Contrast Sensitivity and Contrast Sensitivity The Technical Stuff, Lessons: 3c Colour and 5c Reduced Contrast Sensitivity.
Impaired Colour Vision
The inability to tell similar colours apart, which cannot be explained by reduced contrast sensitivity, or difficulties learning the concept of colours or colour names due to other CVIs alone has been shown experimentally to due to impaired colour vision. There are at present no clinical tests to identify this issue, but it is likely to be impaired in children with more severe cerebral visual impairment. Further reading What Is CVI? The Image, Colour, Lesson 5d Colours, COOL Games (colour concept and naming game).
The word hemianopia means that vision to one side (left or right) is not present for both eyes - the brain only creating half of the picture. Left hemianopia means vision is absent on the left side, and right hemianopia means vision is absent on the right. Look at the picture below, while looking forward, hold your hands up like this on both sides and gradually bring them together to meet in the centre. With hemianopia, only one of your hands would be visible, only coming into view when it comes near to the centre. Many affected people don't realise that half of their vision is missing. Further reading: What is CVI? The Visual Field and Hemianopia. Lessons: 3d Visual Fields and 6a Hemianopia. If you suspect combined Hemianopia and Hemi Inattention (4), below, please also read What is CVI? Combined Hemianopia and Visual Inattention or Neglect.
Quadrantanopia is like hemianopia (above) but only affects an upper or lower quarter of the vision for both eyes, as shown on the diagram below. It is possible to have hemianopia on one side and quadrantanopia on the other, leaving vision present in only one quarter of the visual field, most commonly an upper quarter. This can be easily missed as it can look like either absent vision or inconsistent vision. If identified, which is relatively straight forward to do, visual support can be targeted to ensure that the one quarter of the visual field where the child can see, is best used.
Click here for a more detailed explanation of the diagram below, with further illustrations of visual field deficits due to damage to the brain.
Where vision is absent due to occipital lobe damage causing hemianopia, quadrantanopia or complete occipital blindness, sometimes movement in the blind part of the visual field can be fairly accurately detected by guesswork more often than not, hence the term blindsight. If it can be harnessed, blindsight can be very useful, to learn how to avoid obstacles, and in some affected people it can be given useful conscious awareness. Ability to see things that move in an otherwise blind area of vision is called the Riddoch phenomenon. Further reading, Lesson 6b Blindsight.
In this condition visual attention to one side (left or right) is absent or diminished. This is a bit different to hemianopia described above, although some people with CVI can have both. Here both eyes are working typically, and the brain creates a picture of what is being looked at, but those affected are only aware of things to one side of their body, but not the other. It is related to the body so only a turn of the body to the affected side will reveal what cannot be attended to. Turning the head and eyes to that side does not work. Hemi inattention can vary in degree and can be very easily missed. Left hemi-inattention is where there is absent or reduced visual awareness to the left side of the body. This is commonly profound. Right hemi inattention is where there is absent or reduced visual awareness to the right side of the body. This is commonly less severe. Further reading: What Is CVI? Visual Neglect & Visual Inattention, Lessons 6c Hemi Inattention and Neglect. If you suspect combined Hemi Inattention and Hemianopia (3), above, please also read What is CVI? Combined Hemianopia and Visual Inattention or Neglect.
Lower Visual Field Impairment
This is reduced or absent vision lower down. Imagine drawing a line going from left to right to divide everything you are looking at, into a top and bottom half. This form of CVI affects the bottom half. It varies in degree. For some, nothing can be seen in the whole of the bottom half of the scene. For others only things that are lower down are not seen. For some, nothing is visible in the area of vision affected, for others, their vision in this area is less clear. This makes the resulting visual difficulties unique for each affected person. This is called Lower Visual Field Impairment. Use this video (from Lesson 3d) to find out how to check your lower visual field (demonstration at 4.10 min). Further reading: What Is CVI? Lower Visual Field Impairment, Lessons 3d Visual Fields and 6d Lower Visual Field Impairment.
Upper Visual Field Impairment
As with lower visual field impairment (above) but relating to the top half of the visual field, most commonly one of the top quarters, associated with a temporal lobe injury (see Brain Locations).
Visual Field Constriction
Everything you see, including what you are looking at and everything to the sides and up and below, is called your visual field. Everything to the sides and above and below, where you see less clearly, is called your peripheral vision. Some do not have vision as far out to the sides as well as above and below, as others, so their peripheral vision is reduced from the outsides towards the centre. In extreme cases only a small tunnel of vision in the centre is present. This is called Visual Field Constriction. There are many causes including stroke. It is also associated with Cerebral Palsy. Further reading: Visual Fields What Are They? Diagram Visual Field Deficits - Illustrations
Things that move too fast are missed or not seen clearly. This is true for us all of course. As things move faster, like a speeding car, they become more difficult to see clearly. With this type of CVI, the speed where clear vision is lost or reduced, is slower than is typical. For some this can be a lot slower. So even a person's walking pace may be too fast for clear vision. This is called Dyskinetopsia. A big word, basically meaning that things that move quickly are not seen clearly! Slow things down, right down. Learn the optimal speed for your child - and slow your child down too, because when they move they create movement around them, and this moving scene is more blurred when moving faster. Further reading: What Is CVI? Movement, Lessons 3e Movement and 7a Dyskinetopsia.
Anything that moves is not seen at all. This is very rare, and means that everything, including the person, needs to be as still as possible for things to be seen clearly. It is called Akinetopsia. Further reading: Lesson 7a Dyskinetopsia (Akinetopsia at end of lesson)
This CVI makes reaching for things using vision to accurately guide hand and arm movement difficult, like picking up a cup without knocking it over. Knocking into door frames when going through them can also be an issue. This is a common type of CVI in children . It is called Optic Ataxia, and can make them look clumsy. They may have been told they have dyspraxia or DCD (see Labels). There are other reasons for inaccurate visual guidance of movement for example low visual acuity or impaired contrast sensitivity, and these need to be identified before a diagnosis can be made. Further reading: What is CVI? Optic Ataxia Lessons 3f 3rd Dimension and 7b Optic Ataxia and Level 9 Dorsal (2).
Difficulty seeing more than one thing at a time, especially where there is clutter or a lot of movement and noise. This is a common CVI in children, and can be difficult to pick up, because sometimes it can be as if they see as normal, and other times they don't spot something that is in front of them. It seems as if life is sometimes viewed looking through something as small as a drinking straw - when it is hard to make sense of anything - but other times, especially when somewhere well known, when it is peaceful and quiet, almost everything can be seen. We use the umbrella term Simultanagnostic Vision to describe the range of experiences related to this type of vision. To help, reduce clutter, noise and movement, keep your child calm and help to keep them happy in familiar liked places, where activities will be easier. Further reading: What Is CVI? Simultanagnosia Spectrum. Lessons 3g Visual Attention and 7c Simultanagnostic Vision and Level 9 Dorsal (2). Dorsal Stream Dysfunction CVI Experience.
Recognising people by their faces is something many people can have difficulty with. This can be very embarrassing, and the cause of serious problems around confidence and social relationships. Nearly all of the CVIs listed above can cause facial recognition difficulties. The best approach will be different for different CVIs. However, a person may not be affected by any of the above CVIs and have very good vision, but still have difficulty recognising people by their faces. It is likely they have a condition called Prosopagnosia. It is possible to have CVIs and prosopagnosia, where the visual difficulties caused by the CVI does not fully explain the face recognition difficulties. Facial Recognition difficulties, whether prosopagnosia, caused by other CVIs or both, are often accompanied by difficulties recognising facial expressions. Further reading: Lesson 8b Impaired Facial Recognition and Lesson 8c Impaired Recognition of Facial Expressions.
Shape & Object Agnosia
Think of some shapes. Squares, circles and triangles may come to mind, but we use our ability to recognise things by their shape in almost everything we do. Could you recognise a cup in front of you to drink from, if there was no cup shape? Shape and object recognition is something most of us take for granted and never have to think about, because we can see things clearly, and easily recognise them by their shape. Nearly all the CVIs (above) can cause difficulties learning shapes, even of simple things like a cup. This difficulty can affect all elements of learning, but the approach needs to be matched to the cause, the actual CVI or CVIs creating the difficulty. You can also have good vision but the part of the brain that remembers and recognises shapes may not be working so well, making recognising things by their shapes and recognising objects difficult. This can create significant learning difficulties and is called Shape & Object Agnosia.
Remembering how to get to places, like a walk to your local park or around a supermarket or mall, is actually quite a complicated process. These are called routes, and for most people, simple routes are easy to remember, but more complicated ones might take a few trips to learn, or need extra support like directions or a map. Many of the above CVIs can create difficulties remembering routes. In the home, the places where things are stored and put away are smaller routes, but are still 'routes', and so finding things again is more difficult. The support needs to be matched to the CVI or CVIs causing the difficulties, as it will be different for each one, and different again for combinations. Some people are not affected by any of the CVIs above but still have this problem finding their way around places and remembering routes. They may have Topographic Agnosia. Further reading: Lesson 8d Impaired Route Recognition.
Many of the CVIs can cause reading difficulties, and each requires an individualised approach. There is another condition called Alexia. which is sometimes referred to as word blindness. Very rarely a child with alexia can write, but not read what they have just written. Further reading: Newsletter 18 Literacy (with links to many further resources). LOOK reading tool.
Dyslexia is a reading disorder that has many causes. CVI is a known cause of dyslexia, due to both dorsal and ventral stream disorders. It is complicated though. CVI can cause learning and language delays, that contribute to reading difficulties but this is not dyslexia. Further reading Developmental Dyslexia Paper.
Learning numbers and basic mathematics can be made difficult for a range of reasons owing to different CVIs. There is a condition affecting the learning of numbers called Dyscalculia, explained in the Labels page. Dyscalculia can have many different causes, including an inability to visually recognise numbers.
Acalculia is similar to dyscalculia (above) but is more marked. It is typically due to an acquired brain injury, for example a stroke.
As what you are looking at changes, such as when you move through a room, your brain is constantly refreshing the picture it creates, so what you see is a clear visual representation of everything around you. So, if you look at one of your hands, and then maybe look at a pen, your brain very quickly replaces the image of the hand with the image of the pen. With palinopsia, the previous image lingers. There are different experiences, so with the hand and pen example, for some, when looking at the pen, they still see the hand. For others they see both the pen and hand, one image overlaying the other. Some may be aware of trailing of the previous image. There is a range of different causes.
A hallucination occurs when a person becomes aware of something that is not actually present, for example hearing a voice that is not there. Visual hallucinations relate specifically to the person believing they can see something, that is not there. There are different types, some very detailed and clear like seeing a person who is not present. Other visual hallucinations may be of lights or colours or patterns that are also not real.
Cerebellar visual perseveration
One function of the part of the brain called the cerebellum (which is just below the occipital lobes), is to deal with time, so that the timing in our minds matches the timing of real world events. If the cerebellum is not functioning typically, it may process time more slowly, which can make what a person is looking at less clear. So a ball moving has been described to us as looking more like a long sausage shape, because the time processing by the mind is slower than the movement of the ball.
We perceive our world as three dimensional because we have binocular vision. Binocular vision is achieved through both eyes being perfectly synchronised, so working together, to create the appearance of depth, called stereopsis (see Lessons 3f & 7b). With the Pulfrich Phenomenon, the transmission of visual information from one of the eyes, is slower to get to the brain than the other eye, so the information is not synchronised. This makes the perception of direction of movement of a person or a car toward the side of the affected optic nerve to appear as if there is going to be a collision . This is most commonly identified as an eye disorder related to the optic nerve, but cerebral origins of the Pulfrich Phenomenon have also been recorded in the area of the brain where the temporal and parietal lobes meet. Separate from the cerebral cause, Pulfrich Phenomenon can affect people who have CVI due to Multiple Sclerosis, owing to damage to one optic nerve.
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