subject: Injury to the visual system of the brain [print this page] Injury to the visual system of the brain Injury to the visual system of the brain
The visual cortex is composed of five areas, which are labelled by neuroscientists as V1, V2, V3, V4, and V5. V1 is also known as the striate cortex because of its striped appearance. This is by far the largest and most important area of the visual cortex. It is sometimes called the primary visual cortex. The other visual areas are referred to as extrastriate cortex. V1 is one of the most extensively studied and understood areas of the human brain. The primary visual cortex is an approximately 0.07 inch (2 mm) thick layer of brain with about the area of an index card. Because it is scrunched up, its volume is only a few cubic centimeters. The neurons in V1 are organized on both the local and global level, with horizontal and vertical organization schemes. This part of the visual cortex is tuned to pick up colour, shape, size, motion, orientation, and other visual aspects, which are more subtle. The manner in which this part of the brain is organised means that there are certain cells in the primary visual cortex, which are activated by the presence of colour A, others activated by colour B, and so on.
How does the visual pathway operate? Raw sensory data comes from the eyes as an ensemble of nerve firings called a retinotopic map. The first series of neurons are designed to perform relatively elementary analyses of sensory data a collection of neurons designed to detect vertical lines might activate when a critical threshold of visual "pixels" prove to be configured in a vertical pattern. Higher-level processors make their "decisions" based on preprocessed data from other neurons; for example, a collection of neurons designed to detect the velocity of an object might be dependent upon information from neurons designed to detect objects as separate entities from their backgrounds.
What happens when the visual pathway or visual cortex is damaged?
25% of the human cortex is devoted to processing visual information. With that huge amount of cortex devoted to just one sense, the chances are that any diffuse brain-injury will knock out a significant portion of brain cells dedicated to vision. Is it surprising therefore, that so many brain-injured children experience visual difficulties? Let us take a look at some ways in which those visual difficulties can express themselves. These are some of the distortions of visual processing which have been reported.
Wide spectrum tuning
Imagine a situation where everything within your visual field is competing equally for your attention. In this situation, you would not have the ability to tune out' some elements of your visual field and selectively attend to one or two elements alone. Your brain would be trying to process everything you could see AT THE SAME TIME! The result is chaos for the children who suffer this problem, causing anxiety and high stress. This type of visual oversensitivity was reported by Bruno (2006), who endured brain-injury through a car accident. She described how brain-injury propelled her into a world of psychological perplexity, double vision and incapacitating visual and auditory oversensitivity Many parents report that the visual world is just too much for their children, indeed children have themselves reported a situation where they are unable to focus on a single visual stimulus. Indeed, it seems that apart from inappropriate activity from the brain's tuning mechanism, damage to parts of the brain's parietal lobes can lead to this type of visual difficulty. A child may appear to be competently scanning his visual environment, but cannot attend to particular visual features of that environment! (Carlson, 2007)
The child who suffers this problem will only relax if placed in an under-stimulating, darkened environment. He is only truly at peace when he is asleep. He rarely makes eye-contact, because he has difficulty attending to the single visual stimulus of the eyes, which are competing with other visual stimuli in the environment (does this ring any bells for parents of children diagnosed with autism?). Everything is competing for his visual attention simultaneously, so he finds it impossible to focus on any one person or object. As a consequence of his inability to cope with his visual world, the child with this problem can, in a desperate measure to protect his immature, overwhelmed sensory system, withdraw' into himself.
Narrow spectrum tuning.
In this situation, the child appears not to be aware of most of his visual environment, singling out one object and almost completely focussing his attention on it. This over-focussed' attention can appear to be obsessive behaviour to the outsider. This child plays with one toy and one toy only because he is focussed upon specific features of it. He may be fascinated with movement, such as a spinning top, or wheels moving and will spend hours just looking at this. Rizzo & Robin, (1990), describe this situation perfectly. Apart from a malfunctioning neurological tuning mechanism, injuries to the parietal / occipital lobes of both hemispheres of the brain can also create a situation whereby individuals can only pay visual attention to one object at a time. This is known as Balint's syndrome and it is possible that some of our children who experience narrow spectrum tuning' difficulties, will have injuries in this part of the brain. However, there is also a convincing developmental explanation for narrow spectrum tuning.' Young babies have difficulty in shifting their attention, - this is well known and is a developmental phase. Infants who are less than four months of age will sometimes stare at an attractive object, being unable to shift their gaze. Occasionally this inability to shift their visual attention will make them cry out in distress, (Johnson et al, 1991). It could very well be that some children, who have narrow spectrum tuning' difficulties, never emerge from this phase of visual development.
Over-amplification
This again is a type of visual oversensitivity whereby the sensory tuning system of the child is acting to amplify the visual information, which the eyes are taking in. The child, who suffers visual over-amplification problems, is the child who hates bright lights. He particularly dislikes sunny days and hates anything moving close to his eyes. He may not concentrate on anything at all with his central vision, preferring to view things from the less threatening position of his peripheral vision. We, as healthy individuals are allowed a small insight into the way this child feels when we have a migraine attack and our vision becomes sensitive to bright lights. What is occurring both in migraine sufferers and with our brain-injured children is that specific inhibitory systems of the tuning system within the brain are not activated sufficiently, resulting in overstimulation in the visual cortex. (Mulleners et al, 2001). Unfortunately, this is the visual world, which this child inhabits 24/7. This child will not make eye contact, but for different reasons to the child with wide spectrum tuning difficulties; - this child literally finds eye contact to be very threatening and will avoid the situation at all costs.
Under-amplification
In this situation, the sensory tuning system of the brain is simply not exciting the visual cortex sufficiently and so it is unable to process incoming sensory information. These children are sunworshippers; they find bright lights and visually attractive displays to be absolutely fascinating. Children with this problem, if their motor control allows, can often be found waving their hands in front of their eyes in an attempt to self-stimulate their visual system. Children showing under-amplification problems, like those displaying over-amplification difficulties can seem hard to reach, but for the opposite reason; - they are simply unaware of much of the visual world around them.
Internal tuning
In this phenomenon the visual system is tuned inwardly to visual phenomena, which it itself is creating. Again, it is possible to relate this to what happens to certain individuals who suffer from migraine. Many migraine sufferers, (including myself!), experience a visual display' prior to an attack, where all sorts of shapes and colours occlude the vision. Similarly, the visual system of some braininjured children is capable of producing this effect. These children appear preoccupied with looking at something, which you cannot determine! They appear to be staring into the middistance and it is immensely difficult to break their concentration. As early as 1956, Beck and Guthrie were describing the internally generated visual phenomena experienced by individuals who had suffered brain-injuries, one describing seeing different coloured orbs in their visual field, floating up and down. (p. 6). Is it any wonder that some of our children are fixated upon this self generated visual world?
Other visual problems
The development of vision and the child's ability to use his visual skills in a meaningful way may be, as I have just described, distorted by brain-injury. Visual development therefore will most likely be stopped, or slowed to snail's pace. Injury may interfere with the smooth operation of the visual pathways in the brain, or cause direct injury to the occipital cortex, - the processing centre for vision. Injuries such as these can take a terrible toll. They can take the visual ability of the child back to pre-birth stages, in some cases creating a neurological blindness. - This is a situation in which there is nothing at all wrong with the eyes, they are working as they should, but because of damage to the primary visual cortex and the fact that essential parts of the neural networks, which support visual ability have been damaged, the brain is simply unable to process what the eye can see. I remember one little boy, whose parents came to me, who was totally unresponsive in visual terms. He did possess a pupillary light reflex, (his pupils dilated when in the dark and constricted when in the light). His doctors who had informed the parents that he was probably neurologically blind, were not doing anything to try to remedy the situation. It took the parents two years of patiently stimulating their son's visual development under my direction, to bring his vision to a level where he would visually track an object across a room and visually explore his environment. The most moving moment however, was the first time he looked his mother in the eye and smiled. From there, I instituted further stimulation, which later culminated in the adoption of a reading programme. He made incredible progress!
Other visual problems, sustained by brain-injured children include visual field problems; - Each hemisphere of the brain is responsible for processing visual information from one half of the visual field, (the opposite side), so an injury to part of the occipital cortex in the right hemisphere of the brain can cause a visual deficit in the left visual field and injury to part of the left occipital cortex can cause deficits to the right visual field.
Another phenomenon, which can occur due to injury to the occipital cortex, is that the child may not notice movement within his visual environment. He may pay good visual attention to most aspects of his visual environment and yet fail to detect the sudden movement of an object close by.
Deficits in the ability to perceive colour (cerebral achromatopsia), may also be experienced due to brain-injury. Interestingly, this problem may occasionally be experienced in only one half of the visual field, if the injury to this part of the occipital cortex is only in one hemisphere. Patients with this type of injury in both hemispheres report their vision as being in black and white. (Heywood & Kentridge, 2003).
Having highlighted some of the major effects of brain-injury upon vision, we should now consider how vision develops in normal circumstances, because this is the developmental pathway, down which we wish to lead our children.
The path of visual development
When a child is born, his vision is already at a relatively sophisticated level. He can see quite well, although his vision is a little blurry and he cannot see as far or as clearly as you or I. He does have difficulty switching his focus from one point to another point at a different distance. He is however able to scan his visual field although at this point his eye movements are slow and disjointed.
By the time he is one month of age however, his eye movements are smooth and he is able to scan his visual field more effectively. You may notice that a young baby may appear to have very big eyes in relation to the size of his head; - There is a very good reason for this. The eyes of a young baby are forming a massive number of complex attachments with the brain. If the eyes grew substantially after making those attachments, new nerve fibres, (axons) would have to be grown in order to constantly readjust the attachments between the growing eyes and the brain. This would mean that the brain would have to continually reorganise its structure as the eyes grew. Hence the eyes come ready-made,' full size!
At two months of age, baby is developing pattern discrimination and contrast sensitivity, although at this point, he prefers less complex patterns. (Such as preferring to look at a checkerboard with large squares, rather than one with small squares). Infants are very attracted to looking at high-contrast edges and patterns. Large black and white patterns offer the maximum achievable contrast to the eye and consequently are most noticeable and eye-catching to babies.
By three months of age, baby is able to focus as well as an adult and at four months his pattern discrimination has developed to the point where he prefers to look at more complex patterns and is becoming interested in the internal detail within a shape.
At six to seven months of age, when he is starting to crawl, he is beginning to use his two eyes together and is developing an appreciation of the third dimension and depth perception (stereopsis). There is evidence that the specific neural networks, which are essential for the development of depth perception will not develop unless baby is provided the opportunity to scrutinise objects with both eyes. If baby's eyes are not given practice in moving together properly, he may never develop fully functional depth perception, even if the eye movements are later rectified by surgery on the eye muscles . There is also some evidence that it is crawling, which helps the development of depth perception by helping to mature the relevant areas of the brain and by affording the child, through movement, the opportunity he needs to use both eyes together properly. (Berk, 1997).
Vision continues to develop throughout the preschool years. It is essential that it does so, in order that there are continued improvements in eye/hand coordination and depth perception.
There are many exercises, which can be carried out with brain-injured children to try to achieve these objectives. One of the most important and enjoyable exercises to carry out with young children is to read to them. This encourages the development of robust visualisation proficiency as they "picture" the story in their minds. Just because a child has suffered brain-injury this is no reason to deprive him of this enjoyable activity. Although his vision, or hearing may be impaired to some degree, one never knows how much of the message is actually striking home; - so read to him!
By school age, the child's visual acuity, (the level of fine discrimination of detail, which his vision will permit), is equal to that of an adult.
To what degree does the visual cortex have qualities of plasticity?
It was Payne and Lombar, (2002), who highlighted the plastic qualities of the visual cortex. They pointed out that the consequences of localised injury of the cerebral cortex in the brain of a child differ from the consequences elicited by corresponding damage to the brain of an adult.
"In the young brain, some distant neurons are more vulnerable to injury, whereas others survive and expand their projections to bypass damaged and degenerated structures. The net result is that visual processing can be retained. Experiments using reversible deactivation show that at least two highly localisable functions of normal visual cortex functioning are remapped across the cortical surface as a result of an early lesion of the primary visual cortex. Moreover, the redistribution of connections have spread the essential neural operations for vision from the visual parietal cortex to a normally functionally distinct type of cortex in the visual temporal system. Similar functional reorganizations can explain the retention and recovery of abilities following early lesions in other cerebral systems, and these other systems may respond well to emerging therapeutic strategies designed to enhance the sparing of functions."
Which is why Snowdrop type programmes of developmental stimulation are so important! If your child has visual problems which are caused by brain injury and you want to learn more about Snowdrop's treatment, simply email snowdrop_cdc@btinternet.com, or visit our website at http://www.snowdropcerebralpalsyandautism.com
Further Reading.
Beck, A. T., and Guthrie, T. (1956). Psychological significance of visual auras: Study of three cases with brain damage and seizures. Psychosomatic Medicin, Vol XVIII, no 2,
Berk, L. E. (1997). Child Development. (4th Edition) London. Boston. Allyn & Bacon.
Carlson, N. R. (2007). Physiology of Behavior. London. Allyn and Bacon
Heywood, C. A. and Kentridge, R. W. (2003). Achromatopsia, color vision and cortex. Neurology clinics of North America. (21), 483-500.
Johnson, M. H., Posner, M. I., and Rothbart, M. K., (1991). Components of visual orienting in infancy: Contingnency learning, anticipatory looking and disengaging. Journal of Cognitive Neuroscience, 3, 335-344.
Mulleners, W. M., Chronicle, E. P., Palmer, J, E., Koehler, P. J., and Vredeveld, J. W. (2001), Suppression of perception in migraine: Evidence for reduced inhibition in the visual cortex, Neurology, January 23, 2001; 56(2): 178 - 183.
Payne B, R. & Lomber S, G. Plasticity of the visual cortex after injury: what's different about the young brain? Neuroscientist. 2002 Apr;8(2):174-85.
Rizzo, M. and Robin, D. A. (1990). Simultanagnosia: A defect of sustained attention yields insights on visual information processing. Neurology, 40, 447-455.
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