Sunday, 4 December 2011

Review of Cerebral Visual Impairment - Working Within and Around the Limitations of Vision By Gordon N Dutton

Gordon’s paper is in my view an essential read. Did you know there is more than one vision system? There is reflex vision, which is subconscious and designed to you protect us from danger. So when a parent of a blind child says ‘can my child see?’, the answer may actually be ‘Yes – subconsciously’. Your primitive visual system senses a peripheral movement to the side of your vision and sends a message to the brain before you are even aware of it.  A non ambulant blind child may be able to ‘see’ a moving spoon approaching from the side and open their mouth. This vision is less effective if the movement approaches from straight ahead. It engages the rods spread throughout the peripheral retina. 

Then there is the higher level of vision, where information is split up into two systems, handled in different regions. These two systems are respectively a ‘where’ system, enabling movement through space; and a ‘what’ system, enabling recognition of what is viewed. Over 40% of the brain concerns vision therefore any child with damage to brain tissue can potentially have  visual problems. These can range from blurred vision and eye control to analysing visual information and moving through visual space. 

Gordon’s explanation of the visual system needs to be read and re-read. It shows how it really helps to know where the brain damage occurs. For instance the occipital lobes at the back of the brain (this is the visual cortex) receive electrical signals from the visual pathways and sort the information into all the component parts. Vision is not merely one thing. It consists of colour, detail, orientation, movement, form, distance, perspective and so on. Damage in this area may result in gaps in the visual field. For example there are children with a complete loss of vision on one side, called hemianopia. Damage in the right brain affects the left side of vision; damage to the of the cortex is affects the lower field of vision. 

What is really vital to understand is Gordon’s unpacking of the two information flows, called the dorsal stream and the ventral stream. It takes 0.1 of a second for information to reach the brain from the eyes and a further 0.1 of a second to analyse information in these two systems. The dorsal stream (where stream) connects the occipital lobes with the parietal lobes (top of the brain), the motor cortex (vertical strip of brain in the centre of the cortex) and the frontal lobes (front of the brain cortex). The ventral stream connects the occipital lobes to the temporal lobes (lower cortex). 

The posterior parietal cortex is the region where lots of information is handled at once. Like computer RAM it enables many programmes to be open at the same time and facilitates complex computing tasks. It works with the frontal cortex which makes the decisions about what to look at. Damage here can severely reduce the amount of information a child can handle at any one time. They are seeing the world more like a baby who can only do and see one thing at a time because the functioning of their parietal cortex has not developed into the sophisticated mechanism of an older child or adult. 

Read the full paper HERE

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