As 50%+ of the brain is involved in processing information received via the visual pathways, the amazing sense called vision is far more complex than one would imagine, and because it happens in a split second it is all too easy to underestimate the potential difficulties caused when for whatever reason it is not working properly.
Information received via the eyes is captured by the retina, a layer at the back of the eye which converts the light information into electrical signals. These impulses travel along the optic nerve to the brain where they are interpreted as a visual image. The optic nerves from each eye meet at the optic chasm, or crossover place in the brain. Information received from the right side crosses over to the left side of the brain and vice versa.
The left half of the brain controls what is happening on the right side of the body and vice versa, hence a stroke in the left half of the brain will cause problems on the right side of the body.
The visual information travels via bundles of nerve fibres called the optic tracts to the visual cortex, or occipital lobes at the back of the brain. The image is processed in the occipital lobes, in terms of clarity of vision (or acuity), contrast, colour and the areas which are seen, the visual fields.
In the middle temporal lobes the ability to accurately perceive movement takes place, whilst the posterior parietal lobes or motor cortex are responsible for the accurate visual guidance of movement of arms and hands, legs and feet and body and brain and being able to see lots of things at once and allowing us to give attention to different components of a visual scene.
The processed image information then travels to the frontal lobes of the brain where choices are made about what part of the whole image being looked at, to select from.
This links to the posterior parietal lobes which drive the movements of the body which in turn link to the frontal areas of the brain which drive fast movements of the head and eyes to look at the chosen object.
This dorsal stream is the unconscious system which allows search, and visual guidance of movement. It is linked closely with the ventral stream which brings about recognition.
The temporal lobes are our visual filing cabinet. The left temporal lobes are responsible for our ability to recognise shapes, objects, letters and words and to name colours. The right temporal lobes are responsible for recognising people, facial expressions and routes. This system is called the ventral stream and is conscious.
Either system can be damaged but the dorsal stream is damaged more frequently.
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