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Visual Impairment :
Special Educational Needs
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Nerve impulses from the retinas of the eyes travel along different routes to the left and right side of the brain. The left hand side of the visual field is processed in the right hemisphere and vice versa for the right hand visual field. The central optic chiasma, situated at the base of the hypothalamus, is the area of the brain where the nerves divide and cross over to go to the two hemispheres.

Diagram showing the optic pathways in the brain

The nerve impulses from the eyes are transmitted through the optic pathway to the primary visual cortex which is located in the occipital lobe at the back of the brain. This is linked through neural connections to other parts of the brain which are crucial in interpreting the information received. The information is relayed along two distinct pathways called the ventral stream and the dorsal stream. The dorsal stream takes information to a secondary visual cortex in the parietal lobe, while the ventral stream takes information to the temporal lobe.


Diagram of the visual cortex in the brain

The primary visual cortexes receive the nerve impulses from the optic nerves. The right hand side of the visual field is focused onto the left side of the retina and the nerve impulse travel through the optic pathway to the visual cortex in the left hemisphere of the cerebrum, and vice versa for the left hand side of the visual field. The primary cortexes then transmit information along two primary pathways in the brain. These are the dorsal (top) stream and the ventral (lower) stream.

The dorsal stream travels to the parietal lobe at the top of the brain, which interprets aspects of the information received. It is called the “Where” or “How” pathway. And is associated with motion, the representations of object locations, and control of the eyes and arms. It is important in enabling us to have a detailed map of our visual field, keep an accurate body image, perceive and interpret spatial relationships, and detect and analyse movement.

The ventral stream travels to the temporal lobe and is referred to as the “What” pathway. It is associated with recognising form and objects, and is linked to the long term memory and working memory. Linking what we see to memory is important in recognising and understanding the objects we see and in deciding how to respond. Without this we have no visual recognition and can suffer visual agnosia (which is partial or total inability to recognise objects visually, i.e. the individual sees the object but, without being able to link the information to memory, is unable to identify it).