Injury to one occipital lobe leads to lack or impairment of vision on the opposite side to the injury, with both eyes being affected in the same way. Commonly there is lack of vision to one side, from the centre to the side, but sometimes an upper or lower quarter of the visual field on one side can be missing. Rarely, an 'island' is missing, like a hole in the visual field, again both eyes are affected in the same way.
Visual Field Impairments due to lack of occipital lobe function are called hemianopia (also known as hemianopsia).
In some cases with babies and young infants, the brain develops new pathways to create a full visual field in the occipital lobes, where previously a part was missing. In very young children the brain is at its most plastic, and whilst there is potential to build new pathways throughout life, the optimum time is between the age of 0-3 years. As the person gets older, this level of improvement of vision becomes less likely.
Some people are born with hemianopia and are not aware they have a visual field impairment - it is their 'normal' and they develop instinctive methods to manage. Some even learn to drive and pass their driving tests, including the basic sight test because they use awareness of movement to trigger their eyes to move fast towards any movement that is detected.
The occipital lobes are the only part of the brain that can give an image colour, and with that colour, colour contrast, and visual acuity.
The occipital lobes create the image with colour and print it out (to be carried further into the visual brain). If the image is incomplete here, for example due to a brain injury like a stroke, an incomplete image will be passed on.
This is the case with Mary, only the right occipital lobe works due to an injury to the left one, and this only creates half the picture on her left side.
Because the occipital lobes have not created an image with colour, Mary can never see a colour image in her right visual field.
However we know from Mary's Visual Field diagram (explained in the previous section, image below) that Mary can respond to some visual stimuli on both her left and right sides.
Mary sometimes responds to visual stimuli on her right side, where her occipital lobes have not created a visual image, but only moving visual stimuli are seen. Mary responds by turning both her head and eyes to the moving visual stimuli.
This is because Mary has some blindsight.
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