This problem is called selective blindness.
The image that we see with our eyes is broken down not only into separate colour components but into other components too. It is, rather incredibly, deconstructed into component parts such as horizontal lines, vertical lines, circles and so on. Each of these component parts is sent to a separate area of the brain for processing, with the different components of the scene only merging again when they are unified into what you perceive as the image.
Not only are horizontal lines sent to a different part of the brain to vertical lines, but lines that are at 30º, 45º, 60º and so on go to their own individual areas too, and even movement is processed in its own discrete areas of the brain by its own dedicated brain cells.
When the person whose brain is being scanned is looking at a horizontal line a particular part of the brain lights up, and when looking at a vertical line a different part lights up. And so on.
If a person suffers brain damage to an area of the brain that deals with a particular type of visual information the ability to process that information may be impaired. For instance if the region that deals with horizontal lines is damaged, horizontal lines may drop out of the final image, while vertical ones and angled ones are still seen with no problem. Thus a person with this condition may be unable to see horizontal objects such as bookshelves but be able to see the books that are resting on them because their spines are vertical.
Hope this helps.