Problems in vision, after brain injury, is fairly common. I will focus here on visual field defects, visual inattention, neglect or agnosia. The problems in hearing, touch and smell will be discussed briefly.
First of all, I need to point out that the brain is largely depended on information which it receives from our 5 senses: vision, hearing, touch, smell and taste. Vision is our 'long-range' sense, hearing a good second, and smell a pretty good third. Touch and taste are obviously the short-range senses. Mostly forgotten but equally important is the perception of our inner workings (i.e. organs, joints, limbs) because with this information we keep our body more or less in balance and healthy. With all this sensory information the brain starts to calculate and interpret. It concludes things and it acts upon it. What most people do not know is that our brain has already made his mind up how to act, before we even become conscious of this choice. In other words, consciousness is slower than our information processing and it always lags behind. In this sense, free will does not exist...
Problems in smell do occur largely after traumatic brain injuries in which the nerve that regulates smell is damaged. It lies at the front-base of the brain, a region easily damaged after a frontal car collision. Usually, also taste is damaged because one normally has to smell in other to taste fully. In the following link I'll explain the possible effects in daily life after such type of injury.
Problems in hearing usually amount to nothing other than deafness in one ear but also misinterpretation of sounds can be a problem. This can have devastating effects on the understanding of speech or keeping up with your surroundings. It is also possible that a nice piece of music can now turn into something very unpleasant.
Problems in touch not only concern feeling less, literally, but it can work the other way around as well: feeling too much. Hypersensitivity, as it is called technically, can lead to not being touched anymore by your partner because a gentle touch is felt like a slap or causes increasing pain. Taking showers can become gruesome because the impact of streaming warm water seems as if all your skin is burning.
Because most people are visually oriented, vision is a long-range sense, and it can be studied relatively easy, most research articles and books are written about problems in vision after brain damage. In dealing with vision we have to realize that vision is a multi-layered process and done by the brain, not your eyes. The eyes are just a 'simple' mechanism to transform light waves into electrical signals. They form the basis of our vision. With the output (or product) of your eyes, the brain starts to work and tries to interpret. Only after this interpretation, we consciously can 'see things'. I will give many examples of how we misinterpret our world just because our brain is miscalculating things or is being fooled on another page.
Here, it is important to realize that visual field defects can occur after brain injury. Parts of our visual field can become 'invisible' to us, usually the full left half, sometimes only small parts of our visual field. The main negative effect is that we do not notice things when we walk or drive somewhere. A very serious problem is that a patient normally is not aware of this defect. With 'just' a visual field defect such kind of awareness can quickly be learned so that the patient looks more to his right or left.
Two photographs here show you the situation with a normal vision (picture above) and with hemianopia (picture below). I rearranged these pictures in such a way as to give you an idea how someone with hemianopia 'sees'. It is not so much different from normal vision...but someone with hemianopia will not see the third horse on the left!
However, in the case of a higher order visual problem such as visual inattention (known as neglect, such awareness is extremely difficult to learn. As a consequence, patients with neglect do tend to neglect the left side of their space and are prone to collisions and other accidents.
Another example of impaired vision is the two pictures below in which a street and a cyclist are seen. The first picture shows a normal vision in which clearly the cyclist can be seen. The second picture is the view from someone who has neglect for the left side of the visual field. Clearly, the cyclist is not seen whenever the patient looks at the traffic sign (one-way). Actually, also the trees at the far left will not be seen. Whenever the patient turns his head much more to the left, only then the cyclist will be seen.
A very nice presentation about Neglect can be found on YouTube. In this short movie a lady can not copy the cat correctly. She will miss everything on her left side. Later in this movie the topic is about blindsight: a very interesting phenomenon that tells us that the vision process is multi-layered. In the more automatic, unconscious layers we still process very much about visual stimuli. However, it does not reach our consciousness, so in effect: we do not see it! The link to this YouTube film is:
Link to YouTube movie about visual neglect and blindsight
Finally, the most serious problems in vision are when the brain cannot interpret correctly all the visual output of the eyes. These are called the agnosias: not knowing what you see. Either because the meaningful ideas of a form can no longer be connected to each other, or because a form itself cannot be integrated anymore to a meaningful form. Such kind of damage results in the most interesting studies and articles (i.e. The Man who mistook his wife for a hat) but is less common than one might think.
Problems in the senses after brain injury are quite common but often less noticed by lay people. It can lead to serious problems in daily life. Especially, problems in vision that usually result in seeing less of your surroundings so there is a higher risk of accidents. Visual field defects are possible but these can be compensated for relatively easy because one can become aware of these defects. Higher order interpretation problems in vision, such as in neglect where someone is not aware of the fact that a large (left) part of his visual field is missing, are much more difficult to cure. Visual agnosia is the most debilitating of the visual defects after brain damage because here one may have good eyesight, but without knowing what one sees, normal reactions and activities are hardly possible.
On two other pages I'll explain more about the serious effects of visual problems on daily life and also how such defects can be detected.