Hm, what about the character having two diseases, which have the symptoms you like due to interaction? Or you may locate a specific disorder to a special brain region. I don't want to get too precise, but you could have an stroke affecting regions which normally inhibit others. Missing the inhibition there might cause additional effects. When I remember right, e.g., the savants have their abilities due to missing inhibitions of brain areas.
A desease causing hallucinations is schizophrenia (along with a huge bunch of symptoms). The bad thing about schizophrenia in your case is that most people think about several personalities instead of the 'positive' and 'negative' symptoms this desease comes along with. Just have look at wikipedia, whether you could use schizophrenia for your story:
http://en.wikipedia.org/wiki/SchizophreniaIf you understand German you should take a look at the German version of this article. The positive and negative symptoms are explained in more detail there (
http://de.wikipedia.org/wiki/Schizophrenie).
If schizophrenia is not enough you could add an disorder for example to the prefrontal cortex (
http://en.wikipedia.org/wiki/Prefrontal_cortex) to reach the cognitive deficits you like. You should carefully think about the deficits you want your character to have and then search for the brain region which is responsible for that.
I hope this helped a bit. Please feel free to ask, if you have more questions.
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