When You're Visited By A Copy Of Yourself, Stay Calm
npr.org
npr.org
I have to say that one of my favourite neuroscience books has to be, "The Man who Mistook his Wife for a Hat" by the aforementioned Dr. Oliver Sacks.
http://www.amazon.co.uk/Man-Who-Mistook-His-Wife/dp/03305236...
It's really a great read for anyone interested in neuroscience, psychology or just weird and wonderful stories in general.
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Negative hallucinations are even more common (like not being able to see the car keys when they are right in front you). I see people show signs of negative hallucinations when they walk past litter on scenic paths.