I love telling people what I find fascinating about the human brain. For instance, there is a subset of people in the world who have "prosopagnosia", or face blindness. These people can see most things normally and typically have normal functional lives - however, they have a special form of blindness wherein they cannot see faces. This is really hard to describe, but imagine looking at a face and simply not seeing what is there - they can see the rest of the body and can hear the person's voice - but they literally cannot see the persons face! Here are a couple of You Tube clips about prosopagnosia.
Prosopagnosia stems from damage to a very specific part of the brain the Fusiform Face Area.
Damage to this region results in prosopagnosia - or face blindness. If the damage is quite extensive, like the second video, the person in question is also blind to other animate objects. People with extensive damage to the inferior temporal cortex can have visual form agnosia - or a deficit in the ability to see shapes and objects. For example, someone with visual form agnosia would not only be face blind but they also would not be able to tell the difference between a circle or a square.