Some people with brain damage, or by a quirk of fate, lack a very specific mental function. Studies of these people can tell us much about how the brain works. 

Blindsight

Blindness usually results from damage to the eye, but damage to visual processing areas of the brain can also remove vision. Remarkably, some patients have no conscious vision but can still point at a coloured dot on a screen (they are told just to guess where it might be). This suggests that we can 'see' things without being consciously being aware of them. There appear to be two pathways of visual information in the brain, one linked to conscious awareness and one that bypasses it.

Synaesthesia

When a train pulls into a station, most of us hear a range of noises: squeaky wheels, hissing brakes and so on. Some people, though, see a variety of coloured shapes - each specific for a particular sound. Synaesthetes seem to have unusually wired brains, such that auditory signals, for example, link to visual areas.

Prosopagnosia

People with agnosia lose the ability to recognise particular objects. This can be incredibly specific - they may fail to recognise just tools or animals. They can describe or draw a rake, say, but cannot say what it is. People with prosopagnosia are unable to recognise faces. These conditions imply that there are 'modules' in the brain specifically dealing with these features of the outside world.