People with hemispatial neglect just can’t seem to see one side of space, normally the left. For example, if you ask them to copy a drawing, they’ll only draw the right hand side. Women may only do their makeup on one side of their face and men may only shave the right side of their beard. This isn’t a visual problem; their eyesight is fine.
Hemispatial neglect is caused by damage to one side of the parietal cortex (often from a stroke), an area of the brain that integrates various sensory inputs. Most often damage to the right parietal cortex causes the patient to ignore the left hand side of the world; damage to the left parietal cortex normally has other symptoms, due to the dominance of the left hemisphere in language processing and the right hemisphere in spatial processing. It is thought that neglect is an attention deficit, and patients simply cannot orient their attention towards the contralesional side of space (the side opposite to where the lesion is).
This is not simply a visual deficit as it occurs across many senses – hearing, touch, taste as well. In fact, for many patients it also occurs in their mind: they neglect one side of any dreams, memories and hallucinations they may have. Not being able to imagine one side of the world is called representational neglect. In one famous experiment, Bisiach and Luzatti (1978) asked two patients with representational neglect to imagine standing on one side of the Piazza del Duomo, a famous square in Milan that they knew well. They were asked to name all the buildings they could see from this viewpoint. They only named buildings on the right of where they were standing. When asked to imagine standing at the opposite end of the square and again name all the buildings they could see, they again named the buildings on their right: the opposite buildings to the ones they had said previously. So they knew the buildings were there, they just could not see that side of space, even in their mind.
It’s easy to say that patients with hemispatial neglect can’t see the left side, but “left side of what?” is an important question. Some patients can’t see any of the objects to the left in a room, whereas others can see all objects in a room but can’t see the left hand side of any of the individual objects.
And ‘left’ can be very subjective for patients. Amazingly, some patients may correct an inverted or slanted object and ignore the left side of the corrected image. For example, if shown a mirror image of a map of the world, some ignore the western hemisphere, despite the fact it is on the right because they have corrected the map of the world in their mind.
So the neglected side of space is not ignored completely, the brain seems to process it at some level. This was also shown by Marshall and Halligan (1988) in an influential experiment in which neglect patients were shown drawings of two houses, one of which was on fire on the left hand side. Despite not being able to see the fire (it was on their unattended side), when pushed to say which house they would rather live in, a significant majority of the patients chose the house not on fire though they couldn’t explain why. They clearly processed the left hand side of the image to some level, it just didn’t reach consciousness.
Hemispatial neglect is a fascinating condition that it is almost unimaginable having. It shows us how weird and wonderful the brain is, with each area playing its specific part, even working beyond our conscious awareness.