Perception involves many different processes – and not all of them are conscious.

When Ellen returned home from the hospital, her family immediately noticed that something was wrong. The hair on one side of her head was combed; on the other, it was knotted into clumps. Her shawl hung only over one shoulder; the rest trailed on the floor. And, most disconcerting of all, she was wearing lipstick and mascara on just one side of her face.

Ellen was exhibiting a syndrome called hemi-neglect, which can be characterized as a profound indifference to the left side of the world. It’s not that she was blind, technically. It’s that she was utterly indifferent to anything that happened on the left side of her visual field. It was as if, for her, “leftness” just didn’t exist.

It’s not exactly clear what’s going on in the minds of patients like Ellen. But what we do know is that hemi-neglect only occurs after a patient has suffered a stroke to the right parietal lobe, which indicates that this area plays an important role in perception.

Admittedly, that’s not saying much. There are about 30 areas of the brain involved in perception, and we don’t know what most of them do. Unfortunately, this is another situation where we only figure out how things work when they go wrong. For example, we know that the middle temporal area is responsible for processing movement because a Swiss woman was no longer able to perceive motion after receiving damage to this area. To her, a car driving down the street would appear as a strobe-like series of static images.

So, what’s going on in Ellen’s case?

Well, it seems like the right parietal lobe must be responsible for a kind of perception. People with typical brain function possess an internal “searchlight” that’s able to scan the visual field and fall upon specific objects. This searchlight ability seems to be lacking in hemi-neglect patients.

But, remarkably, studies suggest that many of these patients’ other perceptual processes continue to function. For example, patients faced with a picture of a house that was on fire only on the left side invariably said they wouldn’t want to live in it – even though they couldn’t explain why.

The implication is that not all the processes involved in perception are involved in conscious awareness. It seems parts of our brain are “seeing” without keeping us in the loop.