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The great philosopher Immanuel Kant believed that nothing matters more to our existence than space. Every experience we have—from the thoughts in our heads to the stars we see wheeling through the sky—makes sense only if we can assign it a location. “We never can imagine or make a representation to ourselves of the non-existence of space,” he wrote in 1781.
The nonexistence of space may certainly be hard to imagine. But for some people it is part of everyday life. Strokes can rob us of space. So can brain injuries and tumors. In 1941, neurologists Andrew Paterson and O. L. Zangwill, working in Edinburgh, Scotland, published an account of a 34-year-old patient who had been hit in the head by a mortar fragment. The injury wiped out his sense of the left half of his world. Paterson and Zangwill described how the man “consistently failed to appreciate doors and turnings on his left-hand side even when he was aware of their presence.” He also “neglected the left-hand side of a picture or the left-hand page of a book despite the fact that his attention was constantly being drawn to the oversight.” The patient could play checkers but ignored the pieces on the left side of the board. “And when his attention was drawn to the pieces on this side,” the doctors wrote, “he recognized them but immediately thereafter forgot them.”
This condition, called spatial neglect, challenges our intuitive notions of how we understand the world. But by mapping how people lose some of their sense of space, neuroscientists are gaining new insights into how we build that sense in the first place.
When scientists began to look at the injuries that cause spatial neglect, back in the mid-1900s, the results were crude. They found, for example, that many people with this condition had suffered an injury to the parietal lobe, a patch of cortical brain tissue near the top of the head that is important for many mental tasks, from paying attention to making plans. But the parietal lobe is a huge chunk of neural real estate. Saying that damaging the parietal lobe could cause spatial neglect is about as meaningful as saying that bombing New York City could wreck the financial market.
Making matters more complicated, spatial neglect can take several forms—as Glyn Humphreys, an experimental psychologist at the University of Birmingham in England, and his colleagues recently documented in a study. They gave an exam to 41 people who had suffered brain damage. Each subject received a sheet of paper with 150 line drawings of apples, only 50 of which had a complete outline. Fifty apples had a gap on their right side. The remaining 50 had a gap on the left. Humphreys and his colleagues told their subjects to cross out all the complete apples and leave the incomplete ones alone.
The subjects had five minutes to finish the test. That was plenty of time for some. But Humphreys and his colleagues found that 11 overlooked a number of apples on the left side of the page. Two others ignored apples on the right. And ten people who took the test made a fundamentally different mistake: They crossed out apples on both sides of the page, but sometimes they crossed out apples with gaps, wrongly thinking that the shapes were complete. Eight crossed out apples with gaps on the left, and two crossed out apples with gaps on the right.
The two classes of mistakes made by the subjects indicated two distinct forms of spatial neglect. The first group suffered from a condition called egocentric neglect, unaware of space on one side of their bodies. The second group suffered from allocentric neglect, unaware of space on one side of the objects they looked at.
Once Humphreys had identified people with egocentric and allocentric neglect, he and his colleagues captured high-resolution images of the subjects’ brains. They saw that people with egocentric neglect all tended to have damage in a cluster of regions along one side of the brain (including part of the parietal lobe). People with allocentric neglect had damage in a different network of brain regions. Some of those regions overlapped with the egocentric areas; others extended farther back in the brain.
To probe more deeply into our sense of space, Lee Lovejoy, now a researcher at the Salk Institute for Biological Studies in La Jolla, California, wanted to study the superior colliculus, a region deep in the brain stem. Researchers have long known that this part of the brain is involved in the movement of our eyes. Lovejoy suspected that it might also be important for our awareness of the space around us.
In this case, there were only a handful of living patients with damage to this region. The superior colliculus is located next to parts of the brain stem that keep the heart beating and play other life-sustaining roles. Strokes or other injuries that wipe out the superior colliculus often wipe out those regions, too. Most of the time they don’t just leave people unable to play a game of checkers—they leave people dead. So Lovejoy and his colleagues used monkeys and a drug called muscimol, which safely shuts down parts of the brain by inhibiting neurons where it is injected. They could thus modulate the activity of healthy brains without causing any lasting damage.