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See Inside February/March 2009

Seeing the World in Half-View

Victims of a disorder called neglect just don't get the whole picture

A PATIENT NAMED SALLY recently suffered a stroke that damaged her right parietal lobe without affecting other parts of the brain. The left side of her body—controlled by the right hemisphere—was paralyzed. But she was mentally normal and continued to remain the talkative, intelligent woman that she was before the stroke.

Yet Sally’s father observed other disturbing symptoms to which—oddly enough—Sally herself seemed oblivious. When she attempted to move around the room in her wheelchair, she would sometimes bump into objects on her left.

Further testing confirmed that Sally was largely indifferent to objects and events on her left, even though she was not blind to them; once her attention was drawn to them, she could see them. Her eyesight was normal; her problem was in attending to the left. For example, when she ate, she would consume only the food on the right (a), ignoring the left side of the plate. But if her attention was drawn to the food on the left, Sally could see it perfectly, recognize it and reach for it. Sally’s deficits indicate that she suffers from hemineglect (or simply neglect), which can also occur in isolated form, unaccompanied by major paralysis.

Seeds of Neglect
How do such perturbations of perception arise? Neglect is, fundamentally, a disorder of attention. Although the human brain has 100 billion neurons, only a small subset of them can be active at any time creating meaningful patterns, and this limit results in an attentional bottleneck. That is why you can see either a duck or a rabbit in (b) but never both simultaneously. It also explains why when you are driving, you are not consciously aware of most things going on around you while you focus on the pedestrian in front of you. Seen in this light, the neurological syndrome of neglect is really a floridly exaggerated version of the kind of neglect we all engage in to avoid sensory overload.

To understand neglect, we need to consider some anatomy. Visual input from the retina is sent along the optic nerve and diverges into two parallel pathways called the “old” and the “new,” reflecting when each evolved. The former, sometimes called the “where” pathway, projects into the parietal lobes and is involved in locating and orienting to things around you. The latter projects to the visual cortex, and from there two other pathways emerge called “what” and “how,” which project into the temporal and pa­rietal lobes, respectively. The what pathway is involved in object recognition and identification, whereas the how pathway directs how to attend to and interact with objects. The how and where pathways converge on the parietal cortex and are functionally linked—you must process both where a chair is and how to move to avoid bumping into it. Sally had damage to the how pathway in her right hemisphere, so she was ignoring everything on her left side.

Curiously, neglect is seen only with damage to the right brain. Why doesn’t left damage result in neglect of the right half of the world? Marsel Mesulam of Harvard University proposed an ingenious explanation. The right hemisphere, which has more attentional resources and a preeminent role in spatial vision, can survey the entire visual scene, both right and left hemifields, simultaneously. The left parietal, in contrast, can attend to only the right side of the world. So when the left hemisphere is damaged, the right can compensate. If the right parietal is damaged, however, the left visual field is unattended; in other words, unilateral neglect occurs.

It is fairly easy to diagnose neglect. The patient will tend to look rightward constantly and will not spontaneously look left even if a person approaches from that direction. When tracking an object moving from right to left, she will “lose” the object halfway through its excursion—not following it leftward past her nose. She applies makeup only on the right side of her face. A male patient will shave only his right chin. Or brush only the teeth on the right.

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