Attention: Disorders

Neglect

Neglect is a common and disabling consequence of brain damage. When it is severe, the eyes and head deviate toward the side of the damage, and people may fail to dress or groom the opposite half of their bodies, to eat food from that half of their plates, or to acknowledge anything on that side. Neglect is strongly associated with damage to the right side of the brain, particularly the posterior parietal and superior temporal lobes, so it is usually the left side of space that is neglected. The problem is not one of sensory transmission. The independence of neglect from sensory factors was illustrated elegantly by Eduardo Bisiach and Claudio Luzzatti, who asked two patients to describe a familiar Milanese square from memory. Both described buildings on the right, but not on the left, relative to their imagined viewpoints, showing that even mental images may be neglected.

Neglect is usually understood as a spatial bias of attention, such that items in relatively rightward positions are selected at the expense of those further to the left. Unawareness is not restricted to the visual world. Patients may neglect sounds, touches, and even smells. They may also neglect internal sensations, contributing to a loss of awareness for one side of the body. However, the world is experienced as complete because, by definition, the patient is unaware of the neglected parts. It may thus be difficult for a patient to achieve direct insight into his or her problem, and neglect can be hard to treat. At a theoretical level, the condition offers a unique window on spatial attention. No two patients are exactly alike in their symptoms, and the differences between them can inform us about how attention is organized in the brain. For instance, a patient who neglects visual objects might nonetheless be alert to [Page 82]touches on that side, or vice versa. Such patterns show that attention is not a single function shared across sensory modalities. The mechanisms by which we attend to our different senses must be at least partially separate because they can be disrupted separately by brain damage.

Research into neglect has tended to focus on its visual effects, but even here the symptoms are remarkably varied, suggesting many subdivisions of visual attention. For instance, visual space can be divided conceptually into that which can be accessed directly by reaching and grasping (near space) and that which cannot (far space). This distinction may be of significance for the control of behavior, and neglect research has helped confirm its biological reality. Peter Halligan and John Marshall reported a neglect patient who made large errors in estimating the midpoints of horizontal lines in near space, yet who could accurately bisect lines of equivalent visual extent in far space, by pointing a light-pen or throwing a dart. A further, distinct region of space is personal space, bounded by and immediately surrounding the body. Patients have been reported who can search both sides of external space effectively, yet fail to notice bright markers, such as balls of fluff, attached to one side of their clothing. Visual neglect for personal space may interact with neglect of bodily sensations to destroy awareness for one side of the body.

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