A patient in a rehabilitation hospital wakes up in the morning and proceeds to shave his face. When he puts the shaver down to go eat breakfast, one notices that he shaved only the right side. While eating breakfast, the patient starts to look feverishly for his coffee cup until someone points out that it is slightly over to the left of his dish. At lunch or dinner he may leave the food on the left half of his plate untouched while asking for more, only to be reminded that there is still food on the plate. If asked to draw a clock, the patient will draw a circle correctly but then crowd all the numbers into the right half. If asked to draw a person he will draw only the right side of the body, leaving out the left arm and leg. If questioned about the drawings, the patient states that they look all right to him.

This syndrome, known as neglect syndrome, is observed in stroke or accident victims who have fairly extensive damage to the posterior (parietal or parieto-occipital) regions of the right hemisphere . It sometimes occurs after similar damage to the left hemisphere, but much less frequently and in milder form. The impression one gets in observing such a patient is that he or she behaves as if the whole left side of space, and sometimes even the left side of his/her own body, does not exist.

Several questions have long been asked about the syndrome. Why is there such blatant inattention to one half of space? To what extent is it related to damage in the visual system? Why are patients with damage to the right hemisphere much more likely to show long-lasting neglect symptoms than patients with equivalent damage to the left hemisphere? The answers are still not clear, but the phenomenon of neglect provides some valuable clues about the working relationship between the left brain and right brain.

Although they may be initially unaware of it, many neglect patients are actually blind in their left visual field. Because information from the left half of visual space is initially processed in the visual area of the right hemisphere, damage there can produce a hemianopic (literally, ñhalf-blindî) observer who canÍt see any object to the left of the point of fixation. This half blindness, though, does not by itself explain the inattention in neglect patients.

Many examples can be found of patients who are blind to half of the visual field but do not show neglect of that side of space. Patients in whom damage is restricted to the optic-nerve pathways or to the primary visual areas of either hemisphere typically compensate for their half-field blindness through eye and head movements. Patients with damage to the left hemisphere who are blind in the right visual field rarely display ther kind of persistent functional neglect to one half of space shown by right-hemisphere patients.

Moreover, some neglect patients are not hemianopic at all. In testing situations, they can accurately report simple visual stimuli flashed alone in the left visual field. However, when stimuli are presented, simutaneously in both visual fields, experimentally or in everyday situations, they will report only the items in the right half of visual space. Input from the right field reaching the undamaged left hemisphere appears to interfere with the brainÍs ability to process input from the left field coming into the damaged right hemisphere. The left half of the stimulus is ñextinguishedî by the right, but the patients see the left half of the pattern clearly if it is presented alone.

The extinction effects seen in these patterns explain at least part of the neglect patientÍs inattention to the left half of the world. Events in the right visual field may continuously extinguish information in the affected left field and consequently lead to orientation only to the right. This class of nonhemianopic neglect patients is particularly interesting to investigators of visual perception.

What happens to extinguished left-field information? Is it truly lost? Or is it present in the nervous system but unavailable to conscious experience? Several lines of research have studied this question. Some investigators have found that under certain circumstances the patients are able to report what appeared in the left field even when there was stimulation in the right. If forced to guess from among several choices, they perform much better than chance, although they may never acknowledge having actually seen the pattern in the left visual field .

In a somewhat different line of research, patients who normally extinguished the left-field stimulus when two discrete patterns were presented did not do so when a single large pattern crossing the midline between the left and right fields was used. They were consistently able to identify meaningful drawings like a key or safety pin presented on midline, even though relying only on the information in the right half of the drawing would not have provided them with enough information to recognize it.

These findings can be interpreted in terms of a speculative but intriguing theory of why the neglect syndrome seems to arise from damage to the right hemisphere. The left hemisphere normally processes most of the speech and language skills of the brain. Furthermore, work with split-brain patients has shown that their left hemisphere often confabulates incorrect verbal responses based on that part of the visual information available to it. Perhaps after damage to the right hemisphere, the dominant left hemisphere of the neglect patient acts in an egotistical fashion and assumes that what it sees encompasses everything there is. In other words, the left hemisphere, because it appears to be more self-sufficient than the right on account of its verbal capabilities, is more likely to act as if nothing is wrong in the presence of right-hemisphere damage than would the right hemisphere in the presence of left-hemisphere damage.

In the extinction patients just discussed, the left hemisphere seems very willing to report only information in its visual field as long as it can interpret that information in some comprehensible manner. If it cannot make sense out of what is presented to it, or if there is nothing at all in its field, only then does the left hemisphere make use of some of the information projected to the damaged right half of the brain.

Other explanations as well have been proposed for the asymetrical nature of the neglect syndrome. One possibility is that mechanisms controlling selective attention or even arousal are lateralized to the right hemisphere. Another possibility is that the right hemisphere is more spatially adept in general and thus, in its absence, the left does a poor job of comprehending space.

These last two theories share the idea that neglect involves damage to centres responsible for poor orientation to the world, centres located in or at least controlled by the right cerebral hemisphere . This is in contrast to the notion of the egocentricity of the left hemisphere, an idea postulating that neglect is a result of the left hemisphereÍs inappropriate rationalisations in the face of impaired or inefficient input from the right.

We have stressed the visual problems and anomalies usually accompanying the neglect syndrome. Yet many investigators believe that thinking about neglect in purely perceptual terms does not capture all its manifestations. A subtle aspect of the syndrome may be independant of any sensory processing problems. An anecdote about a neglect patient serves to illustrate the point.

An Italian neglect patient was asked to imagine entering a wellknown plaza in Rome from the north end and to describe what he saw. The patient had been familiar with the plaza prior to his stroke. He proceeded to describe all the buildings to the west - that is, to the right - of where he would have entered, but he failed to mention any of the buildings to the east. He was then asked to imagine entering the plaza from the south and to describe what he saw. The patient proceeded to describe all the buildings in the eastern half of the plaza.

This story suggests that neglect can be independant of visual processing since it even affects the recall of images from memory. If neglect truly is a visual deficit, it must be a high-level one involving complex aspects of perception. The fact that some neglect patients even deny their illness further attests to this point. An understanding of this unusual, lateralised deficit may add in important ways to our knowledge of how the left brain and the right brain contribute to awareness and perception.

Source: Springer,S.,Deutsch,G., Left Brain,Right Brain, W.H.Freeman &Co.,SanFrancisco,1981

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