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Unilateral Spatial or Hemispatial Neglect (SN) is observed in stroke or accident victims with damage to the right parietal or parietal-occipital lobe. Damage to this region of the brain results in "neglect" or misperceptions of space contralateral (opposite) to brain damage. Interestingly enough Spatial Neglect may result from damage to the left hemisphere, but less frequently and in milder forms. The present paper will discuss how misperceptions of space and the asymmetrical nature of Spatial Neglect suggest cerebral dominance and organization in spatial/attention abilities. The present paper will also consider the similarities between symptoms of Spatial Neglect as well as the "blindspot" phenomenon observed in many people.
Spatial neglect patients have information from the left hemispace, but no conscious awareness of this information. As a result, Spatial Neglect patients bump into objects on the left side; fail to shave, groom or dress the left side of the body; fail to orient to events in the left hemispace (visual field); omit details from the left side when copying drawings; fail to read sentences to the left side of the page; ignore auditory information presented in the left ear; and in severe cases fail to move their left limbs (1). Because the above symptoms result from right hemispheric damage, it is assumed that normal perceptive abilities or spatial attention is lateralized to the right hemisphere. Support for this observation is seen in Heilman's (1993) experiment that show "electroencephalogram (EEG) literature, blood-flow studies, and other experiments activate the right parietal region during task conditions involving orienting or attending to events in both the left and right hemispaces (Springer & Deutsch, 1998)."
The left hemisphere (LH) is unable to attend to both hemispaces. However, the fact that the Spatial Neglect patients can attend to right hemispace following parietal lobe damage suggests that the LH is able to modulate some spatial attention. For instance, Kinsbourne (1993) observed that the LH has a strong directionality into the right side of space. Physiological experiments similar to those used in Heilman's study support the observation that LH directs attention to the right hemispace by showing activation of the LH during tasks attending to the right hemispace in Neglect patients.
Therefore the asymmetrical nature and detrimental effects of right hemispheric Spatial Neglect is due to the LH's inability to compensate for the damaged right hemisphere (RH) ability to attend to the left sides of space.
Extinction is a phenomenon also observed in Spatial Neglect patients whereby patients attend to left hemispace stimuli in the absence of right hemispace stimuli (2). For example, visual extinction occurs when the patient fixates on a central cue and can identify stimuli presented on the left and right hemispace respectively, but ignores the stimulus to the left when the right stimuli is simultaneously presented. According to Kinsbourne, extinction occurs in Neglect patients because hemispheric directional tendencies oppose each other and are not equal in strength. The RH is dominant for attending to both hemispaces, competes with the LH's stronger directionality to the right hemispace, and "overrides" the LH's function. Thus, "severe neglect of events on the left occurs after right brain damage because the normal directionality advantage of the left hemisphere into right visual space becomes even more exaggerated." (Kinsbourne, 1993)
In sum, Neglect symptoms as well as further observations made by Heilman and Kinsbourne offer insight into cerebral dominance, "normal" brain organization for spatial abilities, as well as show the similarities between Neglect patients and "normal" individuals with blindspots. To reiterate the RH is dominant in attending to both hemispaces while the LH can only attend to the right hemispace. If the LH parietal region is damaged then the RH can compensate for the lesion and attend to both hemispaces. On the other hand when the RH parietal lobe is damaged the LH cannot compensate for the RH ability to attend to the left side of space, and thus directs attention solely to the right hemispace. However, it can be argued that neglecting the left hemispace is similar to the blindspots observed in the vast majority of "normal" individuals. Like Neglect patients most people are unaware of their misperceptions of space or blindspots, and like Neglect patients most continue their daily lives believing that all their visual hemispatial inputs and perceptions are accurate. But how can we ever be sure that our spatial abilities and perceptions are accurate given the fact that we cannot simultaneously attend to all hemispaces?
Based on the observations in this paper, I argue that our perceptions are never completely accurate. They are simply "less wrong" than Neglect patients. We are able to process a hemispace more than Neglect patients, but does that mean that our perceptions are significantly better than Neglect individuals? For instance, we "normal" people are inhibited from attending to the "rear" hemispace without turning around and looking back. As a result of this, our perceptions are completely limited to the stimuli presented in our conscious visual fields. Therefore, like Neglect patients there is stimuli presented outside of our visual fields that we cannot consciously process because regions in the brain, and visual cortex are either missing or unable to do so.
1)Hemispatial Neglect Syndrome website
2)Neglect (selective inattention) website
3)Contralateral Spatial Neglect on Rutgers University webpage
4)Behavioral Neuropsychology: Perceptual Disorders on National Academy of Neuropsychology Online
K. Heilman, R. Watson, and E. Valenstein, "Neglect and related Disorders," in Clinical Neuropsychology, ed. K. Heilman and E. Valenstein (New York: Oxford University Press, 1993).
M. Kinsbourne, "Orientational Bias Model of Unilateral Neglect: Evidence from Attentional Gradients Within Hemispace," in Unilateral Neglect: Clinical and Experimental Studies, ed. I. H. Robertson and J.C. Marshall (Hillsdale, NJ: Erlbaum, 1993).
S. Springer, and G. Deutsch. Left Brain, Right Brain: Perspectives from Cognitive Neuroscience. 5th ed. R. Atkinson and G. Lindzey (New York, NY: W.H. Freeman and Company, 1998).
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