Dissociating Components of the Attentional Network in Neglect

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DESCRIPTION (provided by applicant): Brain damage following stroke affects many Americans, having a major impact on quality of life and the ability to carry out activities of daily life. To minimize the human and economic impact of stroke, the factors that predict recovery and successful rehabilitation need to be understood. Visual neglect is a common consequence of brain damage to the right hemisphere, and the presence of this symptom (or syndrome) is a major predictor of poor clinical outcomes. Visual neglect is a clinical deficit of attention, in which the person fails to notice, or to respond to items opposite the lesion. So a person with a stroke to the right hemisphere may eat from only the right hand side of their plate, ignoring the food on the left, or may shave only the right side of their face, and so on. Neglect has been studied for many years, in part for its major clinical significance, but also because it has the potential to provide insight into brain mechanisms of attention. The most common, severe and enduring neglect follows damage to the right hemisphere, leading to neglect of the left. The presence of right neglect is more controversial, and of lesser clinical significance. For these reasons, the present study is confined to studies of left neglect due to right hemisphere damage. The two most fundamental questions about neglect remain unanswered, and the subject of deep controversy in the scientific literature. Simply stated, these are: what is the nature of the deficit? and what are the key areas of brain damage that cause neglect? We propose a simple theoretical framework for answering these questions and providing rapprochement for the divergent views of attention. We propose that visual neglect is a complex syndrome that is nonetheless dissociable into sets of deficits that result from damage to, or dysfunction of, four regions of the cortex. These four cortical regions represent discreet, yet interconnected modules in the attentional network of the brain. The intraparietal sulcus is a region involved in determining the global structure of items, and responding to them; damage to this region is associated with bias on the line bisection task. The temporal operculum and insula (a region often referred to as superior temporal gyrus) is involved in orienting to items; damage to this region will lead to errors on the line cancellation task. The temporoparietal junction is involved in assigning salience to items; damage to this region will lead to visual extinction - a failure to report items in the impaired visual field only if there are items in the good field. Finally the frontal eye fields are regions primarily concerned with perceptual inhibition; damage to this region will lead to increased distractibility. We propose that the controversies over the core areas and symptoms are due to variability in sampling patients with damage to these four systems. The present study tests these predictions by converging use of neuropsychology, functional MRI, and transcranial magnetic stimulation in normal observers.