Spatial neglect: a window into spatial cognition and beyond

Unilateral cerebral lesions may bring about a multifaceted pattern of impairment of spatial cognition termed ‘unilateral spatial neglect’. The deficit is more frequent and severe after lesions involving the right cerebral hemisphere, and concerns the left side of space, contralateral to the side of the lesion (contralesional). The main feature of spatial neglect is a disordered perceptual awareness of sensory events taking place in the contralesional side of space, and of the body. The perceptual impairment may occur in isolation, or be associated with a deficit of the planning of intentional movements towards the neglected side of space, suggesting that perceptual and action systems, though closely linked, are discrete processes. Unilateral spatial neglect may be characterized as a disorder of conscious spatial representations: A great deal of evidence suggests that ‘neglected’ events are nevertheless adequately processed by the brain up to the extraction of their meaning, and even with a preserved representation of the metric of space, provided the explicit and aware spatial localization or detection of events is not required. Awareness of events around us appears to involve spatial reference frames. The wealth of selective patterns of impairment shown by neglect patients (e.g., personal vs. extra-personal neglect) indicates that manifold spatial representations exist, notwithstanding our largely unitary phenomenal experience of space. The neural networks supporting spatial representation and attention include the posterior-inferior parietal regions, the temporo-parietal junction, the premotor cortex, and the fronto-parietal connections, as well as the basal ganglia and posterior thalamic nuclei. The neural correlates of spatial neglect do not comprise the primary motor and sensory cortices, suggesting a higher-order, cognitive, deficit of perceptual and action processes. Spatial representations, however, also provide a basic reference frame to elementary sensorimotor loops. This is suggested both by the higher incidence of left-sided sensorimotor hemi-syndromes after right hemispheric damage, and by their amenability to physiological maneuvers (vestibular stimulation, prism adaptation, etc.), which affect spatial representations, and a number of manifestations of the neglect syndrome. Finally, spatial representations are involved in some function monitoring processes and belief systems. This is suggested by the syndrome of unawareness of motor and sensory deficits (anosognosia), and by the wide range of delusional beliefs concerning one side of the body (the somatoparaphrenic symptom-complex), and, less frequently, extra-personal events. Seen through the neuropsychological perspective provided by patients suffering from spatial neglect, spatial awareness is a multi-component process, pervading many areas of cognition.