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. A deficit of the planning of intentional movements towards the neglected side of space may also occur, 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 their aware spatial localization or detection is not required. Awareness of events around us appears to involve spatial reference frames, which also contribute to cognitive processes, such as numerical cognition. The wealth of selective patterns of impairment shown by neglect patients, e.g., “personal” vs. “extra-personal”, “perceptual” vs. “premotor” neglect indicates that manifold spatial representations exist, notwithstanding our largely unitary phenomenal experience of space. The main neural networks supporting spatial representation and awareness include the posterior-inferior parietal regions, the temporo-parietal junction, the premotor cortex, and the fronto-parietal connections, as well as subcortical grey 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 spatial processes. Spatial representations provide a basic reference frame also 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 productive 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 with spatial neglect, spatial awareness is a multi-component process, pervading many areas of cognition.