Bilateral parietal dysfunctions and disconnections in simultanagnosia and Bálint syndrome.

In 1909 Rezsö Bálint published an extraordinary case study of a man with complex visuospatial deficits resulting from bilateral parietal lesions. Despite some controversies over the nature of reported symptoms, in 1954 Hecaen and Ajuriaguerra conceived the term "Bálint syndrome," not only to honor Bálint's influential work but to firmly conceptualize this striking neurologic disorder. Nowadays it is largely agreed that, while Bálint syndrome may result from multiple etiologies, it is principally diagnosed based on the presence of three symptoms: simultanagnosia, optic ataxia, and ocular apraxia. One of the most striking characteristics of Bálint syndrome, perfectly capturing the nature of this remarkable disorder, is that affected patients cannot perceive more than one object at a time or comprehend multiobject visual scenes due to a lack of ability to detect several objects concurrently and to grasp the spatial relationships between them. This chapter gives an overview of the cognitive mechanisms and neuroanatomy underlying Bálint syndrome, which provides key insights into our understanding of the role of parietal cortex in human attention, visual perception, and visuomotor control. This chapter also pays homage to Glyn Humphreys (1954-2016), who pioneered contributions to the knowledge about complexity of visual and spatial deficits associated with Bálint syndrome.