The enigma of Bálint's syndrome: neural substrates and cognitive deficits

In 1909 a Hungarian physician, Rezsö Bálint, published the first report of a striking visual problem in a patient with bilateral parietal lesions (Bálint, 1909). Bálint had studied his remarkable patient for several years, starting in 1903, struggling to fully understand the nature of the complex neurological disorder. The disorder was characterized by problems in simultaneously processing multiple items resulting in poor interpretation of complex visual scenes, sensory inattention, and deficits in visually guided limb and eye movements (Bálint, 1909). While Balint’s subsequent clinical work took him away from neurology to general medicine, the 1909 case study remains his lasting legacy and helped to define a syndrome now named after him (Husain and Stein, 1988; Rizzo and Vecera, 2002). Since Balint’s first publication, several other cases have been published and in its current status the syndrome is described to comprise of several primary symptoms including simultanagnosia (impaired spatial awareness of more than one object at time), optic ataxia (misreaching to visual targets), ocular apraxia (described by Bálint as “psychic paralysis of gaze”) and general visuospatial disorientation (Bálint, 1909; Wolpert, 1924; Rizzo and Vecera, 2002). Affected individuals often behave as if they are blind due to the lack of normal visual awareness of the surrounding world, which for them can appear to be a chaotic compilation of random single objects. The disorder has a severe impact on everyday living. Simple and previously easy tasks such as walking or eating and drinking become very difficult due to the patient perceiving only one object at a time, failing to appreciate the relationships between different items and having problems with reaching and grasping objects. Not surprisingly given what the disorder might tell us about spatial awareness, object perception and action, Bálint’s syndrome has captivated the interest of researchers aiming to understand the underlying functional and neural processes. There are many interesting questions to raise about the syndrome. First, the individual symptoms of the syndrome not only dissociate but also represent very wide categories of deficits often difficult to assess with standard neuropsychological tests and varying significantly between individual cases (Damasio et al., 2000; Rizzo and Vecera, 2002; Jackson et al., 2005). Secondly, while the original case reported by Bálint occurred following bilateral parietal lesions, different lesion combinations have been described in the literature (for review see Rizzo and Vecera, 2002; Chechlacz et al., 2012). Finally, the syndrome has been reported to result from diverse aetiologies including bilateral strokes, intracranial tumors, traumatic brain damage and neurodegenerative conditions including posterior cortical atrophy (PCA) and Alzheimer’s disease (Benson et al., 1988; Hof et al., 1990; Damasio et al., 2000; Rizzo and Vecera, 2002). The syndrome is infrequent and each individual case offers unique perspectives into the cognitive and neural mechanisms underlying the disorder. The aim of this special issue then is to bring together into a single forum current research on the syndrome. The studies collected here cover both the anatomical and the cognitive mechanisms of the different symptoms associated with the syndrome. The most striking and most widely studied symptom associated with Bálint’s syndrome is simultanagnosia. Simultanagnosia was defined by Wopert as an inability to interpret complex visual displays due to problems with processing multiple items and the relations between them (Wolpert, 1924). Dalrymple et al. (2013) provide here a review of processing deficits in simultanagnosia focusing on object-based versus space-based attention. Two further articles in this Special Issue (Meek et al., 2013a; Shakespeare et al., 2013) examine deficits in complex scene perception in PCA—a disorder in which the clinical presentation often encompasses Bálint’s syndrome as its key feature. Simultanagnosia has been frequently associated with deficits in global processing when the integration of multiple local elements into global compound shapes is required (e.g., Karnath et al., 2000; Shalev et al., 2005). Interestingly, though the patients are typically poor at explicitly reporting global compound shapes, there is evidence that global processing may take place implicitly. Here, Mevorach et al. (2013) report a case study of a patient who was able to respond accurately to both global and local targets when they were presented as the salient aspects of the compound letter, but the patient was then unable to report the alternative level. The complex nature of global/local processing and the potential mechanisms contributing to deficits observed in simultanagnosic patients are also addressed here by two other studies, which examine the cognitive and neural substrates of Gestalt perception in healthy individuals (Rennig et al., 2013a,b). The first study by Rennig et al. (2013b) demonstrates the important role of size constancy for hierarchical object processing (the integration of local elements into global objects) using a paradigm based on the Kanizsa illusion. The second paper (Rennig et al., 2013a) compares BOLD responses in chess experts vs chess novices using ROI analyses of data from four fMRI studies. This paper elegantly demonstrates that the temporo-parietal junction (a region frequently reported to be damaged in simultanagnosic patients)