Bálint's syndrome

Simultanagnosia is the inability to perceive simultaneous events or objects in one's visual field.[11] People with Bálint's syndrome perceive the world erratically, as a series of single objects rather than seeing the wholeness of a scene.[12]

This spatial disorder of visual attention—the ability to identify local elements of a scene, but not the global whole—has been referred to as a constriction of the individual's global gestalt window—their visual "window" of attention. People fixate their eyes to specific images in social scenes because they are informative to the meaning of the scene. Any forthcoming recovery in simultanagnosia may be related to somehow expanding the restricted attentional window that characterizes this disorder.[13]

Simultanagnosia is a profound visual deficit. It impairs the ability to perceive multiple items in a visual display, while preserving the ability to recognize single objects. One study suggests that simultanagnosia may result from an extreme form of competition between objects which makes it difficult for attention to be disengaged from an object once it has been selected.[14] Patients with simultanagnosia have a restricted spatial window of visual attention and cannot see more than one object at a time. They see their world in a patchy, spotty manner. Therefore, they pick out a single object, or even components of an individual object, without being able to see the global "big picture."

A study which directly tested the relationship between the restriction of the attentional window in simultanagnosia compared with the vision of healthy participants with normal limits of visual processing confirmed the limitations of difficulties of patients with simultanagnosia.[15]

There is considerable evidence that a person's cortex is essentially divided into two functional streams: an occipital-parietal-frontal pathway that processes "where" information and an occipital-temporal-frontal pathway that provides "what" information to the individual.[16]

Bálint referred to this as "psychic paralysis of gaze"—the inability to voluntarily guide eye movements, changing to a new location of visual fixation. A major symptom of Oculomotor apraxia is that a person has no control over their eye movements, however, vertical eye movements are typically unaffected. For example, they often have difficulty moving their eyes in the desired direction. In other words, the saccades (rapid eye movements) are abnormal. Because of this, most patients with Oculomotor apraxia have to turn their heads in order to follow objects coming from their peripherals.[17]

Optic ataxia is the inability to guide the hand toward an object using visual information[18] where the inability cannot be explained by motor, somatosensory, visual field deficits or acuity deficits. Optic ataxia is seen in Bálint's syndrome where it is characterized by an impaired visual control of the direction of arm-reaching to a visual target, accompanied by defective hand orientation and grip formation.[19] It is considered a specific visuomotor disorder, independent of visual space misperception.

Optic ataxia is also known as misreaching or dysmetria (English: difficult to measure), secondary to visual perceptual deficits. A patient with Bálint's syndrome likely has defective hand movements under visual guidance, despite normal limb strength. The patient is unable to grab an object while looking at the object, due to a discoordination of eye and hand movement. It is especially true with their contralesional hand.

Dysmetria refers to a lack of coordination of movement, typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye. It is sometimes described as an inability to judge distance or scale.[18]

As Bálint states, optic ataxia impaired his patient's daily activities, since, 'while cutting a slice of meat...which he held with a fork in his left hand, ...would search for it outside the plate with the knife in his right hand', or '...while lighting a cigarette he often lit the middle and not the end'. Bálint pointed out the systematic nature of this disorder, which was evident in the patient's behaviour when searching in space. 'Thus, when asked to grasp a presented object with his right hand, he would miss it regularly, and would find it only when his hand knocked against it.[19]

The reaching ability of the patient is also altered. It takes them longer to reach toward an object. Their ability to grasp an object is also impaired. The patient's performance is even more severely deteriorated when vision of either the hand or the target is prevented.[20]

Bálint's syndrome has been found in patients with bilateral damage to the posterior parietal cortex. The primary cause of the damage and the syndrome can originate from multiple strokes, Alzheimer's disease, intracranial tumors, or brain injury. Progressive multifocal leukoencephalopathy and Creutzfeldt–Jakob disease have also been found to cause this kind of damage. This syndrome is caused by damage to the posterior superior watershed areas, also known as the parietal-occipital vascular border zone (Brodmann's areas 19 and 7).[24]

Some telltale signs suggesting Bálint's syndrome following bilateral brain insults may include:

• limitation to perceive only stimuli that is presented at 35 to 40 degrees to the right. They are able to move their eyes but cannot fixate on specific visual stimuli (optic apraxia).
• patient's field of attention is limited to one object at a time. making activities like reading difficult because each letter is perceived separately (simultanagnosia).
• figure/ground defects in which a patient can see either the background but not the object residing somewhere in the whole scene, or conversely can see the object but sees no background around it (simultanagnosia)
• a patient, while attempting to put one foot into a slipper by trying to insert the foot into a nonexistent slipper several inches from the real slipper, even as the patient focuses on the actual slipper (optic ataxia)
• a patient raising a fork or spoon containing food to a point on the patient's face above or below the mouth, and possibly finding the mouth by trial and error by manually moving the utensil on the face (optic ataxia)[25][26]

The validity of Bálint's syndrome has been questioned by some. The components in the syndrome's triad of defects (simultanagnosia, oculomotor apraxia, optic ataxia) each may represent a variety of combined defects.

Because Bálint's syndrome is not common and is difficult to assess with standard clinical tools, the literature is dominated by case reports and confounded by case selection bias, non-uniform application of operational definitions, inadequate study of basic vision, poor lesion localisation, and failure to distinguish between deficits in the acute and chronic phases of recovery.[12]