In 1909, Rudolf Bálint described optic ataxia within the context of a more general parietal syndrome that also included psychic paralysis of gaze, and emispatial neglect. The post-mortem exam of his case revealed a bilateral softening of the parietal lobe.
As Bálint refers, 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…”.
The term optic ataxia was introduced in analogy to tabetic ataxia, which is characterized by a lack of proprioceptive control of movement. It was meant to describe a disorder of hand reaching movements that, in Bálint’s view, was specifically dependent on a defective visual control. The patient, in fact, correctly performed all movements when these were guided by somatosensory information, as when the right hand reached to different parts of his own body. With eyes closed, he could perfectly imitate with the right hand passive postures imposed to his left hand, thus excluding the possibility of apraxic disorders. Crucial to this interpretation was the observation that “…all movements performed defectively with the right hand were executed perfectly or with very little error with the left hand”, which excluded the possibility of a disorder of visual attention.
Thus, Optic ataxia is characterized by an impaired visual control of the direction of arm reaching to visual targets, and is accompanied by defective hand orientation and grip formation as well. In humans, optic ataxia is associated to lesions of the superior parietal lobule (SPL), that also affect visually guided saccades.
In the last 10 years, anatomical and physiological studies of the SPL have shed new light on the role of parietal cortex in the control of combined eye-hand movements to visual targets, and on the underlying distributed network linking parietal to frontal cortex. A main emerging functional feature of SPL neurons seems their capacity to combine in a spatially congruent fashion different directional eye and hand related information, those that any coding scheme so far proposed considers essential for the composition of motor commands for reaching. This integration occurs within the global tuning field (GTF) of parietal neurons, is context-dependent, and involves eye and hand information that share same directional properties. Depending on task demands, this integration of signals can result in the representation of different reference frames for coordinated eye-hand movements. The dynamic operations occurring within the GTFs might depend, at least in part, on the reciprocal sets of association connections linking the superior parietal lobule and the premotor areas of the frontal lobe. From this picture, the SPL emerges both as a main source of visual input to the frontal cortex, and as a key structure for visuomotor integration based on re-entrant signalling, therefore, as a crucial node in the visual control of movement.
It is our hypothesis that in parietal patients the directional errors that characterize reaching are consequence of the breakdown of the combination of directional eye and hand information within the GTFs of parietal neurons. In these patients, the spatial match among information about target location, eye and hand position, and movement direction would be prevented, so as to impair visually-guided eye-hand movements. This breakdown could be, al least in part, dependent on the failure of a re-entrant fronto-parietal signalling, an obvious consequence of the degeneration of the cortico-cortical systems linking parietal and frontal cortex. Cortico-cortical connections are in fact essential for shaping the dynamic properties of cortical neurons.

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