Eye–hand coordination (also known as hand–eye coordination) is the coordinated control of eye movement with hand movement, and the processing of visual input to guide reaching and grasping along with the use of proprioception of the hands to guide the eyes. Eye–hand coordination has been studied in activities as diverse as the movement of solid objects such as wooden blocks, sporting performance, music reading, computer gaming, copy-typing, and even tea-making. It is part of the mechanims of performing everyday tasks; in its absence most people would be unable to carry out even the simplest of actions such as picking up a book from a table or playing a video game. While it is recognized by the term hand–eye coordination, without exception medical sources, and most psychological sources, refer to eye–hand coordination.
Behavior and kinematics 
Neuroscientists have extensively researched human gaze behavior, with studies noting that the use of the gaze is very task-specific, but that humans typically exhibit proactive control to guide their movements. Usually, the eyes fixate upon a target before the hands are used to engage in a movement, indicating that the eyes are used to provide spatial information for the hands. Furthermore, the duration that the eyes appear to be locked onto a goal for a hand movement varies, with the eyes sometimes remaining fixated until a task is completed. Other times, the eyes seem to scout ahead toward other objects of interest before the hand even grasps and manipulates the object. Conversely, humans have been shown to be able to aim eye movements toward the hand without vision, using spatial information from hand proprioception.
Eye-guided hand movements 
The more dominant behavior in humans, studies have shown that when eyes and hands are used for core exercises, the eyes generally direct the movement of the hands to targets. Furthermore, the eyes provide initial information of the object, including its size, shape, and possibly grasping sites which are used to determine the force needed to be exerted by the fingertips for engaging in a given task. For shorter tasks, the eyes often shift onto another task to provide additional input for planning further movements. However, for more precise movements or longer duration movements, continued visual input is used to adjust for errors in movement and to create more precise movements.
For sequential tasks, it has been observed that eye gaze movements occur during important kinematic events like changing the direction of a movement, or when passing perceived landmarks. This is related to the task search oriented nature of the eyes and their relation to movement planning of the hands, and the errors between motor signal output and consequences perceived by the eyes and other senses which can be used for corrective movements. Furthermore, the eyes have been shown to have a tendency to 'refixate' upon a target to refresh the memory of its shape, or to update for changes in its shape or geometry. This has also been shown to be true in drawing tasks with relating visual input and converting it into hand movements to produce a copy of what was perceived. In high accuracy tasks, studies have shown that when acting upon greater amounts of visual stimuli, the time it takes to plan and execute movements, increases linearly as per Fitts's Law.
Hand-guided saccades 
Humans have been demonstrated to be able to aim eye movements toward the hand without vision, using the sense of proprioception, with only minor errors related to the internal knowledge of limb position. It has been shown the proprioception of limbs, both active and passive movements of the limbs are result in eye saccades overshoots when the hands are being used to guide eye movements. These overshoots have been determined to result from the control of eye saccades rather than previous movements of the hands in experiments. As a result, limb based proprioception has been determined to be capable of being transformed into ocular motor coordinates to guide eye saccades, which allows for the guidance of the saccades by the hands and feet.
Neural mechanisms 
The neural control of eye–hand coordination is complex because it involves every part of the central nervous system involved in vision, eye movements, touch, and hand control. This includes the eyes themselves, the cerebral cortex, subcortical structures (such as the cerebellum, basal ganglia, and brain stem), the spinal cord, and the peripheral nervous system. Some other areas involved in eye–hand coordination that have been studied most intensely are the frontal and parietal cortex areas for the control of eye saccades and hand reach control. Both of these areas are believed to play a key role in eye–hand coordination and the planning of movements during tasks.
A more specific area, the parieto occipital junction, is believed to be involved in the transformation of peripheral visual input for reaching with the hands, as found via fMRI. This region in particular has been shown to have subdivisions for reach, grasp, and saccades. In additional to the parieto occipital junction, the posterior parietal cortex is believed to play an important role in relating proprioception and the transformation of motor sensory input to plan and control movement with regards to visual input.
Many of these areas, in addition to controlling saccades or reach, also show eye position signals that are required for transforming visual signals into motor commands. In addition, some of the areas involved in reach, like the medial intraparietal cortex, show a gaze-centered remapping of responses during eye movements in both monkeys and humans. However, when single neurons are recorded in these areas, the reach areas often show some saccade-related responses and the saccade areas often show some reach related responses. This may aid in eye–hand coordination or hint at the ability of cells to wire together as they're used more frequently.
Clinical syndromes 
Numerous disorders, diseases, and impairments have been found to result in disruption to the coordination of the eyes and hands, owing to damage to the brain itself, degeneration of the brain due to disease or aging, or an apparent inability to coordinate senses completely.
Impairments to eye–hand coordination have been shown to occur in older adults, especially during high velocity and precise movements. This has been attributed to the general degeneration of the brain's cortex, resulting in a loss of the ability to compute visual inputs and relate them to hand movements. However, while older adults tend to take more time for these sorts of tasks, they are still able to remain just as accurate as younger adults, but only if the additional time is taken.
Bálint's syndrome 
Bálint's syndrome is characterized by a complete lack of eye–hand coordination and has been demonstrated to occur in isolation to Optic Ataxia. It is a rare psychological condition resulting most often from damage bilaterally to the superior parieto-occipital cortex. One of the most common causes is from strokes, but tumors, trauma, and Alzheimer's disease can also cause damage. Balint's syndrome patients can suffer from 3 major components: optic apraxia, optic ataxia, and simultanagnosia. Simultanagnosia is when patients have difficulty perceiving more than one object at a time. There have been three different approaches for rehabilitation. The first approach is the adaptive or functional approach. It involves functional tasks that utilize a patient's strengths and abilities. The second approach is remedial approach and involves restoration of the damaged central nervous system by training perceptual skills. The last approach is multicontext approach and this approach involves practicing of a targeted strategy in a multiple environment with varied tasks and movement demands, along with self-awareness tasks.
Optic apraxia 
Optic apraxia is a condition that results from a total inability of a person to coordinate eye and hand movements. Although similar to optic ataxia, its effects are more severe and may not necessarily come from damage to the brain, but genetic defects or degeneration of tissue.
Optic ataxia 
Optic ataxia or visuomotor ataxia is a clinical problem associated with damage to the occipital–parietal cortex in humans, resulting in a lack of coordination between the eyes and hand. It can affect either one or both hands and can be present in part of the visual field or the entire visual field. Optic ataxia has been often considered to be a high-level impairment of eye–hand coordination resulting from a cascade of failures in the sensory to motor transformations in the posterior parietal cortex. Visual perception, naming, and reading are still possible, but visual information cannot direct hand motor movements. Optic ataxia has been often confused with Balint's syndrome, but recent research has shown that optic ataxia can occur independently of Balint's syndrome. Optic ataxia patients usually have troubles reaching toward visual objects on the side of the world opposite to the side of brain damage. Often these problems are relative to current gaze direction, and appear to be remapped along with changes in gaze direction. Some patients with damage to the parietal cortex show "magnetic reaching": a problem in which reaches seem drawn toward the direction of gaze, even when it is deviated from the desired object of grasp.
Parkinson's disease 
Adults with Parkinson's disease have been observed to show the same impairments as normal aging, only to a more extreme degree, in addition to a loss of control of motor functions as per normal symptoms of the disease. It is a movement disorder and occurs when there is degeneration of dopaminergic neurons that connect the substantia nigra with the caudate nucleus. A patient's primary symptoms include muscular rigidity, slowness of movement, a resting tremor, and postural instability. The ability to plan and learn from experience has been shown to allow adults with Parkinson's to improvement times, but only under conditions where they are using medications to combat the effects of Parkinson's. Some patients are given L-DOPA, which is a precursor to dopamine. It is able to cross the blood-brain barrier and then is taken up by dopaminergic neurons and then converted to dopamine.
See also 
- Vidoni, E. D.; McCarley, J. S.; Edwards, J. D.; Boyd, L. A. (2009). "Manual and oculomotor performance develop contemporaneously but independently during continuous tracking". Experimental Brain Research 195 (4): 611–620. doi:10.1007/s00221-009-1833-2.
- Johansson, R. S.; Westling, G; Bäckström, A.; Flanagan, J. R. (2001). "Eye–hand co-ordination in object manipulation". Journal of Neuroscience 21 (17): 6917–6932. PMID 11517279. CiteSeerX: 10.1.1.211.9086.
- Liesker, H.; Brenner, E.; Smeets, J. (2009). "Combining eye and hand in search is suboptimal". Experimental Brain Research 197 (4): 395–401. doi:10.1007/s00221-009-1928-9.
- Bowman, M. C.; Johannson, R. S.; Flanagan, J. R. (2009). "Eye-hand coordination in a sequential target contact task". Experimental Brain Research 195 (2): 273–283. doi:10.1007/s00221-009-1781-x.
- Coen-Cagil, R.; Coraggio, P.; Napoletano, P.; Schwartz, O.; Ferraro, M.; Boccignone, G. (2009). "Visuomotor characterization of eye movements in a drawing task". Vision Research 49 (8): 810–818. doi:10.1016/j.visres.2009.02.016.
- Lazzari, S.; Mottet, D.; Vercher, J. L. (2009). "Eye-Hand Coordination in Rhythmical Pointing". Journal of Motor Behavior 41 (4): 294–304. doi:10.3200/JMBR.41.4.294-304.
- Ren, L.; Crawford, J. D. (2009). "Coordinate transformations for hand-guided saccades". Experimental Brain Research 195 (3): 455–465. doi:10.1007/s00221-009-1811-8.
- Gomi, H. (2008). "Implicit online corrections in reaching movements". Current Opinion in Neurobiology 18 (6): 558–564. doi:10.1016/j.conb.2008.11.002.
- Jackson, S. R.; Newport, R.; Husain, M.; Fowlie, J. E.; O'Donoghue, M.; Bajaj, N. (2009). "There may be more to reaching than meets the eye: Re-thinking optic ataxia". Neuropsychologia 47 (6): 1397–1408. doi:10.1016/j.neuropsychologia.2009.01.035.
- Boisseau, E.; Scherzer, P.; Cohen, H. (2002). "Eye-Hand Coordination in Aging and in Parkinson’s Disease". Aging Neuropsychology and Cognition 9 (4): 266–275. doi:10.1076/anec.9.4.266.8769.
- Jackson; Swainson, G. M.; Mort, R.; Husain, D.; Jackson, M. (2009). "Attention, competition, and the parietal lobes: insights from Balint's syndrome". Psychol Res. 73 (2): 263–270. doi:10.1007/s00426-008-0210-2. PMID 19156438.
- Udesen, H. (1992). "Balint's syndrome: visual disorientation". Ugeskr Laeger 154 (21): 1492–4. PMID 1598720.
- Al-Khawaja, I. Haboubi (2001). "Neurovisual rehabilitation in Balint's syndrome". J. Neurol Neurosurg. Psychiatry 70 (3): 416. PMC 1737281. PMID 11248903.
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Further reading 
- Crawford, J. D.; Medendorp, W. P.; Marotta, J. J. (2004). "Spatial transformations for eye–hand coordination". Journal of Neurophysiology 92 (1): 10–19. doi:10.1152/jn.00117.2004. PMID 15212434.
- Johansson, R. S.; Westling, G.; Bäckström, A.; Flanagan, J. R. (2001). "Eye–hand co-ordination in object manipulation". Journal of Neuroscience 21 (17): 6917–6932. PMID 11517279.