Hand Eye Cordination

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. It has been studied in activities as diverse as tea making, the movement of solid objects such as wooden blocks, sporting performance, music reading, online first person computer gaming, and copy-typing. It is a way of performing everyday tasks and 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.

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 over movements in order to guide 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 search 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 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 in order 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 in order 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. Finally, 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.

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 most studied most intensely are the frontal and parietal cortex areas for the control of eye saccades and hand reach 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.