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The Hand Selection Process

The hand selection process refers to the integration of neural activity associated with choosing which hand to perform a task with during any basic reaching or grasping tasks that does not involve the use of any tools. For adults this is primarily an unconscious and rapid decision making process developed over many years of practice and experience. However, for younger children this process may be slower and more deliberate due to their lack of experience with specific motor actions.

In order to make a hand selection decision, the brain must combine a large amount of neural activity considering sensory information regarding the spatial location of the desired object or target, and the motor actions required to complete the movement towards this location. There are several different theories as to how this information biases different potential motor plans within the brain before these biases are then integrated with motor constraints. This integration of information then allows one hand to be preferentially selected alongside a corresponding motor plan based on their combined ability to best perform the desired action.

Sensory Information Biases

There are a number of sensory factors that combine to affect the possibility of either hand being selected during the decision process. These sensory factors bias neural activity toward which hand and which movement plan it should select . Factors include, but are not limited to;

• The efficiency of movement and how much 'cost' or effort goes into the movement when using either hand
• The awkwardness of movement and if it involves an across body reach. If so, does this across body reach allow the movement to finish in an ultimately comfortable position?
• The final position of the hand and arm after the reach or point has been made and whether or not this may hinder future movements
• The grasp needed to hold or pick up the target object and whether that grasp can be better executed by one hand over the other

These factors can have either an excitatory or an inhibitory effect on the neural activation's for each potential movement plan, thus increasing or decreasing its chances of being selected depending on whether that plan is beneficial or not to that specific sensory factor. All sensory bias factors have a cumulative effect on the selection decision, therefore the final decision is not made based on considerations of one aspect of the movement, but on all aspects.

Recent reaching movements also have a large effect on which future movement plans will be selected. For example if the previous action has be performed with the right hand, the next movement is significantly more likely to also be performed with the right hand than with the left hand. This is because the action plan is still within Short-term memory and can easily be recalled and used to also perform this action. Similarly, if the same reaching action is repeated in two or more successive reaches, the time taken to respond to the reach (Reaction Time) the second time, will be significantly shorter than to the first reach.

Neural process hypotheses

Handedness is thought to possibly affect hand selection decisions, therefore the vast majority of academic research examining the hand selection process uses right handed participants only. Typically, the movement of either hand is controlled within the contralateral or opposite hemisphere. Much of the research examining the hand selection process combines behavioural research paradigms with neuroimaging techniques such as functional magnetic resonance imaging (fMRI) to examine the specific regions of the brain involved in the process before applying transcranial magnetic stimulation (TMS) to examine the effects of increasing or inhibiting neural activity within these regions during selection based tasks.

Motor dominance theories

Motor dominance theories suggest that during reaching and grasping tasks people will almost always choose to use their preferred right hand to reach for the desired object or location regardless of whether this location is on their left or right side of space. This theory suggests very little neural processing and even less integration of sensory and motor information to make a selection decision.

Left Hemisphere Lateralised Selection

Left hemisphere only selection based theories suggests that although either hand may be selected to perform basic reaching and grasping tasks, much of the neural processing will occur primarily within the left hemisphere, regardless of whether the left or right hand is being selected.

These theories suggest that predominantly right handed reaches will be used when the desired object lies within the right side of space and left handed reaches will be used when reaching towards the left side of space. These theories suggest that such hand selection decisions are made based on integration of sensory and motor information biases processed in various region of the brain.

• Prefrontal cortex (PFC) biases the different action plans either positively or negatively based on sensory information, making each motor plan more or less likely to be selected.
• Parietal cortex (PC) considers the motor planning constraints of reaching towards the desired target location.
• Premotor cortex, particularly the dorsal stream (PMd), of the Left hemisphere only, processes the final hand selection decision based on information gained from the prefrontal and parietal lobes, regardless of the hand ultimately selected.
  

Competition Process of Hand Selection

Competition based theories of selection also suggest that either hand may be selected to perform the required action during reaching or grasping tasks and that the decision making process occurs through resolving competition between each hands potential action plans.

The Affordance competition hypothesis (ACH) describes the patterns of neural activation associated with the competition process involved in hand selection. Each hemisphere creates several potential action plans for its associated hand and each action plan can be biased by relevant sensory information in either a positive manner, hence increasing activation, or in a negative manner, thus decreasing activation. Activation then builds up until one hemisphere reaches activation threshold and the corresponding hand is selected to perform the action.

This competition process occurs within;

• Bilateral Posterior parietal cortex
• Dorsal Premotor cortex.

Research furthering the ACH, examines when people switch from selecting their right hand to selecting their left hand and when people are equally likely to select either hand. If an object of interest is in the right side of space people will mainly use their right hand to reach for it. The further to their right it is, the more likely they become to use their right hand and the faster the decision is made. The same is true for the left hand and objects in the left side of space. These are more obvious selection decisions and therefore require less neural processing and can be responded to faster than more central locations.

However at central target locations more neural processing is required to decide which hand can make the most efficient response as it may seem that either hand could perform an equally efficient movement. This extra processing time results in slower responses to centrally places objects of interest. The point at which either hand is equally likely to be selected is commonly referred to as the point of subjective equality (PSE). For right handed people, this point most commonly lies slightly into the left side of space as there still exists some bias for the more dominant right hand, but not enough for it to be favoured for all reaches into the left side of space.

Inability to make hand Selection decisions

Damage to the brain, particularly to Parietal , Premotor or Prefrontal cortex may, among many other problems, result in the inability to make selection decisions. Damage may occur as a result of;

• Stroke
• Traumatic brain injury (TBI)
• Brain Tumor

These forms of damage to the brain, that usually only occur in one hemisphere, create Lesions or disruptions to the motor control and selection processes as they prevent the integration of sensory and motor information and therefore cannot bias a specific hand or motor plan. This then leads to the undamaged hemisphere becoming dominant and its associated hand and arm performing significantly more actions than the affected limb, regardless or the target location.

Understanding the Selection Process

By understanding more about how the hand selection process works, new therapies can be developed to aid the rehabilitation of the affected hemisphere after damage. Constraint-induced movement therapy is but one example of a therapy developed to aid with the rehabilitation of the affected hemisphere. This therapy constricts the use of the unaffected limb and forces the patient to use their affected limb instead, thus preventing the undamaged and dominant hemisphere from taking control all of the time. This allows the affected hemisphere to regain activation and recover more effectively.