Previous research suggested that our central nervous system (CNS) contains internal models that make it possible for us to form a representation of our body within our environment. Within these internal models, we differentiate between the forward model and the inverse model. The forward model is used as a predictor of our behaviour, and its consequences. In other words, the forward model uses motor commands as “moving the arm” and predicts the arms new position. The inverse model on the other hand produces motor commands to achieve the desired body position. In this case for example, it would get the environmental input (“how far is the object?; how heavy is it?”) and uses this information to define accurate motor commands (more information by Wolpert et al.). Even though some research suggests that we are aware of our motor actions, it can be assumed, that not all processes of these two internal models are done consciously.
Impairments of these internal models can be
used to explain several motor disorders, some of which I will explain in more
detail.
Optic
ataxia shows general difficulties in grasping an
object, even though the visual abilities of the patient are intact. In other
words, the brain seems to be unable to use information about its environment to
define accurate motor movements. Blakemore et al. therefore, explains the
phenomena with an impaired inverse model, which causes a lack of information
about the object being grasped for an adjustment of the grasping movements.
Another example is explained in a disorder called anarchic hand, where participants complain about their hand moving
by itself. Here, the inverse model
seems to forms the representation and sends the information, but does it
inappropriately and causes involuntarily motor actions.
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