Control of body orientation and stability results from complex interactions among the individual, the environment, and the task being performed. Successful accomplishment of the desired task depends on processing and integrating multiple sensory inputs to determine the relative orientation among body segments and body position relative to the environment. This information is used to plan and execute movements that allow maintenance of balance during task performance. Older models of postural control as parallel sensory reflexes (such as visual, vestibular, and proprioceptive righting responses) are no longer considered useful (Horak et al., 1997; Mergner et al., 2003).
Figure 8.2 shows a contemporary model of postural control (adapted from Merfeld et al., 1993) that outlines how the process of selection and adaptation of balance responses may occur. In this model, the central nervous system uses sensory inputs in combination with knowledge of biomechanical constraints to form internal models of body and sensory dynamics. A postural control strategy is selected to achieve the desired body orientation and a copy of the control strategy is fed to the internal models to yield estimated orientation and expected sensory afferent signals. These expected afferent signals are then compared to actual sensory afferent signals that result from the postural movement performed. A sensory conflict, or difference between expected and actual afferent signals, is relayed to the internal model of body dynamics, the model is changed, and the next postural strategy modified
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