Forecast or Fall: Prediction's Importance to Postural Control

Abstract

To interact successfully with an uncertain environment, organisms must be able to respond to both unanticipated and anticipated events. For unanticipated events, organisms have evolved stereotyped motor behaviors mapped to the statistical regularities of the environment, which can be trigged by specific sensory stimuli. These "reflexive" responses are more or less hardwired to prevent falls and represent, maybe, the best available solution to maintaining posture given limited available time and information. With the gift of foresight, however, motor behaviors can be tuned or prepared in advance, improving the ability of the organism to compensate for, and interact with, the changing environment. Indeed, foresight's improvement of our interactive capacity occurs through several means, such as better action selection, processing, and conduction delay compensation and by providing a prediction with which to compare our actual behaviors to, thereby facilitating error identification and learning. Here we review the various roles foresight (prediction) plays in maintaining our postural equilibrium. We start by describing some of the more recent findings related to the prediction of instability. Specifically, we cover recent advancements in the understanding of anticipatory postural behaviors that are used broadly to stabilize volitional movement and compensate for impending postural disturbances. We also describe anticipatory changes in the state, or set, of the nervous system that may facilitate anticipatory behaviors. From changes in central set, we briefly discuss prediction of postural instability online before moving into a discussion of how predictive mechanisms, such as internal models, permit us to tune, perhaps our highest level predictive behaviors, namely the priming associated with motor affordances. Lastly, we explore methods best suited to expose the contribution of prediction to postural equilibrium control across a variety of contexts.

Keywords: anticipation; balance; fall; postural control; posture; prediction.

Figure 1

Dealing with complex environments often requires behavioral flexibility to maintain postural equilibrium. For example, in cluttered environments it is often necessary to grasp a nearby object to establish a new base of support, while suppressing a highly automatic stepping reaction if an obstacle blocks the foot. The speed and complexity of such sophisticated, goal-directed behaviors necessitates a higher level of control, and implicates a role for advanced preparation based on environmental cues in the control of balance.

Figure 2

Simplified neural networks underlying an affordance to grasp. Black arrows indicate how the brain converts visual information into movement plans for a variety of possible actions. As movements are encoded in frontal and parietal networks, action representations compete with one another. These actions are biased by the basal ganglia and prefrontal cortex at multiple locations in the brain as per the affordance competition model (103, 104). We act when one of the possible actions wins the competition. In this example, we see that the stairs have a supporting railing. The railing affords a grasp and the rope affords a grasp, but since the railing is more stable, the railing grasp is primed. If the grasp is the most salient afforded action, we may execute it in the event of a stumble. Here, it is important to note that such directed arm action would conceivably be prompted by viewing a supportive handle—a handle associated with postural recovery from past experience. Furthermore, such action would only exist as an internal representation until called upon.