Question

How does the internal forward model proactively adjust motor commands to account for the sensory consequences of an action before that action finishes?

Accepted Answer

The internal forward model operates by creating a parallel representation of motor activity to predict how the body will respond to a command before sensory feedback from the limbs actually arrives. When the brain sends a motor command to the muscles to perform an action, it simultaneously sends a copy of that command, known as an efference copy, to the forward model. The forward model uses this efference copy to calculate the expected sensory consequences of the movement, such as how the arm will move through space. This prediction is called the forward state estimate. By comparing this predicted sensory state with the actual sensory feedback received from the body as the movement progresses, the brain calculates a sensory prediction error, which is the difference between what was expected and what actually occurred. If the prediction error is non-zero, the brain can immediately adjust the outgoing motor commands to correct the trajectory before the action is completed. This process is essential because sensory feedback from the muscles and joints travels through the nerves at a relatively slow speed, which would otherwise result in a significant delay. For example, when you reach for a cup, your forward model predicts the position of your hand in real time; if the wind pushes your hand off course, the brain detects the mismatch between the predicted position and the actual visual feedback and adjusts your muscles mid-reach to compensate, ensuring a smooth and accurate movement.

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How does the internal forward model proactively adjust motor commands to account for the sensory consequences of an action before that action finishes?