g., γ dynamic drive, γ static drive, α motor neuron drive) within a forward model of the system can inferences be made to interpret the state of the system from ambiguous afferent

signals. There have been many studies that have investigated whether forward models can be found within the sensorimotor system. However, conclusive evidence for a forward model in the sensorimotor find more system has been very difficult to produce. This is because the output of the forward model, a prediction of a future event, is not a measurable output but, instead, used to guide the control of the motor system (Mehta and Schaal, 2002). Several studies supporting the use of forward models in the sensorimotor system have used different techniques, for example sinusoidal tracking with induced delays (Miall et al., 1993) or virtual pole balancing with feedback blanking (Mehta and Schaal, 2002). In one study the existence of a forward Cell Cycle inhibitor model was probed by asking subjects to report the final hand position at the end of reaching movements that had been physically perturbed without visual feedback (Wolpert et al., 1995). The systematic errors and the

variability in the errors in the estimated positions were indicative of a forward model similar to the Kalman filter. Using saccades during reaching movements to probe the underlying predicted hand position, several studies have provided evidence that estimates of body state use both sensory unless feedback and a model of the world (Ariff et al., 2002 and Nanayakkara and Shadmehr, 2003). They asked subjects to visually track the position of their

hand during full-limb reaching movements. They found that saccades tended to move to a position 196 ms in advance of the position of the hand (Ariff et al., 2002). By disturbing the arm position with unexpected perturbations, they demonstrated that saccades were initially suppressed (100 ms following the disturbance), then following a recalculation of predicted position, the eyes moved to a predicted position (150 ms in advance, suggesting access to efferent copy information). In contrast when the perturbation also changed the external dynamics (i.e., adding a resistive or assistive field), this recalculation was incorrect, and subjects were unable to accurately predict future hand position. This work suggests that the prediction of future hand position was updated using both the sensory feedback of the perturbation and a model of the environment. When the model of the environment was incorrect, the system was unable to accurately predict hand position. On the other hand, when the altered environment could be learned, the saccade accurately shifted to the actual hand position, demonstrating that the model of the environment could be adaptively reconfigured (Nanayakkara and Shadmehr, 2003). Prediction can also be used for perception.