"Sperry (1950) and von Holst and Mittelstaedt (1950) independently suggested that motor commands must leave an image of themslves (efference copy) somewhere in the central nervous system that is then compared to the afference elicited by the movement (reafference). It was soon recognised that the efference copy and reafference could not simply be compared, since one is a motor command and the other is a sensory cue (Hein & Held 1961; Held 1961)." To solve this problem, various models have been proposed.
Merfeld has developed his own model, which he describes as follows: "The primary input to this model is desired orientation, which when compared to the estimated orientation yields motor efference via a control strategy. These motor commands are filtered by the body dynamics (e.g., muscle dynamics, limb inertia, etc.) to yield the true orientation, which is measured by the sensory systems with their associated sensory dynamics to yield sensory signals. In parallel with the real-world body dynamics and sensory dynamics, a second neural pathway exists that includes an internal representation of the body dynamics and an internal representation of the sensory dynamics. Copies of the efferent commands (efference copy) are processed by these internal representations to yield the expected sensory signals, which when compared to the sensory signals yield an error (mismatch). This error is fed back to the internal representation of body dynamics to help minimize the difference between the estimated orientation and true orientation" (2001, p. 189).
Merfeld's model resembles one developed by Gray (1995): "Analogous to Gray's description of his model, this model (1) takes in sensory information; (2) interprets this information based on motor actions; (3) makes use of learned correlations between sensory stimuli; (4) makes use of learned correlations between motor actions and sensory stimuli; (5) from these sources predicts the expected state of the world; and (6) compares the predicted sensory signals with the actual sensory signals" (Merfeld, 2001, p. 190). The distinctive feature of Merfeld's model is that in the event of a mismatch between the expected and actual sensory signals, the mismatch is used as an error signal to guide the estimated state back toward the actual state. In Gray's model, all action halts during sensorimotor mismatch.