Common Coding

Commensurate Representation

Most classical frameworks of interfaces between perception and action rely on separate coding. The classical view holds that representations on the afferent and the efferent side of the interface are incommensurate. While afferent representations refer to patterns of sensory stimulation, efferent representations refer to patterns of motor activation. Bridging the gap between them requires creating arbitrary linkages (mapping between sensory and motor codes).

In contrast, the common coding approach suggests that in addition to such separate codes, there are further levels of representation in which afferent and efferent information have the same format and dimensionality. Common coding is meant to apply to the relationship between representations on the afferent and the efferent side (referring to ongoing events and intended actions, respectively). These representations are commensurate, since they both refer to external events. They permit creating linkages between perception and action that do not rely on arbitrary mappings. In common coding, action planning is conceived of in terms of operations that move from current events to intended future events. These operations involve the matching of event codes and action codes; perception and action may modulate each other by means of similarity.

Ideomotor Principle

The principle of common coding is closely related to the ideomotor theory of voluntary action as advanced by Rudolph Hermann Lotze and William James in the 19th century. In line with this theory, common coding hypothesizes that actions are represented in terms of their perceptual consequences. The representations of actions are thus similar to those of other events, except that they can be generated through bodily movements. When individuals perform actions, they learn what their movements lead to (ideomotor learning). Ideomotor theory claims that these associations can also be used in the reverse order: When individuals perceive events of which they know (from previous learning) that they may result from certain movements, perception of these events may evoke the movements leading to them (ideomotor control). The distinction between such learning and control is analogous to the distinction between forward and inverse computation in motor learning and control. While ideomotor learning leads to the prediction of action outcomes, ideomotor control helps in the selection of actions that will lead to an intended outcome.

Content-Based Interference

A straightforward application of common coding is to compatibility effects in task performance. Compatibility effects indicate that task performance [Page 162]is modulated by similarity, or compatibility, between stimulus and response sets. For example, as has been shown by Marcel Brass and colleagues, common coding can account for movement imitation in stimulus-response compatibility paradigms. Other studies have extended the scope of the notion of compatibility to action effects. Hommel showed that stimulus-effect compatibility may under certain conditions override stimulus-response compatibility. Likewise, Wilfried Kunde showed that response-effect compatibility may be functional as well. These effect-related compatibility effects provide direct support for the claim that action selection engages anticipatory representations of action effects.

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