Abstract
Bach, Frank, and Kunde introduce a hypothesis that encompasses two main claims: (1) motor imagery relies primarily on representations of the perceptual effects of actions, and (2) the engagement of motor resources provides access to the specific timing, kinematic or internal bodily state that characterize an action. In this commentary, I argue that the first claim is compelling and suggest some alternatives to the second one.
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Actions are intertwined with representations of their effects. Accordingly, it is possible to anticipate the effects of an action through “forward modelling”, and it is possible to select an action based on a mental representation of an intended effect through “inverse modelling” (Wolpert et al., 2001). Traditionally, motor imagery is assumed to stem from forward models: it is experienced when the predicted perceptual effects of an action that is prevented from being executed are nonetheless brought to mind.
In contrast, Bach, Frank, and Kunde (BF&K) propose that motor imagery is part of the machinery of inverse models. Their proposal encompasses two main claims. The first and most important is that motor imagery precedes and, in fact, is a prerequisite to action selection. According to the ideomotor approach of action control, voluntary behavior results from a three step process: (1) the selection of a goal defined in terms of perceptual effects, (2) the selection of an action whose predicted effects align with one’s goal, and (3) the execution of the associated movements. To put it simply, BF&K propose that motor imagery emerges when mental representations of the anticipated effects of an action (how their body movements will “look, feel, sound, and how they affect the environment”)—which usually operate unconsciously to guide action planning—are brought to mind. Accordingly, they suggest labeling the resulting experience “effect imagery”. Of course, effect imagery and action execution are tightly associated. The authors illustrate this inextricable link by reviewing evidence that imagined actions elicit activations in the action execution network, interfere with action execution, elicit muscular activity, and vice versa that action execution may facilitate or impede imagery (see the target article for discussion). However, the important point is that “there is no pure “motoric” imagination that is not triggered by a prior imagination of the intended action outcomes”. This proposal is illuminating and the arguments put forth are compelling. As noted by BK&F, this view accommodates easily various types of evidence typically presented in support of the motoric nature of imagery and, in fact, solves several of its theoretical and empirical challenges. Importantly, this hypothesis underscores the need for caution in the use (and interpretation) of motor imagery paradigms to investigate the integrity of the motor system in neurological and psychiatric conditions (De Lange, Roelofs & Toni, 2008; Jeannerod & Decety, 1995; Munzert et al., 2009).
The second claim is that effect imagery may be supplemented by an additional “motoric” route to imagery in situations that require accessing information about the specific timing, kinematic or internal bodily state that characterize an action. This proposal follows from two arguments: (1) action planning based on anticipated effects generally focuses on determining “what to do” without providing detailed instructions on “how to do it” (Hommel et al., 2001; Jeannerod, 1988); (2) motoric imagery is made use of “when the effects usually used to plan a particular action do not correspond to the information required by the current imagery task”. As noted by BF&K, this proposal is compatible with the finding that motoric imagery tends to be more pronounced and easier to detect in tasks requiring participants to access timing and kinematic information (Stinear et al., 2006). It also offers a plausible explanation to the finding that motor disorders, like hemiplegia, dystonia, conversion paralysis, Parkinson’s disease or apraxia may impede patients’ motor imagery (De Lange, Roelofs & Toni, 2008; Jeannerod & Decety, 1995; Munzert et al., 2009).
However, this proposal faces several challenges. The authors note that we “often” or “usually” use mental representations of general or “distal” action–effects to plan actions. However, the critical issue to consider for determining when imagery must draw upon motor resources is not the type of action–effect representations that we often use to plan action, but rather the extent of action–effect information that is available and can be accessed if needed. In addition to distal effects, specific timing and kinematics may also be planned, learned, and controlled. If this information is available, access to fine-grained features of actions may not require motor resources. And indeed, there is evidence that people born without upper limbs are influenced by knowledge of the typical timing and biomechanical constraints of upper-limb movements when performing imagery tasks, such as the hand laterality judgment task (Funk & Brugger, 2008; Vannuscorps et al., 2012; Vannuscorps & Caramazza, 2015) and apparent body movement perception (Vannuscorps & Caramazza, 2016a). On this view, the effect of motor impairment on imagery is interpreted as evidence that representations of action effects simply evolve and change through experience. And although it is true that tasks focusing on finer details appear to recruit the motor system more, this does not imply that these tasks require motor resources. They may be simply more demanding, or some action–effects may be more strongly associated with motor programs than others. At this juncture, it may be more parsimonious to consider that access to low-level action features does not necessarily require motoric imagery.
So, what could be motoric imagery useful for? I see at least three alternative possibilities. First, although it is likely not the main contributor of the effect of mental practice on motor learning (as discussed in the target article), motoric imagery is likely to play a role in motor learning by observation and covert practice (Ruffino et al., 2017; Stefan et al., 2005). Second, motoric imagery might contribute when one is asked to judge or imagine an action or a body movement/posture that we have never performed and seen before. In such cases, relying on effect imagery is not possible, and motoric imagery could fill this gap. Accordingly, in the Hand Laterality Task, indexes of motor imagery are prominent for less familiar postures and often absent when the postures are familiar (Parsons, 1987; ter Horst et al., 2010; Vannuscorps et al., 2012). The influence of familiarity is also supported by the finding that although individuals born without upper limbs are indistinguishable from control participants in judging the laterality of hands depicted in familiar postures (Vannuscorps & Caramazza, 2015; Vannuscorps et al., 2012), they struggle to identify the laterality of complex hand postures involving atypical finger and wrist orientations (Maimon-Mor et al., 2020). Third, motoric imagery may support the ability to maintain meaningless, uninterpreted, body movements and postures in memory (Galvez-Pol et al., 2020). In line with this idea, motor suppression impairs the ability to retain a sequence of observed body postures in memory (Moreau, 2013) and individuals born without upper limbs are significantly less good than controls at maintaining hand postures in memory, even for a few seconds (Vannuscorps & Caramazza, 2016b). As a result, motoric imagery may contribute to performance in any task that benefits from this ability, including the recognition of actions depicted in adverse conditions (Vannuscorps & Caramazza, 2023; Vannuscorps et al., 2013).
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This study is supported by Fédération Wallonie Bruxelles (Belgium), ARC21/26–112.
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Vannuscorps, G. When does imagery require motor resources? A commentary on Bach et al., 2022. Psychological Research 88, 1817–1819 (2024). https://doi.org/10.1007/s00426-023-01917-6
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DOI: https://doi.org/10.1007/s00426-023-01917-6