In motor learning, is mental practice as effective as physical practice?

Mental Practice

One of the most frequently used and advocated off-task methods to promote learning is mental practice, in which the performance of a task is mentally rehearsed, often using imagery techniques, in the absence of overt physical practice. Experimental assessment of mental practice effects usually requires several different groups of subjects, at a minimum (Goginsky & Collins, 1996): All subjects are given a pretest on a task to be learned, followed by the experimental manipulation, then a posttest on the learning task. The mental practice manipulation often entails covert rehearsal of the task, sometimes involving strategies such as imagery. In this case, however, the learning that can be attributed only to mental practice effects cannot be inferred just from a retention test. Rather, one must demonstrate that performance on the posttest exceeded performance in a control group that did not perform intervening practice or that performed practice on an unrelated task. In addition, mental practice is usually compared to a third condition in which a group physically practices the task for the same amount of time as the mental practice group. Some experiments also include combination conditions with alternation between trials of mental and physical practice. Of course, many experiments use other variations of these mental practice manipulations (see reviews by Feltz & Landers, 1983; Feltz, Landers, & Becker, 1988; Lotze & Halsband, 2006).

Is Mental Practice as Effective as Physical Practice?

A nice demonstration of all these various practice conditions and their effects was provided in a complex study by Hird, Landers, Thomas, and Horan (1991). Twelve groups of subjects participated in the experiment. Six groups were asked to learn a pegboard task, inserting pegs of different colors and shapes as rapidly as possible into squares cut in a board. The other six groups performed the pursuit rotor task. For each task, subjects performed a pretest, seven sessions of training (on separate days), and a posttest. During the training sessions the 100% physical practice group performed eight trials on the task while the 100% mental practice group covertly practiced the task for the same amount of time. Three other groups involved combinations of practice, consisting of two, four, or six trials of physical practice combined with six, four, or two trials of mental practice (i.e., 75% physical practice [P], 25% mental practice [M]; 50P:50M; and 25P:75M groups). The control group performed an unrelated task (the stabilometer) for the same amount of time during these training sessions.

The difference in performance between the pretest and posttest for each group in Hird and colleagues’ study is presented in figure 11.7.1 The sets of findings for the two tasks are remarkably similar. The groups given mental practice (100%M) were more effective than the no-practice (control) groups, but not nearly as effective as the groups given the same amount of physical practice (100%P). In addition, the results for the combination groups showed that learning was enhanced with higher proportions of the training trials spent in physical, compared to mental, practice (e.g., compare the 75P:25M groups with the 25P:75M groups in figure 11.7).

The findings of Hird and colleagues (1991) have been replicated (Allami, Paulignan, Brovelli, & Boussaoud, 2008) and concur with the reviews of the mental practice literature conducted by Feltz and Landers (1983; Feltz et al., 1988; Lotze & Halsband, 2006). The results suggest that whenever possible, physical practice is preferable to mental practice for learning. However, when physically practicing a task is not possible, as when an individual is away from a clinical rehabilitation setting, then mental rehearsal is an effective method for augmenting learning (Dickstein & Deutsch, 2007; Mulder 2007).

Hypotheses About Mental Practice Effects

Why then, is mental practice effective for learning a motor skill? Certainly, one of the components of mental practice involves learning the cognitive elements in the task; that is, learning what to do (Heuer, 1985). Given the requirement of rehearsing mentally, the learner can think about what kinds of things could be tried, can predict the consequences of each action to some extent on the basis of previous experiences with similar skills, and can perhaps rule out inappropriate courses of action. This view suggests that not very much motor learning is happening in mental practice, the majority being the rapid learning associated with the cognitive elements of the task. Such a view fits well with data from Minas (1978, 1980), who used a serial throwing task in which subjects had to throw balls of different weights and textures into the proper bins. The main finding was that mental practice contributed to the learning of the sequence (the cognitive element) but did not contribute very much to learning the particular throwing actions (motor elements).

Another view, however, suggests that there is more to mental practice than the learning of the cognitive elements in a task. Some suggest that the motor programs for the movements are actually being run off during mental practice but that the learner simply turns down the “gain” of the program so that the muscular contractions are not visible. Research on so-called implicit speech, in which subjects are told to imagine speaking a given sentence, shows patterns of electromyographic activity from the vocal musculature that resemble the patterns evoked during actual speech. One possibility is that very small forces (not sufficient to cause movements) are produced and the performer receives Golgi tendon organ feedback about them (chapter 5), as the Golgi tendon organs are extremely sensitive to small loads. Another possibility is that the “movements” are sensed via feedforward and corollary discharge (i.e., “internal feedback”), generated when the motor programs are run off (chapter 5). Yet another possibility discussed earlier in this chapter is that planning a movement (which should be part of mental practice) is, in itself, beneficial to learning.

Most recent hypotheses about the effects of mental practice have focused on the specific role of imagery. Many researchers now agree that imagined actions share similarities with the actual movements being imaged. For example, performance times are similar for imaged and physically performed trials of the Fitts reciprocal tapping tasks with different indexes of difficulty (Cerritelli, Maruff, Wilson, & Currie, 2000; Decety & Jeannerod, 1996; Kohl & Fisicaro, 1995; Stevens, 2005). Similar effects are also observed in grasping tasks (Frak, Paulignan, & Jeannerod, 2001) and when a mass either is loaded or is imagined being loaded to a limb (Papaxanthis, Schieppati, Gentili, & Pozzo, 2002). Studies involving brain mapping techniques (chapter 2) also point to similar activation regions in the brain when movements are produced and imagined (Jeannerod, 2001; Jeannerod & Frak, 1999). Together with observations that apraxia patients sometimes fail to inhibit imagined movements (without awareness; Schwoebel, Boronat, & Branch Coslett, 2002), the findings suggest that imagery is a process by which actions are programmed as in normal movements but are inhibited from being executed. According to this view, at least some learning can be attributed solely to the motor programming process, in the absence of movement execution.

Distribution of Practice

Our discussion of on-task conditions of practice begins with one of the variables that instructors and therapists have under their control: the scheduling of periods of work (i.e., time spent in actual practice) and rest (i.e., time not practicing the task). This scheduling can be considered within the constraints of a short time frame, such as the amount of work and rest during a 45 min therapy session. Or the scheduling may be considered in terms of a longer time scale, as when one chooses the length and frequency of sessions per week. The question of importance concerns whether or not the frequency and length of rest periods have an effect on learning the skill being practiced in the work periods. What is the best way to distribute the time spent in work versus the time spent resting—or simply, what is the best practice distribution?

Defining “Massed” and “Distributed” Practice

Research on practice-distribution effects has frequently used the terms massed practice and distributed practice. In one sense, “massing” means to put things together—in this case, running work periods very close together with either no rest at all or very brief rest intervals between periods of work. By default, distributing practice means spacing these periods of work apart with longer intervals of rest. The labels are not truly satisfactory, however, because researchers often use these terms to describe the two extremes of practice distributions within a particular experiment, and because many experimenters use more than two distribution conditions (e.g., Ammons, 1950; Bourne & Archer, 1956). Thus, these terms must be considered relative to the context of other conditions within any particular experiment and relative to the context of other experiments.