The results of Experiment 1 showed that the learning effect was better in the error-mixed condition. In Experiment 2, observing only error models had a higher deviation than the correct condition. These results showed that observing some error actions along with correct ones has an advantage in imitative learning, compared with observing only correct actions.
In Experiment 1, learners performed better when observing the error models along with correct actions than when observing correct models only, which is partially consistent with previous studies. In a previous study, in imitative learning, viewing actions performed by expert and novice performers was better for motor learning than viewing actions by experts only. . The novice model in the previous study was different from the error model in our study in that the novice model achieved the goal successfully with a poor process, while the error model did not achieve the goal. In our study, errors (4 N and 24 N) could certainly be judged as such because the output forces of the error actions were different from the target force (8 N).
Some studies reported worse results from observing novice models than skilled ones [12, 23]. It could be because these studies were conducted with between-subject designs; the participants observed only the novice model; they did not observe the skilled (correct) model. Therefore, the participants might not have been able to ascertain those aspects of the observational action that were worse and those that were done well. As imitation has an automatic characteristic [24–26], it was possible that the participants imitated the worse aspects of the observational action if they did not distinguish the error. In our study, the participants first imaged the correct model by watching the correct models in pre-training session. Then, in main training session, because 50% of the models were correct actions, the participants would have a standard to discriminate the error easily. This view was confirmed in Experiment 2. In main training session, the correct model was not provided to the participants. The retention 2 test showed decreased performance.
The deviation of retention 2 shows that the performance was better in the error-mixed condition, but the deviation of main training session showed that it was worse in the error-mixed condition. This contrary result might cause some confusion regarding the condition that was better for motor learning. However, motor learning is considered to produce relatively permanent changes in the capability for skilled behavior. The motor learning effect could be inferred from the performance, but it is not identical to the performance since performance could easily have been affected by many other factors [27–29], such as, temporary facilitation or inhibition effects from visual or auditory information, fatigue, and so on. As the setting of retention 2 was the same between the two conditions and without any feedback or task-related visual information, we believed that the result of retention 2 could more accurately reflect the learning effect. Additionally, the contrary result might be caused by the difference in execution conditions between the two conditions. This situation occurs because the correct action (congruent with the desired action) can facilitate action execution [30, 31]. In the error-mixed condition, the participants had less direct facilitation of correct observational actions, which might have led to worse temporary performance compared with the correct condition in main training session.
Greater processing of task-related information occurred in the error-mixed condition, which may have boosted the learning effect. According to goal-directed imitation theory, the imitator does not imitate the observed action as a whole, but rather decomposes it into separate aspects [26, 32, 33]. These aspects are hierarchically ordered, and the useful aspect is utilized by the imitator . When the participants observed error actions, they decomposed them into at least three aspects: error aspects (for example, the thickness of the sponge and the velocity of pinching), correct aspects (for example, the pinching direction and how to hold the pinch sensor), and meaningless aspects (for example, the black background). The error aspects were utilized to detect whether the observer had the same errors. If the participant identified the same errors, these errors would be inhibited [26, 35]. This process could be considered to promote the detection of self-generated error, which is related to motor learning [19, 36, 37]. The correct aspects were used to imitate. In addition to the memory representation of the pinching action, the participants achieved the goal. Meaningless aspects would not be processed specifically. When a participant observed the correct action, it would be decomposed into correct and meaningless aspects. The participants would not think more about what errors occurred in the correct action because they could identify the correct aspects from the instruction and the experience of pre-training session. The correct aspects would be utilized to imitate and enhance the representation of the pinching action [38–40]. However, when the participants did not know the tasks well, they may not have distinguished between correct aspects and errors.
In the error-mixed condition, the participants observed three types of actions, while in the correct condition, they observed only one type of action. In other words, the participants had two error actions processes and one correct action process in the error-mixed condition, but only one correct action process in the correct condition. More processes in the mixed condition might be the factor that promoted the learning effect [3, 12]. Some neuroimaging research also indicated that the error-mixed condition had more processes than the correct condition. These previous studies showed that when a participant watches an error action, more cortical activation is found [41–43].
This study had several limitations. First, we only compared the immediate retention learning effect; it will be necessary to investigate the transfer and retention effects of 24 hours or longer in the future. Second, we only assessed the force parameter, and did not assess kinematic parameters. Finally, we had only behavioral data to confirm the processing of errors made by others; thus, a neuroimaging study is also required.