Participants

A total of 50 undergraduate students participated in the study. There were 30 participants (aged 28.5 ± 6.6 years; 13 females, four left-handed) in Experiment 1 and 20 (aged 27.5 ± 3.0 years; 10 females, all right-handed) in Experiment 2. All participants were screened to rule out medication use, a history of neurological or psychiatric disorders, head trauma, substance abuse, or other serious medical conditions. The study was approved by the Ethics Committee of the Medical Faculty of Tohoku University. Written informed consent was obtained from all the participants before the study.

Apparatus and video stimuli

As shown in Fig. 1, a custom-made pinching device was used, which consisted of a sponge (Ishihara Co. Ltd, Japan) and a pinching sensor (H500 Hand Kit; Biometrics Ltd, UK). The pinching device was connected to an E-Link (Biometrics Ltd., UK) that amplified the signal from the force sensor to a Powerlab (Adinstruments Ltd., USA) to transform analog data into digital data. The Powerlab system was used to measure the peak force in each trial using the LabChart software. A 15.6-inch notebook personal computer (Apple 107T5; refresh rate = 85 Hz) was used to present the visual stimuli to the participants. The experiment was programmed and controlled by a handmade software using MATLAB (Version 2010a; Math Works Inc., USA).

Fig. 1
figure1

Three types of observational pinching actions. The images show the modal compaction point of each type of pinching action video. The wave on the right-side image shows the force change during the pinching action on the left side. Actions displayed in the video: upper, 4 N; middle, 8 N; down, 24 N. The force of the 8 N action was used as the target model, while the other actions were used as error models

A total of three types of pinching action at different peak forces (4, 8, and 24 N) were performed by an experimenter and recorded with a high-speed camera (CASIO EX-F1, Max 1200 fps) to create the observational models. Each pinching action could easily be discriminated by the thickness of the sponge compressed by the pinching (see Fig. 1); that is, the compressed thickness under 4 N was almost 3/4 of the thickness of the original sponge, while 8 N was almost 1/2, and 24 N was almost 1/4. The participants were asked to pinch a force sensor for 3 s. The target force was 8 N in all conditions and was almost at 10–15% of the maximum voluntary contraction of pinching action in healthy adults, as assessed in the pre-experiment.

Participants sat on a chair in front of a table on which they placed their right or left hand, with their fingers in the pinching position. The distance from the participant’s eyes to the screen of the personal computer was 65 cm. The height of the chair was adjusted to make the participants comfortable.

Procedures

The experiment consisted of four sessions: “pre-training,” “pre-test,” “main training,” and “post-test” (Table 1).

Table 1 Designs of Experiments 1 and 2

There were three conditions that were different during the main training: error-mixed, correct only, and error only conditions. In the error-mixed condition, the observed action trials consisted of 25% of 4 N, 50% of 8 N, and 25% of 24 N. In the correct condition, only 8 N was used. In the error condition, 4 N and 24 N were 50% each.

Before the experiment, we explained the experimental protocol to the participants in detail. Participants were evaluated several times to make sure they understood the method thoroughly. In the “pre-training” session, the participants were instructed to pinch a force sensor at the peak force of 8 N 10 times while observing the pinching action performed by an experimenter at 8 N on video. In both the “pre-training” and “main training” sessions, an experimenter provided auditory feedback regarding their pinching force, which was “stronger than the target force,” “right force,” or “weaker than the target force” (Fig. 2a).

Fig. 2
figure2

a The protocol used during pre-training and main training session. The participant was instructed to pinch with a peak force of 8 N while they watched the pinching action in the video. They received information regarding their results from auditory feedback when the green bar was shown on the screen. In the pre-training session of both the conditions and the main training session of the correct condition, the observed pinching action was 8 N only. In the main training session of the error-mixed condition, the observational actions were 4, 8, and 24 N, which were shown randomly. b Protocol used during pre-test and post-test session. Participants were instructed to pinch with a force of 8 N as they learn in the training session. No information regarding the results was provided

In the “pre-test” and “post-test” sessions, participants were instructed to pinch at the peak force of 8 N 20 times without viewing a video or receiving feedback about their force (Fig. 2b), to measure the learning effect by comparing the pre- and post-test results.

The “main training” session had three conditions: correct model, error model, and error-mixed model conditions. The “main training” session comprised five blocks, with 20 trials in each block. In the correct model condition, participants were instructed to pinch at 8 N 100 times while observing pinching actions at the peak force of only 8 N. In the error model condition, participants were instructed to pinch while receiving auditory feedback regarding their force while observing pinching at 4 N or 24 N (each force video appeared 50% of the time). In the error-mixed model condition, they pinched while randomly observing pinching at a peak force of 4, 8, or 24 N (see Fig. 2a). The correct range of peak force was from 7.6 N to 8.4 N, which was 8 N ± 5% × 8 N.

There were two minutes between the sessions, and one minute between the blocks. All participants pinched using their left and right hands based on whether they took part in Experiment 1 or 2, and counterbalancing was conducted between handedness (dominant and non-dominant) and the conditions (error-mixed and correct in Experiment 1; error and correct in Experiment 2).

Experiment 1

In Experiment 1, error-mixed and correct conditions were given to the participants in the main training session.

Experiment 2

In Experiment 2, the participants were given the error and correct conditions, as described in Table 1. The experimental setting was the same as in Experiment 1, and pinching at error forces 4 N and 24 N appeared randomly at 50% rate.

Data analysis

To measure the learning effect of the pinching force, we calculated the root mean square of the deviation between the peak and target forces (8 N). For each trial:

$$mathrm{Deviation }= sqrt{{(peakforce-8)}^{2}}(mathrm{Unit}:mathrm{ N})$$

The experimental design was a 2 (timing: pre and post) × 2 (model: either error-mixed in Experiment 1 or error model in Experiment 2 vs. correct model) repeated measures analysis of covariance (rm ANOVA). To analyze the learning curve in the main training session in both Experiments 1 and 2, we also conducted a 2 (models: either error-mixed in Experiment 1 or error in Experiment 2 and correct model) × 5 (times: first to fifth blocks in the main training session) rm ANOVA. In the post hoc test, multiple comparisons with Bonferroni correction were performed; the represented p value was the adjusted value. Data are shown as mean ± Standard error (SE). The significance level was set at p < 0.05.

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