Figure 2 shows the results for Success Figure 2 (A), Fail Figure 2 (B), and Unsuccess Figure 2 (C) rates as performances. An ANOVA was conducted for each rate to ascertain which feedback contributed to task performance. The results Figure 2 (A) showed no significant interaction for the success rate (F = 1.34, p > .05), and the main effects were found for the vibration (F = 9.17, p <.001) and visibility (F = 124, p <.001) conditions. The post-hoc tests indicated that the success rate was significantly higher in the continuous-coded condition than in the other three conditions and that the success rate was significantly higher in the visible condition. However, a significant interaction was found for the FAIL rate (F = 3.61, p < .05) Figure 2 (B). The post-hoc test indicated significant differences between the visible and invisible conditions for each vibration condition (p < .01), but no significant differences were found between the vibration conditions for both visible and invisible conditions (p > .05), although continuous-coded conditions showed a relatively low failure rate. The results of the unsuccess rate showed no significant interaction (F = 0.45, p > .05) Figure 2(C), and the main effects were found for the vibration condition (F = 3.65, p < .05) but not for the visibility condition (F = 2.17, p > .05). The post-hoc test indicated that the unsuccess rate for continuous-coded was significantly lower than that for single-coded (p < .01).

Next, we investigated the trajectories of the holding force to determine whether the continuous-coded condition had an effect on maintaining a stable holding during the trials. Figure 3 (A) shows examples of the trajectories of one participant for each object’s weight in the invisible condition. With the continuous-coded vibration (red lines), the holding force initially fluctuated and later remained within a “stable hold” range (blue ranges). The total time at “stable hold” for trials other than failed trials was extracted as an index of control precision and is compared in Figure 3 (B). The continuous-coded group showed the longest “stable hold” time, and ANOVA showed a significant interaction (F = 5.55, p < .01). The post-hoc test showed that the time for the continuous-coded condition was significantly longer than for the no condition, regardless of visibility conditions (p < .01). The time for the continuous-coded condition was also longer than for the single-coded in the visible condition (p < .01). In addition, the duration of the single-coded condition was significantly longer than that of the no condition in the invisible condition (p < .05). The time in the visible condition was significantly longer than that in the invisible condition for all vibration conditions (p < .01).

To examine the reason why total “stable hold” time increased with the continue-coded, we additionally calculated the rate of returning to a “stable hold” state after transitions from “stable hold” to “overpowered” or “weak hold” (Figure 4). For the return rate from “overpowered” Figure 4(A), no significant interaction was indicated (F = 1.12, p > .05) and both main effects of vibration (F = 11.2, p < .001) and visibility (F = 10.2, p < .01) were confirmed. As a result of the post-hoc test, the continue-coded condition had a significantly high returning rate from “overpowered” than that in the none condition (p < .01), and the visible condition had a significantly higher rate than the invisible condition (p < .01). For the return rate from “weak hold” Figure 4 (B), a significant interaction was found (F = 8.06, p< .001). Although there was no difference in the visible condition, the rate from the “weak hold” of the none condition was significantly lower than that of the continuous-coded condition (p < .01) and the single condition (p < .001) in the invisible condition. For each vibration condition, the rate from the “weak hold” in the visible condition was significantly higher than in the invisible condition (p < .01).

In this study, we demonstrated that the proposed tactile feedback scheme based on continuous-coded vibration stimulation had an enhanced effect on catch-and-hold object task performance in a VR environment. The success rate of the task was significantly higher in the continuous-coded condition than in other conditions. However, the Fail (i.e., insufficient force) and Unsuccess (i.e., excessive force) rates did not show significant differences. The results indicate that the type of vibration stimulation affects the fine-tuning of holding force.

Looking at the force shift during the trial in more detail as shown in Figure 3 (A), the continuous-coded condition showed superior performance for total stable hold time than that for the none and single-coded conditions. In addition, the force was more stable in the stable-hold range under the continuous-coded condition. Therefore, it is considered that the continuous-coded vibration method can assist in quick adjustment to the appropriate state and assist in fine-tuning even within the same state to avoid shifting the state. The return rates to the stable hold from other statuses (weak-hold and overpowered) also showed the superiority of the continuous-coded condition, supporting the concrete effect of the proposed method in fine-tuning holding even under invisible conditions. The fact that single- and single-coded conditions did not have as positive an effect as a continuous-coded condition suggests that repeated stimulation made participants less likely to miss the information presented by the vibration and more likely to keep track of their own holding state. In feedback control, it is important to ensure the robustness of the information to determine the direction of correction. On the other hand, considering the requirements of Success not being an overpowered state, the stimulation method does not only affect the immediate recovery of the state but also the avoidance of state transitions before they occurred. It is possible that the continuous-coded vibration affected th feedforward coordination in the prediction of the initial response and adjustment amount.

The single condition that is based on the event-related method, for which performance improvements have been reported 20, 21, and 22, had only limited positive effects, with only a return rate from weak hold in the invisible condition. In our study, the task was longer and more complex than in previous studies. The differences in the tasks may have affected the attenuation of the support effect for controlling the holding force. Another study also reported a result in which the stimulation method based on the contact event had no significant effect in a task longer than five seconds, but the study reported no significant slip prevention effect when the method did not provide stimulation with an object slipped 28. The positive effect of a return rate from weak hold in an invisible condition with a single condition compared to a none condition Figure 4 (B) indicates that the single stimulation method allows for dealing with the particular situations to be avoided even if the task time is longer when slip events are also notified. On the other hand, the other performances, including the return rate from the overpowered state, did not show a pronounced effect of the single condition. The transition to weak hold and overpowered, in which the same stimulus was presented with a single condition, was difficult to distinguish in the invisible condition. As a result, it is considered that a movement toward recovery from weak hold, which leads to a critical situation of the object falling, was preferentially induced. This suggests that a simple feedback method, such as a single condition, may not be sufficiently effective for complex movements in which simple feedback and responses cannot be handled.

The single-coded condition added the information by vibration patterns to the single condition but did not show improved assistive effects compared to the single condition, contrary to our expectation. However, the condition showed a significant effect on the total stable hold time Figure 3 (B), a more fundamental index, in an invisible situation compared to the no condition. The increase in the total time of the optimal state despite the lack of a clear effect on either immediate correction or advance prediction of control suggests that the single-coded method promotes fine-tuning to prevent state transitions within the same state. Because this role is also observed for the continuous-coded condition, it is considered to be an effect of the stimulation coding method. However, the effect of the single-coded condition is limited to an invisible situation, perhaps because the complexity of the stimulus and low presentation probability result in less reliability for the vibrating stimulus, and the effect Is lost in a visible environment where multiple information presentations are mixed.

In summary, we found that the feedback method with continuous-coded vibration was effective for manipulation in tasks that were not completed by feedforward control. The method of coded vibration stimulation assisted in fine-tuning the manipulation by support in state awareness, and the addition of continuous stimulation to this ensured the robustness of feedback information. The results support both hypotheses of the assist effect of the increased information content of the stimulus and the certainty effect of the continuous stimulation; however, they also indicate that the increased information content does not function well on its own.