Experiment 1: Visuomotor Training Improves Mental Rotation
In Experiment 1, we developed a novel training-transfer paradigm (Fig. 1a) by combining mental rotation (Fig. 1b) and visuomotor training tasks (Fig. 1c) to evaluate whether a separate domain-specific or common domain-general rotational operation drives mental and visuomotor rotation. The domain-specific hypothesis predicts no transfer across domains, whereas the domain-general hypothesis predicts that VMR training will facilitate the efficiency of mental rotation, but other motor tasks without rotation operations will not.
To evaluates these hypotheses, all participants performed four sessions: a pre-test of mental rotation, visuomotor training, a post-test of mental rotation, and visuomotor washout (Fig. 1a). The two mental rotation sessions were identical for all participants, requiring participants to determine whether a tilted asymmetric letter (e.g., R) was a normal or a mirrored letter (Fig. 1b). Therefore, it enabled us to evaluate how mental rotation performance was affected by the different visuomotor training tasks (VMR, control reach and control SRT tasks) (Fig. 1c). The visuomotor washout session was designed to confirm that the visuomotor training effects were maintained during the post-test of mental rotation.
During the visuomotor training session, participants (n = 19) were randomly assigned to the VMR (red border) group or the other two control groups (Fig. 1c): a reach task (blue border) or a sequential reaction time task (SRT; cyan border). These two control groups enabled us to estimate test-retest improvement in mental rotation and parse out contributions led by movements without rotational operation. In the VMR task, participants moved the cursor (small black dot) from the starting base (open circle) toward the target (big black dot). Each target direction was located in 45° increments from 0° (12 o'clock) to 315°, for a total of eight possible target locations. There were two types of trials. In rotation trials, the cursor direction (solid line) was rotated 45° clockwise (CW) from the hand trajectory (dotted line). This perturbation creates movement errors, forcing the brain to learn or update new sensory-motor relationships to reestablish appropriate motor control. In no-rotation trials, the cursor direction (solid line) normally followed the hand trajectory (dotted line). Participants performed one baseline block (80 no-rotation trials) measuring inherent bias in the reaching movement toward each target and four training blocks (80 rotation trials/block). In the control reach task, participants moved the cursor from the starting base toward the target and the cursor direction normally followed the hand trajectory. In the control SRT task, participants pressed a button to indicate the corresponding target location and learned a repeating sequence of button-press (see Methods: Exp.1 Tasks and Procedures for details).
As shown in Fig. 2, we evaluated the motor performance of three groups during the visuomotor training session. In the visuomotor rotation (VMR) group (Fig. 2a: red), reach error was reduced across trial blocks in the visuomotor training (F(5.52, 99.43) = 40.72, p < .001, η2 = .55), suggesting enhancement in visuomotor rotation. As a control, the control reach task without the rotated visual feedback (Fig. 2a: dark blue) showed uniformly small reach error (F(6.60, 118.7) = 0.75, p > .25). In the control SRT group (Fig. 2b), reaction time was reduced across blocks in the training session (F(2.55, 45.92) = 7.18, p < .001, η2 = .07). A post-hoc linear trend analysis of reaction time across blocks was also significant (slope = -9.37, R2 = .96, p < .001). These results indicated learning of the sequential motor skills.
Of primary interest was whether the mental rotation performance was differentially affected by VMR task training compared to the other control training tasks. To confirm all training groups performed the mental rotation task reasonably well, we first calculated the mean accuracy of the mental rotation task separately in the pre- and post-test sessions across the VMR (pre vs. post-test: 88.9% ± 1.4% s.e., vs. 90.2% ± 1.1%.), control reach (88.5% ± 1.2%.vs 91.4% ± 1.2%) and SRT groups (90.6% ± 1.6% vs 92.0% ± 1.6%). In a two-way repeated measures ANOVA, all participants showed higher accuracy in the post-test compared to the pre-test session (F(1, 54) = 16.14, p < .001, η2 = .02) regardless of visuomotor training groups (F(2, 54) = 0.516, p > .250), with no interaction effect (F(2, 54) = 1.309, p > .250). Therefore, the accuracy was equated across three groups. In addition, we confirmed that the accuracy of pre-test across the three groups were equated by a one-way ANOVA (F(2, 54) = 0.62, p > .250).
Then, we shifted our focus to the mean reaction times of the mental rotation task in the pre- and post-test sessions (Fig. 2c) and a RT difference between the two test sessions (Fig. 2d). Larger RT reduction indicates larger mental rotation improvement. As shown in Fig. 2c, all participants performed the mental rotation task significantly faster in the post-test compared to the pre-test session (F(1, 54) = 121.4, p < .001, η2 = .09) regardless of visuomotor training groups (F(2, 54) = 1.17, p > .250). More importantly, as shown in Fig. 2d, the VMR group (red) appeared to show significantly greater mental rotation improvement than the control reach (dark blue) and SRT (cyan) groups, leading to a significant interaction effect (F(2, 54) = 3.62, p = .034, η2 = .01). We confirmed such observation by a one-way ANOVA: RT reduction across the three groups showed a significant main effect (F(2, 54) = 3.62, p = .034, η2 = .01). Post-hoc independent t-tests show that the VMR group showed significantly larger RT reduction than the control reach (t(54) = 2.33, Dunnett-adjusted p = .022, d = .67) and SRT (t(54) = 2.33, Dunnett-adjusted p = .022, d = .72) groups. In addition, we confirmed that the RT of pre-test across the three groups were equated by a one-way ANOVA (F(2, 54) = 1.70, p = .193).
Note that by analyzing the motor performance of the three groups in the visuomotor washout session, we confirmed that participants maintained their motor learning until the end of mental rotation post-test. In the VMR group, the mean reach error of first trial block in visuomotor washout was − 25.77° ± 1.78° s.e. (Fig. 2a: red), demonstrating a strong after-effect of VMR adaptation and indicating that participants still maintained partial adaptation to the 45° tilted visual feedback until the end of the mental rotation post-test. In the control reach group, the reach error of first trial block was .30° ± .47° s.e. (Fig. 2a: dark blue), which was equivalent to reach error of last trial block (-.17° ± .40° s.e.) of the visuomotor training session (t(18) = 1.00, p > .025). This indicates that the control reach group maintained the same performance as in the visuomotor training session. In the SRT task, the averaged RTs for the repeated trials in visuomotor washout was 307.89 ms ± 14.23 ms s.e. (Fig. 2b), which was significantly shorter than the averaged RTs for the last training block (334.53 ms ± 13.47 ms s.e.) in the visuomotor training session (t(18) = 2.81, p = .006, d = .44) and the averaged RTs for the following random trials in visuomotor washout session (t(18) = 5.30, p < .001, d = 1.15). This indicates that the visuomotor learning in the SRT task was maintained until the end of mental rotation post-test.
In sum, we observed that after a short session of visuomotor rotation training, participants became faster in the mental rotation task compared to before, whereas training on the two control motor tasks without the operation of rotation resulted in significantly weaker transfer to the mental rotation task. Thus, Experiment 1 uncovered that VMR training could lead to improvements in mental rotation, in accord with the domain-general hypothesis.
Experiment 2: Mental Rotation Training Improves Visuomotor Rotation
In Experiment 2, we reversely examined whether mental rotation training enhances visuomotor rotation, while comparing it to other cognitive tasks without a rotation judgment. Such a bidirectional transfer between VMR and mental rotation provides strong converging evidence supporting a common domain-general operation. However, although we observed transfer from visuomotor rotation training to mental rotation in Experiment 1, if we do not observe the transfer in the opposite direction, it could indicate that rotation transfer is unidirectional from motor to cognition, or alternatively, visuomotor rotation requires more complex movement control beyond a rotational operation, and mental rotation training is not sufficient to modify the adaptative performance required for visuomotor rotation.
Here, all participants performed four sessions: pre-test of VMR, visuomotor washout, visual training, and post-test of VMR. During the visual training session, participants were randomly assigned to perform the mental rotation or the control color-discrimination task (see Methods: Exp.2 Tasks and Procedures for details). The two VMR sessions and visuomotor washout were identical for the two visual training groups. The washout session was designed to remove the adaptation to the 45° tilted visual feedback and brought back visuomotor performance at the baseline.
We confirmed that only the mental rotation task but not the control color-discrimination task involved mental rotation processes (Fig. 3a). In the mental rotation group, the RTs increased linearly with the rotation angles, suggesting that participants mentally rotated the letters from the rotation angle to upright. However, the RTs of the control color-discrimination task were the same across all letter angles, indicating that the task did not involve the mental rotation process. We performed a linear regression on the averaged RT across the absolute letter-rotation angles regardless of the rotation directions (clockwise vs. counterclockwise) for the mental rotation and control color-discrimination group, respectively. The linear regression slope is significantly different from zero in the mental rotation group (slope = 1.65, R2 = .12, p < .001) but not in the control color-discrimination group (slope = − .04, R2 < .001, p > .025). The accuracy of the mental rotation (90% ± 1.84% s.e.) and the control color-discrimination (91.54% ± 1.09% s.e.) task did not show a significant difference (t(34) = .72, p > .25).
Of interest was whether VMR performance is differentially affected by the two visual training tasks. Based on our previous work, the learning process of visuomotor rotation mainly happened in the first 10 trial blocks. We first calculated the reach errors of this period for the mental rotation group (Fig. 3b) and the control color-discrimination group (Fig. 3c) in the pre- and post-test sessions. Then we calculated the learning rate (LR) based on the reach errors in the pre- and post-test sessions (Fig. 3d) and the LR gain as a LR difference between the two test sessions (Fig. 3e). Larger LR gain indicates larger VMR improvement. The mental rotation group showed significantly larger LR gain than the control color-discrimination group.
A two-way group x test session repeated measures ANOVA shows a significant interaction effect (F(1, 34) = 5.15, p = .030, η2 = .06), but no main effect of group (F(1, 34) = 1.53, p = .224) or test session (F(1, 34) = .81, p > .25). An independent t-test of LR gain showed that the mental rotation group had a significant higher LR gain than the control color-discrimination group (t(34) = 2.27, p = .015, d = .76). Post hoc paired t-tests show that only mental rotation group (t(34) = 2.48, Šidák-adjusted p = .018, d = .84) had LR improvement between pre- and post-test sessions but not for the control color-discrimination group (t(34) = .73, Šidák-adjusted p > .250). By a post hoc independent t-test, we confirmed that the LR in the pre-test session between the two groups were not significant different (t(68) = 0.87, Šidák-adjusted p > .250).
We also compared visuomotor washout performance across the two groups to assure that the better VMR post-test performance in the mental rotation group was not induced by the less visuomotor washout effect. We calculated the washout effects based on the level of performance at the end of the washout block (see Methods: Exp.2 Data analysis for details). By an independent t-test, we confirmed that the washout effects between the mental rotation (4.33° ± 3.12° s.e.) and the control color-discrimination (6.26° ± 4.32° s.e.) groups were not significant different (t(34) = 1.54, p = .133).
In sum, we observed that after a short mental rotation training, participants became faster in visuomotor adaptation compared to before, whereas training on the control color-discrimination task without the rotation operation did not facilitate the learning rate of visuomotor rotation. Such selective VMR improvement by mental rotation training provides strong converging evidence supporting the domain-general rotational operation hypothesis with Experiment 1.