© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience

B R I E F C O M M U N I C AT I O N S

Random presentation enables
subjects to adapt to two
opposing forces on the hand
Rieko Osu1, Satomi Hirai1,2, Toshinori Yoshioka1 &Mitsuo Kawato1
Studies have shown that humans cannot simultaneously learn
opposing force fields or opposing visuomotor rotations, evenwhen provided with arbitrary contextual information, probably
because of interference in their working memory1–6. In
contrast, we found that subjects can adapt to two opposing
force fields when provided with contextual cues and canconsolidate motor memories if random and frequent switchingoccurs. Because significant aftereffects were seen, this study
suggests that multiple internal models can be acquired
simultaneously during learning and predictively switched,depending only on contextual information.
Over two consecutive days, ten subjects learned reaching movements toeight targets located radially from a central start position. The movements took place in either a clockwise or a counterclockwise velocitydependent rotational force field1,2,5 (Fig. 1), presented in a random order.
Each force field was repeated no more than 5 times (average, 1.76 times).
This randomized sequence is different from previous studies in which
two force fields were presented either in alternating blocks of many trials1
or alternating trial by trial5,7. Before reaching in the clockwise force field(RF), subjects were presented with a red background, a red windmill-like
diagram showing the direction and magnitude of rotational forces, and ahigh-frequency beep. Before reaching in the counterclockwise force field(BF), they were presented with a blue background, a blue windmill-like
diagram and a low-frequency beep. After a 2-s presentation of these cues,
one of the eight targets was randomly presented. Subjects were required
to start within 1 s and reach the target within 225 ± 50 ms (time between
exiting the start circle and entering the target circle) with straight anduncorrected trajectories. The distance between the starting point andeach target was 12.5 cm. Visual feedback of the hand position was suppressed during movements, but the entire hand path was shown after themovement terminated. One cycle consisted of 16 trials, including randomly ordered movements to all eight targets in the two force fields. On
both days, the subjects executed a block of 14 cycles. On day 1, anotherblock of 16 cycles was added, during which force fields were randomly(twice per cycle) turned off while audiovisual cues were retained, toexamine aftereffects2. Before testing, subjects were familiarized with thetask during a block of 192 trials without any force fields (null force field,NF; Fig. 2a).

Figure 1 Experimental procedure. See Supplementary Note online fordetails of force field (RF, BF) generation. ATR’s ethics committee approved
the experiments; subjects gave informed consent before participation.

During their initial exposure to the force fields, subjects’ hand trajectories were highly distorted and curved in the direction of the applied force(Fig. 2b). However, as they practiced, the curvature of the trajectory
decreased in both force fields (Fig. 2c). In the aftereffect trials, trajectory
deviations were generally in the direction opposite to those registered initially (Fig. 2d), suggesting that subjects compensated for imposed
dynamics in a predictive and feed-forward manner after 2 d of training.
Adaptation to each force field was quantified by the directional error,
which is computed as the signed area between the actual hand path andthe line joining the start and target centers, indicating the direction andamount of hand-path deviation from the straight line (deviation counterclockwise, +; clockwise, –). These directional errors were first averaged
across 16 movements within two consecutive cycles for each force fieldand each subject, then averaged across subjects3,6. We compared the temporal evolution of these averaged directional errors in the two force fields(RF and BF) with those of the aftereffect trials (Fig. 3a). On day 1, subjects initially produced large negative (clockwise) errors in the RF andlarge positive (counterclockwise) errors in the BF. After 11–12 cycles,
magnitudes of directional errors decreased, approaching those in the NF(black cross). When the same subjects were retested the next day, the initial directional errors were closer to those in the NF than those for early
cycles on day 1, indicating retention of learning effects.
To confirm statistically significant learning and retention, we computed the difference errors under the RF and BF over cycles 2 and 3 aswell as cycles 13 and 14 on the two days for each subject3,6. The difference
error is defined as the difference in the directional errors between the RFand BF (BF – RF), and indicates the average area enclosed by hand pathsin RF and BF. We averaged difference errors across all subjects (Fig. 3b).
A significant difference was found across the four difference errors1ATR Computational Neuroscience Laboratories, 2-2-2, Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan. 2Nara Institute of Science and Technology, 8916-5,Takayama-cho, Ikoma, Nara 630-0101, Japan. Correspondence should be addressed to R.O. (osu@atr.jp).

Published online 25 January 2004; doi:10.1038/nn1184

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Figure 2 Hand paths of a typical subject. (a) In the NF. (b) The second
cycle on day 1. (c) Last cycle on day 2. (d) Aftereffects. Red, RF; blue, BF.

(ANOVA F3,36 = 4.46; P = 0.0092), and a post-hoc test showed that the difference error of cycles 2 and 3 on day 1 were significantly larger than theother three (Tukey’s HSD test, P < 0.05), supporting the idea that learning occurred. No significant differences were found between the difference errors of the last two cycles on day 1 and cycles 2 and 3 on day 2,supporting retention of motor memory.
For each force field, the direction of hand-path deviation in the aftereffect trials was opposite to that during learning (Fig. 3a). We compareddirectional errors in the NF, the RF aftereffects (NF with RF cues), and theBF aftereffects (NF with BF cues). Although the force field was the samein these three conditions, the directional errors were significantly different (ANOVA F2,27 = 13.58; P < 0.0001). The post-hoc test showed that themean directional error was more counterclockwise for the RF aftereffectand more clockwise for the BF aftereffect, as compared to the NF (Tukey’s
HSD test, P < 0.05).
Simultaneous learning under alternating blocked schedules1–4,6 is possible if the orientation of the arm is altered between fields such that themapping between motor commands and force changes and arbitrary
visual or proprioceptive cues are ineffective1. Learning is very slow when
two force fields are alternated trial by trial, even when cues are provided7.
We evaluated whether richer contextual cues or random trial-by-trial
switching contributed more to simultaneous learning. The difference
error was significantly reduced after two learning days for six subjects
who were given only the color background cue (t-test, P = 0.025; Fig. 3c),suggesting little contribution from the richer contextual cues. To determine whether randomness, rather than frequency, of cue changes isimportant, six other subjects were exposed to two force fields alternatedtrial by trial with the full audiovisual cues. The difference error was not
significantly reduced after two learning days (t-test, P = 0.075; Fig. 3d).
No significant difference was seen for directional errors in the NF, the RFaftereffects or the BF aftereffects (ANOVA F2,15 = 1.68; P = 0.2199;
Fig. 3e). Subjects did not adapt well to alternating, and therefore frequent, but predictable force-field changes. Based on a previous study1and our failure to find a significant difference in errors between consecutive trials when the same force field was repeated two or three times(ANOVA; P = 0.807 for day 1, P = 0.832 for day 2), it is unlikely that having several repetitions of the same force fields within the random schedule contributes to the learning. We conclude that the random order,
which necessitates immediate feed-forward switching dependent on
audiovisual cues, was mainly responsible for successful learning.
Nevertheless, simultaneous learning was more difficult than separate
learning2. Four subjects who learned the two force fields separately
achieved an error level, after only 4 cycles, similar to that after 11–12
cycles in the main experiment.
By providing cues before movement, and by denying on-line visualfeedback, we demonstrated that subjects can predictively switch between
acquired motor programs. Simultaneous learning of two mutually conflicting tasks has been reported to be difficult3–6,8 or very slow7,9 or toinvolve subjects readapting9,10 rather than switching predictively.
Predictive switching has been demonstrated11, but not for multiple conflicting environments. With random-order presentation, humans canadapt to two conflicting environments not merely by using feedback control9,10 or impedance control12,13 but at least partly through simultaneous acquisition of multiple internal models and their predictiveswitching. The results support the MOSAIC model14 (see discussion15),
which proposed that many controllers are selected and learned while
gated partly by contextual information.
Note: Supplementary information is available on the Nature Neuroscience website.
ACKNOWLEDGMENTS
This research was supported by the Telecommunications Advancement
Organization of Japan and the Human Frontier Science Program. We thankT. Milner for improving the manuscript.
COMPETING INTERESTS STATEMENT
The authors declare that they have no competing financial interests.
Received 5 November; accepted 1 December 2003
Published online at http://www.nature.com/natureneuroscience/Figure 3 Learning curves. (a) Average directional errors and s.e.m. (dashed line)
for RF (red) and BF (blue) averaged across two cycles, and aftereffect trials (redand blue) vs. NF (black) in the main experiment. The directional error for NF
was positive as a result of the dynamics of the arm and PFM. Values averaged
across all subjects. (b–d) Average difference errors across all subjects ands.e.m. for main experiment (b), task using only background color cue (c) andtask using alternating schedule (d). (e) Average directional errors and s.e.m. foraftereffect trials (red and blue) vs. NF (black) in task using alternating schedule.

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