SUPPLEMENTAL MATERIAL: Colored context cues can facilitate the ability to learn and to switch between multiple dynamical force fields TouriaAddou, NedialkoKrouchev, John F. Kalaska Groupe de recherche sur le système nerveux central (FRSQ), Département de physiologie, Faculté de Médecine, Université de Montréal, C.P. 6128, Succursale centre‐ville, Montréal, Québec, CanadaH3C 3J7 Successful performance of catch trials Each subject experienced 320 catch trials out of the 5120 total successful trials in their 20 training sets.Half of the catch trials (160) occurred at the first trial of a 4‐trial block and 160 at the end of a block. In RC and URC subjects, half of the catch trials occurred in Va‐field blocks and the other half in Vr‐field blocks. For IRC subjects, half occurred when the monitor was the same color as for the corresponding field in RC and URC subjects, and half with the opposite color‐field pairing. Although the subjects unexpectedly encountered the N‐field during catch trials, they could still complete catch trials successfully if they were able to respect the time constraints required to attain and hold the target (movement duration 300‐400ms; target hold time 200ms). The probability of a successful catch trial was strongly dependent on the nature of the force field, the subject group and the position of the catch trial in the block (Table S1). These trends provide further evidence of how subjects adapted to the force fields, in particular the default‐response strategy of URC subjects in the first trial after the monitor changed color. The six IRC‐Va subjects successfully completed 303/960 catch trials (31.6%) encountered in blocks in which the Va field was paired with a green monitor screen. The success rate was equal for catch trials in the first trial position (154/480; 32.1%) and the last trial position (149/480; 31.0%). In contrast, IRC‐Vr subjects completed only 59/960 (6.1%) of their catch trials, again with similar low success rates in the first‐trial (30/480; 6.3%) and last‐trial (29/480; 6.0%) positions.The performance of the two IRC subject groups was qualitatively and quantitatively similar in the trial blocks with the opposite color‐field pairings (data not shown). The similarity of catch‐trial successes in the first trial and last trial of a block is not surprising since that distinction is completely arbitrary for IRC subjects, who only experienced one field and the monitor color changes were otherwise irrelevant. It is noteworthy, however, that IRC‐Va subjects were 5 times morelikely to complete an N‐field catch trialsuccessfully than IRC‐Vr subjects, even though both groups adapted quickly to their respective fields. Despite the fact that the initial catch‐trial movement component of IRC‐Va subjects was slower than normaldue to the absence of the expected assistive Va field, the movement duration window may have still allowed them to enter the targetbefore the end of the maximum 400ms movement timeand hold the target long enough to complete the trial successfully. The biphasic response of the velocity and force profiles of the catch‐trial responses of IRC‐Va subjects (Fig 6,10,12 of the main article) also suggested that they intervened voluntarily after a brief delay to compensate for the absence of the Va field, increasing the possibility of success. In contrast, the very rapid movement that IRC‐Vr subjects made in catch trials without the resistive Vr field (Fig 6,10,12 of the main article)may not have given them enough time to slow down the movement before attaining the target in less than the minimum 300ms movement duration allowed, or to hold the target for at least 200ms, or both. In summary, the performance constraints imposed by the task may have favoreda greater likelihood of a successful catch trial in the Va field environment compared to the Vr field. Another potential factor is the inherent instability of the Va field, in contrast to the stable Vr field.Subjects can alter the stiffness and impedance of their limb in response to unstable force fields (Franklin et al 2007)and can make adaptive changes to the gain of long‐latency reflex circuits (Akazawa et al 1983; Nichols et al 1999; Perrault et al 2008; Pruszynski et al 2009; Krutky et al 2010; Shemmell et al 2010). As a result, IRC‐Va subjects may have acquired an enhanced ability to use peripheral feedback to make rapid adjustments to their movements in the unstable Va field compared to IRC‐Vr subjects, further increasing their ability to complete catch trials successfully. RC subjects showed similar qualitative trends as the IRC subjects, with some small quantitative differences. RC subjects successfully performed 240/960 catch trials in the Va field (25.0%), with nearly equal success rates in the first‐trial (130/480; 27.1%) and last‐trial (110/480; 22.9%) positions. Although the RC subjects eventually performed the Va‐fielded trials almost as well as IRC‐Va subjects, they still showed a somewhat lower probability of successful Va‐block catch‐trial performance. Like IRC‐Vr subjects, the RC subjects showed a low catch‐trial success rate in Vr‐field blocks(101/960; 10.5%), including 77/480 (16.0%) first‐trial successes and only 24/480 (5.0%) last‐trial successes. Although they were more than twice as likely to make a successful first‐trial catch trial as IRC‐Vr subjects, their success rate was just as low as IRC‐Vr subjects in last‐trial catch trials, suggesting some adjustment of their motor output during the Vr block. This is consistent with the small changes in their velocity and force profiles observed between first‐trial and last‐trial catch trials of a Vr block (Fig 10e, 12e of the main article). The catch‐trial behavior of URC subjects was the most complex. Like IRC‐Va subjects, they showed a high success rate in Va‐block catch trials (288/960; 30.0%), with slightly more successes in first‐trial catch trials (161/480; 33.5%) than in last‐trial catch trials (127/480; 26.6%). They showed a lower success rate (187/960; 19.5%) in Vr‐block catch trials, but this was still substantially higher than IRC‐Vr or RC subjects in Vr‐field blocks. Furthermore, the large majority of the successful Vr‐block catch trials of URC subjects occurred in first‐trial catch trials (142/187successful catch trials; 75.9%). The much higher success rate in the first‐trial catch trials of Vr‐field blocks compared to the last‐trial catch trials, as well as in the first‐
trial catch trials of the Va‐field blocks, are most likely the result of the default‐response strategy of the URC subjects whenever the monitor changed color. As a result, the kinematics of their movements were not as perturbed by the unexpected presence of the N‐field in the first trial of a block after the monitor changed color compared to all the other subjects and they were more likely to complete the first‐trial catch trials successfully in either the Va‐field or Vr‐field blocks than RC subjects.The much lower catch‐
trial success rate in the last trial of a block, similar to that of IRC‐Vr and RC subjects, indicates that the URC subjects made Vr field‐specific changes only after the first trial in each Vr‐field block. Akazawa K, Milner TE, Stein RB. Modulation of reflex EMG and stiffness in response to stretch of human finger muscle.J Neurophysiol 49: 16‐27, 1983. Franklin DW, Liaw G, Milner TE, Osu R, Burdet E, Kawato M. Endpoint stiffness of the arm is directionally tuned to instability in the environment. J Neurosci 27: 7705‐7716, 2007. Krutky MA, Ravichandran VJ, Trumbower RD, Perreault EJ. Interactions between limb and environmental mechanics influence stretch reflex sensitivity in the human arm. J Neurophysiol 103: 429‐
440, 2010. Nichols TR, Cope TC, Abelew TA. Rapid spinal mechanisms of motor coordination.Exerc Sport Sci Rev 27: 255‐284, 1999. Perreault EJ, Chen K, Trumbower RD, Lewis G. Interactions with compliant loads alter stretch reflex gains but not intermuscular coordination. J Neurophysiol 99: 2101‐2113, 2008. Pruszynski JA, Kurtzer I, Lillicrap TP, Scott SH. Temporal evolution of “automatic gain‐scaling”.J Neurophysiol102: 992‐103, 2009. Shemmel J, Krutky MA, Pearreault EJ. Stretch sensitive reflexes as an adaptive mechanism for maintaining limb stability. ClinNeurophysiol 121: 1680‐1689, 2011. Table S1 Data sets, including only the 480 catch trials in each trial position in the same‐color trials for IRC groups: Field Va Vr Trial 1st last 1st last IRC‐Va 154 149 ‐ ‐ IRC‐Vr ‐ ‐ 30 29 RC 130 110 77 24 URC 161 127 142 45 Block‐wise learning curves To examine the time course of adaptation in greater temporal resolution, we calculated the mean error rate and velocity variability for each subject in each successive 4‐trial block in each field and then averaged across the six subjects in each group (Supplemental Figures S1 and S2).With the sole exception of URC subjects in the Vr field (see below), all subject groups showed progressive block‐wise improvements in task performance in the two training runs of the first day, further intra‐day improvements in the second day, and then progressively less or little intra‐day improvements in later training days, despite a gradual improvement across later training. Va field: IRC‐Va subjects showed clear intra‐day learning and inter‐day forgetting of the Va adaptationfor the first three training days, especially for the velocity variability (Figure S2a). Intra‐day improvements in performance were much less evident in the remaining training days. Nevertheless, the IRC‐Va subjects continued to start each day with 1‐2 blocks of trials with notablyhigher error rates and velocity variability across all 10 training days. This indicated that the IRC‐Va subjects always required a few trials of practice (“warm‐up”) in the unstable Va field each training day to recall the Va field‐appropriate motor skill, after completing the baseline N‐field set. A Va‐field warm‐up effect was also evident in the RC subjects but was less pronounced than in the IRC‐
Va subjects. This suggests that they were better able to recall the Va field‐appropriate motor skill at the start of the first training set in each training day, even though they were performing those blocks in alternation with Vr‐field blocks. Intra‐day improvement was evident only in the first two training days, while the only trend over the remaining training days was a gradual overall decrease in mean error rates and velocity variability across training days. The Va‐field warm‐up effect in URC subjects was intermediate between that in IRC‐Va and RC subjects, and was most evident in the velocity variability. As was the case for the other two subject groups, the URC subjects showed the largest intra‐day improvement in the Va field in the first training days, but continued to show intra‐day improvement for the first 5 days and even later. This was partly masked by greater inter‐block variability in URC subjects in any given day compared to RC and especially IRC‐Va subjects. Vr field: Warm‐up effects were far less pronounced in the Vr field for all subject groups (IRC‐Vr, RC, URC), indicating that they were better able to recall the Vr‐appropriate motor skill at the beginning of the first training set in each training day, than the Va‐appropriate skill. IRC‐Vr subjects showed clear intra‐day improvement in error rates in the firsttraining day and much more gradual improvements in later training days. They showed intra‐day improvement in the velocity variability during the first two training days and only very gradual decreases over the remaining training days. However, this was largely due to one subject who showed very high velocity variability during the first 2 days. The other 5 IRC‐Vr subjects showed very rapid adaptation during the first few blocks of the first training session and then continual gradual improvement over subsequent training sets (data not shown). In contrast, RC and URC subjects showed intra‐day improvements in error rates for the first two training days but showed improvements in velocity variability primarily only in the first day. In particular, URC subjects showed progressive improvement in velocity variability primarily during the first half of the first training session, but relatively little systematic reduction of velocity variability for the remainder of the training. Figure S1 Mean error rate for all successive 4‐trial blocks in the Va field (a‐c) and Vr field (d‐f) for IRC‐Va (a), IRC‐Vr (d), RC (b,e) and URC (c,f) subjects. Leftward triangles: first 4‐trial block in a daily training session. Rightward triangles: last 4‐trial block in a daily training session. Figure S2 Mean velocity variability for all successive 4‐trial blocks in the Va field (a‐c) and Vr field (d‐f) for IRC‐Va (a), IRC‐Vr (d), RC (b,e) and URC (c,f) subjects. Leftward triangles: first 4‐trial block in a daily training session. Rightward triangles: last 4‐trial block in a daily training session.