Observation of an expert model induces a skilled movement coordination pattern in a single session of intermittent practice

Abstract

We tested how observation of a skilled pattern of planar movements can assist in the learning of a new motor skill, which otherwise requires rigorous long-term practice to achieve fast and smooth performance. Sixty participants performed a sequence of planar hand movements on pre-test, acquisition, post-test and 24 h post-training blocks, under 1 of 4 conditions: an observation group (OG), a slowed observation group (SOG), a random motion control group (RMCG) and a double physical training control group (DPTCG). The OG and SOG observed an expert model's right hand performing the study task intermittently throughout acquisition, RMCG observed random dots movement instead of a model. Participants in the DPTCG received extra physical practice trials instead of the visually observed trials. Kinematic analysis revealed that only in conditions with observation of an expert model there was an instant robust improvement in motor planning of the task. This step-wise improvement was not only persistent in post-training retests but was also apparently implicit and subject to further incremental improvements in movement strategy over the period of 24 hours. The rapid change in motor strategy was accompanied by a transient within-session increase in spatial error for the observation groups, but this went away by 24 h post-training. We suggest that observation of hand movements of an expert model coaligned with self-produced movements during training can significantly condense the time-course of ecologically relevant drawing/writing skill mastery.

Figure 1

(a) All trajectories (blue lines) and mean trajectory (black line) from representative subjects from the four groups (columns), for the five tests (rows). All graphs have the same scale. The x axis corresponds to left-right movements, while the y axis corresponds to forward-back movements. The red trajectories in the second row correspond to the expert’s video recordings shown to the OG and SOG groups. (b) Time-normalized tangential velocity profiles (blue lines) and mean tangential velocity (black line) from representative subjects from the four groups, for the five tests. All graphs have the same scale. The x axis is normalized time, while the y axis is the tangential velocity. The red velocity profiles in the second column correspond to those from the videos shown to the OG and SOG groups. Velocity profiles with 4 peaks indicate that the movement can be segmented into four movements between the corresponding points, whereas those with three peaks indicate that movements from A→B and B→C show a large degree of overlap or coarticulation.

Figure 2

Time-course of changes in movement duration. (a) The mean (across subjects) of the median of absolute movement duration per block (in s) and the distribution of individual scores at four time points – pre-training, training, post-training and 24 h post-training. Bars – SE; Dotted line – movement duration of the slowed down expert model (note, that all experimental groups produce faster movements than the slowed model), Dashed line – movement duration of the expert model (note that even participants of the OG do not reach the short movement duration of the expert model). (b) Same data as in (a) but normalized to the value from the first test session. (c) Block-by-block absolute duration during training. The slopes of the movement durations during training were not significantly different from 0. (d) Trial-by-trial changes of the movement duration in the first block. Data for all subjects is shown in Supplementary Fig. S1. Bars – SE.

Figure 3

Time-course of changes in accuracy. (a) median (across subjects) of the median of spatial error per block and the distribution of individual scores at four time points – pre-training, training, post-training and 24 h post-training. (b) Block-by-block spatial error during training. (c) Evolution of the spatial error during the first training block (median). Data for all subjects is shown in Supplementary Fig. S2. Bars – IQR.

Figure 4

Simulation of movements with varying overlaps and subsequent values for the coarticulation score and the curvature measure. Simulations of 551 overlap values were performed, at equally spaced intervals from 0 overlap to 55% overlap. The movements were generated by assuming the superposition of four minimum jerk submovements, with a given amount of overlap between the first and second, and third and fourth submovements. The location of the first and third intermediate targets was set using non-linear optimization such that the superposition of the trajectories passed through the required points. The code for generating this simulation can be found. In (a) and (b), every 10^th trajectory is shown. (a) Trajectories of the simulation at varying levels of submovement overlap, ranging from 0 overlap (in blue) to 55% overlap (in red). (b) Normalized tangential velocity profiles, following the same colour scheme. (c) Predicted coarticulation measure as a function of movement overlap. (d) Predicted curvature measure (in cm) as a function of movement overlap.

Figure 5

Time-course of changes in coarticulation ability, using three measures. (a) Mean (across subjects) of the absolute coarticulation score per block and the distribution of individual scores at four time points – pre-training, training, post-training and 24 h post-training. Dashed line –coarticulation score of the expert model. The grey circles indicate data for all subjects. (b) Evolution of the absolute coarticulation score block-by-block during training. The slopes were positive and significantly different from zero only for the OG and SOG groups. (c) A further zoom in on the first block. Data of all subjects can be found in Supplementary Fig. S2. (d) Mean (across subjects) of the path offset measure, (e) Zoom in. on the first block of the path offset measure, (f) A further zoom in on the first block, data for all subjects can be found in Supplementary Fig. S3. (g) The same data as in (a), but normalized to the value from the first test session, and the distribution of individual scores. (h) The same data as in (d), but normalized to the value from the first test session. (i) Mean number of velocity peaks. Dashed line - the number of peaks produced by the expert model. In all graphs, bars – SE.

Figure 6

Performance at two transfer conditions (mirror-reversed and scaled layout) in comparison to trained condition performance at the 24 h re-test. (a) Relative improvement of the movement duration. (b) Relative improvement in the coarticulation score. (c) Relative improvement of the path offset measure. (d) Mean number of velocity peaks.

Figure 7

Experimental protocol. Day 1 included: Pre-test of performance, Training and Post-test of performance, Day 2 included: Consolidation performance test and two transfer tests – Mirror Reverse order of targets and Scaled (reduced distance between targets) tests.

Figure 8

Explanation of coarticulation and computation of the coarticulation score. (a–c) Show three examples of overlap of submovements (in blue). The observed movement is the sum of these submovements (dashed black line). When there is some overlap (b), the height of the trough increases. With sufficient overlap (c), the trough disappears, although we still observe an inflection point (marked as a triangle). (d–f) show three representative performance trials from different stages of learning. The coarticulation score was defined as the ratio of the mean value of the tangential velocity troughs (marked by squares) to the mean value of the tangential velocity peaks (marked by circles). When a peak/trough “disappears” as in (f), the peak and trough are replaced in the calculation with the inflection point. In example (d), showing a single pre-training trial, there is very little coarticulation (score = 11), whereas in example (e), showing a single training trial, there is a much higher amount of coarticulation (score = 46). Example (f) shows a single post-training trial featuring only 3 peaks, i.e., greater coarticulation (score = 66).