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The cost of simplifying complex developmental phenomena: a new perspective on learning to walk - PubMed

The cost of simplifying complex developmental phenomena: a new perspective on learning to walk

Do Kyeong Lee et al. Dev Sci. 2018 Jul.

Abstract

Researchers can study complex developmental phenomena with all the inherent noise and complexity or simplify behaviors to hone in on the essential aspects of a phenomenon. We used the development of walking as a model system to compare the costs and benefits of simplifying a complex, noisy behavior. Traditionally, researchers simplify infant walking by recording gait measures as infants take continuous, forward steps along straight paths. Here, we compared the traditional straight-path task with spontaneous walking during 20 minutes of free play in 97 infants (10.75-19.99 months of age). We recorded infants' footfalls on an instrumented floor to calculate gait measures in the straight-path and free-play tasks. In addition, we scored videos for other critical aspects of spontaneous walking-steps per bout, shape of walking path, and step direction. Studying infant walking during free play incurred no cost compared with the straight-path task, but considerable benefits. Straight-path gait was highly correlated with spontaneous gait and both sets of measures improved with walking age, validating use of the straight-path task as an index of development. However, a large proportion of free-play bouts were too short to permit standard gait measures, and most bouts were curved with omnidirectional steps. The high prevalence of these "non-canonical" bouts was constant over development. We propose that a focus on spontaneous walking, the phenomenon we ostensibly wish to explain, yields important insights into the problems infants solve while learning to walk. Other areas of developmental research may also benefit from retaining the complexity of complex phenomena.

© 2017 John Wiley & Sons Ltd.

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Figures

Figure 1
Figure 1

Distribution of infants’ walking age for (A) all 136 test sessions and (B) the subset of 30 infants in A who were tested repeatedly (69 sessions). The diagonals denote sessions with gait measures collected on the instrumented floor and verified with video. Bars with left diagonals denote sessions (N = 130) in which infants contributed useable gait data in the traditional straight-path task. Bars with right diagonals denote sessions (N = 57) in which infants contributed gait data during spontaneous walking in free play. Bars with both right and left diagonals denote sessions (N = 56) in which infants contributed gait data to both the straight-path and free-play tasks. Bars with no diagonals denote sessions (N = 5) in which infants did not contribute useable gait data on the instrumented floor in either task. Bar color denotes measures scored only from video during free play: Green bars denote sessions (N = 40) for which we scored bout length (steps/bout), path shape (straight/curved), and step direction (forward/not forward). Purple bars denote sessions (N = 29) for which we scored only bout length. Gray bars denote sessions (N = 67) for which we did not score bout length, path, or step direction during free play. Note, bar color refers only to the free-play task because coders scored bout length, path shape, and step direction for all 135 sessions in which we had useable data in the straight-path task.

Figure 2
Figure 2

Procedure and measures of walking. (A) Free-play task. Illustration of one infant’s spontaneous walking in the laboratory playroom filled with toys, couch, and elevations. Part of the floor (shown by rectangle) was instrumented. Infants were free to move or not as they wished; parents (shown sitting) were told to interact normally and mind infants’ safety. An experimenter (shown standing) followed discretely from a distance to record infants’ movements with a handheld camera. Footprints denote two sequences of the infant’s steps on the instrumented floor and smoothed lines show the infant’s path on the remaining floor area during free play. Play on elevations is not shown. (B) Straight-path task. Illustration of procedure for testing infants in the straight-path task on the instrumented floor. The experimenter placed infants at one end of the mat while parents at the other end urged infants to walk straight over using toys and snacks as incentives. The task was repeated as many times as necessary to obtain several useable sequences of continuous, forward steps along a straight path. (C) Path shape. Example of (#1) path coded as straight. Examples of curved paths due to: (#2) a twisting, serpentine change in direction, (#3) turning and stepping in place, (#4) angular, zigzag change in direction, and (#5) an otherwise straight path that began with a hook. To be coded as “curved”, infants had to change direction over at least two consecutive steps or step in place. Red brackets show curved paths containing straight segments with at least four steps. Note, for illustrative purposes, we built examplars in the figure from actual data from infants stepping on the instrumented mat. However, all path shapes were scored only from video because many bouts were not on the mat or were only partially on the mat. (D) Step direction. Examples of forward, backward, and sideways steps. Sequences could contain any combination of steps. Note, for illustrative purposes, we built exemplars in the figure from actual data from infants stepping on the instrumented mat. However, all step direction data were scored only from video because many bouts were not on the mat or were only partially on the mat. (E) Calculation of gait measures for sequences with at least four continuous steps. Step length is the front-to-back distance between steps; step width is the side-to-side distance between steps; and speed (not shown) is the time from the first to last steps. For the straight-path task, only forward steps were selected for calculating gait measures. For the free-play task, steps could be in any direction so distances were calculated as the absolute value.

Figure 3
Figure 3

Number of steps per bout. (A) Frequency distribution of bout length in free-play and straight-path tasks. Height of the blue bars denotes the number of walking bouts during free play, N = 69 sessions. Note, the x-axis excludes bouts between 31 and 105 steps. Height of the orange bars denotes walking bouts during the straight-path task (including short bouts not useable for gait measures); N = 135 sessions. (B) Average proportion of bouts of various lengths in free play task. Data are represented for novice (1–3 months), intermediate (4–6 months), and expert groups (7–10 months) by walking age; N = 69 sessions. (C) Frequency distribution of bout length (after processing) used for analyses of gait measures in free play and straight-path tasks. The y-axis shows the number of bouts. Height of the blue bars denotes bouts used for calculating gait measures during the free-play task. The left side of the distribution begins abruptly at 4 steps due to data processing constraints. The height of the orange bars denotes bouts used for calculating gait measures during the straight-path task. The distribution begins at 4 steps due to processing constraints and ends at 22 steps due to the length of the instrumented mat. Note, the distribution for free-play gait (N = 57 sessions) is partially hidden behind the distribution for straight-path gait (N = 130 sessions); the height of the hidden blue bars is denoted by the horizontal blue lines.

Figure 4
Figure 4

Mean proportion of curved and straight bouts during free play (N = 40 sessions). (A) Left-most bar shows all bouts and two right bars show bouts with 4–10 steps and 11+ steps, respectively. Note, bouts with 1–3 steps were too short to be scored for path shape as represented by the empty spot along the x-axis. Error bars denote standard errors. Blue bars denote curved paths and orange bars denote straight paths. (B) Scatterplot showing proportion of curved (blue symbols) and straight bouts (orange symbols) by walking age. Each symbol represents the proportion of curved or straight bouts in that session.

Figure 5
Figure 5

Mean proportion of bouts in the free-play task containing all forward steps, a mixture of forward and non-forward steps, and no forward steps (N = 40 sessions). (A) Left-most bar shows all bouts. Middle three bars show bouts with 1–3 steps, 4–10 steps and 11+ steps, respectively. Two right-most bars show step direction for free-play bouts scored as curved paths and straight paths, respectively. Error bars denote standard errors. (B) Scatterplot shows proportion of free-play bouts with all forward, mixed, and no forward steps by walking age. (C) Procedure and effects of progressively eliminating steps at the beginning and end of each free-play bout on proportion of omnidirectional steps.

Figure 6
Figure 6

Gait measures for (A) speed, (B) step length, and (C) step width. Scatterplots in left panel show improvements with walking age in the free-play task (blue symbols, N = 57 sessions) and straight-path task (red symbols, N = 130 sessions) for all bouts (open symbols) and the two fastest (closed symbols) bouts in each task of spontaneous walking and the straight-line paradigm; each symbol represents the average for an individual infant’s session. The scatterplots highlight the wide range in individual differences across walking age. Box plots in the middle panel show the 25th to 75th percentiles (box size), median (horizontal line), and ranges (whiskers) for each measure. Line graphs in the right panel show linear curve fits and correlation coefficients for the data in the scatterplots; blue lines denote free play, red lines denote the straight-path task, solid lines represent all sequences, and dashed lines represent the two fastest sequences.

Figure 7
Figure 7

Scatterplots showing correlations between straight-path and free-play tasks for (A) speed, step length, and step width with all sequences selected, and (B) speed, step length, and step width with the two fastest sequences selected. N = 56 sessions where infants contributed gait data to both tasks.

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