Locomotor sequence learning in visually guided walking

Abstract

Voluntary limb modifications must be integrated with basic walking patterns during visually guided walking. In this study we tested whether voluntary gait modifications can become more automatic with practice. We challenged walking control by presenting visual stepping targets that instructed subjects to modify step length from one trial to the next. Our sequence learning paradigm is derived from the serial reaction-time (SRT) task that has been used in upper limb studies. Both random and ordered sequences of step lengths were used to measure sequence-specific and sequence-nonspecific learning during walking. In addition, we determined how age (i.e., healthy young adults vs. children) and biomechanical factors (i.e., walking speed) affected the rate and magnitude of locomotor sequence learning. The results showed that healthy young adults (age 24 ± 5 yr,n= 20) could learn a specific sequence of step lengths over 300 training steps. Younger children (age 6-10 yr,n= 8) had lower baseline performance, but their magnitude and rate of sequence learning were the same compared with those of older children (11-16 yr,n= 10) and healthy adults. In addition, learning capacity may be more limited at faster walking speeds. To our knowledge, this is the first study to demonstrate that spatial sequence learning can be integrated with a highly automatic task such as walking. These findings suggest that adults and children use implicit knowledge about the sequence to plan and execute leg movement during visually guided walking.

Keywords: human; learning; locomotion; vision; walking.

Fig. 1.

A: visual feedback was projected on a screen in front of the treadmill. A 12-marker setup was used for motion capture. MT, metatarsal. B: target (open square) and foot position of the swing leg (filled circle) were displayed during the walking task. The targets moved down the screen at a speed corresponding to the treadmill speed (arrows). Stance leg foot position (open circle) was not visible on the display. The vertical distance between the current target (red square) and the next target (gray square) indicated the desired step length (1). The position of the swing leg appeared after ipsilateral toe off (2). The current target turned from red to white color on a successful hit (3). Scores were displayed on the upper right corner of the screen. The current target and foot position disappeared after ipsilateral heel strike (4). C: the order of random (R1–R3) and sequence blocks (S1–S4) in the sequence learning paradigm. Each block consisted of 100 steps. Sequence-specific learning was calculated as the difference in performance between S3 and R3 (solid line); nonspecific learning was calculated as the difference between R2 and R3 (dotted line).

Fig. 2.

Locomotor sequence learning. Each sequence consisted of 6 steps, and each block consisted of 16 complete sequences. A: performance in a typical subject over 7 blocks. Score (left y-axis, maximum score = 100) is the total number of hits in each block (top x-axis). Success rate (right y-axis) is the fraction of hits within each 6-step sequence (bottom x-axis). Score (bar graph) and success rate (line graph) for the same subject are overlaid on the plot to show variability within each block. B: mean success rate for each sequence averaged across subjects (n = 10). Shaded area of the curve represents standard error (SE). C: mean score (number of hits) over 7 blocks of testing averaged across subjects (n = 10). D: nonspecific learning (R3 − R2) and sequence-specific learning (S3 − R3) for individual subjects. E: group average mean error (n = 10) in the anterior-posterior (AP) direction, calculated as the foot position (5th MT) relative to the target position. Error was normalized to each subject's leg length. F: group average cadence (n = 10). *P < 0.05; ns, not significant.

Fig. 3.

Locomotor sequence learning in healthy adults vs. children. A: mean success rate for each sequence averaged across children in two age groups: 6–10 yr (n = 8) and 11–16 yr (n = 10). Shaded area of the curve represents SE. B–D: group average mean score (B), normalized AP error (C), and cadence (D). Bars represent average data across 7 blocks of testing within each age group: 6–10 yr (n = 8), 11–16 yr (n = 10), and adults (n = 10). E–G: individual data for each child plotted against age. Pearson's correlation (r) is shown for baseline score (E), nonspecific learning (F) and sequence-specific learning (G). *P < 0.05.

Fig. 4.

Locomotor sequence learning at different walking speeds. A: mean success rate for each sequence averaged across subjects for two walking speeds: slow (2.3 ± 0.16 km/h) and fast (3.2 ± 0.16 km/h). B–D: group average mean score (B), normalized AP error (C), and cadence (D). Bars represent average data across 7 blocks of testing at 2 different walking speeds: slow (n = 10) and fast (n = 10). *P < 0.05.