General Materials and Methods
All subjects were normal adults recruited from among the staff and students of the institutions at the Texas Medical Center, with no history of otologic, neurologic, or orthopedic disorder. Subjects who wore corrective lenses used them during training and testing and had at least 20/40 vision with their corrective lenses. These experiments were approved by the Institutional Review Board for Human Subject Research for Baylor College of Medicine and Affiliated Hospitals. Subjects gave informed consent prior to participation.
In both experiments, subjects were randomized to training groups in which they wore either: 1) three different visual distortion lenses, 2) a single pair of visual distortion lenses, or 3) sham (clear) lenses. The lenses were all set in lightweight, black, plastic, safety goggles that accommodated eyeglasses but eliminated 25° of peripheral vision to each side. In both experiments, each group received a different type of visuomotor transformation during training by wearing the lenses. The lenses were as follows: 1) clear plastic with no special optical properties (sham), 2) X 2.0 magnifying lenses, 3) X 0.5 minifying lenses, 4) up/down reversing lenses.
In both experiments subjects were pre-, post-, and retention-tested with the Functional Mobility Test (FMT), using an obstacle course in a 5.5 X 6.7 m room. The base of the course was 10 cm thick, medium density foam (5 lb/ft3
, Sunmate foam with skin-soft coating, Dynamic Systems, Inc., Leicester, NC). The compliant foam changes continually as the individual stands on it, making the support surface unreliable. The foam was used to make proprioceptive information unreliable during ambulation. It had an added benefit for safety: if anyone had fallen it would have provided a soft landing. The use of an obstacle course has been validated for clinical testing in evaluation of elderly people with balance impairments (20
Obstacles included two pleated fabric curtains suspended from the ceiling, pairs of Styrofoam blocks (96.5 cm (height) X 40.6 cm (width) X 10.2 cm (thickness) per block), 8 pairs of colorful, inflated, polyethylene, sand-weighted children's punching bags, (also called “bop” bags), 0.9 to 1.4 m high and 0.3 m diameter, four 15-cm spots on one side of the course that gave auditory cues (buzzers) when subjects walked on them, and a low Styrofoam block (193 cm X 20.3 cm X 10.2 cm) placed on either end of the foam section in which the buzzers were located. Instructions were to walk through the course as rapidly as possibly without touching any obstacles but to step on the buzzers. The dependent measures were the time (sec) to traverse the course timed with a stopwatch (time), and the number of obstacle errors, i.e. the number of obstacles touched plus the number of buzzers missed (obstacles). This test was slightly modified from one that has been described elsewhere (21
). See .
Figure 1 Plan view of the FMT obstacle course. Modified from Moore et al (18). Used with kind permission of Springer Science and Business Media.
For both experiments two pre-test trials were done without lenses. (A trial was one round of walking through the obstacle course.) Subjects were tested on two post-test trials, 2 to 4 days after the last training session, and two retention trials, two weeks after the post-tests. During the post-test and retention trials subjects wore 20° shift right lenses that they had never worn before, to expose them to a novel visuomotor transformation. Technicians who were blinded to subjects' group assignments administered all tests. Inter-rater reliability, using 10 subjects who did not otherwise participate in the study, was: time, r = 0.98, p< 0.0001; number of obstacles touched, r = 0.9, p< 0.001.
Experiment 1: Treadmill Training
In daily life, people must adapt their motor skills to novel situations for which they have not trained. Therefore, this study trained subjects on a locomotor task different from the test task. After pre-tests subjects were trained by walking on a treadmill. This training task differed from the test task in 3 important respects. 1) The surface of the obstacle course was compliant but the surface of the treadmill was noncompliant. 2) When traversing the obstacle course the subject followed a complex spatial trajectory, including turns, but the treadmill training involved no such trajectory. 3) Visual cues during the FMT were salient to the task. During locomotor treadmill training, visual stimuli were not matched to walking velocity. The use of a treadmill also had a practical basis; many training environments have space for a treadmill and safety harness but not for a walking track with a safety harness on a moving track. This experiment tested the idea that treadmill training would carry over to over ground locomotion.
Eighty subjects -- 54 females, 26 males, mean age 31.4 yrs (S.D. 10.1) -- were randomly assigned to one of 5 training groups: sham lenses (n = 20), multiple lenses with the magnifying, minifying, and up/down lenses (n = 20), single lenses with magnifying lenses (n = 20), single lenses with minifying lenses (n = 10) and single lenses with up/down reversing lenses (n = 10). After pre-tests subjects had five 20-min training sessions, given over one week, on the treadmill (Quinton Model Q55) at 3 km/hr, and simultaneously watched videotapes on a screen at eye level 2 m in front of them. The videotapes were popular films chosen by subjects. While watching videotapes they wore lenses for three 6-minute intervals with two 1-minute inter-interval breaks. During the break subjects in the multiple lens group switched lenses. Subjects in the other groups stopped walking and lifted up their lenses.
Experiment 2: Wobble board training
Experiment 2 was done to determine if adaptive generalization could still be elicited when the locomotor aspect of training was eliminated and balance training was constrained to standing on a wobble board. This question is practical. Many clinicians who provide balance therapy do standing balance training to improve walking balance skill, which may not be the best strategy. Instead of treadmill walking subjects stood on a wobble board, which challenged their balance on a noncompliant surface but without locomotion.
Sixty-three subjects -- 34 females, 29 males, mean age 27.4 yrs (S.D. 6.9) – had not participated in Experiment 1. They were randomized to 3 groups with 21 subjects per group: sham, multiple lenses with the magnifying, minifying and up/down lenses, and single lens practice with magnifying lenses. The experiment used a plastic wobble board (Flaghouse Balance Disk, 9 cm high, with a rounded base such that it tapers gradually from 39.5 cm diameter on top to 3 cm diameter at the base), which has multiple degrees of freedom. The lack of a stable support point challenged the subject's dynamic standing balance during training. The subject stood on the board with both feet, wearing a safety harness in case of falls, but the harness did not provide support. The training schedule, lenses, videotapes and FMT from Experiment 1 were retained.
Learning effect between trials
Paired t-tests were performed across the two trials for time and obstacle errors (obstacles) during each of the three test sessions (pre-test, post-test, and retention test) to determine if the first and second trials performed at each test session differed significantly. All statistical tests were performed with the SPPS statistical software v11.0 and significance level was set at alpha = 0.05.
Effects of experimental manipulations
A univariate ANOVA was performed for the pretest obstacles and time to determine if these variables differed significantly for the two between-subject factors, Experiment and Lens Group (Experiment had two levels – treadmill and wobble board; Lens Group had three levels –sham, single lens group with magnifying lenses, variable lenses). Significant differences at the pretest would have warranted use of the pretest value as a covariate in further analyses.
Repeated measures analyses of variance (RMANOVA) were used to test the time and obstacle data (one within subject factor – Session, three levels: pretest, post-test and retention; two across subject factors: Experiment and Lens Group: Experiment had two levels – treadmill and wobble board; Lens Group had three levels – sham, single lens group with magnifying lenses, variable lenses). and give descriptive statistics for Experiments 1 and 2, respectively.
Data from Experiment 1: Treadmill training.
Data from Experiment 2: Wobble board training.
Effect of single lens training over multiple lens training
To determine if training using different single lenses was as beneficial as training with variable lenses repeated measures ANOVAs for time and obstacles were also performed separately for the treadmill training experiment (Within subject factor – Session, three levels: pretest, post-test and retention; Between subject factor – Lens Groups: five levels – sham, single lens group magnifying, single lens group minifying, single lens group up/down, and variable lens group).