Learning Effects of Repetitive Administrations of the Sensory Organization Test in Healthy Young Adults

Article Outline

• Abstract
• Methods
  • Participants
  • Posturography Test Procedure
  • Testing Procedure
  • Data Analysis
• Results
  • Learning Effect
  • Test-Retest Reliability
  • Retention
• Discussion
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Wrisley DM, Stephens MJ, Mosley S, Wojnowski A, Duffy J, Burkard R. Learning effects of repetitive administrations of the Sensory Organization Test in healthy young adults.

Objectives

To evaluate the learning effect of multiple administrations of the Sensory Organization Test (SOT) on performance and to begin to establish clinical meaningful change scores for the SOT.

Design

Descriptive case series.

Setting

University-affiliated clinic.

Participants

Healthy young adults (6 men, 7 women; mean age, 24±4y).

Intervention

All subjects performed the standardized SOT using the SMART EquiTest 5 times over a 2-week period, and 1 month later.

Main Outcome Measure

Composite and individual SOT test condition standardized equilibrium scores.

Results

Test-retest reliability (intraclass correlation coefficient model 2,3) of the composite (.67) and equilibrium score (range, .35–.79) were fair to good. Repeated-measures analysis of variance revealed a significant (P<.05) increase in the composite and equilibrium scores for conditions 4, 5, and 6 over the 5 sessions that plateaued after the third session, and were retained at 1 month. The 95% confidence interval for the composite score change from session 1 to session 4, the plateau of the learning effect, was 3.9 to 8.1.

Conclusions

Although the findings of this study would indicate that multiple baseline measures are desirable for the more challenging conditions, a composite change of greater than 8 points would indicate change due to rehabilitation.

Key Words: Balance, Learning, Posture, Rehabilitation, Reproducibility of results

HUMAN POSTURAL CONTROL has been studied in both healthy subjects and patients with a variety of pathologic conditions to understand the neural mechanisms (sensory, motor) involved in a relatively automatic, yet complex, task, and to develop training and rehabilitation tools to avoid or correct deficits in the system. One means of taxing the postural control system is through changing the characteristics of the support surface. Platform perturbations allow for the investigation of both the sensory influences and interactions on balance control and the motor outcomes.¹, ², ³, ⁴ Deviations in performance seen in those with pathologic conditions has allowed the development of rehabilitation techniques in order to increase stability in these populations and therefore decrease the likelihood of falls.⁵

The ability to maintain an upright posture within a given sensory environment (balance), is dependent on sensory information to detect the position and movement of the body so that appropriate movement responses can be generated.⁶ The Sensory Organization Test (SOT) of Computerized Dynamic Posturography was developed in order to identify the relative contribution of the 3 main sensory systems involved in balance (vision, vestibular, somatosensory).⁶ The test attempts to isolate the various sensory contributions by either removing or distorting (via sway-referencing the visual surround or the surface platform) the visual and/or somatosensory inputs to the postural-control system. The resultant 6 conditions progress from the most stable (eyes open, solid support surface) to the least stable (sway-referenced vision and surface). The 6 conditions of the SOT are described in table 1. Moderate to good test-retest reliability for the SOT over 2 sessions 1 week apart has been shown.⁷, ⁸, ⁹ Scores on the SOT allow the differentiation between subjects with normal and abnormal vestibular function, as well as identifying older adults at risk for falls.⁸, ¹⁰

Table 1. The 6 Conditions of the SOT

The SOT paradigm has been used to show the altered use of sensory input in multiple populations, including older adults,¹¹, ¹² young children,¹³, ¹⁴ and people with vestibular dysfunction,¹⁵, ¹⁶ peripheral neuropathy,¹⁵ and Parkinson’s disease.¹⁷ Clinically, several studies have used this protocol pre- and postintervention during different treatment scenarios. Improved SOT scores have been shown in patients with Parkinson’s disease¹⁷ and osteoporosis¹⁸ after balance training, and in patients with central and peripheral vestibular dysfunction after vestibular rehabilitation.¹⁹

However, it has been reported that repeated exposure to a given perturbation of the postural control system allows for learning of more efficient postural strategies to maintain balance both within a session and over time.²⁰, ²¹, ²² Even in simple quiet stance with eyes closed, a progressive reduction in sway area and sway path was seen with repetition,²³ with the authors suggesting that the body learns to move to a “safer” position that results in less energy expenditure by reducing the amount of sway. This has also been shown in more complex balance tasks, such as standing with a narrow base of support,²⁴ or with vibration to the ankle tendons.²⁰, ²⁵ If repeated exposure to given perturbations leads to an improvement in postural stability, this may serve as a confound to the use of the SOT to measure improvement in balance in intervention studies.

Ford-Smith et al⁷ evaluated the test-retest reliability of the SOT in noninstitutionalized older adults. They found moderate test-retest reliability (intraclass correlation coefficient [ICC], .66 for composite score) when 2 sessions of the test were administered 1 week apart. The ICC for the individual conditions ranged from .26 to .68 with improvements from the first to second session of up to 18%. Tsang et al⁹ found moderate to good reliability of the individual SOT conditions with ICC scores ranging from .72 to .93. No reliability statistics were presented for the composite score. This moderate reliability of individual conditions and the composite score illustrates that additional study is needed to identify the true test-retest reliability of the SOT, and to differentiate the normal learning of the task over time from improvements in balance due to interventions. Therefore, the purpose of this study was to see what learning effects are seen in SOT scores over multiple test sessions in healthy young adults and to determine if this improvement is retained over time. An additional purpose was to begin to establish a framework on which the clinically significant change in SOT scores could be determined.

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Methods 

Participants 

We recruited 13 young healthy subjects (6 men, 7 women; mean age, 24±4y) for this study. Young adults were included in this study because we anticipated that they would show the smallest learning effect and would allow us to estimate the minimum change necessary for determining change due to rehabilitation. The sample size of 13 participants provide a power of .95 for detecting a large effect size (F=.40) using repeated-measures analysis of variance (ANOVA) with a correlation of 0.5 between sessions.²⁶ All participants were university students ranging in age from 21 to 36 years old. All subjects met the following inclusion criteria: (1) no reported history of neurologic, orthopedic, or muscular injury, (2) normal bilateral lower-extremity strength assessed via a manual muscle test, (3) able to stand independently for 20 minutes, (4) able to stand on toes and heels, and have normal functional range of motion in ankles, knees, and hips, (5) have normal somatosensory function in the feet as measured by the Weinstein Enhanced Sensory Test²⁷ and vibration sensation at 128Hz, (6) no history of neck injury, whiplash, or current complaints of neck pain, and (7) not currently on any medication that might affect balance. Subjects were asked to refrain from alcohol consumption for 48 hours before any test session and alcohol consumption was monitored through subject self-report. The study was approved by the Health Sciences Institutional Review Board and informed consent was obtained from each person. All subjects completed a general health questionnaire, the Dizziness Handicap Inventory (DHI)²⁸ and the Activities-specific Balance Confidence (ABC) Scale²⁹, ³⁰ to ensure that they met the inclusion criteria. The DHI is a 25-item questionnaire that quantifies a person’s perceived handicap due to dizziness. It is scored on a 0 to 100 scale with higher scores indicating greater perceived handicap. The DHI has high reliability and validity.²⁸ All subjects included in this study scored a 0 on the DHI. The ABC is a 16-item questionnaire that requires the subject to rate how confident they are that they will not lose their balance on 16 tasks ranging from walking around the house to walking on icy sidewalks. The ABC is scored on a percentage scale, with 100% indicating that subjects are completely confident that they will not lose their balance and 0% indicating that they feel they will lose their balance during that task. ABC scores for the subjects participating in this study were all greater than 95%, with a mean of 98%±2%.

Posturography Test Procedure 

All subjects completed the standardized SOT on the NeuroCom Smart EquiTest.^a The SOT consists of 6 sensory conditions (table 1). Subjects completed 3 trials for each of the 6 sensory conditions during each session. Subjects stood on the platform in bare feet with the feet placed 5.7cm apart and the medial malleolus aligned with the axis of platform rotation. Foot position was marked on the platform to allow consistency between trials and sessions. Subjects wore a harness that attached overhead and prevented falls but did not limit sway. Subjects were asked to stand quietly with their arms across their chest and their eyes open or closed (depending on the condition). An examiner remained stationed behind each subject for safety throughout the test.

Testing Procedure 

Each subject completed 5 testing sessions over a 2-week period with a retention test 1 month later (1 subject completed only the initial 5 testing sessions and did not return for the 1-mo follow-up session). Subjects had a mean of 1.9±1.1 days of rest between the 5 test sessions and a mean of 29.2±4.4 days between the test sessions and the retention test. Subjects completed the SOT according to the manufacturer’s instructions, with 3 repetitions of each condition in each session. Each trial lasted 20 seconds. The conditions were performed in subsequent order 1 through 6 for each of the 6 sessions. No subjects fell during the performance of any condition. A computer-generated equilibrium score was calculated for each trial using the formula provided by NeuroCom. The equilibrium score is a percentage that compares the subject’s anteroposterior center of pressure sway (in degrees) with the theoretical limits of stability, that is, maximum peak to peak sway of 12.5° (8° forward, 4.5° backward).³¹ The equilibrium scores range from 0% to 100%, with 100% indicating perfect stability and 0% indicating a fall. The 3 trials for each condition were averaged for an equilibrium score for each condition. A composite score, the mean of the average equilibrium score for all trials of conditions 1 and 2 and the 3 trials of conditions 3 through 6,³¹ was also computer generated based on a formula from NeuroCom for each session and was used for data analysis. Therefore, the composite score is a weighted average that emphasizes the more difficult balance conditions.

Data Analysis 

We analyzed differences in equilibrium and composite scores across the 6 sessions using repeated-measures ANOVA. In order to perform post hoc testing and to determine the effect of trial, a multivariate analysis with trial and session as factors, and subject and days of rest as co-factors was also performed. Post hoc tests included the Bonferroni test to determine differences between sessions and the Dunnett t to determine which sessions were significantly greater than the first session. Significance level was set at P less than .05. Test-retest reliability was quantified using intraclass correlation coefficient model 2,3 (ICC_2,3). ICC_2,3 was used as 3 trials of each condition were averaged for the individual condition and the composite scores. Using the Fleiss criteria,³² we define ICC values of less than 0.4 having poor reliability; .40 to .75 fair to good reliability, and scores above .75 as having excellent reliability. Paired t tests were performed between scores for session 5 and 6 to determine if the learning effect was retained at 1 month. All analysis was completed using SPSS.^b

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Results 

Learning Effect 

A significant difference was found in equilibrium scores between all 3 trials in all sessions for conditions 4, 5, and 6 but not for conditions 1 through 3. All subjects achieved normal scores on the SOT and all subjects had an increase in their composite score in session 5 compared with session 1. The mean composite score and the 95% confidence intervals (CIs) across the 6 sessions are displayed in figure 1. A significant learning effect was shown across the 6 sessions for SOT conditions 4 through 6 equilibrium scores and the composite score, but not for conditions 1 through 3. Mean and standard deviations (SDs) for the equilibrium scores of individual conditions and the composite scores are listed in table 2. On post hoc testing for condition 4 and 5, significant differences were found between sessions 1 and sessions 3 through 6, and sessions 2 and sessions 3 through 6; for condition 6, significant differences were found between session 1 and sessions 2 through 6; and for the composite score, significant differences were found between session 1 and sessions 2 through 6, and session 2 and all other sessions. Equilibrium scores were significantly greater in sessions 3 through 6 than in session 1 for conditions 4 and 5 and significantly greater in session 2 through 6 than in session 1 for condition 6 (table 3). The composite score was significantly greater in sessions 2 through 6 than in session 1. To determine the amount of change necessary to indicate improvement beyond learning of the task, 95% CIs of the change from session 1 were calculated for each condition and the composite score (see table 3) for each session. The upper level of the CI for the composite score change for session 4, the plateau of the learning effect, was 8.1, so an improvement of greater than 8 points on the composite score would be considered a change greater than the learning of the task.

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• Fig 1. 

  Change in SOT composite scores over the 6 sessions, which plateau at the fourth session. Note that the y axis has been collapsed.

Table 2. Mean Equilibrium Scores^⁎ on Individual SOT Conditions and Composite Scores^† for Each Session

NOTE. Values are mean ± SD.

Table 3. Increase in Equilibrium and Composite Scores Between Session 1 and Subsequent Sessions

Condition	Session
	2	P	3	P	4	P	5	P	6	P
1: Eyes open; firm surface	0.1±1.6(−0.9to1.1)	.81	0.9±1.9(−0.4to2.1)	.26	0.6±1.5(−0.3to1.6)	.37	0.7±1.7(−0.4to1.8)	.36	0.4±1.8(−0.8to1.5)	.58
2: Eyes closed; firm surface	1.4±2.0(0.1to2.7)	.19	1.5±2.3(0.0to3.0)	.14	0.7±2.1(−0.6to2.0)	.60	0.9±1.7(−0.2to2.0)	.33	1.1±2.4(−0.5to2.6)	.35
3: Sway reference vision; firm surface	1.5±4.4(−1.2to4.3)	.22	1.1±4.2(−1.7to3.8)	.42	1.9±4.6(−1.0to4.9)	.09	1.9±4.7(−1.1to4.9)	.10	2.5±4.4(−0.2to5.3)	.04
4: Eyes open; sway reference surface	3.1±7.4(−1.7to7.8)	.09	4.6±5.9(0.8to8.3)	<.01	5.3±6.6(1.1to9.4)	<.01	6.6±6.2(2.7to10.5)	<.01	5.7±6.3(1.7to9.7)	<.01
5: Eyes closed; sway reference surface	2.8±3.5(0.6to5.0)	.07	6.9±3.6(4.6to9.2)	<.01	9.4±4.3(6.6to12.1)	<.01	9.2±4.3(6.5to11.9)	<.01	9.3±4.6(6.4to12.2)	<.01
6: Sway reference vision and surface	7.0±6.2(3.0to10.9)	<.01	9.7±5.8(6.0to13.3)	<.01	10.5±5.8(6.8to14.1)	<.01	12.5±6.8(8.2to16.8)	<.01	13.1±6.9(8.7to17.5)	<.01
Composite score	3.0±2.8(1.2to4.9)	<.01	4.8±2.5(3.2to6.4)	<.01	6.0±3.3(3.9to8.1)	<.01	6.4±3.3(4.3to8.5)	<.01	6.8±3.4(4.6to8.9)	<.01

NOTE. Values are mean ± SD (95% CI). Significance level (P value) from the Dunnett t that scores are significantly greater than the score on that condition for session 1.

Test-Retest Reliability 

Fair to good test-retest reliability was found for the SOT composite score from session 1 to session 2 with an average of 1.7±0.9 days in between testing was .67 (ICC_2,3). Individual equilibrium scores for all conditions except condition 3 (stable surface, sway referenced vision) were also fair to good, with scores ranging from .43 to .79 from session 1 to session 2. Condition 3 showed poor test-retest reliability with an ICC_2,3 of .35.

Retention 

The majority (10/12 [83%]) of subjects either retained or improved their performance on the SOT composite score when tested 1 month later (session 6). Five subjects retained their score, 5 subjects improved their score (average, 1.6 points) and 2 subjects had a decrease in score (average, 2 points). All composite SOT scores were greater in sessions 5 and 6 than in session 1. The subject and number of days rest between sessions as covariate yielded no statistical difference. No statistical differences between session 5 and 6 in individual conditions or composite score were found using paired t tests.

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Discussion 

This study showed, for the first time, the learning effect seen with repetitive SOT administrations. To account for this learning effect, it could be recommended that clinicians administer multiple baseline sessions to establish a steady performance prior to administering an intervention in order to document changes due to rehabilitation. However, the reality of clinical practice may not support the use of multiple baseline administrations and the use of SOT testing may act as training for patients with balance dysfunction. The administration of multiple baseline trials may also promote the learning effects shown in this study. Therefore, we have attempted to establish change values that would represent the amount attributable to learning. This is important, because it provides guidelines for interpreting the improvement in SOT scores after an intervention. Based on the results of this study we recommend using the criterion of 8 point improvement in the composite score for indicating improvement beyond the learning of the task. If previous outcome studies are reexamined using this criterion, we find greater support that balance function improved after intervention. Badke et al¹⁹ show a mean change of 8.5 points (from 42.6 to 51.1) in the composite score after vestibular rehabilitation in people with peripheral and central vestibular dysfunction. This change was not found to be statistically significant (P=.08), but using the criteria presented here it would show minimal change slightly beyond that expected for learning. Sinaki and Lynn¹⁸ found changes of 13 to 37 points in 3 women with osteoporosis after an exercise and posture training support intervention whereas the 2 women who received only the exercise intervention showed a change of −2 and 3 points in their SOT scores.

The clinical relevance of this change score needs to be established. Further research is needed to determine the magnitude of change that would correlate with improvements in functional status. The magnitude of the learning effect may also be different for various populations. We expect that older adults will show a greater learning effect when performing the SOT and that they may require additional sessions for the learning effect to plateau.

Significant learning effects occurred for SOT conditions 4, 5, and 6 over the 5 sessions, whereas conditions 1 through 3 did not. Conditions 4 through 6 are more complex postural tasks and are thought to require vestibular information to maintain upright. Other studies have shown that with repeated exposure to a given perturbation or balance activity, performance on that task improves; this improvement is greatest in more complex tasks such as standing with narrow base of support and with sensory conflict such as vibration.²⁰, ²¹, ²², ²⁴ Greater learning may have been seen in conditions 4 through 6 because of the complexity of the task, the novelty of the task, or the dependence of those tasks on vestibular information. It would be interesting to determine if the same level of learning is seen with people with visual or proprioceptive deficits or in people with highly trained vestibular systems (ie, ice skaters or dancers).

It has been suggested that at least 2 processes of adaptation occur with learning a novel postural task.²⁰, ²⁵ One is short-term adaptation that reduces the postural sway by either changing the postural strategies (ie, increasing the stiffness in the ankles) or through reweighting of sensory information. Short-term adaptation was observed in this study with the equilibrium scores of individual trials improving within each session. Another form of adaptation is the long-term process; the profound effect previous experience of the postural task has on the development of a strategy for maintaining postural control.²⁵, ³³, ³⁴ The majority of the subjects in this study retained or improved their composite scores on the 1 month follow-up session. The fact that 42% (5/12) of the subjects improved their performance after a 1-month rest indicates further adaptation.

The SOT composite and equilibrium scores showed fair to good test-retest reliability according to the criteria by Fleiss³² when tested in healthy young adults with 1 to 3 days between tests. This is in agreement with the Ford-Smith et al⁷ results from repeated administrations of the test 1 week apart in noninstitutionalized older adults, but is lower than the test-retest reliability found by Tsang et al⁹ when testing 12 older adults 1 week apart. Surprisingly, the 2 conditions with the highest test-retest reliability are the hardest conditions for subjects to perform and have the greatest variability. The ICCs for the other conditions may have been lower due to a lack of variability. Although not ideal, this level of test-retest reliability has been considered acceptable for clinical tools.⁹, ³⁵, ³⁶ When used in combination with the improvement criteria discussed above, this reliability should allow for accurate interpretation of the SOT scores.

The test-retest reliability of the SOT composite score between sessions 4 and 5 and sessions 5 and 6 were excellent, further substantiating that the plateau in performance of the SOT occurs after the fourth administration of the test and that the improvements are retained even after a month. Variability was seen across trials and testing sessions, most likely due to learning. This learning effect manifested itself as lower standard deviations for conditions 4 through 6 across sessions and in lower ICC values from session 1 to 2 compared with the ICCs from session 4 and 5 or 5 and 6.

Study Limitations 

One limitation of this study is that it was performed with healthy young adults. It is interesting that even active young adults can improve their balance function on this clinical assessment tool. However, the study findings need to be expanded to other groups. Older adults and people with sensory and motor pathologies may show a different learning effect. Additional studies are also needed to determine the sensitivity and specificity of the criteria introduced.

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Conclusions 

Healthy young adults show a learning effect when provided with 5 repetitions of the SOT over a 2-week period. The learning effect appears to plateau around session 3 and 4, and occurs primarily in the composite score and conditions 4 through 6. Multiple baseline measures of the SOT should be administered in order to document change due to rehabilitation. As an alternative, improvements of more than 8 points in the composite score indicate recovery beyond the effect of adaptation to the SOT itself. This improvement criterion provides an additional method for assessing outcomes in intervention studies.

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36. VanSwearingen JM, Paschal KA, Bonino P, Yang JF. The modified Gait Abnormality Rating Scale for recognizing the risk of recurrent falls in community-dwelling elderly adults. Phys Ther. 1996;76:994–1002
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• a NeuroCom International Inc, 9570 SE Lawnfield Rd, Clackamas, OR 97015.
• b Version 13; SPSS Inc, 233 S Wacker Dr, 11th FL, Chicago, IL 60606.