DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 1
Running Head: DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS
Dual Task Cost in Persons with Multiple Sclerosis: A Systematic Review
Anna Jordan Wetzel
University of Illinois Urbana-Champaign
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 2
Introduction
Multiple Sclerosis (MS) is one of the most prevalent neurodegenerative diseases in young adults
(What is Multiple Sclerosis? 2010). There is a scientific interest in Multiple Sclerosis and its associated
complications due to its degradation of neurological function and the cascade of events that follows.
The disability associated with MS is believed to result from the myelin sheath, or protective fat covering
nerve cells, being destroyed by the body’s own nervous system leading to the formation of scar tissue,
or sclerosis, and damage is additionally caused to the nerve itself that leads to a series of associated
complications. The destruction of nerve cell bodies causes an interruption of signals sent through the
body from the brain to the spinal cord adversely impacting cognitive and motor function.
Cognition and MS
In Neuropsychology of Multiple Sclerosis, Calabrese (2006) focuses on how there is a notably
large cognitive cost apparent in individuals diagnosed with MS. The emergence of new technology, such
as brain-imaging techniques that permit more accurate lesion quantification, has allowed several new
studies to be conducted that examine the cognitive degradation experienced by MS patients. More than
half of MS individuals complain of decreases and alterations in their typical cognitive function with
noticeable changes specifically in their attention and information processing, memory, mental flexibility,
and visuoconstruction (Calabrese, 2006).
Depreciation in attention and information processing is detectable by a delay in reaction times
to various stimuli. Explanation for these delays is controversial, but proposed possibilities include too
excessive of a demand on the MS patient’s memory load and/or a limited capacity for information that
can be stored in their short. Most importantly, these delays and deficits seem to appear quite early in
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the progression of disease. They are thought to be an important cause in further problems and
complications that arise as a result of the disease worsening.
Perhaps the most commonly affected area of cognition impaired by the neurological
deterioration from MS is a decrease in memory functioning. An important thing to note about memory
impairment caused by MS is that only certain components of memory are markedly affected. Encoding,
which encompasses recognition memory, is not nearly as deteriorated as the ability of the MS patient to
recall or retrieve information. Impairment occurs in working memory, specifically in regards to verbal
skills, everyday memory function including long-term memory, and in the performance of demanding
effortful tasks. However, demanding automatic tasks, implicit learning, short-term memory, and
recognition memory as previously noted have not been found to be weakened by MS at this point in
research (Calabrese, 2006).
Finally, damage to the typical function of MS patient’s cognitive or mental flexibility is
associated with MS. The term mental flexibility is an umbrella expression for the areas of planning,
feedback employment, concept development, and problem solving skills. Calabrese (2006) tested these
areas and found impairment of all of the mentioned with the most noteworthy effect on conceptual
analysis.
Balance/Gait and MS
Balance and walking are also impaired from MS. It is estimated that about 85% of MS patients
have some level of gait disturbance and more than a third of these individuals lose the ability to walk
within a two year time span (Givon, Zeilig, & Achiron, 2009). A wide-range of studies have been
conducted to examine exactly how and what areas of gait are impacted by the exacerbating effects on
the central nervous system caused by MS. Although different tests were implemented in multiple
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experiments, knowledge of the most commonly used assessment of gait with Multiple Sclerosis, EDSS, is
vital. The Expanded Disability Status Scale is designed specifically for MS individuals to assess
measurements of motor impairment. This form of evaluation is used both clinically and experimentally
(Martin, 2006).
A multiplicity of symptoms caused by MS contribute to an impairment of gait. In a study
conducted by Givon et al. (2009) that looked at independent walkers with MS for a short duration of
time, motor weakness, sensory disturbances, ataxia, and spasticity were deemed the main contributors
to gait degradation. An EDSS assessment further revealed that individuals diagnosed with MS had a
general decrease in their velocity, cadence, and step length (Givon, 2009). Additionally, they were found
to have asymmetrical gait, their flexion of the knee and hip were increased, as well as their overall
sagittal range of motion within the hip (Givon, 2009). Finally, the MS patients also had a decrease in
sagittal ankle range of motion according to the EDSS (Givon, 2009). These deficiencies are likely due to
the deterioration of motor neurons from the disease destroying the protective myelin sheath and
damaging the nerve cell body itself. Coordination is hindered causing the delayed motions and shorter
steps seen through testing. Conclusively, there was a direct and positive correlation between disease
length and the extent of gait deterioration (Givon, 2009).
Impairment in walking can be caused by a combination of pyramidal track, cerebellar, and
proprioception involvement in MS patients (Thoumie, 2005). An important distinction exists between
these causes in Multiple Sclerosis because of the different effects each has on a patient’s gait. Givon et
al. (2009) found that differences exist between the groups in gait and motor production based on
whether they were affected by pyramidal track or cerebellar involvement. Martin, Phillips, Kilpatrick,
Butzkueven, Tubridy, Mcdonald, & Galea (2006) observed that there was evidence of pyramidal
involvement even in subjects that were clinically tested as negative for this attribute. Therefore, data is
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inconclusive towards the exact causes of each area, but it can be concluded that they each have a varied
effect on the individual diagnosed with MS that is in need of further investigation.
Even at the onset of the disease when neurological effects are rather mild, MS patients will
exhibit slowed gait and stride length, as well as difficulty in recruiting the proper ankle muscles leading
to restricted motion at this joint (Martin, 2006). In fact, mildly impaired MS patients were described as
displaying a decrease in their hip, knee, and ankle strength, as well as a slower gait velocity than a
healthy control group individual (Thoumie, 2005). Highly impaired patients had a further degeneration
in their flexor strength, but their decreased gait function was primarily accounted for by a decrease in
the strength of the extensors (Thoumie, 2005). Throughout all stages of MS progression, it was noted
that bilateral impairment existed in some form. The gait velocity and overall ability of the patients is
highly correlated to the termination of the ability to send muscle commands because of damage to
nerve cells.
The inability of the muscle to deliver these commands negatively correlates with the strength of
an individual’s muscles. The previously mentioned study by Thoumie (2005) found that extensor and
flexor muscles were quite pivotal as compensatory mechanisms as muscle strength and ability
degenerated throughout the course of the disease. The use of flexor and extensor muscles helped an
MS patient maintain a higher level of gait function, and as these muscles deactivated, gait would
inevitably dramatically decrease (Thoumie, 2005)
Cognition, Gait, and MS Interaction
There is growing evidence that there are significant interactions between cognitive and motor
dysfunction in MS. The functional outcomes of dual task cost are still unknown, but the general
assumption is that it is related to adverse events such as falls. One method to assess the cognitive-motor
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interactions is with a dual task paradigm, which involves performing cognitive and motor tasks
separately and simultaneously. A decrease in performance of either task is considered dual task cost.
Due to the negative impact that MS has on visual, vestibular, and somatosensory input integration, the
likelihood of dual task interference is high, and thus the risk for falling is elevated. This report is a
systematic review of research concerning dual task costs in the MS population.
Dual Task Cost and Posture
Negahban, Mofateh, Arastoo, Mazaheri, Yazdi, Salavati, and Majdinasab (2011) discuss the
importance of both the cognitive and motor impairment caused by MS while investigating for the
interaction that exists in a dual task scenario. 23 individuals with MS, mean age 32.7 (+/-7.9) years,
were compared to a healthy gender, age, height, BMI, education level and MMSE score-matched control
group, mean age 31.4 (+/-7.9). Disability was evaluated with EDSS with a mean of 2.5 in the sample.
Cognitive function was evaluated with the MMSE with a mean of 29.0 in the MS sample and 29.3 in
healthy controls. Four postural tasks with increasing difficulty were tested. In the easiest condition
where participants stood barefoot on a force platform, on a rigid surface with eyes open, a rigid surface
with closed eyes, and a foam surface with closed eyes. COP data was recorded. The cognitive task
utilized was silent backward counting. For this task, a random number was given and backward counting
was performed. The score for the cognitive portion was based on the final number reported by the
subject and the number of subtracting items determined the score.
The MS group had higher postural sway, mean total velocity, sway area, and variability of sway
compared to control in single and dual task conditions,. In rigid surface open eyes and foam surface
closed eyes with a concurrent cognitive task, two conditions that limit visual and somatosensory
systems, a significant decrement in sway velocity variability (p <.01) existed in the anterioposterior
(Rigid/eyes open SD velocity (cm/s): 1.36 vs. 1.43, Foam/eyes closed SD velocity (cm/s): 2.23 vs. 2.54)
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 7
and mediolateral planes (Rigid/eyes open SD velocity (cm/s): 1.51 vs. 1.69, Foam/eyes closed SD velocity
(cm/s): 2.35 vs. 2.56) within the MS subjects compared to single task condition. This was not observed in
the control group. The results suggest that different responses occur in persons with MS compared to
healthy individuals during a dual task and these differences are more pronounced in the most difficult
postural conditions. In order to determine the DTC for this experiment, we computed the DTC for sway
velocity for the MS group and control group. Overall, it was found that dual task cost was higher in MS
compared to control for rigid closed (MS: 23.2% vs. Control:10.5%) and foam closed conditions (MS:4.9%
vs. 2.1%). This suggests that people with MS have a higher DTC for more posturally challenging tasks and
thus a greater potential for falls compared to healthy controls.
Kalron, Dvir, and Achiron (2011) investigated how early postural control impairment from
somatosensory degradation occurred in MS and its effect on dual tasking. 52 subjects with Clinically
Isolated Syndrome (CIS) who had their first neurological event for one month and were within three
months of diagnosis, (mean age 35.2 (+/-1.3) years) were compared to 28 healthy age and gender
matched subjects (mean age 32.8 (+/- 1.2). The mean EDSS score of the MS group was 1.7(+/- 2). Center
of pressure (COP), or the point on the body where the sum of pressure acts producing force, and sway
rate was used to quantify stability while subjects had their eyes open, closed, or while performing a
cognitive task. Subjects performed the modified stroop test and visually focused on a dot placed in front
of them. The cognitive task utilized was the modified stroop test which consisted of reading the true
color of 50 colored words (i.e. the word “blue” written in red) on a printout as fast as possible. Each
condition was performed three times with COP parameters collected. 50% of the experimental group
performed normally compared to the healthy controls. Sway rate (7.92 vs. 4.72 mm/s) and COP in
lateral, anterior-posterior and plane were significantly higher in the CIS group in open-eye conditions
compared to healthy controls. This was also observed in closed-eye conditions (11.32 vs. 7.26 mm/s)
and with the stroop test (11.25 vs. 6.25 mm/s). During the stroop test condition, sway rate was slightly
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elevated in both groups compared to the open-eyes condition. Therefore, balance control was found to
be affected from one to three months of MS onset.
The MS subjects had a higher sway rate and plane standard deviation compared to the healthy
controls with the added cognitive task, suggestive of a dual task cost associated with early MS. We
found that the sway velocity DTC was elevated in the MS group (42.1%) compared to healthy controls
(32.1%). Thus, there is a higher risk for falls due to increased sway implying less stable balance as seen in
various other COP parameters collected for people with MS.
Boes, Sosnoff, Socie, Sandroff, Pula, and Motl (2011) investigated the effects of dual task on
posture while standing in individuals with MS with mild versus moderate disability. Decrements in
postural control were predicted to increase with increasing disability, and dual task cost was expected to
be more elevated in individuals with higher disability. This higher DTC would suggest a greater risk for
falls in this population if present. 45 ambulatory persons with MS were divided into mild disability (EDSS
2.0-3.5 and n=19) and moderate disability groups (EDSS 4.0-6.5 and n=26)). The cognitive task involved
naming items in a category (word list generation) with a semantic and then a phonetic task to decrease
the potential of a learning effect. Sway speed and area, as well as displacement were used to quantify
postural control.
Postural control was found to decrease with increasing disability level. This relationship was
deciphered by differences in sway rate, which were much more elevated in higher disability (295.5 vs.
182.4 mm2), as well as anterioposterior displacement (6.5 vs. 4.5 mm). High dual task was found via
decreased postural control across both groups with an added cognitive task. This was determined by an
increased sway area (277.3 vs. 200.6 mm2), anteriorposterior velocity (8.6 vs. 6.8 mm/s), and
mediolateral velocity (11.9 vs. 8.5 mm/s), and mediolateral displacement (6.8 vs. 5.7 mm). However,
dual task deficits were not different between disability groups. Consequently, it was concluded that
disability level was not related to dual task impairment in individuals with Multiple Sclerosis.
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In calculating the overall dual task cost between the groups, it was found that for anteriorposterior sway the mild individuals had a greater DTC (46.2%) compared to the moderate group (13.2%).
The same phenomenon was observed in medio-lateral sway where the mild MS individuals had a DTC of
68.4% versus 20.5% observed in the moderate group. Therefore, a linear positive relationship does not
appear to exist between increasing disability and risk for falls from the data produced in this study.
Another investigation aimed at deciphering postural effects of dual tasking in Multiple Sclerosis
was conducted by Jacobs and Kasser (2011). Jacobs and Kasser hypothesized that decreased postural
control in the MS group would exist and would consequently lead to more changes in the MS group
during the dual task condition. In terms of the confounding factors, it was hypothesized that increased
information processing demands, decreased information processing capacity, and modified attentional
strategies would exist. 13 MS subjects (EDSS 0-4.5) with no uncorrected visual or hearing impairments
were compared to 13 age (within 2 years) and gender matched healthy controls. Three postural control
tasks (step initiation, forward leaning in the limits of stability, and postural reaction to a rotating
surface) were performed in both groups on a force plate that collected COP data singly and in a dual task
paradigm. An explicit time response cognitive task, an auditory Stroop Test, was used in the dual task
condition. Results of the cognitive task were quantifiable and based on speed and accuracy of the verbal
response given.
The most notable differences were found in step initiation where delayed postural adjustment
in MS existed in single and dual task conditions compared to the healthy controls (single: MS-531
ms/Healthy-447 ms; dual: MS-551 ms/Healthy-475 ms). Additionally, step length from single to dual task
increased with MS subjects, and decreased in healthy controls. Fatigue had an effect on postural
adjustment and the onset of foot lift in the MS group in the single compared to dual task condition,
identified by the Modified Fatigue Impact Scale. This score was also associated with the forward leaning
condition, where variability in leaning onset timing and leaning position occurred. Jacobs and Kasser’s
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hypothesis was partially supported in that differences did exist between subjects with and without MS
for all tasks, however dual task was most notable in step initiation between the two groups. This study
also suggests that dual task cost is associated with fatigue levels. The authors believe that this
investigation underestimates the MS-related costs in dual tasking due to methodology, thus further
work is needed on this subject matter. Although the author’s maintain that step initiation dual task cost
is great in individuals with MS, closer examination of the data does not reveal such a trend.
When we computed the exact dual task cost, we found that a notable dual task cost existed for
COP lean variability in the MS group compared to healthy controls. The healthy controls did not have
any deficit in cognitive and motor dual tasking versus the MS participants whom had an effect of 11.8%.
This decrease in COP lean control could be indicative of a greater likelihood for falling within this
population when performing a paired cognitive and motor task.
When looking at the results of all postural and dual task cost studies, the MS group dual task
cost ranges from 4.85% to 68.4% with a mean of 32.3% with distinct postural control variables in each
investigation. For the control group, DTC ranges from 2.08% to 32.4% with a mean of 22.1%.
Therefore, the overall effect of MS on DTC is moderate (d=-0.56) based on Cohen’s guidelines. Due to
the existence of an elevated DTC in people with MS, further work needs to be done to decipher factors
related to such a phenomenon to consequently decrease the chance of falling.
Dual Task Cost and Walking
Walking impairment during cognitive and motor dual tasking in people diagnosed with Multiple
Sclerosis may exist due to the nature of the disease. Hamilton, Rochester, Paul, Rafferty, O’Leary, and
Evans (2009) examined dual task cost by the manipulation of task demand in a sample of 18 MS subjects
with minimal to moderate walking impairment using a cross-sectional study. The mean EDSS score was
2.74 (+/- 1.59) and all had a relapsing-remitting diagnosis. 18 age and gender matched healthy
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individuals were used as controls. Single cognitive and motor tasks and dual tasks were performed. The
gait task was choosing a self-paced speed on the 4.57 meter GAITRite while spatiotemporal parameters
were collected. The cognitive tasks consisted of the participants remembering a sequence of numbers.
They did two versions of the task in the fixed condition 7 digit long sequences were given, and a titrated
task, where digit length was determined by a previous assessment specific to each subject. Five trials
were conducted including walking alone, fixed digit task, titrated digit task, walking with fixed digit, and
walking with titrated task.
The experimental group had larger decreases in their cognition as seen in the digit task
assessment (Titrated: MS-50.50 % correct/ Control-67.94 % correct], Fixed: MS-29.28 % correct/Control62.78% correct), gait velocity (Titrated: MS-99.57 cm/s/ Control-136.72 cm/s), Fixed:MS-97.35
cm/s/Control-135.74 cm/s) , and swing time variation (Titrated: MS-3.93/Control-3.23 %,Fixed:MS-4.00
%/Control-3.00 %). From the data in this study, there seems to be more support for decrements in dual
task due to divided attention and not as a result of overload of working memory, as previously
mentioned as a potential explanation for cognitive deficits associated with MS, meaning that increasing
cognitive load does not appear to increase dual task cost.
In our calculations of dual task cost, we found a dual task cost of 10.5% in MS versus the healthy
controls that had only a 2.7% effect for gait velocity reduction. A slowing in gait velocity may be
suggestive of more difficulty in performing a motor and cognitive task simultaneously for people with
MS, suggesting they are potentially at more of a risk for falls.
Kalron, Dvir, and Achiron (2010) conducted a similar study to Hamilton et al. (2009) study with
even lower baseline impairment due to MS. The goal was to determine if dual task effects during
walking were elevated in in CIS stage MS patients. 52 CIS patients with a mean age of 35.2 (+/- 1.3) years
were compared to 28 age and gender match healthy individuals with a mean age of 32.8 (+/- 1.2) years.
The average EDSS score in the experimental group was 1.7 (+/-.2), and they had all undergone a
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neurological event with the degree of demylenation determined by both a neurologist and brain MRI.
The motor tasks performed were normal and fast walking on the 4.6 meter GAITRite. The cognitive and
motor dual task was a word list generation cognitive task (recite as many words as possible in specific
category) paired with normal walking on the GAITRite. Each of the three conditions was repeated three
times and spatiotemporal parameters were collected.
It was found that CIS subjects had a generally wider base of support (Control: R:10.1/L:10.2 vs.
R:8.4/L:8.4 cm), were slower (125.5 vs. 133.4 cm/s), and more asymmetrical in comparison to healthy
controls outside of the dual task paradigm. In the dual task condition, the experimental group had a
prolonged double support phase (R:17.3/L:17.3 vs R:14.5/L:14.3 %GC) with reduced velocity (116.2 vs.
132 cm/s). Instability in MS subjects was attributed to deficits in medio-lateral postural control causing a
widened base of support as a means of compensation. It was concluded that dual task interference
occurs very early in MS based on deficits in multiple gait measures such as gait velocity in comparison to
healthy controls. This study was unique in mandating a one to three month period following the first
neurological event.
Via our computations of dual task cost, the MS subjects had a great DTC compared to controls for
gait velocity. Specifically, the DTC for MS was 7.4% and only 1.1% for the controls. Due to an
approximately seven times more elevated deficit in walking speed in persons with MS, there is reason to
believe that these individuals struggle more in dual task situations and may consequently be more prone
to associated falls.
Another study conducted by Sosnoff, Boes, Sandroff, Socie, Pula, and Motl (2011) found that the
amount of disability scales walking impairment in Multiple Sclerosis. Like previous studies, Sosnoff et al.
(2011) studied spatiotemporal parameters of walking during cognitive loading in MS subjects. 78 people
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with MS with three levels of disability, mild (EDSS 2.0-3.5), moderate (EDSS 4.0-5.5), and severe (EDSS
6.0-6.5) were studied. The sample reflected typical disability seen in MS populations. Subjects walked on
26 foot GAITRite four times and completed a concurrent cognitive task in the last two trials. Each trial
calculated overall FAP score, velocity, stride length, step time, base of support, and percentage of gait
cycle in double support and swing phase for both legs. Cognitive effects were quantified with change in
spatiotemporal parameters and word number. Subjects performed semantic word list generation
followed by phonetic WLG to minimize the learning effect.
Differences in walking function between disability groups was found with the severe disability
walking the slowest with shorter steps and greater percentage of the gait cycle in double support.
Moderate to severe disability groups had dual task cost two to three times greater than mild disability
seen in gait velocity, cadence, and FAP scores.
The dual task cost that we specifically found for increasing disability was 7.2% for mild, 13.4% for
moderate, and 13.6% for severe for gait velocity. Not only is this indicative of a positive linear
correlation between DTC and disability level, but is also suggestive that people of higher disability in MS
(particularly moderate or severe) are at a greater risk of falling.
Across all of the gait and dual task cost studies, the MS groups dual task cost ranges from 7.2% to
13.6% with an average of 9.3% . For the control groups, dual task cost ranges from 1.05% to 2.74% with
an average of 1.5%. The overall effect of MS on dual task cost during walking was large (d = -4.06) based
on Cohen’s guidelines. With notable group differences in gait and DTC studies, there needs to be further
work to uncover what may lead to such a occurrence in order to lessen the elevated risk for falling that
person with MS seem to have.
Predictors of Dual Task Cost
Several factors may increase the likelihood of a dual task cost in people with Multiple Sclerosis.
It is pivotal to understand these factors in order to increase the safety of those with MS in terms of
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 14
reducing their risk for falling, which we assume to be elevated from the existence of DTC across current
research compared to healthy controls.
Disease severity may play a role in determining the dual task cost that one experiences. Various
studies have begun to decipher the potential relationship. Hamilton et al. (2009) measured the cognitive
and dual task cost in people with low disability with a large range of disease severity. Additionally,
Jacobs and Kasser (2011) conducted sub group analysis with different types of MS and different
disability levels in terms of severity and found that personal characteristics were a predictor of dual task
cost. Kalron et al. (2010) proposed subdividing disease severity based on symptoms. However, it is
important to recognize that these studies were not specifically examining the effect of disease severity
on DTC. Sosnoff et al. (2011) and Boes et al.(2012) both use disability level separation to study dual task
cost. The more severe the impairment could lead to the higher the dual task cost because more
cognitive and muscle function impairment could exist. Despite a substantial amount of work being done,
the effect of disease severity is still unknown and no effect has currently been found.
Years since diagnosis may be another predictor for dual task cost in the MS population. A longer
diagnosis may be associated with higher dual task cost because a further progression could be indicative
of more neurological degradation and a cascade of other associated cognitive and muscular effects.
Limited research has been done to determine this factors effect and no relationship has currently been
uncovered.
Motor ability is degraded due to the nature of the disease. Thus, muscle strength may be a
predictor of dual task impairment with less muscle strength potentially increasing dual task cost because
muscle function is essential to stable gait and this may place an increased cognitive demand on the
individual in an attempt to compensate. Cognitive ability is also impacted because of the impact MS has
on the central nervous system. Therefore, a lower level of cognitive function could potentially increase
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 15
dual task cost because the added cognitive load of a motor task may have a larger effect. Hamilton et al.
(2009) mentions this as a possible confounding factor and recommends that baseline cognitive ability be
evaluated for valid comparison of subjects across research studies.
Fatigue is either noted or determined as a predictor of dual task cost by most of the authors of
work done on the cognitive-motor interaction that is associated with MS. Higher fatigue could lead to a
greater dual task cost because the individual may be unable to meet the additional attentional demands
that a motor task potentially has. Initially, Hamilton et al. (2009) mentions the probability of such an
effect by noticing a high score on CFQ question 9 being associated with dual task cost. This was later
substantiated by a positive correlation found in Jacobs and Kasser’s postural dual task cost study (2011).
However, the effect found could not be generalized as a predictor because a larger sample was
necessary to see the differences between MS and healthy individuals. Consequently, Jacobs and Kasser
(2011) emphasize the importance of interventions to modify fatigue to lessen the dual task cost.
Finally, assistive devices are yet another associated factor that could elevate dual task cost. This
is due to the presentation of a potential cognitive challenge from an increase in cognitive load that could
increase dual task cost by an attentional overload (Hamilton, 2009).
Several other predictors may exist that could increase dual task cost increasing the fall likelihood
of person with MS. These factors need to be deciphered and targeted to refine interventions to provide
the MS population with the most adequate support possible for safety and lifestyle benefits.
Future Investigations
In order to better understand the cognitive-motor interaction that may exist in people with MS,
future studies need to modify current methods and fill several gaps in research. Boes et al. (2012)
discusses the importance of the potential for improvement in the MS population in terms of lessening
dual task cost and whether this would be via cognitive or specifically dual task improvements. Early
interventions should be tested to see effectiveness and determine degenerating areas that are more
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 16
malleable in terms of increasing function to decrease fall likelihood (Kalron, 2010). To further decrease
falling likelihood, functional consequences in daily lifestyle need to be identified (Boes, 2012).
As previously discussed, the potential predictors of dual task cost need to be screened for and
targeted in interventions. Consequently, better tools for assessment are necessary. Hamilton et al.
(2009) and Kalron et al. (2010) identify the current absence of clinical tools to assess dual task
components independently, which would provide a more accurate depiction of dual task cost. The
development of improved screening mechanisms would allow interventions to be more effectively
targeted and successful.
Also, it is important to recognize the difference in cognitive task type used across these various
investigations. For postural dual task cost, Negahban et al. (2011) uses silent backward counting, Kalron
et al. (2011) uses a modified stroop test, Boes et al. (2012) uses word list generation, while Jacobs and
Kasser (2011) use an auditory stroop test. For gait dual task cost, Hamilton et al. (2009) uses fixed and
titrated digit span assessment, Kalron et al. (2010) uses modified word list generation, while Sosnoff et
al. (2011) utilizes a world list generation. Different cognitive task types may challenge different areas of
cognition (Jacobs & Kasser, 2011). The great degree of variation in methodology could potentially distort
results and does not allow for straightforward comparison between studies.
The lack of uniformity across dual task in MS studies has been acknowledged by various studies
in an attempt to decipher how to fill such gaps in research and to determine if a significant effect exists.
One potential characteristic of a cognitive task that may skew dual task results is the level of articulation
required for an individual. For example, Yardley, Gardner, Leadbetter , and Lavie (1999) looked at a
cognitive task requiring greater articulation. Greater postural sway was found in dual task situations
with this type of cognitive task. Hyndman, Ashburn, Yardley, and Stack (2006) instead used a non
articulation requiring task. It was found that subjects had a decreased sway velocity and amplitude was
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 17
found. Thus, there seems to be a connection between articulation and dual task cost, with higher
articulation cognitive tasks leading to more deficits in postural control.
The effect of cognitive task type has been investigated even further with the manipulation of
tasks that recruit different memory components, including working versus semantic, to see the overall
consequence on dual task cost. In a study conducted by Beauchet, Dubus , Aminian, Gontheir, and
Kressig (2005), cognitive task type in a gait dual task situation was studied in the elderly. A verbal fluency
task that utilizes semantic memory (word list generation-anima names) and a backward counting task
(simple arithmetic) that uses working memory, or executive control, were given to subjects. Mean stride
time was found to be consistent regardless of the walking-associated cognitive task, but other
spatiotemporal parameters appeared to be dependent on cognitive task type. Therefore, there appears
to be some noteworthy effect of cognitive memory task type on dual task cost.
Potential sources of the effect of this cognitive task type manipulation have been put forth. For
example, more competitive interactions for executive functions during gait could perhaps explain why
the gait variability was higher with the executive control type tasks. The arithmetic task could also have
increased gait changes due to more articulation demand seeing that there are more numbers than
animal names in the word list generation task (Yardley, 1999). The enumeration task is a rhythmic task,
which has been shown to increase interference in concurrent tasks (Taga, Yamaguchi, &Shimizu, 1991)
and could serve as an explanation for the observed stride-time variability. Regardless of the mechanism,
there appears to be a difference between executive control tasks and tasks that involve more crystalized
knowledge, such as word list generation. These differences needed to be accounted for by the
researcher when investigating dual task cost with a concurrent cognitive and motor task in MS.
In sum, this previously recognized effect is not easily explainable. Woollacott and ShumwayCook (2002) have formerly established that dual task gait changes increase with cognitive task difficulty
level. However, in this particular study the cognitive task difficulty was perceived to be the same and
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 18
therefore cannot explain the variation in gait performance in dual-task. Thus, more work needs to be
conducted on memory task type’s effect on dual task cost to expose the basis of such a phenomenon.
One other area of cognitive task type effect on dual task cost that may potentially skew results is
the level of sensitivity offered by currently existing methodology. Hamilton et al. (2009) acknowledges
the lack of sensitivity offered by the digit task assessment and suggests that in order to clearly establish
the cognitive root of dual tasking, of divided attention or capacity, task demand needs to be
manipulated. This provides even further support for the need for dual task cost effect o be assessed and
studied in much more detail to decipher a clear effect on results due to its potential implications for
those with MS.
Various motor skill areas also need to be evaluated in future work. Dual task cost in high level
activities is still in need of assessment (Kalron, 2010). Also, postural effects and dual tasking must be
looked at with more precision by measuring both static and dynamic balance, and the changes
experienced in daily lifestyle outside of a laboratory ( Kalron, 2010).
Conclusion
It is important to recognize that dual task cost exists among both control and persons with MS and
its effect is greater in postural tasks than gait. Specifically, dual task cost during posture is approximately
4 times greater than DTC during walking. However, differences between persons with MS and controls
are less apparent in maintaining a static stance than during walking. There are not notable differences
between MS and control groups in terms of dual task cost.
A limited amount of research has been conducted on dual tasking and its effect on postural control
and walking in the Multiple Sclerosis population. Consequently, the functional outcomes of dual task
cost are still unknown. However, the general assumption in is that it is related to falls. More work needs
to be done in this area to substantiate this proposed relationship to help prevent falls in those with MS.
DUAL TASK COST IN PERSONS WITH MULTIPLE SCLEROSIS 19
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