Archives of Clinical Neuropsychology
20 (2005) 705–718
Modality-specific aspects of sustained and divided
attentional performance in multiple sclerosis
M. McCarthy a,b,∗ , J.G. Beaumont a,b , R. Thompson b , S. Peacock b
a
School of Human and Life Sciences, Roehampton University, Whitelands College,
Holybourne Avenue, London SW15 4JD, UK
b
Department of Clinical Psychology, Royal Hospital for Neurodisability, London, UK
Accepted 5 April 2005
Abstract
While the prevalence of cognitive impairment in multiple sclerosis is well documented, few studies
have systematically investigated the profile of attentional abilities. In the current study, 30 MS participants were assessed on measures of sustained and divided attention and compared to a sample of 30
neurologically intact healthy controls. Performance on visual and auditory unimodal and bimodal trials
were conducted for measures of both forms of attention. A three-factor mixed measures analysis of
variance (group × task × modality) was conducted. MS participants were impaired relative to controls
on all measures of speed and accuracy across unimodal and bimodal trials and more impaired on measures of divided attention than on sustained attention measures. Performance on the bimodal trials was
also significantly compromised relative to the unimodal trials especially on the divided attention task.
The theoretical and clinical implications of these findings are discussed.
© 2005 National Academy of Neuropsychology. Published by Elsevier Ltd. All rights reserved.
Keywords: Attention; Multiple sclerosis; Neuropsychology
1. Introduction
It is estimated that cognitive deterioration occurs in 45–65% of patients with multiple
sclerosis (MS), (Rao, 1995). The pattern of cognitive impairment appears as variable as the
disease trajectory itself and typically includes mild impairments of language, normal recognition memory, immediate recall and rates of forgetting, but with clear impairment in retrieval
∗
Corresponding author.
E-mail address: [email protected] (M. McCarthy).
0887-6177/$ – see front matter © 2005 National Academy of Neuropsychology. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.acn.2005.04.007
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M. McCarthy et al. / Archives of Clinical Neuropsychology 20 (2005) 705–718
from long-term memory. Problems with attention and concentration are also common, as are
slowed rates of information processing and both abstract and conceptual reasoning deficits
(McCarthy, 1996).
Given the importance of an intact attentional system for the execution of many cognitive
activities, it is somewhat surprising that the profile of attentional dysfunction in MS has not
been more systematically investigated (Higginson, Arnett, & Voss, 2000). Sullivan, Edgley,
and Dehoux (1990) surveyed 1180 people with MS, and of the 38% who reported cognitive
difficulties in at least one area of cognitive processing, 22% of these reported difficulties with
attention. This is even more interesting set against the 23% of MS patients who considered
themselves to have memory problems. There is also some evidence that MS participants sometimes confuse attention and memory processes and interpret attentional deficits as memory
impairments (Lezak, Whitham, & Bourdette, 1990). Of those studies that have investigated
the nature of attentional dysfunction in MS, results remain ambiguous and the extent or nature
of attentional dysfunction in MS is still unclear.
Attentional dysfunction has been reported by Callanan, Longsdail, Ron, and Warrington
(1989), who investigated a range of functions including visual and auditory attention and reported impairments of attention as the most prominent cognitive abnormalities in the group.
Filley, Heaton, Nelson, Burks, and Franklin (1989) also found MS patients were more impaired on measures of visual and auditory sustained attention than a cohort of patients with
dementia of the Alzheimer’s type (DAT). Cohen and Fisher (1989) reported that MS patients
showed particular difficulty on measures of sustained attention and Kujala, Portin, Revonsuo,
and Ruutiainen (1995) found cognitively impaired MS participants to be slower on all tests of
attention compared to subgroups of cognitively intact MS participants. In examining aspects of
working memory, D’Esposito et al. (1996) reported MS patients to show a greater decrement
than controls in the performance of dual condition tasks compared with single task conditions. Conversely, Beatty, Paul, Blanco, Hames, and Wilkbanks (1995) reported performance
on the Digit Span subtest of the WAIS-R to be in the average range for patients with MS
although scores were significantly lower than control participants matched for age and education. DeLuca, Johnson, Beldowicz, and Natelson (1995) also found that MS patients were not
different from controls on measures of digit span. Van den Burg, Van Zomeron, Minderhoud,
Prange, and Meijer (1987) failed to find any trace of attentional abnormality in a group of MS
patients with mild disabilities.
Such performance anomalies are not unexpected given the disease variability in MS. Additionally, such disparity in findings might reflect the diversity of measures used to measure
attention within this clinical population. While the current cognitive literature boasts a huge
selection of paradigms to measure distinct processes in attention (Higginson et al., 2000), so
far, the construct validity of these measures have not been psychometrically tested and it is
unclear whether many of the measures used in studies cited above are indeed measuring the
same mental construct at all. This contrasts directly with measures of language or memory
where standardised neuropsychological measures of the distinct processes of each system have
been appropriately validated.
Closer inspection of the memory-based research in MS raises some suspicion that such
memory problems could in fact be secondary to a primary impairment of attention. For example, Coolidge, Middleton, Griego, and Schmidt (1996) reported memory performance to
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707
be significantly more affected by the presence of interference in comparison with a noninterference learning condition. In this context, it is possible that the presence of interference
compromises patients’ initial acquisition of information, which in turn adversely affects their
recall. Grafman, Rao, Bernardin, and Leo (1991) investigated effortful and automatic processing in MS and control participants and reported equivalent performance on automatic
processes, but reported MS participants to be significantly more impaired than controls on
measures of effortful processing. Again, this failure to use attentional resources for more effortful processing could be interpreted as a primary impairment of attention. Finally, a study
by Goldstein, McKendall, and Haut (1992) reported that gist recall was a preserved feature
of memory functioning in patients with MS. Indeed one might speculate that this attention to
critical features might indicate a covert response strategy for dealing with a reduced attentional
reserve, thereby precluding the processing of surface details.
Furthermore, if attention is conceptualised as a system supported by multiple anatomical
regions, both cortically and sub-cortically (Mesulam, 1990; Posner & Petersen, 1990), then
the diffuse lesion profile associated with MS pathology is consistent with a disruption of
attentional efficiency.
In the current study, the specific nature of attentional dysfunction in MS is considered.
Measures of sustained and divided attention were developed based on a review of 300 papers
of experimental methods for the assessment of attention in cognitive psychology spanning the
last 40 years. Visual and auditory unimodal and bimodal trials were compared across both
forms of attentional measures. Bimodal trials were included as there is still controversy in
the normative literature as to whether attention is organised in a multi-modal or modalityspecific way (see Driver, 1998 for review). In addition, many of the previous studies that
have investigated attention have generally only compared attention in the visual or auditory
modality and a measure of attention across modalities might be considered useful both for the
clinical assessment and disability management of patients with multiple sclerosis.
2. Method
2.1. Participants
Thirty MS participants (determined by neurological examination) and 30 controls individually matched for age and education took part in the study. The MS group consisted of 18
females and 12 males all of whom had received a definite diagnosis of MS. Ages ranged from
27 to 78 (x = 51.83) with a mean of 11.95 years in education (range of 9–20 years). The mean
number of years since diagnosis was 14.17 (range 1–49 and S.D. = 2.40). MS participants
ranged from 1 to 8.5 on the Kurtzke (EDSS) Expanded Disability Status Score (x = 5.95). This
20-point scale ranges from 0 (normal neurologic examination) to 10 (death due to MS) and
represents increasing levels of disability. An average of 5.95 in this cohort represents “intermittent or unilateral constant assistance required to walk about 100 meters with or without
resting” (Kurtzke, 1983, p. 1451). Participants in the MS group were further classified as
chronic progressive (n = 5), relapsing remitting (n = 13), stable (n = 10), or unknown (n = 2) on
the basis of disease history and current symptom status. Interesting, no significant differences
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in terms of age (F(3,26) = 1.617, p = 0.210), years of education (F(3,25) = 0.875, p = 0.467),
years since diagnosis (F(3,26) = 0.468, p = 0.695) or EDSS (F(3,25) = 0.875, p = 0.467) were
observed between these sub-groups. The control group consisted of 12 males and 18 females
and were matched to the MS group on the basis of age and number of years spent in education.
Age ranged from 26 to 78 (x = 52) and the mean number of years in education was 12.33 (range
9–17 years). Whilst no formal measures of visual acuity were undertaken, all participants who
wore spectacles for corrected vision were encouraged to wear them throughout testing.
2.2. Attention measures
The development of two new measures of sustained and divided attention is an important
part of this study. One of the critical features of these tasks is that their execution actively
requires the participant to continuously monitor all stimuli, as the nature of the target stimulus
changes over the course of the task. This task characteristic contrasts with previous measures
of sustained attention where the participant has been required to repeatedly detect the same
target stimulus embedded in a range of distractor stimuli. Equally, the divided attention task
in the current study has been designed to force participants to divide their attention between
the processes required to remember and to attend. Again, this task characteristic contrasts
with more conventional measures of divided attention where dual task methodologies have
been employed and performance decrements on a primary task are measured when the
participant is required to simultaneously perform a secondary task. The current measure
forces participants to actively divide their attention between the processing required to retain
the first stimulus presented while simultaneously attending for the presentation of the next
stimulus.
2.3. Sustained Attention Task (SAT)
This task was designed to assess participants’ ability to sustain attention on a single task
over an extended period of time and was presented on a desktop computer. Participants were
required to attend to a series of numbers and to respond when they thought that specific target
numbers had been presented. Target numbers changed over the course of the task; participants
were instructed to respond initially when number ‘one’ was presented, then, after they had
responded to number one, to respond next when number ‘two’ was presented, and so on up to
number 10. The task required participants to maintain a specific response set and to attend to
a single target at any one time, but avoided the potential problems of automatic responding or
the procedure becoming more of a vigilance task.
There were 300 trials in total, each trial corresponding to the presentation of a single digit.
Each trial lasted for 2 s. Each trial was presented for the first 300 ms with an interstimulus
interval of 1700 ms. Non-target trials were presented in a pseudo-random order, as any number
either side of the target number was not presented in the sequence of non-targets (e.g. a
participant waiting for the target number six would not be presented with the numbers ‘five’
or ‘seven’ as non-targets). This task was presented three times using three different modes
of presentation, auditory, visual and bimodal. In all conditions, participants heard a warning
tone if they missed one of the target numbers. This indicated that the current target had been
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709
missed and they should now attend for the next highest digit. The warning tone was to minimise
potential cumulative errors, which could occur in a situation such as a participant being unaware
that they had missed an early target and going on to miss all subsequent targets.
2.4. Divided Attention Task (DAT)
In this task, participants were presented with pairs of digits and asked to respond when
target pairs (defined as any pair of digits where the members were consecutive in either an
ascending or descending direction) were presented. On this task the identification of targets
was determined wholly within a particular trial. Any pair of consecutive numbers was defined
as a target pair regardless of its relationship to proceeding pairs (so that the first target could
be 5–6, the second 8–9 and the third 4–3). To avoid possible masking effects, the members of
pairs were not presented simultaneously but were separated by a short gap. The task consisted
of 60 trials in total (30 target pairs and 30 non-target trials pairs). The presentation of target
or non-target trials was again not truly random as presentation was constrained so that no
more than three of either type were presented consecutively. Each trial lasted for 4 s: the first
member of the pair was presented for the first 300 ms followed by a 200 ms interval followed
by the second member of the pair for 300 ms. A 3 s interval separated the presentation of each
stimulus pair.
Visual, auditory and bimodal presentations were conducted. In addition to being separated
by a time gap, members of auditory and visual pairs were also spatially separated. In the
auditory condition, the first member of the pair was presented to the left ear and the second
to the right ear. In the visual modality, the first member of the pair was presented so that the
centre of number was 45 mm to the left of the monitor screen centre, the second so that its
centre was 45 mm to the right.
In the bimodal condition all pairs consisted of one auditory member and one visual member,
with the presentation was balanced across modalities. Presentation order was random with the
constraint that no more than three auditory–visual or three visual–auditory trials should occur
consecutively.
2.5. Design and procedure
A mixed measures factorial design was employed with one between subjects factor, Group
(control and MS participants), and two-within subject factors, Task (sustained and divided
attention measures) and Modality (auditory, visual and bimodal). Response time and response
accuracy were the dependent measures.
The presentation order of the auditory and visual conditions was counterbalanced, so that
the bimodal condition was always presented as the final condition. Participants in both groups
first completed the SAT followed by the DAT. While all of the control participants completed
both measures in a single session, 25 of the 30 MS participants completed the measures in two
sessions with a 1-week interval. This was arranged to minimise any effects of fatigue amongst
the MS group.
Participants were only given practice trials on the SAT if they were unable to understand the
standard instructions. On the DAT all participants received at least five practice trials (or until
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performance was at ceiling) before starting each of the conditions. Both tasks were presented
continuously from initial onset.
3. Results
Response time and response accuracy scores were collected from both types of task. Response times were measured from onset of the stimulus on the SAT, and from onset of the
second member of the stimulus pair on the DAT (this was intended to provide some equivalence
with response times on the SAT). Following convention, all response times ± 2.5 standard deviations from the mean were removed. This was completed for independent design cells and
resulted in the removal of 1.67% of scores from the control group and 3.33% of scores from
the MS group. All response time data reported below comes from these trimmed samples.
Table 1 presents the summary data of the MS and control group for both measures.
Measures of response accuracy included the number of target trials responded to (hits), the
number of non-target responded to (false positives), and a combined measure of the percent of
trials accurately responded to (calculated by subtracting the total number of incorrect responses
[false negatives − false positives] from the total number of correct responses, dividing the result
by the total number of possible trials and multiplying by 100). Tables 2–4 present the summary
data of the MS and control group for both measures of sustained and divided attention.
The results were further analysed using a three-way (group × task × modality) mixed measures analysis of variance. Logarithmic transformations (log base 10) were performed on all
reaction time data and all further analyses used these logarithmic transformed values.
Table 1
Presents means and standard deviations for response times (ms) on both measures across all conditions
Sustained attention task
Visual
Auditory
Bimodal
Divided attention task
MS group
Control group
MS group
Control group
778.14 (215.53)
743.27 (278.98)
758.24 (221.18)
535.03 (56.73)
594.58 (73.91)
637.49 (65.50)
845.10 (228.22)
737.61 (226.51)
881.90 (270.76)
585.92 (73.37)
673.40 (112.21)
642.76 (95.84)
Note. Standard deviations in parentheses.
Table 2
Presents means and standard deviations for total number of target hits on both measures across all conditions
Sustained attention task
Visual
Auditory
Bimodal
Divided attention task
MS group
Control group
MS group
Control group
9.03 (1.45)
9.20 (1.32)
8.83 (1.66)
9.97 (0.18)
9.83 (0.46)
9.90 (0.31)
29.07 (1.96)
28.90 (1.83)
28.03 (4.06)
29.80 (0.61)
29.70 (0.75)
29.53 (1.11)
Note. Standard deviations in parentheses.
M. McCarthy et al. / Archives of Clinical Neuropsychology 20 (2005) 705–718
711
Table 3
Presents means and standard deviations for number of false positives on both measures across all conditions
Sustained attention task
Visual
Auditory
Bimodal
Divided attention task
MS group
Control group
MS group
Control group
1.47 (2.39)
1.17 (2.09)
1.30 (2.37)
0.20 (0.48)
0.37 (0.67)
0.13 (0.43)
1.33 (2.51)
1.10 (1.92)
1.97 (3.40)
0.57 (0.86)
0.93 (1.17)
1.20 (1.92)
Note. Standard deviations in parentheses.
Table 4
Presents means and standard deviations for overall percentage of correct responses on both measures across all
conditions
Sustained attention task
Visual
Auditory
Bimodal
Divided attention task
MS group
Control group
MS group
Control group
99.19 (1.05)
99.34 (1.00)
99.18 (1.18)
99.92 (0.17)
99.82 (0.24)
99.92 (0.19)
96.22 (6.89)
96.33 (5.04)
93.44 (10.37)
98.72 (1.84)
97.94 (2.30)
97.22 (4.60)
Note. Standard deviations in parentheses.
A significant main effect for group was reported for all four measures: number of target hits
(F(1,58) = 10.46, p < 0.01); number of false positives (F(1,58) = 6.44, p < 0.05); percent correct
responses (F(1,58) = 5.68, p < 0.05); and reaction time to targets (F(1,58) = 21.11, p < 0.001).
As expected, the control group had significantly more hits, fewer false positives, and a greater
percentage of correct responses and faster response times than MS participants.
As already noted, significant main effects of task are difficult to interpret for the response
accuracy measures given the differences in design between the two tasks. However, a significant
main effect for task was observed for the response time measures (F(1,58) = 26.11, p < 0.001).
Post hoc analysis were significant using the Least Significant Difference (LSD) procedure (see
Table 5) and resulted from significantly faster responses on the sustained attention tasks than
on the divided attention task.
There were significant main effects for modality which included: number of target hits
(F(2,116) = 3.21, p < 0.05); number of false positives (F(2,116) = 1.23, p > 0.05); percent
correct responses (F(2,116) = 5.07, p < 0.01); and response time to targets (F(2,116) = 9.22,
p < 0.001). No significant main effect for modality was reported for the number of false positives values. This illustrates parity of difficulty across modalities. Where a significant main
effect for modality was observed, post hoc analyses using the LSD procedure (see Table 5)
showed the bimodal condition was associated with significantly slower response times, and a
significantly lower percentage of correct responses than both the auditory and visual conditions in isolation. Although there was a main effect of modality for the number of target hits,
there were no significant differences between the specific pairs of conditions. No significant
differences were observed for comparisons between the auditory and visual conditions on any
of the response measures.
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Table 5
Presents post hoc tests for significant main effects for each of the four response measures using the Least Significant
Difference statistical procedure
Response time
to targetsa
Number of
hit targets
Modality
Auditory–visual
Auditory–bimodal
Visual–bimodal
−0.0001
−0.030***
−0.030***
−0.058
0.333
0.392
Task
Sustained–divided
−0.033∗
Group
MS–control
0.104***
Number of
false positives
−0.153
0.919∗
1.072∗
−19.71***,b
−0.944**
Percent correct
responses
2.915***,b
0.822∗
−1.641∗
a
Differences between response times are given in logarithmic units.
Differences should be interpreted with caution.
∗
p < 0.05.
∗∗
p < 0.01.
∗∗∗
p < 0.001.
b
In addition to these main effects, several significant interactions were observed. A significant
interaction between modality and task on the percentage of correct responses (F(2,116) = 5.14,
p < 0.01), suggests that stimulus modality differentially affected performance on the sustained
and divided attention measures. Fig. 1 presents the mean scores for all modality conditions for
MS and control participants on both tasks.
It is clear that performance in the visual and auditory modalities is roughly equivalent
and that performance in the bimodal condition was poorer in both. This reduction in the
percentage of accurate responses for the bimodal condition is far more pronounced for the
divided attention task than it was for the sustained attention task in relation to the relevant
unimodal conditions. No further significant effects were observed for this interaction either
Fig. 1. Presents interaction for modality and task for overall percent of correct responses on both measures across
all conditions.
M. McCarthy et al. / Archives of Clinical Neuropsychology 20 (2005) 705–718
713
Fig. 2. Presents interaction for modality and group for overall reaction time on both measures across all conditions.
for the two other accuracy measures (number of target hits and number of false positives) or
for the response time measure.
A significant interaction of modality × group for the response time measure was also observed (F(2,116) = 20.56, p < 0.001). As seen in Fig. 2, the overall profile of performance across
modality conditions differs as a function of group. If one assumes that the control group represents a normative pattern of performance, then the results suggest that the MS group showed
relatively normal patterns of response for auditory stimuli, an exaggeration of a normal deficit
associated with bimodal presentation and a grossly impaired performance on visual stimuli.
No further significant interactions of modality × group were observed.
Finally, a significant interaction of group × modality × task for the response time measure
was also observed (F(2,116) = 5.08, p < 0.01) and this interaction is graphically represented
in Fig. 3. This further interaction with task illuminates the greater differences in response
Fig. 3. Presents interaction between group, modality and task for overall reaction time on both measures across all
conditions.
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M. McCarthy et al. / Archives of Clinical Neuropsychology 20 (2005) 705–718
speed between the two groups across the three-modality conditions as a function of task.
Immediately apparent from Fig. 3 is that the differences in response times between the two
groups are more pronounced on the divided attention task than on the sustained attention task.
Additionally, the overall profile of performance across modalities was different for both groups
again as function of task. For the divided attention task, the greatest decrement in performance
between the two groups was where MS participants had to divide their attention in either
the bimodal or visual conditions. For the sustained attention task, the performance decrement
between the two groups was more pronounced for the visual condition than it was for the
auditory and bimodal conditions.
In summary, the MS group showed a consistently poorer performance than the control group
across all three modality conditions for both tasks, with a lower percentage of correct responses
and higher response times. The relationship between the different modalities is less transparent.
The most consistent pattern appears to be poorer performance (i.e., lower accuracy scores or increased response times) in the bimodal condition, especially on the divided attention task where
the poorest performance is observed for the MS group on all four measures and for the control
group for all but one of the measures (response time). Another interesting pattern is the dissociation between the MS group and the control group with respect to performances in auditory and
visual conditions. On both tasks, MS participants performed better in the auditory than in the
visual conditions whereas this profile was reversed in the control group and performance was
consistently better in the visual condition. It is likely that visual sensory impairments in the MS
group may have contributed to this effect, but since no formal measures of visual acuity were
recorded it is difficult to be certain at this point. For future research studies it would be prudent
to employ a visual screening measure, such as The Cortical Vision Screening Test (CORVST;
James, Plant, & Warrington, 2001) to eliminate any ambiguity in findings due to visual sensory
loss. In terms of task differences, a consistent pattern of slower responses in both groups
on the divided attention task was observed. Given the differences in numbers of targets and
non-targets, no meaningful comparisons between tasks can be made for the response accuracy
measures.
4. Discussion
The current study was designed to investigate the performance profiles of MS and control
participants on measures of divided and sustained attention across visual, auditory and bimodal
trials. The results suggest that the performance of MS participants was slower and less accurate
than matched controls on both measures of attention. The three-way interaction of group, task
and modality would suggest that this finding was not simply a result of a general motor
slowing, or a reduction in information processing speed efficiency in the MS cohort, but
instead suggested differences in performance related to task and modality. Indeed, the findings
fit comfortably with previous positive findings of an impairment of attentional processes in
MS (Beatty et al., 1995; Callanan et al., 1989; Cohen & Fisher, 1989; D’Esposito et al., 1996;
Filley et al., 1989; Higginson et al., 2000; Kujala et al., 1995).
Of additional interest was a more refined understanding of the nature of attentional impairment in MS. One notable finding was that response time measures were significantly slower
M. McCarthy et al. / Archives of Clinical Neuropsychology 20 (2005) 705–718
715
on the divided attention task than on the sustained attention task for MS participants. To some
extent these findings are contrary to what might have been expected given the earlier studies
investigating attention in MS, where impairments of sustained attention were thought to be the
more prevalent (Cohen & Fisher, 1989; Filley et al., 1989). However, many of these studies
failed to control for patient fatigue and used more vigilance-style sustained attention measures
than the type of measure used in the current study. For this reason it is difficult to know, retrospectively, the extent to which patients’ impaired efficiency in processing information might
have affected their performance.
It is possible that such performance differences could be related to a general lack of parity
in task difficulty but this is unlikely to be the case since response times for control participants
were almost equivalent across these same measures. However, it is the case that qualitative
differences between the two tasks provide a closer insight into the specific nature of attentional dysfunction in MS. For example, the divided attention task required a much higher rate
of responding (30 target trial pairs) than the sustained attention task where the detection of
only 10 target stimuli was required. It also is likely that both tasks enlisted the use of working memory (WM; Baddeley & Hitch, 1986) for the temporary retention and rehearsal of
relevant information, though it is difficult to quantify the precise contribution made by the
WM components. On the divided attention task, each stimulus had to be retained and then
actively compared to each consecutive stimulus if a target pair was to be accurately detected.
For the sustained attention task, only the previous target stimulus had to be retained to enable
the next target stimulus to be identified. For this reason, differences in performance between
the divided and sustained attention tasks might be attributable to faulty mechanisms in the
WM system. This proposition supports previous findings of Rao et al. (1993) and Ruchkin
et al. (1994) both of whom reported the performance of MS participants to be impaired on
measures of verbal working memory, the component of working memory (otherwise known as
the phonological loop) thought to be responsible for the temporary rehearsal and retention of
verbal information. More recently, D’Esposito et al. (1996) reported that MS participants were
impaired relative to controls when performing a dual task paradigm, which they interpreted
to be indicative of an impairment of the central executive component of working memory. It
is probable that either of these deficits in isolation or indeed some combination thereof could
have affected MS participants’ performance on the divided and sustained attention measures
currently employed.
Also of interest was the performance of MS participants across modalities, although some
caution ought to be exercised against over-interpreting these significant effects due to the
prevalence of visual sensory deficits in MS, and the lack of any formal measures of visual
acuity in the current study. A significant interaction among group, modality and task on the
response time measures highlighted the differential pattern of performance in MS and control
participants across tasks as a function of modality. While the control group generally performed
better in the visual modality than the auditory modality, the converse pattern was true for MS
participants. As discussed, given the prevalence of optic neuritis in MS symptomology, it is
likely that this effect is due to impairment in visual processing rather than any true attentional
effect. More interesting, however, are the differences in performance on the bimodal trials
across tasks. While MS patients followed a similar pattern of performance to controls on the
sustained attention task, performance on the bimodal trials of the divided attention task were
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disproportionately slower relative to unimodal trials. Of course, this finding could again be
explained by MS participants’ impaired visual processing, but if this were likely then a similar
disproportionate increase in response time processing ought to have been witnessed on the
sustained attention task, and this was not the case. For this reason, the findings of the current
study suggest that the MS participants were more impaired on divided than sustained attention
measures of the current investigation.
These findings have important theoretical implications for the way in which attentional
dysfunction is clinically assessed, both in MS and in other neurological conditions. Attention
is a highly sophisticated, multi-functional and multi-component mental construct and many
conceptual frameworks have been proposed to understand it (see Webster & Ungerleider,
1998 for review). The hope of finding a unitary definition to explain the entire capacity of
the attentional system has long been abandoned in the experimental literature in favour of
theories which consider the micro-mechanisms implicated in the execution of the various
components which make up the entire system of attention. For example, Posner and Petersen
(1990) have identified the three processes of orienting, disengaging and re-orienting of attention
as critical mechanisms involved for execution of visual attention processing. The theoretical
fractionation of attention into more highly specified sub-components is in line with other
cognitive systems such as language and memory, where individualised components have been
singled out both in terms of definition and in terms of measurement. However, the clinical
assessment of attention lags far behind the recent theoretical advances that have been made
and single unitary measures of attention (such as the digit span component of the Weschler
Adult Intelligence Scale—revised (WAIS-III)), the Stroop, or paced auditory serial attention
task (PASAT) are still used clinically to make inferences about the capacity and functioning
of the entire attentional system. Until such time as a more comprehensive framework for
understanding the totality of attention is developed, the clinical assessment of attention will
continue to be unsatisfactory (Higginson et al., 2000).
The findings of the current study are also useful for building a framework for understating
the overall profile of cognitive impairment in MS and for considering the manner in which
impairments of attention affect the performance of other cognitive systems. The findings
contrast with previous studies which suggest that impairments of sustained attention are the
most prevalent and highlight the need to assess attention in as many sensory modalities as
possible in order to gain a reliable understanding of participants attentional potential.
There is also preliminary evidence that attention retraining can positively affect performance
on other neuropsychological tests. Plohmann, Kappos, and Brunnschweiler (1993), reported
tentative evidence that attention retrained participants showed less errors and a reduction in
performance fluctuations over time than non-trained participants.
Future research initiatives need to consider in more detail the prevalence of attentional
abilities in MS with particular emphasis on suitably validated attentional measures. Also
desirable is the need to select more cognitively homogenous subgroups of MS participants.
In practice, however, this can often be quite difficult to achieve since participants disease
status can change dramatically even during the course of a research investigation and disease
duration has not been found to be positively related to cognitive status (see Hutchinson, Burke,
& Hutchinson, 1996, for review). In addition to these methodological considerations, future
research also needs to unpack the complex relationship between attentional and memory
M. McCarthy et al. / Archives of Clinical Neuropsychology 20 (2005) 705–718
717
functions. It is not yet established whether MS patients presenting with specific impairments of
memory also present with specific impairments of attention. Similarly, it is not clear the extent
to which performance on memory measures might be compromised by specific impairment of
attention. Beatty et al. (1996) examined acquisition and retrieval processes in MS and identified
specific profiles of memory impairment, which could not in isolation be explained by either
faulty retrieval mechanisms or by slowed acquisition. It is now agreed that multiple cognitive
processes contribute to memory performance, one of which is attentional ability. There is also
evidence that MS participants often confuse attention and memory problems when reporting
on the nature of their cognitive problems (Lezak et al., 1990). As Hutchinson et al. point
out “care must be taken to distinguish between attention deficit and problems with memory
proper” (p. 5). Future research therefore, should invest in making this relationship between
attention and memory more conceptually transparent.
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