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Given the broad range of functions thought to be impaired in patients with MS it is rather surprising that little effort has been devoted to investigating whether attentional mechanisms might be impaired. This might be considered to be a grave omission given that attention is critical to the efficient performance of all intelligent behaviour. It might, for instance, be the case that

the prevalence of memory deficits reported in such populations is secondary to a more fundamental impairment in attention. This is certainly plausible given the findings of DeLuca et al. (1994) which suggest that memory impairment in MS is caused by a deficit in the initial acquisition of information. It might also be possible that the impaired speed of processing efficiency common to MS patients is inextricably linked to a disruption of the fundamental processes of an attentional system. Many studies explain deficits of cognition in MS patients in terms of an inability to deal with information rapidly. If indeed it is the case that attention is disturbed in MS patients then it may be worthwhile to consider whether attentional deficits can be disentangled from information processing deficits. It would also appear to be the case given the pathological course of MS that attention might at the very least be expected to be compromised. Filley

at al. (1988) comment "White matter diseases, which are typically diffuse or multi-focal, could disrupt the global attentional system at several points". (p.249). Caltagirone (1991) comments on evidence reported from autopsy studies "that ventricular enlargement, peri-ventricular and pre-frontal plaques could all produce damage to fibre tracts interconnecting frontal lobes and other cortical and sub-cortical areas of the CNS" (p. 138). In the light of Mesulam's (1981; 1985) neuronal network of attention hypothesis linking parietal, frontal, limbic and brain stem structures, it might be plausible to expect impairment on a variety of attentional measures in MS.

While some studies investigating attentional impairment in MS have been reported, the findings remain largely inconclusive and serious methodological issues need to be addressed.

Elsass and Zeeberg (1983) compared 50 MS patients with 105 non neurologically impaired patient controls using a continuous reaction time paradigm. The authors reported that the MS patients had significantly delayed

reaction times in comparison to controls and this correlated significantly with degree of disability. However, it is not clear from the results of the study whether a primary motor deficit could account for patients' slowed reaction times. One might even hypothesise that reaction times might be expected to be slower in MS patients; the crucial point therefore being real differences between conditions rather than between patient s.

DeLuca et al. (1995) examined attention and concentration in a group of MS, chronic fatigue syndrome (CFS) and depressed patients. In this study attention and concentration were assessed using forward and backward digit span tests and variations of PASAT (paced auditory serial addition tests). Their results showed that while CFS and MS patients exhibited difficulties on measures of information processing efficiency, MS patients were not different from matched controls on measures of Digit Span. They interpreted these findings as indicative of the fact that MS patients did not manifest attentional problems. The findings of this study could be challenged on the grounds that DeLuca at al. failed to provide subjective measures of fatigue, which may have grossly affected the performance of the Chronic Fatigue group thereby masking some of the fundamental problems known to be impaired in MS patients. One serious methodological factor appears critical to the interpretation of their findings. They report "The tests and questionnaires were given in one session lasting about 150 minutes" (p.39). This translates to a session lasting nearly 3 hours which given the characteristics of the clinical groups employed seems extremely inappropriate.

Van den Burg at al. (1987) employed a variety of measures to investigate attentional problems in a group of MS patients with mild physical disability. They report that their results "did not reveal a trace of attentional deficits in patients with MS" (p.497).

A more recent study by Callanan et al. (1989) investigated a range of functions including visual and auditory attention in a group of 48 patients with clinically isolated lesions of the type seen in MS. They reported that "attention deficits are the most prominent cognitive abnormalities in these patients" (p.370). Visual attention was assessed by recording the time taken to cancel a target letter from an array of mixed letters. It is hardly surprising that patients did poorly on such a measure given the emphasis on speed of performance, a function known to be disrupted in MS patients. Auditory attention was assessed using a variant of the traditional vigilance task. Five groups of letters were read at a speed of 1 letter per second and embedded among them was the alphabet in the conventional order. The subject's task was to signal when each letter of the alphabet was presented. The total number of errors was recorded as a measure of auditory attention. Callanan at al. reported the cohort to be significantly impaired on this measure. In fact they proceed to argue that "measures of visual and auditory attention were impaired in patients with clinically isolated lesions whose performance on a number of other tests was normal, suggesting that this may be one of the sensitive measures of cognitive impairment in MS" (p.371).

Of course, the extent to which the performance of patients on single tests of visual and auditory attention can be taken as a global index of the state of current functioning of their entire attentional system is questionable. This of course is a conceptual issue and is relevant to all of the studies described in this section. There is no disputing that specific attentional measures are impaired in these patients but one must guard against over-interpretation. Impairments on specific measures of attention cannot be taken as an accurate indication of the current status of their entire attentional system. Fatigue and psychomotor slowing, characteristic consequences of MS pathology, are also relevant variables and should be considered when interpreting performance

scores.

Cohen and Fisher (1989) reported that MS patients showed difficulties on measures of sustained attention, as well as a greater inconsistency across trials than normal controls, which they attributed to fatigue. They suggest that the deficit could not be entirely explained in terms of slowed processing as the inter-stimulus interval was sufficiently large to allow subjects to make their response.

Cohen adds "although the problems with sustained attention that have been noted with MS may be partly related to psychomotor slowing, it would be a mistake to assume that impaired psychomotor functioning indicates that attention is intact, or that psychomotor slowing precludes the existence of a fundamental attentional disturbance" (p.257). They continue with the argument that psychomotor slowing may in fact reflect the basis of attentional disturbances in MS and that perceptual-motor slowing and attentional difficulties may reflect a common source of dysfunction.

More systematic investigation of attention is critical and has only recently been undertaken in other neuro-degenerative disorders. Sprengelmeyer et al. (1995) examined the pattern of attentional deficits in Huntington's disease. They reported patients' performance to be severely impaired on divided attention and inter-modal integration tasks. Similarly, Bennett et al. (1995) investigated covert visual spatial attentional mechanisms in Parkinson's disease. They concluded that while orienting attention to an expected source of stimulation was not compromised in their population of Parkinson's disease patients, the patients' ability to modulate their attentional focus or manage more than one attentional task at a time was impaired.

A further reason for suspecting attention to be impaired in MS is linked to the idea that MS is so often classified as a sub-cortical dementia, in which impairments in sustained attention are frequently reported. Filley et al. (1989) administered an extensive neuropsychological battery of tests to a cohort of patients with dementia of the Alzheimer's type (DAT) (cortical disease classification) and MS patients (sub-cortical disease classification). They reported that MS patients were more impaired on measures of sustained visual and auditory attention while DAT patients were worst on tasks exploring memory and language functions. Filley etal. (1989) interpreted the findings as support for the hypothesis suggesting differential profiles of cognitive impairment in dementias caused by cortical (DAT) and sub-cortical (MS) pathological changes. Others, such as Beatty et al. (1988), would dispute such a pure distinction since they report findings which suggest that the cognitive impairment in MS shares features common to both cortical and sub-cortical dementias. Filley et al. (1989) nevertheless interpreted the attentional deficit described in MS patients as resulting from some disruption of a diffuse attentional system arising from the brainstem structures and /or a damage to pathways originating in the prefrontal cortex and terminating in posterior cortical areas. This is very convincing especially in the context of Mesulam's (1981;1985) neural network of attention hypothesis.

There is clearly one obvious reason for the paucity of research into attentional impairments in MS. Our understanding of any impaired cognitive process remains critically dependent upon a properly constructed model of processing in normals. In psychology, attention has only recently again become an interesting and challenging area of investigation. Unlike other cognitive systems such as language or memory, the mechanisms underlying attention are unclear and incomplete. For this reason no formal measures of attention have been available and deficits of attention in other neurologically

impaired patients have been based on measures of digit span or on measures such as PASAT or Stroop tasks. Although these measures are sensitive to some aspects of attentional processing, it has to be realised that enormous advances have been achieved in the theoretical understanding of attentional mechanisms (even since the beginning of this project four years ago), but the application of this experimental work to the development of more sensitive assessment tools has lagged behind. Fortunately a test of everyday attention (TEA) has recently been developed, (Robertson et a!., 1994) based on the empirical findings of Posner et al. (1990). This assessment tool will be described in more detail in a later section as it is to be employed in the remaining clinical experiments of this study.

4.8 SUMMARY

In summary, MS patients present with impairments of memory, conceptual reasoning, sustained attention, visuo-perceptual abilities, and on tasks requiring rapid processing of information all within the context of generally preserved language ability. This pattern of impairments has led some investigators to classify MS as a sub-cortical dementia. It contrasts sharply with dementias of the cortical gray matter which are characterised by prominent amnesia, aphasia and agnosia (e.g. DAT). It is unclear from the literature whether such a classification is of any clinical benefit since some studies propose that MS shows characteristics of both cortical and sub-cortical dementia. (See Rao et a/., 1991 for a more detailed discussion)

A substantial number of studies have investigated the relationship between cognitive impairment in MS and variables such as disease course, degree of disability, duration of illness, medication usage and emotional status. The relationship is controversial with many of the studies reporting conflicting results. However, the majority of investigators have found no association

between degree of disability, duration of illness or medication usage and cognitive dysfunction. Some studies have reported that cognitive performance in depressed MS patients is more impaired than in non-depressed MS patients although this discrepancy is rarely significant. Others have attempted to link poor performance with patient's fatigue and consequent low motivation. It is well established that low motivation can adversely affect cognitive performance,

but this is unlikely to account for all of the cognitive difficulties found in MS. As mentioned, in the majority of studies disease course was found to be unrelated to cognitive impairment. Grossman et al. (1994) systematically investigated whether relapsing-remitting (RR) or chronic progressive (CP) MS patients showed significantly different patterns of cognitive impairment. In general, CP patients were more severely impaired than patients in the RR group, but the overall pattern of deficits was not grossly dissimilar in the two groups.

Finally, many studies have investigated the relation between cognitive performance in MS patients with quantified lesion scores derived from MR imaging. As expected, the extent of cerebral demyelination correlates significantly with severity of cognitive impairment. Swirsky-Sacchetti (1992) employed 'lesion severity variables' such as total lesion area (TLA), ventricular-brain ratio and size of corpus callosum as well as ratings of lesion site (e.g. frontal, temporal, and parieto-occipital regions) and correlated them with performance on a broad range of neuropsychological tests. They found neuropsychological test scores to be highly related to all measures of degree of cerebral involvement, with TLA being the best predictor of neuropsychological deficit. They reported mean lesion area for the cognitively impaired group to be 28.30cm^ and mean lesion area for the cognitively intact group to be 7.41 cm^. Lesion sites also correlated significantly with impairment on specific cognitive tests.