The “posture second” strategy: A review of wrong priorities in Parkinson's disease
Introduction
Life is about priority, evident even in mundane tasks such as driving a car and using a mobile phone at the same time. Most people can achieve this during regular driving, but the conversation is likely to cease temporarily when a busy crossing is approaching. This “secondary task” (or dual task, as it is more commonly termed) interference is at play during everyday tasks, including seemingly simple acts such as walking. In the field of balance and gait research, perhaps one of the most influential publications of the last decade was the description of the “stops walking while talking” (SWWT) test by Lundin-Olsson et al. [1]: an inability to walk and talk at the same time had a good predictive value for the occurrence of falls in the next 6 months. This observation was made in a mixed group of elderly persons, many of whom were depressed, had dementia, or both. Apparently, the two concurrent tasks of walking and talking competed within the central nervous system. The inference was that those unable to talk while walking had a restricted central processing capacity, permitting them to do only one task at a time to avoid a system overload. This restricted central processing capacity could also explain the association between “stops walking while talking” and dementia or depression; whether depression or dementia itself is in any way causally related to dual tasking limitations has never been demonstrated. Another consequence of this finding — with a potentially far-reaching implication — was that gait was not a simple automatic task that is governed solely by subcortical structures, but in fact represents a much more complicated job requiring conscious attention and perhaps some ongoing cognitive processing.
The Lundin-Olsson paper left many questions unanswered. For example, it is theoretically possible that some persons purposely stopped walking while talking, simply because they considered this to be unsafe. If so, then such people who stop walking while talking should perhaps have the lowest risk of falling, not the highest, because they opt for a safe behaviour. Clearly, this was not true for the group as a whole, where the overall fall risk was increased, but individuals within the group may have chosen different strategies. In other words, dual tasking difficulties may well be solved in different ways by different populations, depending on such factors as age, disease status, or prior experience. This would set limitations to the generalisation of dual tasking problems. Also, some may have stopped walking while talking out of politeness to face the person you are talking to, which in effect deprives the person of visual feedback of the gait trajectory? This by itself may be enough reason for persons who rely heavily upon visual feedback to stop walking. And what about the nature of the secondary task, which was actually poorly defined in the original description as “maintaining a routine conversation”? Might a more complex and more demanding secondary task be able to predict falls even better? Is the dual task interference restricted to “cognitive” secondary tasks, or is a secondary “motor” task also able to jam the system? And, finally, what about dual task problems in patients with neurodegenerative diseases characterised by cognitive decline, restricted central processing capacities, or frequent falls?
Because of these many unanswered questions and the potentially important implications, the “SWWT principle” was widely followed and extrapolated to a host of other tasks — including balancing and a range of other secondary or even tertiary tasks — and to a range of pathological conditions, including e.g. Alzheimer's disease [2], [3], Parkinson's disease (PD) [4], [5], [6], stroke [7], vestibular disorders [8] and peripheral neuropathy [9]. Adverse effects on balance were noted not only for attention-demanding secondary tasks with cognitive loading, but also for types of secondary tasks that stressed the motor systems. For example, difficulty with carrying a glass of water while walking can also predict falls in the elderly [10]. Here, we will briefly review our own work in this field, which mainly concentrated on patients with PD, with some extensions to the ageing processes. The results provided some new insights into the mechanisms underlying “failure” to execute multiple tasks simultaneously — and in particular on the role of priority processes.
Section snippets
Background
Falls are very common in patients with PD. Depending on the duration of follow-up and the method of falls ascertainment, prospective studies identified an incidence of persons with at least one fall from 39 to 68%, and from 25 to 50% for recurrent (twice or more) fallers [11], [12], [13], [14]. The impact of these falls is considerable, due not only to the associated injuries, but also because of the secondary immobilisation caused by a fear of renewed falls [13], [15], [16]. Prevention of
Getting the priorities entirely wrong
We have suggested that young healthy subjects are well able to cope with complex situations by adopting “safe” strategies (prioritising balance over other concurrent tasks), and that such behaviour is less often seen in elderly persons and, in particular, in patients with PD. Interestingly, however, even young healthy subjects may occasionally get their priorities wrong. In a nicely designed study, Bhateni et al. [47] suddenly perturbed upright standing young persons who held a cane (or, in
Implications for treatment
Recognition of dual tasking limitations and their impact on the risk of falls may have treatment implications. It is conceivable that safer dual tasking strategies can be trained by physiotherapists, for example by instructing patients to avoid secondary tasks during complex walking or balancing activities [56]. Another possibility is the use of cognitive rehabilitation [57]. Such treatment possibilities and their effect on everyday performance have thus far not been investigated, and this
Conclusions
Evidence is beginning to accumulate that healthy subjects may correctly perceive the difficulty of multiple task performance, and purposely lend priority to execution of one part of a complex task, at the expense of other elements. Which particular type of strategy is chosen may depend on the preference of individual subjects. Some persons will adapt their behaviour by decreasing the walking speed and thus avoid the risk of a fall. Others may lend priority to the walking task at the expense of
Acknowledgements
Dr. Bastiaan R. Bloem was supported by a research grant of the Prinses Beatrix Fonds.
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