Performing two tasks simultaneously (dual task performance) is a frequent activity in human life. However, performing dual tasks can be difficult. When people attempt to perform dual tasks, performance is generally impaired, manifested by increased errors or reaction times compared with when the tasks are performed individually. Such a deterioration of performance is defined as dual task interference.¹

The underlying mechanism of dual task interference is still unclear. It has been described as a competition for attentional resources,² or competition for information-processing mechanisms.³ Three of the most influential explanations are capacity-sharing, bottlenecks, and cross-talk.¹ These are ‘attentional’ models, with the term ‘attentional’ referring to the focus of mental activity on a task. The capacitysharing model is based on the assumption that attentional resources are limited. When people perform two tasks simultaneously, resources must be divided between the tasks. How attention is divided between the two tasks relies on several factors, including task complexity, familiarity, and importance. According to this model, dual task interference occurs only if the available resource capacity is exceeded, resulting in a decline in performance on one or both of the tasks.^2–4 The bottleneck theory refers to the idea that certain critical mental operations must be carried out sequentially. A bottleneck arises when two tasks require a critical mental operation at the same time-point.5 In contrast, the cross-talk model assumes that task similarity reduces dual task interference because the use of the same pathway increases the efficiency of processing by using less attentional resource capacity.^4–7 Patients with Parkinson’s disease (PD) commonly have difficulties in performing movements. This problem becomes more prominent during their performance of complex movements, including dual tasks. Schwab et al.⁸ described that PD patients have particular difficulty executing two motor tasks simultaneously. This problem is more obvious when patients perform different motor acts with each hand. For example, a patient may be unable to draw a triangle with his or her dominant hand while squeezing a bulb with the other hand, or he or she may be very slow in performing simultaneous tasks, such as flexing the elbow and pinching the thumb and index finger at the same time. Benecke et al.^9,10 found that when PD patients were asked to perform rapid elbow movements combined with a simultaneous or sequential hand movement, they showed a marked slowing of movement greater than that seen in each task individually. By contrast, there was no decrement of performance in normal subjects when two tasks werecombined. This deficiency correlated better with clinical measures of bradykinesia, and improved more impressively after administration of L-Dopa than the slowness in each component movement.¹¹ Similar observations have been described in several subsequent studies.^12–14

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