Interference in dual-fluency tasks after anterior and posterior cerebral lesions

Abstract

According to earlier findings, letter fluency, repetitive pattern drawing and figural fluency are more sensitive to anterior than to posterior brain lesions. The aim of this study was to demonstrate that the percentage impairments from the single-task results are more pronounced after anterior than after posterior lesions when letter fluency is performed simultaneously with pattern drawing or figural fluency. The single-task results showed no significant differences between the patients with anterior and posterior lesions. The patients with anterior lesions, especially those with left-anterior lesions, had more pronounced percentage dual-task impairment than the other patients in letter fluency but not in pattern drawing or figural fluency. The results did not confirm the prediction that the average of the percentage decrements of the concurrent performances (the combined dual-task cost) or the larger of the two decrements would be more pronounced after anterior than posterior lesions. However, the patients with left-hemisphere lesions were inferior to those with right-hemisphere lesions in the single letter-fluency task, and the combined dual-task cost was more pronounced after left-hemisphere lesions.

Introduction

Frontal lobes are considered to be important in the co-ordination, integration and control of specialised cognitive skills during goal-directed or controlled activity [14], [30]. These executive functions are needed in novel tasks which cannot be accomplished directly with earlier acquired stimulus-driven or automatic skills [29]. Although, there is wide agreement on the importance of frontal-lobe activity for executive functions, not all frontal lesions cause executive dysfunction [5], and various frontal areas may make different contributions. Moreover, non-frontal brain structures might also be involved in executive functions. Comparison of the effects of focal anterior and posterior lesions on measures of executive function is one way to study this problem.

Simultaneous performance of two tasks has been utilised to study the efficiency of executive functions [3], [6]. The component tasks should not be habitually performed simultaneously and should not depend on the same special-purpose processors [13]. Several studies have shown that dual-task performance is more sensitive to diffuse brain pathology than separate performance of the component tasks [2], [4], [8], [11], [15], [21], [32]. However, not all studies have confirmed this hypothesis [10]. Based on their experiments and meta-analysis, Park et al. [24] argued that people with traumatic brain injury are impaired on divided-attention tasks requiring controlled processes on episodic or semantic memory, but not on divided-attention tasks with perceptual or motor components not depending on the controlled retrieval of stored information.

Experiments with healthy people have shown that the interference caused by a simultaneous task is more pronounced on letter-fluency performance, which is sensitive to frontal-lobe lesions, than on category-fluency performance, which is not sensitive to frontal lesions [20], [23]. However, direct evidence of the sensitivity of dual-task performance to frontal-lobe pathology is less clear. Vilkki et al. [32] found more severe dual-task decrement in patients with acute closed head injury than in non-brain-damaged control patients, but patients with focal frontal-lobe lesions did not differ from patients with non-frontal lesions or from control patients. Baddeley et al. [5] demonstrated that frontal-lobe-damaged patients with behavioural dysexecutive syndrome showed more pronounced dual-task decrement than frontal patients without the syndrome, but they did not study patients with non-frontal lesions. Functional magnetic resonance imaging revealed prefrontal activation when healthy participants were simultaneously making semantic judgements and spatial rotations, but only posterior activation was observed when the tasks were performed separately [13]. This finding strongly suggests that the contribution of prefrontal functions is important in dual-task performance.

If two tasks are sensitive to the effects of frontal-lobe lesions and require the same executive resources when performed separately, simultaneous performance of these tests should considerably increase their sensitivity to frontal-lobe pathology. Letter-fluency [22], repetitive-pattern-drawing [18], [19] and figural-fluency [16], [28] tasks have been widely used in the neuropsychological assessment of executive functions and of the consequences of frontal-lobe pathology [17], [18], [27]. We predicted that the patients with anterior brain lesions would show more pronounced percentage impairment from the corresponding single-task results than the patients with posterior lesions when letter fluency was performed simultaneously with pattern drawing or figural fluency.

In a dual task, participants may be asked to consider both tasks equally important, or to concentrate primarily on one of them. The latter instruction usually improves the performance of the attended-to task and impairs the performance of the other. The ability to deliberately divide attention between concurrent performances depends on the type of tasks. For instance, a concurrent task during encoding causes profound impairment of memory, but similar interference during retrieval does not considerably reduce memory [7]. Craik et al. [12] confirmed this result and demonstrated that free recall causes more pronounced impairment of concurrent performance (visual-motor reaction-time task) than encoding. The authors suggested that ‘retrieval processes are in some sense obligatory, or are protected, with the result that attentional resources are demanded and redeployed for their execution.’ Evidently, attentional resources required by free recall could not be deliberately restricted and allocated to concurrent performance.

For the present study, the participants were told that the concurrent tasks were equally important. They were requested to carry out the simultaneous tasks so that neither performance would be notably more impaired from the single-task performance than the other because only the larger impairment of the two percentage changes would be taken into account as a measure of dual-task decrement [32]. Consequently, the participants had to deliberately control and balance the division of resources between the two tasks as accurately as possible.