Sustained attention deficits in pat ients with right frontal lesions
Introduction
During the last two centuries there has been a great deal of speculation as to the nature of the functions performed by the frontal lobes (for reviews, see 21, 23, 42. Despite the numerous studies that have examined patients with frontal lesions, the frontal lobes have remained the most enigmatic part of the brain. Hypotheses regarding their function have included working memory 14, 33, strength of associations between elements in working memory [20], inhibition of prepotent responses 8, 33, a ‘supervisory attentional system’ 39, 40, internal representation of contextual information [2]and ‘managerial knowledge units’ [15]. While the deficits exhibited by patients with frontal lobe lesions (FLLs) on a wide variety of tasks seem to support, to some extent, any of the above hypotheses, it is unclear whether all of their deficits can be attributed to a single mechanism, or whether different regions of the frontal lobes may subserve different functions.
Several investigators have implicated the frontal lobes in the performance of tasks requiring sustained attention. In support of this hypothesis, FLL patients have been reported to show deficits on tasks requiring sustained attention 32, 37, 43. Furthermore, numerous studies using normal subjects have shown an increase in blood flow in the frontal lobes during performance of many different types of tasks requiring sustained attention 3, 7, 28, and frontal lobe dysfunction has been reported in children with attention deficit hyperactivity disorder [41].
Since the ability to sustain attention is necessary for adequate performance of many of the tasks for which FLL patients have been reported to show deficits, including the Tower of Hanoi 13, 39and the Wisconsin Card Sorting Test (WCST) [25], it is necessary to rule it out as the cause of these deficits before ascribing them to higher-order cognitive functions, such as planning.
Unfortunately, in many studies implicating the frontal lobes in sustained attention, ‘attention’ has not been well defined. It is usually defined as performance on tasks that are relatively boring and repetitive. However, virtually all tasks involve the ability to sustain attention to some extent, and it is very difficult to quantify, a priori, the degree to which a particular task will require, and can measure, sustained attention.
The study of sustained attention in normal subjects, in the form of ‘vigilance’ tasks, has a long history 1, 6, 18, 22, 27. Vigilance tasks typically involve monitoring a display for the presentation of intermittent targets, which are interspersed with a greater number of non-targets. Hundreds of studies have shown that performance declines with time on task; reaction time (RT) and proportion of targets missed increase, and there is often a similar increase in the number of false positive errors. Thus, one possible operational definition of sustained attention is the extent of decline in performance over time.
Vigilance studies in normal adults have shown that overall performance, as well as the decrement with time, is dependent on many factors, including the rate of target presentation. Generally, subjects detect a greater proportion of targets when the rate of presentation is relatively fast 4, 17, 19, suggesting that frequent presentation of the target helps to maintain subjects' alertness. Frequency of target presentation may be an important variable to control when assessing the performance of FLL patients on vigilance tasks; Wilkins and his colleagues [43]reported that patients with anterior lesions were inferior to those with posterior lesions on a target counting task and that the difference between groups was greater when the rate of target presentation was relatively slow. This suggests that the anterior patients' performance was especially bad when they were required to sustain attention over a period of time in the absence of other external stimulation to maintain their arousal.
The present study was conducted to measure differences between patients with right FLLs, left FLLs, and control subjects in the change in performance over time on several tasks requiring sustained attention. One task assessed changes in simple RT over time, another, more complex task required subjects to respond to some stimuli and refrain from responding to others. The effect of varying the rate of target presentation was also investigated. A third task was designed to assess sustained attention under more ‘natural’ conditions by presenting a short story to subjects and having them respond to certain targets. Some of the targets were predictable within the context of the story, and some were unpredictable. This task would allow us to determine whether patients with FLLs would benefit from contextual cues.
Section snippets
Subjects
Subject groups are described in Table 1. Patients were divided into two groups based on the predominant side of lesion. Lesion localization was based on CT and/or MRI scans, using templates developed by Damasio and Damasio [5]. In all patients, the majority of the lesion was located in the frontal lobes (anterior to the central sulcus and superior to the Sylvian fissure). In three there was some damage to the parietal lobes, and in five there was some temporal damage. Although the damage was
Acknowledgements
The authors would like to acknowledge Dr Bryan Fantie for help in arranging the collection of pilot data at American University, and Caroline Hollnagel, Kim Clark, Krista Wilde, Judith Wachs, David Alway and Nevada Winrow for administering neuropsychological tests. We would also like to thank two anonymous reviewers for their comments on an earlier version of the manuscript.
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