Using resource allocation theory and dual-task methods to increase the sensitivity of assessment in aphasia

Background: Quantifying the severity of language impairment and measuring change in language performance over time are two important objectives in the assessment of aphasia. The notion of cognitive effort as understood from a resource allocation perspective provides a potentially useful complement to traditional constructs employed in aphasia assessment. Aims: The series of experiments described in this paper used resource allocation theory and dual-task methodology (1) to assess whether a language comprehension task (Story Retell Procedure) and a visual-manual tracking task trade performance under dual-task conditions, and (2) to investigate the potential utility of these methods in clinical assessment of aphasia. In Experiment 1, the validity of a difficulty manipulation of the SRP was investigated. In Experiments 2 and 3, the reliability and validity of the visual-manual tracking task were evaluated. Experiment 4 investigated whether the two tasks trade performance under dual-task conditions. Methods & Procedures: In Experiment 1, 20 normal participants listened to and retold stories presented by a normal speaker and speakers with mild, moderate, and severe aphasia. Participants' comprehension performance was measured by calculating the amount of information retold per unit time. In Experiment 2, root mean square (RMS) tracking error data were collected under fixed joystick displacement conditions. In Experiment 3, 20 normal participants performed single-task tracking across 12 trials at each of three difficulty levels, and performance was evaluated in terms of RMS error. In Experiment 4, three groups of 20 normal individuals performed the tracking task while listening to stories told by the normal speaker and speakers with aphasia. Story retell performance was evaluated between subjects across three tracking difficulty levels and tracking performance was evaluated within subjects across story difficulty (normal, mild, moderate, and severe aphasia). Outcomes & Results: The results of Experiments 1-3 supported the reliability and validity of the difficulty manipulations for the story retell and tracking tasks. In Experiment 4, tracking performance was found to vary significantly across story difficulty, with subjects demonstrating better tracking performance while listening to stories told by a mildly aphasic speaker than during stories told by a speaker with moderate aphasia. There was no effect of tracking difficulty on story comprehension as measured by subsequent story retell performance. Conclusions: The results provide qualified support for both a resource allocation view of language performance in normal individuals and the potential utility of these methods in the assessment of aphasia. These conclusions, however, are mitigated by the finding of only a unidirectional (as opposed to bidirectional) performance trade, and by the fact that the effect of story difficulty on tracking performance was observed across only two levels of aphasia severity.