Estimating the amount of mental effort required for independent mobility: Persons with glaucoma

PURPOSE. To validate the use of the secondary-task method to estimate the amount of mental effort required for independent travel and to determine how the amount of mental effort varies with characteristics of the environment. METHODS. Reaction time (RT) to a secondary task (randomly presented vibrations) was obtained in 28 persons with glaucoma as they walked in four different environments: hallway, high-pedestrian area, approach-to-stairs area, and stairs. Cronbach's α was used to assess the reliability of the secondary-task RT measures from the split-half measures of the high-pedestrian area. The construct validity of the RT measures as an estimate of mental effort was assessed by (1) the association between the RT scores and the item measure scores for similar environments derived from a Rasch analysis of perceived difficulty ratings obtained in a separate group of 83 persons with glaucoma and (2) the association between the RT scores and the severity of visual field loss. RESULTS. Reliability was supported by a Cronbach's α coefficient of 0.89. Construct validity was also supported by the data: (1) Log RT scores were linearly related to the Rasch analysis item measures for comparable mobility situations (P = 0.008), and (2) the log RT, averaged across the four environments, was linearly related to the mean deviation score of the Humphrey Visual Field Analyzer (estimate of visual field loss; P = 0.003; Carl Zeiss Meditec, Inc., Dublin, CA). A comparison of the log RT scores obtained in the four environments showed significantly higher log RT scores in the stairs area than in the high-pedestrian and hallway environments, whereas the log RT scores in the high-pedestrian and hallway environments were not significantly different. CONCLUSIONS. The findings in this study demonstrate the reliability and construct validity of the secondary-task RT measure as an estimate of the amount of mental effort required to travel independently. It is sensitive to environmental characteristics and the loss in a walker's visual field. The method could be an objective way to document those who may benefit from professional intervention and to evaluate the effectiveness of an intervention.