Metabolic and cardiorespiratory measures of mental effort: the effects of level of difficulty in a working memory task.

24 participants (12 female) performed a continuous memory task during which metabolic, cardiorespiratory, performance, and subjective mental workload measures were taken. Task difficulty was varied using two manipulations in a within-subjects factorial design: memory load (one or three items) and temporal demand (interstimulus intervals of 2, 3, or 4 s). Males and females differed in initial metabolic rate, but did not differ in their response to the task. Memory load affected all measures, while temporal demand affected only respiration rate, performance, and subjective mental workload. Metabolic, Respiratory, Cardiovascular, and Subjective/Performance components were identified in a principal components analysis (PCA), and the Respiratory and Subjective/Performance components were affected by the task manipulations. When performance quality was examined, the Metabolic component revealed that poor performers had greater energy expenditure during the task than good performers, and the Cardiovascular component revealed that good and poor performers differed in their response to memory load and temporal demand. Cardiac and metabolic changes during mental work were not a function of overall mental effort, but were specific to the effort due to memory load and to the individual differences among participants in their ability to perform the task. However, respiration was sensitive to the mental effort associated with both memory load and temporal demand, but was not sensitive to individual differences.