The present study tested predictions from resource-depletion (Gluckman, Warm, Dember, & Rosa, 1993) and effort-regulation (G. Matthews & Desmond, 2002) models of performance effects associated with transitions from a high to a low level of task demand. The former is supported when transitioned participants do more poorly on the post-shift task than non-shifted controls; the latter is supported in the opposite case. A compensatory tracking study by Ungar (2005) suggested that the absolute level of task difficulty is a critical factor to consider when testing these models. His data supported the resource-depletion model when the task was difficult and the effort-regulation model when the task was easy.
The present study was carried out to determine if these effects also extend to the vigilance or sustained attention domain. Demand transition was accomplished by shifting participants from a dual-task condition in which they shared a vigilance task with a concurrent tracking task to a single-task condition in which they performed the vigilance task alone. The absolute level of task difficulty was manipulated by varying signal salience, high (easy), low (hard). Task type, simultaneous (SIM: comparative judgment) or successive (SUC: absolute judgment) was employed as a potential moderator variable. The results with the SUC task duplicated the earlier findings of Ungar (2005)- the resource exhaustion model was supported with low salience signals and the effort-regulation model was confirmed with high salience signals. In the context of the SIM task, no evidence was found to support the resource-exhaustion model; the effort regulation model was supported regardless of signal salience. Thus, the present study extends Ungar's (2005) findings to the vigilance domain and also shows that task type is a key factor to consider when studying demand transitions in this area. In addition to these findings, measurements made with the Dundee Stress State Questionnaire (DSSQ; G. Matthews et al., 2002) indicated that task type is also a critical factor in task-induced stress in vigilance; loss of task engagement during the course of the study was greater in the context of the SIM than the SUC task.