Working Memory and Persistent Inhibitory Set: An Individual Differences Investigation

Inhibition of task-irrelevant interference is important component of executive attention. In this study, the participants were divided into high working memory (WM) and low WM groups and the persistent inhibitory task inertia after the Stroop color-naming task was tested by measuring naming latencies for neutral colored stimuli. The high WM group tended to show persistent stronger inertia than the low WM group. It is suggested that the inhibitory control is an involuntary process that is mobilized when attention is voluntarily directed to a task that involves automatic activation of competing responses like the Stroop effect. Working memory (WM) is an important mental faculty, enabling us to stick to our current task in spite of so many intrusions or allurements. It is believed to be main part of executive control of attention (Engle, 2002). The executive control is, at least partly, made possible by suppressing interferences from task-irrelevant stimulations from surrounding environment. Exclusion of task irrelevant signals is made possible by inhibition exerted on them when we have to concentrate on our task. That exclusion of irrelevant signals is part of the working memory capacity is demonstrated by the study reported by Conway, Cowan, and Bunting (2001). In this study, these researchers divided their participants into two groups according to their WM capacity and asked them to attend to one message presented to one ear while ignoring another one coming from the other ear. While they were repeating a message (the procedure is called shadowing) presented to the attended ear, their names were presented to the other, unattended ear unknowingly to them. It has been known that an important message like one's own name can break through the barrier set by attention. Thus, about one third of the participants were reported to be able to detect their own name presented to the unattended ear (Moray, 1959). However, when participants were divided into high and low WM groups, 65% of the low WM participants could detect their own name in the unattended message, while only 20 % of the high WM participants did so. These results suggest that those with high WM capacity are less vulnerable to the task-irrelevant intrusions from both outside world and from within their own mind (Teasdale, Dritschel, Taylor, Proctor, Lloyd, Nimmo-Smith, Baddeley, 1995). What is less clearly understood is whether inhibition is a voluntary process under the control of executive attention or it is an automatically mobilized by-product beyond its control. For example, when they attend to one ear, do people impose inhibitory barrier to the other ear by their own voluntary control? Or is it automatically set in place when they voluntarily attend to the message presented to the attended ear? In this study, we explored this issue of voluntariness of the inhibitory control using the inhibitory task set inertia. The inhibitory task set inertia is the persistent residual cost that accompanies task switch (Wylie and Allport, 2000). Task switch is a manifestation of executive control encountered when one task is switched to another, like the one you undergo when you respond to phone-call during writing sentences with word processor. Just after the switch, performance of the new task declines usually for a short period after the switch. Eventually it will return to the level of operation performed before such a switch. However, there is a residual effect of inhibition lasting much longer than the usual task switching cost. This is called the task-set inertia and ascribed to proactive interference arising from the performance of a prior, competing task (Wylie and