The notion that the human information processing system has a limit in resource capacity has been used for over 100 years as the basis for the investigation of a variety of constructs and processes, such as mental workload, mental effort, attention, elaboration, information overload, and such. The dual task or secondary task technique presumes that the consumption of processing capacity by one task will leave less capacity available for the processing of a second concurrent task. When both tasks attempt to consume more capacity than is available, the performance of one or both tasks must suffer, and this will presumably result in the observation of degraded task performance. Consider, for example, the amount of mental effort devoted to solving a difficult arithmetic problem. If a person is asked to tap a pattern with a finger while solving the problem, we might be able to discover the more difficult parts of the problem solving process by observing changes in the performance of the secondary task of finger tapping. While a participant is reading a chapter of text in a book or on a Web browser, we might be able to use this same technique to find the more interesting, involving, or confusing passages of the text. Many implementations of the secondary task technique have been used for more than a century, such as the maintenance of hand pressure (Lechner, Bradbury, & Bradley, 1998; Welch, 1898), the maintenance of finger tapping patterns (Friedman, Polson, & Dafoe, 1988; Jastrow, 1892; Kantowitz & Knight, 1976), the performance of mental arithmetic (Bahrick, Noble, & Fitts, 1954; Wogalter & Usher, 1999), and the speed of reaction time to an occasional flash of light, a beep, or a clicking sound (e.g., Bourdin, Teasdale, & Nourgier, 1998; Owen, Lord, & Cooper, 1995; Posener & Bois, 1971). In using the secondary task technique, the participant is asked to perform a secondary task, such as tapping a finger in a pattern, while performing the primary task of interest. By tracking changes in secondary task performance (e.g., observing erratic finger tapping), we can track changes in processing resources being consumed by the primary task. This technique has been used in a wide variety of disciplines and situations. It has been used in advertising to study the effects of more or less suspenseful parts of a TV program on commercials (Owen et al., 1995) and in studying the effects of time-compressed audio commercials (Moore, Hausknecht, & Thamodaran, 1986). It has been used in sports to detect attention demands during horseshoe pitching (Prezuhy & Etnier, 2001) and rock climbing (Bourdin et al., 1998), while others have used it to study attention associated with posture control in patients who are older or suffering from brain disease (e.g., Maylor & Wing, 1996; Muller, Redfern, Furman, & Jennings, 2004). Murray, Holland, and Beason (1998) used a dual task study to detect the attention demands of speaking in people who suffer from aphasia after a stroke. Others have used the secondary task technique to study the attention demands of automobile driving (e.g., Baron & Kalsher, 1998), including the effects of distractions such as mobile telephones (Patten, Kircher, Ostlund, & Nilsson, 2004) and the potential of a fragrance to improve alertness (Schieber, Werner, & Larsen, 2000). Koukounas and McCabe (2001) and Koukounas and Over (1999) have used it to study the allocation of attention resources during sexual arousal. The notion of decreased secondary task performance due to a limited-capacity processing system is not simply a laboratory curiosity. Consider, for example, the crash of a Jetstream 3101 airplane as it was approaching for landing, killing all on board.
Adjustment of cognitive control to the frequency of dual-task interference