Pickin', Singin', and Attendin': Possible Limitations on Human Processing at the Highest Level of Accomplishment

Psychologists have long investigated whether humans can carry out more than one task at once. In music, the classic example of a dual-task performance is singing while playing an instrument. That is, if a vocalist accompanies him- or herself on guitar, successful performance requires the simultaneous maintenance of pitch and rhythmic accuracy on two separate instruments. Laboratory studies investigating dual-task performance, though usually involving the tracking of visual stimuli, have found typically that performance on either task can suffer (for review see Pashler, 1998). More recent studies of musicians have shown that there are instances in which complicated tonal and rhythmic processing can be coordinated quite well (Keller & Repp, 2008), but that the coordination of two musical tasks is difficult and relevant to the cognition of human dual-task performance.To account for the difficulty in combining two tasks, theorists have postulated processing limitations of various sorts in human cognition. One such limitation is a bottleneck: a structural limit in the cognitive system. Another processing-limitation view postulates a finite pool of resources available for carrying out cognitive activities. If tasks demand more of those resources than are available, performance on one or more task will suffer. Recent studies have provided support for several different sorts of predictions emanating from the bottleneck and resources views (see Kamienkowski, Pashler, Dahaene, & Sigman, 2011; Levy & Pashler, 2001).The study of dual-task processing has not been without controversy, however. Although the processing-limitation perspective has been dominant, there has been interest in the possibility that humans can carry out multiple tasks concurrently, and without error. Allport, Antonis, and Reynolds (1972) found that music students could accurately sight-read music while shadowing verbal material (see also Shaffer, 1975). The Executive Processing Interaction and Control model of Meyer and Kieras (1997a, b), postulates no structural limitations to human processing (but see Anderson, 2007). Deficits in dual-processing can occur in three ways: (a) interference due to stimulus or response overlap, which can be eliminated by using tasks from different domains; (b) lack of practice in combining tasks, which can be eliminated by practicing tasks conjointly; and (c) participants' strategies, arising from instructions giving one task priority, which can be eliminated by placing emphasis on both tasks. Studies have demonstrated equivalent levels of performance by individuals carrying out two tasks concurrently versus separately (Hazeltine, Teague, & Ivry, 2002; Oberauer & Kliegl, 2004; Schumacher et al., 2001; but see Levy & Pashler, 2001). Hirst, Spelke, Reaves, Caharack, and Neisser (1980) showed that dual-task performance can be increased by practice in carrying out tasks conjointly, although their participants were not able to reach the point of being able to carry out two demanding tasks without error (see also Oberauer & Kliegl, 2004).Understanding multiple-task performance promises to illuminate basic aspects of human information processing, and the findings may facilitate our understanding of situations in which humans try to carry out multiple tasks, beyond musical examples. However, questions can be raised concerning recent studies that have found errorless dual-task performance. First, a number of more recent studies have produced results that can be interpreted as supporting the idea that there are limitations to human processing capacity that may be extremely difficult if not impossible to overcome (Kamienkowski, Pashler, Dahaene, & Sigman, 2011; Levy, Pashler, & Boer, 2006; Pashler, Johnston, & Ruthruff, 2001; Ruthruff, Johnston, & Van Selst, 2001; Van Selst, Ruthrauff, & Johnston, 1999). A second problem with demonstrations of errorless dual-task performance comes from the fact that that conclusion depends on acceptance of the null hypothesis-no differences are found between dual-task versus single-task conditions-which requires that we have confidence in the sensitivity of our design to detect processing limitations. …