Dual-task interference

Much of my research relates, in one way or another, to the limitations of dual-tasking – the decrements in performance observed when people try to do two things at the same time.  Dual-task situations are becoming increasingly prevalent in modern life, as, for example, people operate cell phones or ipods as they drive, talk, or write papers.  I believe that studying the interactions between concurrently performed tasks provides a powerful tool for examining the processes and representations that govern our behavior.  I view my general approach as analogous to a particle physicist who studies the results of collisions of particles too small to directly observe: by examining the interactions between simultaneously performed tasks, I attempt to probe the structure of response selection processes.  The patterns of dual-task costs I have observed in a number of studies are not accounted for by any of the existing theories of response selection and therefore provide an opportunity to break new theoretical ground.

The starting point of this line of research is a study examining whether practice allows individuals to perform two tasks at the same time.  We (Hazeltine, Teague, and Ivry, 2002) set out to determine whether the absence of dual-task costs could be accounted for by models that assumed that responses were selected in a one-at-a-time, serial fashion.  We added a short interval between the stimuli for the two tasks and found that, after practice, this interval affected the time between the two responses but not the response times (i.e., the interval between the stimulus and the response).  Thus, it appeared that individuals were able to select two responses at the same time.  However, a follow-up study (Hazeltine, Ruthruff, & Remington, 2006) showed that dual-task costs strongly depended on the specific combination of tasks.  When a task using auditory stimuli and manual responses was paired with a task using visual stimuli and vocal responses, dual-task costs were robust throughout training.  In contrast, the costs were essentially eliminated after training when a task using auditory stimuli and vocal responses was paired with a task using visual stimuli and manual responses.  The difference in dual-task costs was particularly striking because the stimuli and responses were highly similar for the two groups and because the single-task reaction times were nearly identical. 

To further illuminate how the relationship between the tasks interacts with practice to determine dual-task costs, we are currently performing a series of experiments in which we systematically manipulate whether the stimuli for the two tasks occur on the same modality and whether the responses occur on the same modality.  The results indicate that the complete elimination of dual-task costs reported by myself and others occurs only under a fairly narrow set of conditions.  Moreover, the findings suggest that early in practice, task difficulty plays the dominant role in determining dual-task costs.  However, after several sessions of practice, it appears that the overlap in the stimulus-response pairings across the two tasks drives the dual-task costs (Hazeltine, Ruthruff & Wifall, in prep). 

Given these findings, I have proposed a new model of response selection in which performance early and late in practice involve distinct processing strategies (Hazeltine, Ruthruff, & Remington, 2006).  Early performance is controlled in the sense that central operations are performed serially.  In contrast, after practice, central operations can take place in parallel.  However, this does not mean that the central operations for the two tasks do not interact.  When the two tasks involve similar central codes, interference between concurrently performed operations can occur, leading to persistent decrements in performance. 

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