1  Introduction


Whether people talk to their co-driver while driving a car, browse the web while using other computer programs or cook a meal while monitoring their children’s activities – humans are permanently confronted with a multitude of information streams and options to behave. Multi-tasking seems to be part of the everyday life in most of us.

However, there exist fundamental limitations in our ability to do multiple things at the same time. In fact, there is strong evidence from experimental psychology that humans are generally not even able to do two simple things at the same time (Pashler, 1994; Welford, 1952). Even when trying to make two easy decisions simultaneously, severe performance costs emerge – that is, people are slower and make more errors as compared to a situation where both decisions are made sequentially. While this may be acceptable in some situations (e.g. being slower in browsing the web) it can have disastrous consequences in other situations (e.g. not braking in time for a child running on the street).

In any case, we need to control the temporal order of our actions in order to perform optimally and in accordance with our internal goals. Recent theories on dual-task processing assume that there exist control processes in the human cognitive system which coordinate the processing stream of multiple tasks and thus deal with the seemingly inherent multi-tasking deficit expressed in dual-task performance costs (Meyer & Kieras, 1997; Logan & Gordon, 2001; Sigman & Dehaene, 2006).


The most prominent neuroanatomical structure associated with the control of human behavior is the lateral prefrontal cortex (lPFC) (Duncan, 2001; Fuster, 2000; Miller & Cohen, 2001). The lPFC is known to be involved in the maintenance of information and the attentional selection and coordination of relevant and irrelevant information enabling goal-directed behavior.

It has been shown with functional Magnetic Resonance Imaging (fMRI) that the lPFC is also involved in the processing of dual tasks (D’Esposito et al., 1995; Erickson et al., 2005a; Schubert & Szameitat, 2003). These studies identified dual-task-related regions by comparing the processing of dual tasks with the processing of single tasks. Such a comparison, however, might reflect any difference between the two types of tasks. Only a few recent studies attempted to specify the type of interference and the control processes related to the dual-task-related activity in the lPFC (Dux, Ivanoff, & Marois, 2006; Herath, Klingberg, Young, Amunts, & Roland, 2001; Jiang, 2004; Szameitat, Schubert, Mueller, & von Cramon., 2002; Szameitat, Lepsien, von Cramon, Sterr, & Schubert, 2006). However, the precise functional role of the lPFC for the control of dual-task processing and the neural mechanism of dual-task coordination are still widely unknown. In particular, three important questions concerning the functional role of the lPFC in dual-task processing remain open. These questions concern:

  1. the generality of the lPFC involvement in dual-task processing across different tasks and different types of dual-task coordination
  2. the functional neuroanatomical dissociability of different control processes involved in dual-task processing in the lPFC
  3. the interaction of the lPFC with posterior task-relevant brain regions serving the control of the dual-task processing stream.


The three fMRI studies presented in this dissertation aim at specifying the functional role of the lPFC in interference processing in dual tasks with respect to these three issues. All three issues are of relevance not only for the understanding of the neural implementation of dual-task processing in the lPFC but also for the understanding of the functionality of the lPFC in general.

In the following section, first, a short background on cognitive theories on dual-task processing will be provided. Then, the functionality of the lPFC for cognitive control in general will be described in order to derive hypotheses about the functionality of the lPFC in dual-task processing. After describing the methodological approaches used in the three studies of this dissertation, an overview of the research questions and the obtained results of these studies will be given. The obtained findings will then be summarised and some future directions will be outlined. Each study is presented in detail as original article.

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