Attention and speech-processing related functional brain networks activated in a multi-speaker environment.

Human listeners can focus on one speech stream out of several concurrent ones. The present study aimed to assess the whole-brain functional networks underlying a) the process of focusing attention on a single speech stream vs. dividing attention between two streams and 2) speech processing on different time-scales and depth. Two spoken narratives were presented simultaneously while listeners were instructed to a) track and memorize the contents of a speech stream and b) detect the presence of numerals or syntactic violations in the same ("focused attended condition") or in the parallel stream ("divided attended condition"). Speech content tracking was found to be associated with stronger connectivity in lower frequency bands (delta band- 0,5-4 Hz), whereas the detection tasks were linked with networks operating in the faster alpha (8-10 Hz) and beta (13-30 Hz) bands. These results suggest that the oscillation frequencies of the dominant brain networks during speech processing may be related to the duration of the time window within which information is integrated. We also found that focusing attention on a single speaker compared to dividing attention between two concurrent speakers was predominantly associated with connections involving the frontal cortices in the delta (0.5-4 Hz), alpha (8-10 Hz), and beta bands (13-30 Hz), whereas dividing attention between two parallel speech streams was linked with stronger connectivity involving the parietal cortices in the delta and beta frequency bands. Overall, connections strengthened by focused attention may reflect control over information selection, whereas connections strengthened by divided attention may reflect the need for maintaining two streams in parallel and the related control processes necessary for performing the tasks.

Shielding and relaxation in multitasking: Prospect of reward counteracts relaxation of task shielding in multitasking.

Performing two similar tasks at the same time requires the shielding of the prioritized Task 1 from interference of additional Task 2 processing (between-task interference). In the present study we tested how motivational factors such as prospect of reward might drive shifts between increased proactive control, enabling task shielding, and reduced proactive control resulting in relaxed task shielding. In Experiment 1 an instruction-induced prioritization of Task 1 over Task 2 resulted in initially reduced between-task interference. With increasing time on task, however, between-task interference continuously increased, presumably because participants engaged less in proactive control resulting in reduced task shielding. In Experiment 2 the prospect of reward activated proactive control as indicated by reduced between-task interference in the Reward than in the No reward condition. In Experiment 3, we directly compared the performance of a Reward and a No reward group in a between-subject design. Whereas between-task interference again continuously increased over time in the No reward group, indicating a relaxed mode of task shielding, the Reward group displayed constant small between-task interference over time, suggesting maintained high levels of task shielding. Together these findings speak in favor of an impressive flexibility in regulating cognitive control engagement in multitasking situations. This not only shows the capacity for optimization of multitasking performance by motivational incentives but also further supports assumptions of the strategic nature of assumed processing limitations (bottlenecks) in dual-task performance.

The role of feedback delay in dual-task performance.

Doing two things at once is hard, and it is probably hard for various reasons. Here we aim to demonstrate that one so far barely considered reason is the monitoring of sensory action feedback, which detracts from processing of other concurrent tasks. To demonstrate this, we engaged participants in a psychological refractory period paradigm. The responses in the two tasks produced visual action effects. These effects occurred either immediately or they were delayed for the first of the two responses. We assumed that delaying these effects would engage a process of monitoring visual feedback longer, and delay a concurrent task more, as compared to immediate effects. This prediction was confirmed in two experiments. We discuss the reasons for feedback monitoring and its possible contribution to dual tasking.

Functional split brain in a driving/listening paradigm.

We often engage in two concurrent but unrelated activities, such as driving on a quiet road while listening to the radio. When we do so, does our brain split into functionally distinct entities? To address this question, we imaged brain activity with fMRI in experienced drivers engaged in a driving simulator while listening either to global positioning system instructions (integrated task) or to a radio show (split task). We found that, compared with the integrated task, the split task was characterized by reduced multivariate functional connectivity between the driving and listening networks. Furthermore, the integrated information content of the two networks, predicting their joint dynamics above and beyond their independent dynamics, was high in the integrated task and zero in the split task. Finally, individual subjects' ability to switch between high and low information integration predicted their driving performance across integrated and split tasks. This study raises the possibility that under certain conditions of daily life, a single brain may support two independent functional streams, a "functional split brain" similar to what is observed in patients with an anatomical split.