Although the human brain is a very complex structure, it's still not big or efficient enough to process every single thing we see, hear, and do. Sometimes this limitation is a good thing, since it forces our brains to filter out minor details that we don’t need to dwell on. Other times it's more of a hassle, since it interferes with how we process information.

Researchers previously thought that different processes in the brain were subject to separate bottlenecks. For example, the process of reading a map might be limited by a very different mechanism than the act of evaluating ideas and forming opinions. However, new research in PNAS suggests that a "unified" bottleneck may restrict our ability to perform very different cognitive processes simultaneously.

The paper focuses on two tasks. Recognizing new information in your surroundings and incorporating it into your view of the world is referred to as "encoding," while reacting to stimuli and making decisions is termed "response selection." They're distinct processes, but they utilize some of the same parts of the brain, so it's possible that the brain can have trouble if asked to perform both of these processes at once or in very close succession. In this study, the researchers used fMRI data to determine whether there is a common bottleneck for both perception and decision making.

Subjects were asked to perform several tasks while inside an fMRI machine. In the first part of the experiment, they had to respond vocally to a tone, respond manually to an image, or perform both of these tasks at once. Not surprisingly, the participants responded faster and more accurately when they only had to concentrate on one task. When the participants had to do both at once, the brain was overloaded; structures that the fMRI revealed to be particularly active at this point were potential bottlenecks for the encoding process.

Then, the researchers performed a similar test to look for bottlenecks in response selection. In these experiments, the participants were quickly presented with one or more letters that they had to recall after fourteen seconds. Here, the overload occurred when they had to remember several letters as opposed to just one; researchers searched for areas of the brain that limited this process.

Several parts of the brain met the criteria for bottlenecks, in that they were active during both the encoding and response selection tasks, indicating that one or more of these structures may be a unified bottleneck that limits in both processes.

To zero in on these candidate regions, participants were asked to perform both an encoding task and a response selection task. They were much slower and less accurate in the response selection task when it immediately followed the encoding task. Four different parts of the brain—the bilateral insula, the left interior frontal junction, the anterior superior medial frontal cortex, and the left intraparietal sulcus—were identified as parts of the bottleneck that decreased performance on this dual task.

So, not only does doing two things at once decrease your speed and accuracy, but it appears that one bottleneck may be responsible for this effect even if the tasks are different. Of course, this is probably not the only neural bottleneck we have; other structures, alone or in combination, likely limit one or more different processes as well. However, this is good evidence that very different cognitive tasks may interfere with each other, thanks to limitations of specific areas in the brain.

The upshot? Next time you make a mistake while multitasking, you can blame the unified bottleneck.

PNAS, 2011. DOI: 10.1073/pnas.1103583108 (About DOIs).