You can pick any two tasks or fly an monitor one instrument at a time; but only two tasks!

Science. 2010 Apr 16;328(5976):360-3. doi: 10.1126/science.1183614.
Divided representation of concurrent goals in the human frontal lobes.
Charron S, Koechlin E.
Institut National de la Santé et de la Recherche Médicale, Paris F-75654 Cedex 13, France.
Abstract
The anterior prefrontal cortex (APC) confers on humans the ability to simultaneously pursue several goals. How does the brain's motivational system, including the medial frontal cortex (MFC), drive the pursuit of concurrent goals? Using brain imaging, we observed that the left and right MFC, which jointly drive single-task performance according to expected rewards, divide under dual-task conditions: While the left MFC encodes the rewards driving one task, the right MFC concurrently encodes those driving the other task. The same dichotomy was observed in the lateral frontal cortex, whereas the APC combined the rewards driving both tasks. The two frontal lobes thus divide for representing simultaneously two concurrent goals coordinated by the APC. The human frontal function seems limited to driving the pursuit of two concurrent goals simultaneously.
PMID:
20395509
[PubMed - indexed for MEDLINE
http://www.ncbi.nlm.nih.gov/pubmed/20395509

Translation for non-neuroscientists:

Often we compare current computer processors with human brain, we consider human brain to be superior at certain things: It's ability to learn, adapt and react. But how does our brain react to multitasking?
We might have adopted quad cores on the desktops, but a new study reveals that human brain is capable of processing only two tasks at a time, something close to a HyperThreading on a single core processor.
When faced with two tasks, a part of the brain known as the Medial Prefrontal Cortex (MFC) divides so that half of the region focuses on one task and the other half on the other task. This logical partitioning allows a person to get hold of two tasks at a time without affecting much performance. (of course it varies by individual skill). And when we attempt to throw a third task to the brain, its easy to have poor performance in two or all of them.
For instance we can talk on phone while walking in a street or even while working on something on computer. As per the study researcher Etienne Koechlin of the Université Pierre et Marie Curie in Paris, France:
"What really the results show is that we can readily divide tasking. We can cook, and at the same time talk on the phone, and switch back and forth between these two activities. However, we cannot multitask with more than two tasks."
For the very same reason its discouraged to drive and talk on phone where both tasks use Brain heavily and context switching can cause delays, sometimes. However, listening to music and driving is much safer since listening to music hardly uses the brain.
Multitasking in the brain
Scientists knew that a region at the very front of the brain, called the anterior prefrontal cortex (APC), was involved in multitasking. But they weren't sure how the MFC was involved.
The research experiment
Koechlin and his colleagues conducted an experiment to study human brain activity in reaction to various situations. The test involved 32 subjects who were give a letter-matching task while they had scanners attached to their head. These scanners were essentially functional magnetic resonance imaging (fMRI). The subjects were given a set of uppercase letters and task was to determine whether those letters were presented in the correct order in order to spell out a certain word. For every successful attempt, they were awarded with cash.
It was observed, that higher the monetary reward, the more activity there was in the MFC. (That proves the greed lies in Brain and not heart )
The next test made the task more difficult by introducing lowercase letters in addition to uppercase. As a result, brain had to switch back and forth between matching the uppercase letters to spell out, say, aSmEOwE (Awesome)
During this dual task, the MFC did its job and divided it. One hemisphere of the brain encoded uppercase letter task, and so showed activity during that task, while the other region encoded the reward associated with the lowercase task, Koechlin said.
To test the human brain's response to three tasks, the researchers introduced a third letter-matching task. The accuracy dropped drastically. Logically, once each hemisphere was occupied with managing one task, there was nowhere for the third task to go. Clearly, The human brain perform as if they systematically forget one of the three tasks.
Brain's Decision-making
The results also explain why its harder for human brain to make choices between three objects as compare to two.
The human brain is good at remembering two things at a time hence efficiently compare them and make decisions. When options are multiple, human brain discards the choices by considering two (at max, at a time) until they are left with two and then one.
Read mo http://geeknizer.com/human-brain-mul...#ixzz3GvHU0KYf