Dr Adam Gazzaly,  director of the Neuroscience Imaging Center at the University of California, San Francisco, answers the question of how the brain multitasks on the brainfacts.org website:

Multitasking is an attempt to engage in more than one goal at the same time. When two tasks demand competing attention, there is generally a switching that occurs between the neural processes involved, rather than concurrent processing as may be expected with true multitasking. Of note, recent research suggests that it may be possible for the brain to split two demanding tasks.

The prefrontal cortex has been frequently implicated as a brain region that mediates multitasking and the switching processes. Multitasking is commonly shown to impair cognitive performance, as each switch results in a reduction in performance compared to doing one task at a time. However, there is growing evidence that the ability to multitask can be trained with repetitive and adaptive practice. Multitasking abilities have been observed to decline as we age.