According to Lin Edwards
New research has shown that the brain handles two tasks at once by dedicating half the brain to one task, and the other half to the second. This means it may not be able to effectively handle more than two complicated tasks simultaneously.
as Koechlin explained, if you try to tackle three jobs at once, the frontal cortex will always neglect one of them
According to Walter Kirn
Multitasking messes with the brain in several ways. At the most basic level, the mental balancing acts that it requires—the constant switching and pivoting—energize regions of the brain that specialize in visual processing and physical coordination and simultaneously appear to shortchange some of the higher areas related to memory and learning. We concentrate on the act of concentration at the expense of whatever it is that we’re supposed to be concentrating on.
What does this mean in practice? Consider a recent experiment at UCLA, where researchers asked a group of 20-somethings to sort index cards in two trials, once in silence and once while simultaneously listening for specific tones in a series of randomly presented sounds. The subjects’ brains coped with the additional task by shifting responsibility from the hippocampus—which stores and recalls information—to the striatum, which takes care of rote, repetitive activities. Thanks to this switch, the subjects managed to sort the cards just as well with the musical distraction—but they had a much harder time remembering what, exactly, they’d been sorting once the experiment was over.
Even worse, certain studies find that multitasking boosts the level of stress-related hormones such as cortisol and adrenaline and wears down our systems through biochemical friction, prematurely aging us. In the short term, the confusion, fatigue, and chaos merely hamper our ability to focus and analyze, but in the long term, they may cause it to atrophy.
See also on my blog “Amount of time spent on a project isn’t what counts — it’s the amount of uninterrupted time“, and elsewhere by David Glenn “Divided Attention: In an age of classroom multitasking, scholars probe the nature of learning and memory“.