MANY young people have music playing in their earbuds even as they surf the Internet, update Facebook and Twitter, fire off an SMS, and keep an eye on the television - all the while doing some real work.

At first blush, this media multitasking allows you to get a few things done in the same time period, which must mean higher productivity and so a good thing.

Yet, true multitasking may be largely unachievable. What the young are doing may simply be switching between multiple tasks - without doing any particular one too well.

A study from the Chinese University of Hong Kong just published polled undergraduates on their media use, grouping them into light and heavy multitaskers, depending on how many media types they were simultaneously exposed to habitually.

They were then asked to look at 48 short red or green lines scattered at random angles on a screen. While 47 lines changed colour simultaneously, one remained unchanged. When asked to identify the latter, heavy multitaskers consistently did worse.

So habitually heavy multitaskers focused less well on a particular task requiring much attention, even when instructed to do so.

Other studies have also shown that once people get into the habit of heavy multitasking, some problematic cognitive styles become ingrained. For example, a 2009 University of Oregon study showed that those who habitually consumed information from several media simultaneously were unable to distinguish between relevant and irrelevant information for the task at hand.

This was because they were so used to distributing their attention among different sources of information, whether needed for the particular task or not. They tended to focus their attention on information in their surroundings more than that which was most relevant to the task at hand.

So instead of focusing on writing their essay on the causes of World War II, for example, their attention is spread among their perpetual Facebook and Twitter updates, answering e-mail messages and SMSes, the music pulsating deep in their ears, the Google results of their search listing not only causes of World War II but also all movies set in the period, complete with photos of actors starring in them.

In other words, irrelevant information distracted heavy multitaskers more, making them less able to concentrate on just one task.

If this becomes an ingrained cognitive style, working memory and learning could be adversely affected. Working memory is that part of the memory that holds information needed for a person to reason and understand something. The amount of information it holds is limited.

A 2009 Stanford University study even found that, when asked to switch their focus from one task to another, habitually heavy media multitaskers responded slower than non-habitual ones.

The study also confirmed that this was because they were less able to ignore irrelevant information. Their working memory was less well organised as well.

(In the Hong Kong study, if a beep was added at the instant of colour change, heavy multitaskers actually did better. So their style of broad attention spread may, at times, pick out different sensory information that could be randomly useful to a task. This, however, is very little bang for the buck.)

In sum, what looks like multitasking may really be klutzy task switching. The underlying problem is that there is a finite capacity to every function in the brain.

In particular, there is the executive function located in the very front of the brain at the forehead. This function manages attention and coordinates information while also being responsible for moral judgment, intellect, insight, decision-making and self-control.

The human brain has only a finite amount of attention. So if we are focused broadly on lots of general information, and are trying to collate the different pieces and relate them to one another, we are not going to be able to bring sustained attention to any one issue. In short, there will be a lack of depth in the attention we give.

In sum, research on media multitasking suggests that, far from speeding things up, it slows one down; affects one's ability to learn; and can result in poorer reasoning and judgment.

This might explain why Facebook comments and tweets can often be so immoderate or ill-considered: not just because they are composed on the run, so to speak, but also because they might have been composed by someone multitasking on several other activities.

If so, the remedy is simple: before hitting 'Send', stop all other activities and re-read your comments soberly and with single- minded attention. That allows the executive centre to devote 100 per cent attention to your response, which is likely to improve your cognitive processing and your judgment. You can then hit 'Delete' instead if a comment appears, on review, ill-advised.

So when two or more mental tasks - especially high-level intellectual ones - use the same cognitive resources from the same part of the brain, it stands to reason that they are hard to execute simultaneously. Thus you cannot toggle between a calculus problem set and a critique of the Arab Spring. Or, answer an e-mail and a phone call simultaneously.

That is, the brain cannot parallel process two or more mental tasks of the same nature. Each time the executive function is called upon to switch attention to another task, you are using up part of some finite brain resource. The end result is that one task interferes with the other.

The fact that you can drive and talk at the same time does not disprove this. It is easier to carry out two sufficiently different tasks - like driving and talking - because tasks like driving or walking become automatic with practice. Such tasks use the subconscious parts of the brain and do not call on the executive function.

For example, the brain stem (which is located at the bottom of the brain and connects the two cerebral hemispheres to the spinal cord) controls automatic functions like breathing, heart beat and reflexes. It also does most of the processing for daily walking.

With learning and practice, some complex tasks can also become automatic. The cerebellum, (the region of the brain at the lower back of the head) controls balance and posture.

Once it learns to drive a car or golf ball, it takes over from the motor cortex in the middle, top part of the brain which controls fine motor skills. These tasks then become automatic.

By contrast, speech involves Broca's Area on the left brain further in front and also Wernicke's Area further at the back of the head, behind the temples. These two areas are connected by a special bundle of nerves.

Since talking and walking call on different parts of the brain, and only speech requires intensive cognitive processing, you can walk and talk at the same time.

But different tasks on the various new media all require intensive cognitive processing, such as composing texts. Given the brain's finite capacity for such processing, what looks like multitasking may actually be just task juggling and none too adroitly either.

If so, those not young enough to multitask may take solace from the fact that the young who do so aren't very good at it anyway.