When Multitasking Goes Wrong


L. M. Carrier, 2009


My colleague asked me the other day about a problem called “thrashing” that might occur when people are multitasking too much. It’s basically when you are spinning your wheels on multiple tasks but feel that you are getting nowhere and nothing done. It seems that thrashing is a common term in computer work to describe situations that are similar but apply to the computer processor, not to humans. For example, you might find that your computer is “thrashing” when you are running multiple programs and they are competing for memory and for hard drive space. Basically, the computer drastically slows in response to your commands and at the same time seems to be doing nothing.

Can this happen to humans when we try to do multiple tasks at once? I have not seen anything in the scientific literature specifically on human thrashing. However, it has been known for quite some time by psychological scientists that multitasking can lead to several negative effects:


1. Attempting to do two or more tasks at once can slow down your progress on or more of the tasks. In other words, in most cases, doing extra tasks slows you down. This happens even with very simple tasks as shown in many research studies. Here are some examples of three different studies in which trying to perform two tasks at once slowed down responses to the task that started a little after the first (see the “Task 2 Reaction Times”) below. The “Stimulus Onset Asynchrony” is the amount of overlap of the two tasks. As the overlap gets close to zero (i.e., as you move to the left on the x-axis), the slowing of the second tasks gets worse.





Borrowed from Van Selst & Jolicoeur (“Decision and Response in Dual-Task Interference”) (http://human-factors.arc.nasa.gov/cognition/papers/mvselst/daridti/daridti.html)



2. Trying to switch rapidly between tasks causes there to be a “switch cost” which means that it takes extra time to restart one task after switching away from another task. The graph below shows an idealized representation of switch costs. When the time between two tasks get smaller (the “RSI”), the extra time to start the second task gets longer (Switch Cost). Even when there is a fairly long time between the two tasks (the right end of the x-axis), there still is a cost to switching.





Borrowed from Wong (“Research”) (http://www.math.princeton.edu/~kfwong/research)



3. Doing more than one task at a time can lead to errors or underperformance in one or more of the tasks. So, not only are you likely to be slower to complete a task, but you also are likely to be less accurate. Here are the results of a study in which people had to perform a seemingly very simple task, which was to watch an image on a screen and notice when there was a change in color. Their ability to do this task by itself is shown by the lines labeled “Blank” on the graphs. Then, look at the lines labeled “Attend.” In this case, people had to do another task at the same time (either judge whether a scene, the left graph, or a word, the right graph, was animate or inanimate). It’s clear that having to “attend” to another task at the same time decreased Accuracy in the color change detection task.





From Makovski, Shim, & Jiang (2006) (“Interference from filled delays on visual change detection”) (http://www.journalofvision.org/6/12/11/article.aspx)



            There also is a much-cited study in which it was shown that doing a second task while learning changed how the brain processed the information to be learned (Foerde, Knowlton, & Poldrack, 2006). When learning took place while doing another task at the same time, the learned information was stored in a different brain location and through a different brain system than when learning took place without distraction. All of the types of mistakes, errors, or mental states that occur when humans multitask probably have not yet been sketched out by scientists. If you are interested in studying this, or if you have some interesting experiences of your own related to multitasking, let me know (lcarrier@csudh.edu).




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