I’ve reported, often, on the evidence that multitasking is a problem, something we’re not really designed to do well (with the exception of a few fortunate individuals), and that the problem is rooted in our extremely limited working memory capacity. I’ve also talked about how ‘working memory’ is a bit of a misnomer, given that we probably have several ‘working memories’, for different modalities.
It follows from that, that tasks that use different working memories should be easier to do at the same time than tasks that use the same working memory. A new study confirms that multitasking is more difficult if you are trying to use the same working memory modules for both tasks.
In the study, 32 students carried out a visual pattern-matching task on a computer while giving directions to another person either via instant messaging (same modalities — vision and motor) or online voice chat (different modality — hearing).
While both simultaneous tasks significantly worsened performance on the pattern-matching task, communicating by IM (same modality) led to a 50% drop in visual pattern-matching performance (from a mean of 11 correct responses to a mean of 5), compared to only a 30% drop in the voice condition (mean of 7).
The underlying reason for the reductions in performance seems to be in the effect on eye movement: the number and duration of eye fixations was reduced in both dual-task conditions, and more so in the IM condition.
Note that this is apparently at odds with general perception. According to one study, IM is perceived to be less disruptive than the phone. Moreover, in the current study, participants felt they performed better in the IM condition (although this palpably wasn’t true). This feeling may reflect the greater sense of personal control in instant messaging compared to chat. It may also reflect an illusion of efficiency generated by using the visual channel — because we are so strongly practiced in using vision, we may find visual tasks more effortless than tasks using other modalities. (I should note that most people, regardless of the secondary task, felt they did better than they had! But those in the IM condition were more deluded than those in the chat condition.)
The finding also explains why texting is particularly dangerous when driving — both rely heavily on the same modalities.
All this is consistent with the idea that there are different working memory resources which can operate in parallel, but share one particular resource which manages the other resources.
The idea of ‘threaded cognition’ — of maintaining several goal threads and strategically allocating resources as needed — opens up the idea that multitasking is not all bad. In recent years, we have focused on multitasking as a problem. This has been a very necessary emphasis, given that its downsides were unappreciated. But although multitasking has its problems, it may be that there are trade-offs that come from the interaction between the tasks being carried out.
In other words, rather than condemning multitasking, we need to learn its parameters. This study offers one approach.
 . Behavioral performance and visual attention in communication multitasking: A comparison between instant messaging and online voice chat. Computers in Human Behavior [Internet]. 2012 ;28(3):968 - 975. Available from: http://www.sciencedirect.com/science/article/pii/S0747563211002925