Trying to do two tasks at once, especially if they are similar, often results in both tasks being done less well.

This is called ‘dual-task interference’. It’s thought to be proof that our brains have a limited capacity to process information when it’s of a similar type. Not all tasks interfere with all other tasks. For example, studies suggest that listening to the radio or audio books whilst driving doesn’t affect driving ability, but conversations on either a handheld or hands-free mobile phone do interfere. In fact, mobile phone use is associated with being twice as bad at detecting traffic signals and also with slower reaction times in response to signals that are spotted.

Interference is perhaps most famously shown by the Stroop Effect, measured by an experiment I’d hazard to say that all psychology undergraduates do. The Stroop test gets people reading colour names aloud, which are written either in the same colour ink as the colour word itself, or in a different colour ink. E.g. ‘blue’ written in blue (congruent) or ‘blue’ written in red (incongruent). A bit like this… (but neater):

Reaction times are longer when the colour word is written in a different coloured ink. Stroop published this study in 1935, but it is such an elegant design that it has been replicated hundreds of times using different variations of tasks.

The original Stroop test was conducted in three parts. The first asked 70 university students to read 100 colour names aloud, either printed in black ink or printed in a colour that was different from that named by the word. E.g. ‘red’ would be printed in non-red ink. Here’s another messy mock-up to show you what I mean:

People took an average of 5.6% longer to read the 100 colour names printed in incongruent colours than they did to read them in black. Not a massive difference.

The second experiment asked 100 college students to name 100 colours, presented either as a square block of colour or as a colour word written in ink, where the student had to name the ink colour regardless of what the written word was. E.g. If the ink was red but the colour word written down was blue the student should say red. Comme ça:

The average time to respond to 100 stimuli was a massive 74% longer when people were asked to name ink colours of incongruent colour words, rather than simply naming a colour block.

The third and final experiment was carried out with 32 students, and was very similar to experiment two, except that instead of using blocks of colour it used swastikas, to allow some white to be present in the pattern (because the words had bits of white in them so it would be fairer). It aimed to measure effects of practice on interference and it asked the participants to repeat the experiment on several different days. Practice decreased the amount of interference but didn’t get rid of it entirely, suggesting practice can help reduce interference but only up to a point.

More modern adaptations of the Stroop include a picture-word version, where a word is printed inside a congruent or incongruent picture, e.g. the word ‘hand’ written inside either a hand or a foot. There’s also number Stroop, animal Stroop and even musical note Stroop with musicians, where they are asked to name a note word (do, re, me etc) written either on the correct part of the musical stave (congruent) or on an incorrect area (incongruent). All of these tasks have shown reliable effects of dual-task interference.

Knowing about cognitive interference like this means we can be smart about what we choose to multi-task with. If you’re trying to write something, listening to a radio show or even to songs with words can get in the way. In fact London-based record label Bigo And Twigetti recently created a non-interfering playlist without words just in case you want some music to work to. If you’re driving, talking on a mobile, even if it’s hands free, is not good news, and I’ll let you draw your own conclusions about whether using a smartphone at the same time as having a conversation might interfere with your ability to perform either task… Tom Brosseau certainly has an opinion in his song ‘Cradle Your Device‘.

So much for multi-tasking then, or at least don’t try to do it with things which are too similar.