health - Can't find your keys? Your brain's out of sync

Not under here (Image: Jonatan Fernstrom/Getty)

YOU'RE running late for work and you can't find your keys. What's really annoying is that in your frantic search, you pick up and move them without realising. This may be because the brain systems involved in the task are working at different speeds, with the system responsible for perception unable to keep pace.

So says Grayden Solman and his colleagues at the University of Waterloo in Ontario, Canada.

To investigate how we search, Solman's team created a simple computer-based task that involved searching through a pile of coloured shapes on a computer screen. Volunteers were instructed to find a specific shape in a stack as quickly as possible, while the computer monitored their actions. "Between 10 and 20 per cent of the time, they would miss the object," says Solman, even though they picked it up. "We thought that was remarkably often."

To find out why, the team developed a number of further experiments. To check whether volunteers were just forgetting their target, they gave a new group a list of items to memorise before the search task, which they had to recall afterwards.

The idea was to fill each volunteer's "memory load", so that they were unable to hold any other information in their short-term memory. Although this was expected to have a negative effect on their performance at the search task, the extra load made no difference to the percentage of mistakes volunteers made.

To check that the volunteers were paying enough attention to the items they were moving, Solman's team created another task involving a stack of cards marked with shapes that only became visible while the card was being moved. Again, they were surprised to see the same level of error, says Solman.

Finally, the team analysed participants' mouse movements as they were carrying out a similar search task. They discovered that volunteers' movements were slower after they had moved and missed their target (Cognition, DOI: 10.1016/j.cognition.2011.12.006).

Solman's team propose that the system in the brain that deals with movement is running too quickly for the visual system to keep up. While you are rummaging around a messy house to find your keys, you might not be giving your visual system enough time to work out what each object is. Since time can be costly, sacrificing accuracy on occasion for speed might be beneficial overall, Solman thinks.

The slowing of mouse movements suggests that at some level the volunteers were aware that they had missed their target, a theory that is backed up by other studies that show people tend to slow down their actions after they have made a mistake, even if they don't consciously realise the mistake. Solman reckons this reflects the brain's "attempt to slow down the motor system", to allow the visual system to catch up and conscious perception to occur.

"What's really interesting is the notion that the motor and perceptual system are decoupled. They're both trying to help you find [your keys] but they're not coordinating," says Todd Horowitz, at Harvard University. "There are implications for social search, such as a doctor looking through an X-ray or [security] looking through luggage."

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