First author

Have you ever noticed that when you buy, say, a red convertible, every other car on the road suddenly seems to be a red convertible? According to findings by Marius Peelen and his colleagues in the psychology department of Princeton University in New Jersey, neural mechanisms in the visual cortex are responsible for the phenomenon. When the researchers tested neural responses in the visual cortices of volunteers who had been instructed to look for certain objects in photographs that flashed up on a screen, they discovered that people are extremely adept at spotting objects of interest, even in situations in which the items are difficult to find (see page 94). He tells Nature why.

How did you conduct your experiment?

We tested neural activity in volunteers' visual cortices during two tasks using functional magnetic resonance imaging. In the first task, the volunteers were shown isolated images of cars and people; in the second, they were asked to try to spot a car or person during brief viewings of complex real-life photos. Their visual-system responses were very similar in both situations. This tells us that the visual cortex can filter out, or ignore, irrelevant visual information.

What was the most striking result?

We flashed four photos up on the screen simultaneously and told volunteers to look for cars or people in two of the photos, and to ignore the other two. But the volunteers' cortical activity was high regardless of whether the specified objects were in the photos that they were supposed to be watching or in those they had been told to ignore. Thus, they visually registered the cars or people in photos they were trying to ignore, even though they didn't consciously realize it.

What does this mean for real-life situations?

That you can quickly detect things you are looking for by biasing the processing of a scene in favour of those objects. For example, after buying a red convertible, you may select items in a scene that are similar to your new interest, red convertibles. Looking for someone in a crowd may follow the same basic mechanism. Knowing what you are looking for primes the visual system in favour of objects similar to your 'search template'.

Is there a downside to this phenomenon?

It might result in your missing useful things that you are not actively searching for. For example, you might walk up and down supermarket aisles looking for sugar until you find it. Then, if you also want coffee, you might have to check all the aisles again, even though you may have passed the coffee minutes earlier.