Why it’s so hard to find a blue banana

We like to think we see things how they are. Often, however, what we actually see is based less on the outside world and more on what our brain expects to be there, like a kind of mentally-generated virtual reality.

Now psychologists in Germany have demonstrated this in an elegant experiment involving people's memory for the colour of everyday fruit and vegetables.

Karl Gegenfurtner and colleagues presented 14 participants with strangely coloured fruits on a computer screen – for example a pink banana – against a grey background. The participants' task was to adjust the colour of the onscreen banana until it blended exactly with the grey background. It sounds easy, but the participants couldn't do it because as they adjusted the colour, they compensated not just for the banana's actual pink pigmentation, but also for a yellowness that only existed in their mind, thus leaving the banana with a slight bluish hue. That is, their memory for the typical colour of a banana was interfering with their performance.

By contrast, the participants didn't have any trouble adjusting the colour of anonymous spots of light to make them blend in with the grey background – thus suggesting it wasn't some quirk of the experimental set-up that was causing the participants difficulties with the fruit and veg.

Moreover, when presented with a banana that had been correctly adjusted to perfectly blend in with the grey background, the participants reported that it looked slightly yellow – a percept generated by their own mind, not by the actual colour of the banana.

"Our results show a high-level cognitive effect on low-level perceptual mechanisms", the researchers said.

Further reading

Hansen, T., Olkkonen, M., Walter, S. & Gegenfurtner, K.R. (2006). Memory modulates colour appearance. Nature Neuroscience, DOI:10.1038/nn1794.