We all know that bananas are yellow, strawberries are red and oranges are…well orange. But what about a black and white banana? Or a grayscale green bean? Well it turns out our brain still processes these images in the colour we think they should be in.
In a study carried out earlier this year, neuroscientists showed there was a link between our memories, our knowledge of object colour and the way neuronal networks in the brain perceive the object when presented in black and white. Things with strong colour associations are known as colour-diagnostic, and the colour they’re associated with is their memory colour.
The experiment involved a group of participants who were asked to look at a series of rotating colour-diagnostic images presented in black and white and to say which way they thought the object was moving. However, the movement of the images was just a distraction, because the researchers didn’t want the participants to focus their attention on the colour (or lack of colour) of the objects. This happened whilst the participants underwent a special type of brain scan called functional magnetic resonance imaging (fMRI) – an imaging technique which shows which parts of the brain are most active during a task on account of the blood flow to those areas.
In the next part of the experiment, participants were shown a series of concentric circles where each image was a different colour to the next. Again, participants were asked to say which way they thought the images were rotating.
From these two short tests, it was found that areas of the brain which were activated upon stimulation with the colour images were also activated upon stimulation with black and white. This meant that when the participant was shown a black and white image, the part of the brain that deals with colour became activated. By combining the fMRI recordings from the two experiments, it was then possible to predict the colour of the object presented even if it wasn’t in colour, through mapping out which activity pattern corresponded to which colour.
Specialised computer programmes called classifiers were then used to narrow down which specific areas of the brain were most active upon stimulation of different colours. Surprisingly, they discovered that the colour-sensitive region of the brain (V4) wasn’t the only area doing all the hard work. Another area called V1 also comes into play. V1 is also known as the primary visual cortex, an area of the brain through which all visual signals first pass. It was thought that colour processing was too difficult a task to be dealt with by this region of the early visual cortex, so how on earth can we predict colour from this area?
Well, it’s all to do with what we know how we and remember what objects look like. In reality, V1 acts as a sort of meeting point between memory in the higher areas of our brain (top-down signals) and visual input (bottom-up signals). The two inputs are then compared to encode colour based on an achromatic (black and white) input.
This relaying of previously learned information, from the complex memory centres of our brain back to the primary visual cortex, is how our brain gives colour recognition to a black and white picture, filling in the gaps even if we don’t see it.
Bannert MM, & Bartels A (2013). Decoding the Yellow of a Gray Banana. Current biology : CB PMID: 24184103