In other words, even though pictures of things like bananas and strawberries were presented in black-and-white, the brain seems to be encoding the colour that the objects should be. Then, by looking at activity in three areas of the brain – the primary visual cortex (V1), a colour-processing area (V4), and an object-processing area called the lateral occipital cortex (LOC) – they were able to predict the memory colour of the black-and-white images. In Bannert and Bartels’ study, they trained classifiers to distinguish between patterns of fMRI responses to real colours. Once they’ve learned the correct associations, the classifiers can then be used to predict what a person was looking at, based on their brain activity. The neat thing about this sort of analysis is that you can use this information to train computer algorithms, called classifiers, to learn the associations between patterns of activity and real stimuli. MVPA is different, in that it looks for patterns of activity that incorporate both strong and weak responses. Providing you’ve run a well-controlled study, then you might conclude that if areas of the brain show activation in the motion trials but not the static trials, then they’re involved in motion perception in some way.īut this discards a vast amount of data – any areas of the brain that show weak activation aren’t considered for further analysis, as they’re assumed to not be substantially involved in whatever it is that you’re measuring.
#Picture of the brain series
So say you run a study where you present participants with a series of basic objects, such as squares or triangles, and in some trials they’re moving, and in others they’re staying still. For example, fMRI detects which areas of the brain oxygenated blood is being directed to, and the idea is that those areas need more oxygen because they’re being used in the task at hand. Traditionally, fMRI data analysis techniques try to detect activation of structures within the brain that might correspond to a given behaviour or respond to a certain type of stimulus. To look at this, they performed a multi-voxel pattern analysis (MVPA). What they were really interested in was whether the brain was encoding the colours of objects that were presented in greyscale. Framing the experiment as a motion perception task was a ruse – Bannert and Bartels wanted to make sure the participants focused their attention on the task, but weren’t concentrating on anything other than the movement of the objects. Later on, the participants were shown rings that were coloured red, green, yellow or blue. In the first part of the experiment, they were shown rotating black-and-white pictures of bananas, broccoli, strawberries and other objects that had strong associations with a certain colour, and asked which way the pictures were moving. Michael Bannert and Andreas Bartels from the University of Tübingen in Germany took 18 participants and asked them to perform a motion perception task while undergoing an MRI scan. But what happens when we see these sorts of objects in black-and-white?Ī recent study published in Current Biology looked at exactly this question. This idea is called ‘ memory colour’ – bananas are yellow, broccoli is green, strawberries are red. It’s mainly due to a phenomenon called colour constancy, but for objects that have a particularly strong colour association, it can also in part be influenced by the fact that we know what colour those objects should be. Bright white fluorescent light, a deep red sunset, the dull grey of dusk – it doesn’t matter, we still see a yellow banana. When we look at a banana for instance, it will look yellow regardless of the conditions we’re viewing it in. In this sense, colour is a very subjective property, and it begs one of the all-time great, deep philosophical questions – is your red the same as my red?ĭespite this, our brains do a pretty good job of keeping colours constant for us. A green plant isn’t actually green, it’s just that the plant absorbs all of the wavelengths of light apart from those in a specific range, and you have colour receptors in the back of your eye that interpret that reflected wavelength as being green. For the most part though, it’s all in your head. Flowers of reds, yellows and greens explode under a sea of blue light from the sky. You’re looking out on a vast plain of crops of all different colours, shapes and sizes.