Colour After-Effects

Following on from the brainteaser I posted a couple of weeks ago (see here), this post is about another type of optical illusion – colour after effects.

Take a look at the image below, taken from Thompson & Burr (2009). Focus your gaze at the black spot in the centre for at least 30 seconds. Then immediately look at the blank square below:



You should have seen coloured circles against the white background – green where the red circle was, red where the green circle was, and the blue and yellow circles also reversed. The reason these colours are seen is that they are the opposite on the colour spectrum, a common example used to illustrate this is the fact that you cannot imagine a reddish green, or a yellowish blue. When you stare at the image above, the different cone cells in your eye which respond maximally to red, green, and blue light respond to the image and send information to the optic nerve, which projects to the visual cortex. As you are looking at the image for a long period of time, the cone cells in the retina become fatigued. Then when you then look at white light (which contains all colours of light), these cells do not respond, meaning you see the opposite colour. For example, a cone saturated after viewing red light will not fire when the individual views white light, so we see green.

Thank you for reading, check back for more next Thursday!

Rubber Hand Illusion

Another for the Brainteaser series now – The Rubber Hand Illusion.

This involves tricking your brain into believing that a rubber hand is actually your own – sounds a bit weird I know!

Botvinick & Cohen (1998) found that participants experience ownership of a rubber hand after they have viewed it receiving the same tactile stimulation as their own hand. Tactile stimulation is a fancy was of saying the hands were touched in the same way.

During the illusion, the participants’ own hand is hidden from them by a screen, and a rubber hand is placed on a table where the actual hand would be. The experimenter then uses a brush to stroke both hands in the same way, at exactly the same time. During this, the participant looks at the rubber hand. After a few seconds, the participant starts to feel as if the rubber hand is actually their own, so much so that they jump when the experimenter hits it with a hammer!

This illusion shows how powerful visual information is – in this case it overrides our sense of proprioception: the knowledge of where our limbs are in space. It is also evidence for neural plasticity – the flexibility of the brain to incorporate new information.

For a demonstration of this illusion, click the video below!