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!

Changing Colours?

Today’s Brainteaser is a quick into into an important topic in perceptual psychology – how we see colour.

The light which reflects off objects into our eyes is made from a variety of different wavelengths, which are interpreted by cells in our eyes and brain. This is fine in daylight, but when the light fades, our brains have to make judgements about the colour represented by these wavelengths.

For example, take a look at this image below of two cubes shown under different wavelengths of light:


As you can see, the colours of the squares look very different to each other in the left and right pictures. However, despite this, we still know that the top left had corner square is blue. This is due to calculations carried out in our visual cortex.

This process is much more difficult if contextual cues are removed, for example in the image shown below:


This is exactly the same image as the one above, and I’ve coloured in all but one patch of the same square. So these two dots are actually the same colour, but with the context removed, we see them as different.

Hope you’ve enjoyed this brainteaser, which links in nicely with a debate about a certain dress which took place last year! If you want this explained by science, then check out the video below!

Color Vision

How is it that we are able to see in colour?

The process starts with light hitting the photoreceptors in the retina. As you might remember from my last post, there are two different types of photoreceptors: rods and cones. Rods are used when are eyes are adapted to the dark, while cones are used in daylight, and are involved in distinguishing different wavelengths of light which represent colour.

There are three different types of cone cell, each most sensitive to a different wavelength of light. These sensitivities can be plotted to give ‘spectral sensitivity curves’ like the ones shown below.

As you can see, there is a cone which responds mostly to short wavelengths of light, which are blue. The medium wavelength cone responds more to green, while the long wavelength cone responds mostly to red light. The black curve shows the spectral sensitivity of rod photoreceptors.

Colour Blindness:

Have you ever wondered why the two most common colours to be affected by colour blindness are red and green or why this condition is more common in men than women?

Basically, the gene which determines the sensitivities of the cones is on the X chromosome. As you can see from the spectral sensitivity curves above, the red and green cones are quite close to each other on the spectrum, and are also thought to originate from a single ancestral pigment gene. They are about 96% alike, and the combination of these factors means that alterations are likely to occur. As females have 2 X chromosomes, then it is unlikely that both with be altered, whereas males only have one. This means men have a higher chance of being colour blind.

The signals from the photoreceptors are processed further in the retinal ganglion cells.The majority of these neurons are colour-opponent cells: a response to one wavelength in the centre of its receptive field can be cancelled by showing another wavelength in the receptive field surround. Two types of opponency are found: red versus green and blue versus yellow. For example, a cell with a red ON centre and a green OFF surround will fire if a red light in shone on the centre and the response to red is only cancelled by green light in the surround.

This diagram shows how signals from the cones are processed in the colour-opponent ganglion cells:

There is also an area in the visual cortex in the brain that is specialised for processing colour (the red area in the diagram below). It is known as V4 as has been shown to be active when people are processing coloured stimuli.

Hopefully you now know are bit more about how we are able to see in colour – if you have any suggestions for other topics you’d like me to write about then please let me know in the comment box below 🙂