The McCollough Effect

It’s always a good feeling when I find out about a new illusion that I can share on here – this week’s Brainteaser is another about colour after-effects: The Mccullough Effect.

What makes this different to other colour after-effects is that the stimuli used in adaptation is simpler than the final illusion. This effect was discovered by Celeste McCollough in 1965, and involves alternating black and white lines (known as ‘gratings’) which are viewed as coloured after a period of adaptation. Try it for yourselves here:

First, stare at these images for a minute or so then look at the grid below

McCollough Effect 1

McCollough Effect 2

What you should notice is that these gratings now look coloured, when they are in fact black and white! The vertical lines should look red, whilst the horizontal ones look green.

What is so interesting about these after effects is that unlike others (e.g. here), this effect lasts not just for a few minutes but for hours, or even days. Some studies (e.g. Jones & Holding, 1975) have shown that adaptation for 10 minutes can lead to after effects months later!

Scientists are still not certain which part of the visual system is responsible for this effect or why it is so long-lasting. One theory is that it takes place due to neurons in V1 – the first part of the visual cortex which receives information from the optic nerve via the Lateral Geniculate Nucleus. Only neurons in early visual cortex are sensitive enough for this type of adaptation to occur. A possible reason why this effect lasts for so long could be simply that the adaptation stimulus is rare, so is not seen in the environment for us to de-adapt, whilst others believe this shows a form of associative learning. However, the exact mechanisms are still up for debate.




Face Illusion

Hi everyone,

I haven’t done a Brainteaser for a while, so I thought I’d share with you this clip from QI to see how you get on!


What you should notice, is that when the face is pointed towards you, it appears to be 3D and facing you, regardless of whether or not you are seeing the concave side of the image or not.

This illusion is known as ‘cognitively impenetrable’ because no matter how hard we try to see the face as pointing away from us, our brain always shows us the opposite – in this illusion having knowledge of how it works does not affect it at all.

This therefore shows how powerful the illusion is, and as Stephen Fry mentions in the clip, it shows how automatically we perceive faces, and how biased we are to perceive stimuli as faces. (For more information on this topic, check out my post here)

He also mentions that this ability is thought to be innate – babies only a few hours old have shown to prefer to look at images of faces rather than other stimuli (e.g. Batki et al 2000) and look at whole faces, rather than faces with scrambled features (but otherwise identical) e.g Goren et al (1975).

These results show that infants must have some sort of knowledge about faces and social interactions when they enter the world. Further evidence to support this is shown in a famous study by Meltzoff & Moore (1977) in which young infants – only a few weeks old were about to imitate facial expressions – shown in the well-known image below. This shows how important social interactions are to us at such a young age, as they provide the basis for our further development.

Classic77 Meltzoff_legend (3)



The Bouba/Kiki Effect

Another brainteaser this week, on a test which any psychology student would recognise.

First, have a look at these shapes. Which one would you call ‘bouba’ and which one would you call ‘kiki’? Once you’ve done this, scroll down to see the answer.



In fact, these shapes have not been officially named, but 95% of people asked this question name the shape on the left ‘kiki’ and the shape on the right ‘bouba’. This is amazing considering that this is a free choice, with a 50/50 answer. It seems to be that each shape is better suited to a certain name. But why?

The experiment shown above was carried out by Ramanchandran & Hubbard (2001) as part of their work on synaesthesia – a condition in which people’s senses overlap e.g. hearing sounds as viewing colours. If you’d like to read more about synaesthesia check out my blog post. Their hypothesis as to why synaesthesia occurs is that it is caused by extra connections in the brain between senses.

The authors proposed that the reason most people identify the spiky shape as ‘kiki’ and the rounded shape as ‘bouba’ because the shape of the objects mimic the phonetics of the sounds of the names when spoken. This is therefore evidence that most people, even though they do not have synaesthesia, have connections in their brain between the different senses.

Although this might just seem like a fun, but pretty trivial experiment, the results actually have profound implications for our view on how language began in the first place. As 95% of people map the same sounds to these objects, the authors suggest that there might be ‘natural constraints’ on how sounds and objects are linked. They also suggested that the representation of a word in the motor cortex (to form the shapes to speak the word) and auditory areas are linked to the visual appearance of the object, which again shows these cross-modal connections at work.


The Stroop Test

This week’s brainteaser is about a psychometric test that you might have seen before as it was used in the Nintendo ‘Brain Training’ game a few years ago! You might not recognise the name though, but it is actually called the Stroop test.

In this test, participants are told that they must “say aloud the colour of the word shown, not what the word actually says”. It is named after John Stroop, who discovered the effect in 1935. In his original experiments, he compared the time it took for participants to read words printed in incongruent colours (e.g. the word black written in red ink) with the time it took them to name the colours of the words. He found it took participants 74% longer to name the colours than read the words.

The Stoop Effect shown by participants is as follows: they are quicker to read words shown in the congruent (same) colour to what the word says, and are slower when the word is printed in a different colour to what it says – this is known as the incongruent condition.


This test therefore shows that are brains are slower at responding when shown conflicting information than when stimuli are easily recognised. There are a few hypotheses for why this effect occurs. One hypothesis is that we are naturally faster at recognising words than colours, so we are able to read the word faster than identifying the colour. Another hypothesis is that reading is ‘automatic’ and doesn’t require conscious effort, unlike naming colours which requires our attention.

Since the Stroop test has been developed, it has been used to assess several psychometric functions. A common use is to measure inhibition – as the test requires you to inhibit your natural response to read the word. For example, in developmental psychology, this is part of several tests used to test executive functions (cognitive abilities including switching between tasks, working memory, planning, and inhibiting responses) in children with ADHD.

Different variations of the Stroop test have been developed, underlying the same principle that participants will identify congruent words faster than incongruent ones. One of these variations is the Emotional Stroop test, where instead of being colour words, emotionally-charged and neutral words are presented in different colours, and participants have to name the colour. The Stroop effect is that participants are slower to name the colour of the emotionally-charged words. An Emotional Stroop test was used by Bentall & Kaney (1989) to test patients with delusions (to read more about the background of delusions see here) and compare their responses to healthy controls. Participants were presented with neutral words, words with negative meaning, and words related to paranoia. They found that compared to control participants, participants with delusions look longer to name the colour of the words related to paranoia, suggesting they are more easily distracted by emotional stimuli.

So, how do you think you’d perform on the stroop test? Have a go here and find out!

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!

Are we logical?

If I asked you if you were logical, you would probably say yes. But how rational actually are we? According to some studies.. most people are pretty irrational – let’s see how you get on.

Wason Card Selection Test (Wason, 1966)

Each of these cards has a letter on one side and a number on the other. Your task is to decide which of these cards (choosing the minimum number possible) needs to be turned over to find out whether the following rule is true or false.

Rule: If the card has an A on one side, then there is a 4 on the other side.

Here are your cards, have a look and then scroll down for the solution…


The logical way to solve this problem is to try to find a counterexample to the rule – for example an A card which doesn’t have a 4 on the back. Therefore, the only way to test this rule is to turn over the A card. Only 4% of participants tend to choose this option, suggesting that we aren’t very rational! Instead, most people choose to turn over the A and 4 cards, which is an example of confirmation bias – they look to show that the rule IS true, and don’t look for an example of one of the variables without the other.

So how did you get on?!

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!

Seeing is believing?

Another brainteaser for you!

First of all, watch this video. Make sure you pay attention to the instructions at the beginning – you’ll be asked questions about it at the end.


What did you think?


And importantly – did you see the gorilla?!

Selective attention tests like these show how focused our attention can be, to the extent that we ignore something which should be obvious. This shows that our vision can actually be quite narrow, and might not necessarily be accurate.

But this isn’t necessarily a bad thing – when there is a lot going on around us, it is useful to be able to focus on one aspect of our environment e.g. crossing a busy road, without being aware of other competing stimuli. Attention has been compared to a ‘spotlight’ or a ‘zoom lens’ in that it can be focused on a wider or more specific area of the visual field, depending on what is needed for the current task.

So, do you believe that what you see is true?

Try this test out on your family and friends – and check out my previous Brainteasers post here.


Capgras Syndrome, Phantom Limbs & Synaesthesia

I’m back with a video from the psychologist who inspired me to study this subject at uni – V.S. Ramachandran. This TED talk looks at what makes us human and how studying neuroscience can link questions from philosophy to psychology. It also mentions Capgras syndrome – when patients with brain damage to the area of the brain important for emotion believe their loved ones have been replaced by ‘imposters’; a novel treatment for phantom limb pain and what makes humans creative.

So what are you waiting for? Click the link below 🙂


N.B – if you wanted to read more about phantom limbs then why don’t you check out my blog post