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.





The Mind’s Eye

For the vast majority of you, if I asked you, right now, to imagine a beach – all golden sands and blue sky then I doubt it would be a problem. We ‘see’ the beach with our mind’s eye, even though there is no beach in front of us, and it’s most likely still grey and raining outside. The fact that our mind can generate images this lifelike is extraordinary, and something which I will explore more in this week’s post.

Something also interesting about our ability to generate these mental images, is that we can manipulate them in our minds. We do not see a flat, 2D object, but something we can move, or interact with. This was investigated in one study by Shepard & Metzler (1971) which used novel shapes so that previous experiences could not affect the results. Participants were shown pairs of shapes like the ones in the image below, and were asked to decide whether they were the same shape, or mirror images of each other, as quickly as possible.

As you can see, this requires some concentration! The researchers found that there was a strong linear relationship between the time it took for participants to respond and the angle of rotation. From this, they concluded that people rotate mental images at about 60º per second.

Interestingly, other studies have shown that this rotation speed of mental imagery is affected by the laws of physics – which at first glance seems improbable. I mean if you’re imagining something, why can’t you move it however you’d like? Parsons (1987) found that people find it difficult to rotate a mental image if it is physically difficult for this to happen – they used the example of imagining a foot rotating from someone’s ankle.

This effect is thought to occur as mental imagery, such as the examples above, rely on motor imagery – neuroimaging studies have shown that the motor cortex is active when we perform mental image rotation. As physical movement is constrained by the laws of physics, so are our transformations of mental images.

Mental imagery is also strongly related to visual perception, as shown in this early experiment by Perky (1910). Participants were asked to imagine the image of a given object on the dark screen in front of them, however they didn’t know that a faint picture of that object was also projected onto the screen. They found that although the participants do not notice the projected picture, they report that the image they’re imagining has the same properties as this picture e.g. the same rotation and size. This result suggests there must be some overlap between our mental imagery and our perceptions. More recent studies have also shown our mental imagery has some of the same properties as our visual perception, such as increased sensitively to the lower visual field.

Now, go back to your mental image of the beach, and try to imagine what life would be like if you kept really trying to conjure up that image, but were always unable to. Although most of us take our ability to have mental imagery for granted, a small percentage of people are unable to visualise mental images. This is known as aphantasia, and people with this condition are unable to visualise aspects of a memory, although they will be able to describe it. Scientists who have studied this condition believe that there is a spectrum of the vividness of which people experience mental imagery, with aphantasia being at the bottom.

Thanks for reading, and please subscribe if you don’t want to miss out on a new post every 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!


Can you imagine only being able to see one thing at a time?

This is the struggle faced by patients with Simultanagnosia – an extremely rare visual disorder. The reason this is so rare is that it is caused by damage to the posterior parietal cortex (shown in light blue below) but on BOTH sides of the brain.




The damage to the brain is thought to cause deficits in grouping stimuli, although this is thought to be due to impaired attention rather than impaired perception. 

For example, Luria (1959) found that if patients were shown a star of David with one triangle blue, one red, they only reported seeing one of the triangles, however if the triangles were the same colour, they saw the whole shape.




This result is interesting, because both shapes take up exactly the same amount of space on the retina, which shows that this disorder isn’t caused by a restricted visual field.

Patients also show a bias for local rather than global stimuli – this means they focus on small details rather than the overall shape.

This was shown in an experiment by Shalev et al (2008). The stimuli used were big letters made up of little letters e.g. lots of little H’s forming one big H. Healthy participants are quicker at identifying the big letter when it is made up from little letters that are the same, compared to when they are different, e.g. a big H made of little D’s.

However, Simultanagnosia patients do not show this pattern of results – instead they are faster at identifying the small letters regardless of whether they are the same as the big one.



As you can see from this graph, Simultanagnosia patients get a high percentage of single letters correct, but are impaired for the global (big) letter when it is made of smaller ones, regardless of whether they are the same or not.

One way to help patients see more than one item is to group items together.

For example, although patients cannot read text, they can read single words as the letters are grouped together. Hall et al (2001) found that familiarity of words also helps them be identified, as patients are bad at reading non-words. They also presented patients with acronyms (e.g. NHS) presented in capitals (familiar form) or in lower case (unfamiliar form). They found that Simultanagnosia patients could only recognise the acronyms when they were in capitals, showing that familiarity helps patients recognise the global form.


Thank you for reading!

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.


Visual Neglect

This is a neurological condition that is caused by damage to one if the hemispheres of the brain. This damage causes the patient to be unable to pay attention to one half of their visual field – they just ignore everything in it.

This condition – sometimes known as unilateral neglect, is most common when the right hemisphere is damaged as this hemisphere is involved in distributing attention. Damage to the right hemisphere causes neglect in the left visual field.

The area of the brain thought to be damaged in most cases of neglect is the posterior parietal cortex – shown on the diagram of the brain below:

The following examples are from a patient with unilateral neglect who has been asked to copy some images:

As you can see, they ignore the left side of the drawings. Patients will also ignore the food on one half of their plate and can also neglect one side of their body.

Neglect syndrome can occur after a stroke which damages the right hemisphere – the good news is that with occupational therapy, the patient’s condition can improve over time. One method which has been shown to help patients attend to their neglected side is to use prism glasses, which direct their eyes to their left visual field.

That’s all for now, check back soon for more posts + thanks for reading 🙂


The visual system

How is it that we can see the world around us? It’s quite a bit more complicated than most people think – not simply light hitting the retina and an inverted picture being turned the right way round. Here’s an overview of how we are able to see.

First of all, light enters the eye and hits the retina at the back of our eye.

The retina is made up of several layers of different cells which detect and then start to process the visual input. The cells which respond to light and colour are called photoreceptors. There are two different types of photoreceptors: rods for detecting light and dark, and cones, which detect colour. Cones are concentrated in the fovea, which is the area of the eye used for fixating on stimuli.

The diagram above shows the organisation of cells in the retina. The ganglion cells take the output of the retina to the brain via the optic nerve. The optic disk (see diagram of the eye) is where these projections leave the eye – another name for this area is the blindspot as there are no photoreceptors in this area.

The information from the retina is projected to the Lateral Geniculate Nucleus (see below). From there, it travels to the striate cortex in the occipital lobe – this area is also known as the primary visual cortex or V1.

From the primary visual cortex, the visual information travels through other areas in the occipital lobe, where aspects such as colour and motion processing occur.

After processing in the visual cortex, the information is projected to other areas of the brain. There are then two ‘streams’ in the brain which are specialised for different aspects of vision:

1. the dorsal stream projects towards the parietal lobe and is important in identifying object location.

2. the ventral stream projects towards the temporal lobe and is important in identifying and recognising objects.

These were identified by Goodale and Milner in 1992.

I hope you liked this post on the visual system, check back soon for my next posts about visual disorders and colour vision 🙂