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!


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!

Visual Extinction

Visual Extinction is a condition caused by damage to the parietal lobe, and is similar, although distinct from Visual Neglect.

It is characterised by the ability to see stimuli in the opposite visual field to the brain damage, but only when there is no competition from other stimuli in the visual field on the same side as the brain damage. If there are stimuli in both visual fields then only the one which is projected to the intact side of the brain will be seen.

It is diagnosed using confrontation testing:
– the experimenter wiggles their left/right fingers or both in the air while sitting opposite the patient
– patient can detect each finger when they are presented separately
– however, if both are presented then they can only detect the finger on the left (assuming the right parietal lobe is damaged)

The video below shows a variation of this technique:

However, there are some circumstances in which extinction can be reduced.

Riddoch (2003) presented patients with pairs of objects, which were correctly or incorrectly presented for action. For example, a corkscrew pointing towards the cork in a wine bottle (correct), or at the bottom of the bottle (incorrect).
The results showed that they were better at reporting both items when they were correctly presented for action. When one item was extinguished, they were more likely to report the active item, even if it is in the impaired visual field.
Therefore, which object they reported was influenced by the interaction between them.
This finding was important as it suggests that extinction occurs quite late, in higher-order visual areas and there is some unconscious processing of extinguished items.

Like Neglect, extinction can also occur in motor actions, not just vision.
For example, several case studies have shown that patients can use both arms equally well separately, but become much worse at using their bad arm when doing so at the same time as the good arm.

Hope you enjoyed this post – don’t forget to check back soon for more!

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 🙂


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 🙂

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 🙂