Checker Shadow Illusion

July 29, 2014

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In this optical illusion, we see what appears to be a checkerboard of two different shades of gray. We also perceive a shadow coming from the cylinder. In reality, the two different shades of gray, as seen in square A and square B, are in fact the same shade. This can be proven by printing out the illusion, cutting out the two shades, and putting them side-by-side. It will then be seen that they are in fact the same shade of gray.


According to Edward Adelson, who created this Checker Shadow Illusion, because there is a shadow created by the cylinder, the visual system has to determine where the shadows are and how to compensate for them. Local contrast is something that the visual system uses to determine what effect the shadow has on the image. “In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.” So really this is not a failure of the visual system to demonstrate, it is actually the success of the visual system doing what it was meant to do.


This optical illusion really shows how the visual system is really able to work out and process things in a very meaningful way. The visual system, in this case, was able to break down the information given from the illusion and put it back together in a way that the eye and brain would be able to perceive it.

The Müller-Lyer Illusion

July 29, 2014

In the Müller-Lyer Illusion, three lines are shown with arrows pointing inwards or outwards at both ends of the line. When we look at these arrows (in the top picture) and try to figure out their lengths, the line in the middle seems to be longest, followed by the third line, and the first line is the smallest. However, this is not true. All of the shafts of the lines are actually the same exact size, as the bottom picture shows.

This is a popular illusion that has been the subject of various theories and psychological experiments. However, there does seem to be a neurological basis to this illusion. This illusion occurs because our brain is used to perceiving depth in the 3D world. When the brain must switch to a 2D view of this image, it perceives the image as 3D. This makes the brain think that the shafts of the lines are different sizes, as the size constancy mechanism does not function the same for 2D images as it does for 3D images. Another simpler explanation is that since the lengths of the lines including the fins of the arrows is different, the brain automatically thinks the lengths of the shafts are different.

This optical illusion teaches us that the visual system can be easily tricked, and it does not perceive 2D and 3D images the same way.

Interestingly, it has been shown that the perception of this illusion varies between cultures and age groups.

Einstein or Monroe?

July 29, 2014

This optical illusion works with a number of different faces, but the most common version is seen with the faces of Marilyn Monroe and Albert Einstein. A hybrid image is constructed by combining two images. For instance, looking at the picture from a short distance, one can see a sharp image of Einstein, with only a hint of blurry distortion hinting at the presence of an overlaid image. Viewed from a distance in which the fine detail blurs, the unmistakable face of Monroe emerges.

A hybrid image is perceived one of two ways, all depending on distance. Optical illusions such as these function by combining low spatial frequencies of one picture with the high spatial frequencies of another picture. The concept of binocular rivalry illustrates this optical illusion. Rivalry greatly suppresses activity in the ventral pathway and attenuates visual adaptation to form and motion. Inhibitory and excitatory circuits considered within a hybrid model might account for the paradoxical properties of binocular rivalry.


The Tower – Optical Illusion

July 29, 2014

This is a work of art created by graphic designer and painter István Orosz. He has produced many other illusionary images like this one.Screen Shot 2014-07-28 at 7.01.41 PM In this image, the observer sees a cylinder that does not appear to have to same orientation from every angle. The brain cannot transform this 2D image into a 3D model, so we are not able to follow the cylinder with our eyes for a full rotation without it seeming like the visible face of the wall switches the way it is curving. To add to the confusion, you are almost forced to look at the wall a specific way based on the way the people are positioned in the openings, and the different ways that the people are sitting also do not sit in a position that would make it seem like a true cylinder.

This happens due to the constraints of the brain to produce 3D mental representation based on a 2D image we look at. The brain assigns a depth to every point of the 2D image. Looking at only a small section of the image, that small section is consistent with spatial perspective even though each section suggests a certain orientation or direction of continuation of the cylinder. But, when you try to look at the image as a whole, a spatial paradox results. This indicates that our visual system is limited in constructing 3D models or pictorial representations.

Edge Enhancement and Lateral Inhibition

July 28, 2014

I chose to research the edge enhancement phenomenon. This is where colors are perceived differently based on there relativity to other colors that are lighter or darker. The more light that hits the photoreceptors, the stronger the stimulus. This stimulus is perceived by the brain. Since photoreceptors inhibit the stimulation of their neighboring photoreceptors, the stronger the stimulus in one photoreceptor, the more it inhibits the neighboring photoreceptor. This is called lateral inhibition. If all neighboring cells receive the same stimulus, there will be equal perception of the signal within the brain.
Edge enhancement occurs when receptors that receive stronger stimuli inhibit their neighbors more strongly. It also occurs when receptors that receive weaker stimuli do not inhibit their neighbors as strongly. When a neuron is inhibited more than its neighbors, it results in the perception of a darker color. When a neuron is inhibited less, it results in the perception of a lighter color. This gives an enhanced contrast between light and dark colors/images side-by-side.

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In this photo, lateral inhibition is occurring in the retina. The white areas next to each other cause inhibition of photoreceptors in between. This gives the illusion of slightly darker squares in the middle. These are the grey spots in each corner.

Galileo’s optical illusion

July 28, 2014

Galilleo illusion

In this illusion, the white coloured square against the black background, appears to be larger than the black coloured square on the white background. In actual fact, they are the same size. Our response to light makes it difficult to see that these squares occupy exactly the same area. The Italian astronomer Galileo first made a similar observation when looking at the planets. He found that Venus looked far larger than Jupiter (since it was the brighter of the two planets). Despite this, at second glance, looking through a telescope it is obvious that Jupiter is clearly the largest of the two planets.

This illusion can be explained by light sensitive neurons. A recent study showed that when the electrical signals between neurons were recorded using electrodes,  the neurons sensitive to light object gave a disproportionately greater response than those neurons sensitive to dark things. The heightened activity of these light sensitive neurons explains why our attention is shifted to the right, and why the white square appears much larger. Scientists working on this research believe that these light sensitive cells are within the eye itself, rather than in the brain. These cells are responsible for changing our perception of the image we are viewing. Research into this area has also sparked interest in problems with vision, which could potentially be explained by the imbalance in activity of these neurons.

 With regard to our visual system, this optical illusion tells us that under different conditions, things can be perceived much differently. It also tells us that sometimes our eyes and our brain are not always in complete agreement! What is interesting about this illusion is that the observed effect can  be essential for survival (particularly for predators in the wild).


Rotating Snakes Illusion

July 28, 2014


In the Rotating Snakes illusion, subjects see what appears to be spontaneously rotating circular snakes. This type of illusion has been named a “Peripheral Drift” illusion, as the motion only occurs in your peripheral vision. When you focus on a circle, the rotation will cease. There hasn’t been evidence to show that a colored version is more effective than a black and white image. The main building block to allow this illusion to be effective is a sequence of four  elements of varying luminance; this would be a series of four colors beginning with a black (or darkest color) and ending with white (or the lightest color). Large scale organization is critical as the individual blocks to not evoke a sense of movement.

It is believed that the  four elements of luminance generate local motion signals. Studies have been done that measure the shape of temporal impulse response (TIR) function corresponding to the brightness of the stimuli. With the relative contribution of the transient component falling with decreasing brightness, researchers believe that transient factor of the TIR caused the strength of the illusion.

Retinal ganglion cells carry information to the retina to many different locations within the brain. There are two main categories of these cells, X and Y. X cells have an momentary response initially, but do not show sustained response to stimulus. Y cells respond faster and show more transient responses to the same stimulus. The receptive fields of these cells allow the cells to potentially decipher contrast within the illusion, but not the raw luminance.

Many participants of studies reported that if they focused on the imagine, the illusion failed. This shows that there is a need for the image to be refreshed by blinking or moving the eyes to sustain the illusion.



info from: Understanding the Rotating Snake Illusion by Martin O’Reilly

Stepping Feet

July 28, 2014

Stepping Feet illusion:

  • Describe your optical illusion, what do we see/not see?
  •                  This illusion is to colored blocks traveling across a black and white striped back ground. By looking directly at on of the blocks you can see that it is moving non-stop across the background. But if you do not focus on the blocks but rather look somewhere on the background, you will see the blocks stopping and starting again at each black stripe.
  • Describe the neural bases of how your illusion works.
  •                 When there is a background with contrasted edges (black and white) our perception of the speed/ motion of an object over top of it is changed. But if we focus on the object instead of the back ground we see the constant motion.
  • What does your optical illusion tell us about the visual system?
  •                 This optical illusion tells us that our visual system is probably confused about what we are seeing. Our visual system is confused because the leading and trailing edges of the colored block are in two different backgrounds. So we cannot correctly perceive its speed.

The Leaning Tower Illusion

July 27, 2014



The Leaning Tower Illusion

Two identical pictures of the leaning tower of Pisa are displayed side by side. The towers are parallel. However, it seems that the picture on the right is leaning more than the picture on the left. Rather than looking parallel, the two towers seem to diverge farther apart in the distance. The explanation for this illusion lies in how our visual system interprets depth. We have 2-dimensional eyes, but we need to interpret 3-dimensional space, so our brain has to makes sense out of a distorted picture sent by the retina.

When our retina sees two parallel lines going off into the distance, the lines appear to converge. For instance, stand in the middle of a road and look down the road. The left and right edges of the road seem to converge in the distance. So, when our visual system sees two lines converging in the distance, our brains are hard-wired to interpret them as parallel. In the case of this illusion, our visual system sees two lines (towers) that look like they DON’T converge in 3-d distance. Thus, our brains actually interprets that these towers are diverging apart.

This illusion demonstrates how our brains have evolved to understand 3-dimensional space. Understanding space is crucial to navigating around an environment in search for food and whatnot. Thus, we are able to understand and sense depth in a 2-d image provided by the retina. When we see converging lines, our brains interpret the visual distortion and think parallel. However, this also means we can be tricked. The leaning towers are 2-d but they look 3-d. That causes the illusion to occur.


Alas, poor Yorick!

July 27, 2014

In the late 1800s, Pears Soap started a successful advertising campaign using an optical illusion. Named ‘Yorick’s Skull’ (for the jester who’s skull Hamlet famously holds) the illusion makes use of the effects of an afterimage.

Aside from being a staple in many optical illusions, afterimages are commonly seen in daily life (think of those dark spots in your field of vision after a bright camera flash). Afterimages are a result of our brain adapting to overstimulation. When exposed to a bright or unchanging stimulus for a length of time, the active photoreceptors in our retina become fatigued and lose sensitivity. As a result, when looking away from the stimulus, our retina takes a few seconds to adjust, causing the afterimage.

Some scientists also argue that prolonged exposure to an unchanging stimulus can cause the brain to expect it to remain unchanging. Hence, when it does change, we see the old image for a few seconds.

In the case of Yorick’s Skull, focusing on the ‘x’ in its right eye causes photoreceptors to lose sensitivity as they become fatigued. Additionally, the static nature of the image may cause the brain to believe that the image will not change. Hence, when we abruptly look away from the image, we can clearly see the skull’s afterimage for a few seconds.

(How this helped to sell soap remains ambiguous…)