Optical Illusions
Optical illusions are images formed by misinterpreted sensory signals. They are characterized by visually perceived images that differ from objective reality. (Wikipedia) Gestalt psychologists who study form and configuration say perceptual interpretations people make of any visual object depends not only on the properties that specific element, but also the properties of the features surrounding the visual image. The visual system accomplishes the interaction of the visual object and its surroundings by processing sensory
information about the objects including shape, color, distance, and movement of objects. The brain takes this information and makes certain assumptions about what should be seen in the world. These assumptions are expectations that derive from past experience and from built-in neural wiring for vision. (Kandel)
Dir Journal
Dir Journal

In the image to the left, there are two flower-shaped images both containing orange circles in the middle surrounded by blue circles. Which orange circle is bigger? The correct answer seems to be the orange circle in the right flower, but this is incorrect. Both circles are actually the same size. The orange circles appear to be different sizes because of how the brain perceives the surrounded objects. The left flower has large blue circles surrounding a smaller orange circle, while the right flower has small blue circles surrounding a larger orange circle. The brain makes the assumption that since the orange circle is dwarfed by its surrounding that it must be smaller than the circle on the right. This is what the Gestalt psychologists meant when they said the brain makes assumptions on what should be seen in the world.

Types of Illusions
Optical illusions can be divided into different types. Richard Gregory's article Knowledge in Perception and Illusions states that there are two main types of illusions. There are physical illusions and cognitive illusions. Gregory states that physical illusions are either due to the disturbance of light between the objects and the eyes, or due to the disturbance of sensory signals of the eyes. Cognitive illusions occur because of the brain misapplied knowledge perceived by the brain to interpret or read sensory signals. Although Gregory only classifies two types of illusions, there are other sources that break Gregory's physical illusions into two categories. Wikipedia and BoomerYearBook both break physical illusions into literal and physiological illusions. Literal illusions are optical illusions that depict an image that is not what the actual image means to portray. Physiological illusions occur because of excessive stimulation of a specific type of information. The left shows a literal illusion. Middle has a physiological illusion. The right shows a cognitive illusion. These illusions comes from the Boomer Year Book, NKU, and Dir Journal websites, respectively.
external image 1176564286_elephantlegs.gif external image HermannGrid.jpgexternal image kanizsa.png

Literal Illusions
The picture of the elephant is a classic example of a literal optical illusion. The artist of this picture drew the top portions of an elephant's legs without connecting the legs with feet. Also, the artist drew feet without connecting them to legs. The artist portrays the elephants body parts by using a shading technique. Although we know elephants are supposed to have four legs, this elephant appears to have more than that. The brain is confused here because our it has a tendency to use edges to identify objects. The artist cleverly drew the shaded legs and shaded feet next to blank spaces knowing that the human brain will still recognize an object even without shading or color.

Physiological Illusions
The middle illusion, next to the elephant, is known as the Hermann grid illusion. This is a physiological illusion, which causes the viewer to see gray dots appear on the white lines even though there are no gray dots drawn in this image. Wherever the viewer's eyes are focused, that portion of the image will appear as white lines with a black background. However, the viewer will notice to the sides of the focus point, gray spots appear but disappear when the viewer looks to that area. This illusion can be understood with a basic explanation of on-center receptor fields. On-center receptor fields are part of an important portion part of the visual process called the ganglion cell, but ganglion cells will be discussed in more detail later. On-center ganglion cells are located in the retina. An on-center receptor field can be picture as the shape of a doughnut. The hole of the doughnut is the on area and it is activated by light. The outside of the doughnut shape is the off area. The off area creates an inhibitory effect when light hits it. The activity of the center portion can be countered by inhibition due to light hitting the outside portion of the receptor field.


In the Hermann grid illusion, gray dots only appear in the intersections. The picture to the right provides a helpful visual for understanding this concept. In receptor field A, the on area portion of the receptor field is active because it falls under light (the white band). However, there is a large portion of the off area region of the receptor field falling under light. This causes an inhibitory response overpowering the activity of the on area. This is a process referred to as lateral inhibition. A gray dot appears because there are more off area receptors being affected by light than on area receptors. The viewer will also notice that a gray dot does not appear in the horizontal white bands such as in receptor field B in the picture on the right. This is simply because there is not enough off area receptors being hit by light to counteract the on area activity. Another interesting thing to note is that gray dots only appear in the peripheral visual field. As stated before, the fovea has high visual acuity. The focus point in this illusion will not produce a gray dot because the receptor fields in the fovea are very small. The minute size of the receptor field causes the image to be much clearer. On the contrary, receptor fields in the periphery are much larger creating less visual acuity and causing more off area receptors to inhibit on area receptors. For more helpful images, and further detail about on/off center cells and lateral inhibition, go to http://www.nku.edu/~issues/illusions/HermannGrid.htm.

To understand the concept of on center cells better, it may be beneficial to understand their role in the visual pathway. On-center ganglion cells are a significant part of the process of vision, but they are a very small portion of the process. An image shines on the retina, which is where the photoreceptors, rods and cones, are located. The photoreceptors go through a process where an image is converted into electrical signals. Horizontal cells are laterally connected with photoreceptors and help regulate the output of photoreceptors. The photoreceptors then pass information on to bipolar cells, which then transfer information to both amacrine cells and ganglion cells. The information tranferred to ganglion is also regulated by amacrine cells, thus playing a similar role to horizontal cells. However, amacrine cells differ because they transfer signals to the ganglion cell as well. The ganglion cells can be off-center or on-center cells, but only on-center ganglion cells are used to understand the Hermann grid illusion. Take a look at this helpful image for a better understanding. From the ganglion cells, the signals exit through the optic nerve to enter into the bran.

Dir Journal
Dir Journal

After-images are another popular form of physiological illusions. In the above image, there is an oddly colored American flag. Stare at this image for thirty seconds and then look to the blank space to the right of the flag. In the blank space, a red, white, and blue American flag will appear. Shinsuke Shimojo explains in his article, Afterimage of Perceptually Filled-in Surface that negative after-images occur due to bleaching of the photoreceptors or because of neural adaptation. For neural adaptation, the photoreceptors adapt (tire out) and stop responding once they have focused on an image for a certain period of time. In the flag illusion, there are bluish-green stripes and a yellowish square. The retina contains blue, green, and red cone cells. In this image, the blue and green cone cells are firing when viewing the turquoise stripes. Since the blue and green cone cells adapt and tire out, the only cone cells left to fire are the red cone cells. This is why red stripes are seen where the turquoise one were when looking at the blank space. When viewing the yellow square, a combination of red cone cells and green cone cells are adapting. Therefore, when they tire out, only blue cone cells are able to fire while looking at the blank space. The same principle applies for rod cells, except rod cells are able to detect white and black colors. When viewing black for an extended period of time, the after-image will produce a white color. Bleaching is very similar to neural adaptation. The difference is instead of photoreceptors getting tired (adapting), the visual pigment become damaged due to the excess light. When light hits a rod cell, rhodopsin, the visual pigment in rod cells, changes its configuration and allows a hyperpolarization process to occur in the rod cell. However, an excess amount of a light damages the rhodopsin, preventing the hyperpolarzation. The same process occurs in cone cells, but the visual pigment in cone cells is known as photopsin.

Cognitive Illusions
The image to the right of the Hermann grid illusions is an example of a cognitive illusion. A cognitive illusion occurs because the brain organizes visual information and combines it to form a meaningful whole. The above illusion is known as the Kinazsa triangle. In this illusion, the viewer will perceive a white triangle even though there is not a white triangle drawn in the image. This triangle is seen due to the brains tendency to fill in blanks to create a more familiar picture.

Eye Tricks
Eye Tricks

Cognitive illusions can also occur due to the brains tendency to form an object and a background. In the visual system, only part of an image can be selected as the focus of attention. While this certain portion of the image becomes the focus, the rest will become background. The background and focus can be switched back and forth, but one will always be the focus while the other is always the background. The image to the right is an example of this figure-ground illusion. If the green arrows become the focus point, then there appears to be a yellow background and vise versa. Although the viewer may know that there are both yellow and green arrows, both arrows cannot be perceived at the same time because the brain perceives images in a figure-ground manner.

There are two streams that image information can travel to the brain. If information is focused on the fovea, it will travel through the dorsal stream pathway. The fovea contains a large amount of cone photoreceptors, which allows identification of the image. Since the fovea has a high visual acuity, the dorsal stream is known as the "what" pathway. The background of the image is not identifiable as arrows because this portion of the image is not focused on the fovea. If signals are not being focused on the fovea, then the information will travel through the ventral stream, which is known as the "where" pathway. This image will not easily be identified, but the viewer will be able to detect where it is located.

Cognitive illusions also utilize the brains ability to make assumptions by contrasting colors such as in the spiral illusion spiral illusion. In this illusion, the viewer will notice a blue spiral and a green spiral with magenta and orange lines encircling the entire image. The amazing part of this illusion is that green and blue spirals, are actually the same color. This illusion is created because the orange lines only travels through the "green" portions and not the "blue" portions. Gestalt psychologists explain that the brain does this because it is programmed to group things together and make assumptions. The brain automatically compares the orange lines to the blue spirals causing the viewer to perceive green. Beau Lotto explains it very well when he says, "The visual cortex is adapted to relate the retinal image to behaviour given the statistics of its past interactions with the sources of retinal images: the visual cortex is adapted to the signals it receives from the eyes, and not directly to the world beyond." (Lotto) Take a look at Lotto's presentation for further understanding on how contrasting colors cause illusions. Beau Lotto Video

All of the illusions talked about so far, have been still images, but illusions can also be caused by motion effects. Motion illusions fall under the category cognitive illusions because they are created by misapplied information from the visual cortex. Throughout a person's life he or she sees motion, which is picked up by the retinas and processed all the way to the visual cortex in the brain. It is beneficial to have a general understanding of how electrical signals reach the visual cortex. As stated before, visual information is turned into electrical signals by the photoreceptors and then transferred to ganglion cells. From the ganglion cells, the signal exits the retina through the optic nerve. The optic nerve crosses a path at the optic chiasm and travels to either the thalamus, superior colliculus, or pretectal region, but only the thalamus in significant when considering motion illusions. Once the signal reaches the lateral geniculate nucleus of the thalamus, information then splits into two streams (dorsal and ventral) via optic radiations. These optic radiations terminate in different parts of the visual cortex. Motion is interpreted in both the primary visual cortex and also the middle temporal region of the visual cortex. Similar to other illusions, these areas of the brain are able to make assumptions of the objects seen based on past experience.

After understanding the pathway of vision and knowing where motion is detected, it is easier to understand how a motion illusion can be created. A common theory for motion illusions in known as the phi phenomenon. Take a look at this Stroboscopic Alternative Motion illusion. It appears as if the two dots are moving back and forth, but they are technically not moving back and forth because the two dots are not the same dots in the top and bottom rows. When the dot in the top row disappears a different dot that is identical in appearance appears in the bottom row. This tricks the brain primary visual cortex and middle temporal portion into thinking the dots from the top row is moving to the bottom row. This false perception of motion is known as the phi phenomenon.

Motion illusions can also occur in still images. This false motion is not caused by the phi phenomenon, but the same areas of the visual cortex are responsible for the false perception of the image. In the image below, the leaves are drawn in a pattern that the primary visual cortex and middle temporal region assumes to be the start of motion, so the viewer actually perceives motion.

Dir Journal
Dir Journal

Illusions and the Brain
When thinking about illusions it is important to realize the eyes have little biological value unless the brain can identify the image. (Gregory) We can see an object as having many characteristics such as it being hard, heavy, or sharp, but these traits are not properties of the image that hits the retina. The human brain automatically makes these connections based on experience because the brain is able to store information it encounters in the past. The brain's parietal and temporal regions are both involved in vision. The parietal region is concerned with spatial relationships while the temporal region deals with object recognition. Visual information is conveyed from the retina to the visual cortex located in these two regions of the brain. After experiencing properties of certain objects, the brain is able to store this information and now it is able to make assumptions on what properties objects possess. Illusions, errors of perception, occur due to stored knowledge being inappropriate or misapplied.

Glossary of Terms
Optical illusion - A deceptive image that is misperceived by the visual system.
Gestalt - A German term for configuration or form.
Literal illusion - Image that is different from the the objects that make them.
Physiological illusion - Image caused by excessive stimulation of a specific type of information on the eyes.
Cognitive illusion - A incorrect perception of an image due to assumptions made by the brain about that image.
On-center receptor cell - A receptive cell in the retina that is excited when light hits the center portion and inhibited when light hits the outer portions.
Off-center receptor cell - A receptive cell in the retina that is excited when light hits the outer portions and inhibited when light hits the center portion.
Lateral Inhibition - A process when receptors inhibit the action of their neighbors. Lateral inhibition can differ throughout parts of the body, but for illusions, lateral inhibition refers to the receptor cell when light hits the inhibitory part of the field and a decrease in the excitation.
Fovea - The central part of the retina where visual acuity is high because of the high amounts of receptive fields and the small size of the fields.
Photoreceptor - A type of neuron found in the retina that is capable to converted light into electrical signals.
Rod - A photoreceptor that is sensitive to light and play a major role in night vision. Rods are high concentrated in the periphery of the eye.
Cone -A photoreceptor that detects colors. Cones are able to perceive finer detail than rods and they are highly concentrated in the center of the eye.
Horizontal cell - A laterally connecting neuron in the retina that connects with photoreceptors and regulates their output.
Bipolar cell - The neuron that receives an electrical signal from photoreceptors and transfer the information to a ganglion cell.
Amacrine cell - Similar to horizontal cells, but these neurons not only connect with bipolar and regular their output, but they also transfer the signal to a ganglion cell.
Ganglion cell - The final neuron in the process of sending an electrical signal received by a photoreceptor to the optic nerve.
Optic nerve - The route electrical signals travel to exit the retina and reach the brain.
Optic chiasm - The part of the brain where the two optic nerves' tracts cross.
Lateral geniculate nucleus - The primary processing area for visual information. It is located in the thalamus.
Rhodopsin - The visual pigment in rod cells. Rhodopsin is made up a protein opsin and retinal, which absorbs light.
Photopsin - The visual pigment in cone cells. Photopsin is also made up of opsin and retinal.
Hyperpolarization - A decrease in a cell's membrane potential. It is the opposite of depolarization.
Dorsal stream - This stream is known as the "where" pathway. It is responsible for determining the location of objects in space.
Ventral stream - This stream is know as the "what" pathway. It is responsible for object identification.

1. What are the three types of optical illusions?
a. Physical, Physiological, Cognitive
b. Literal, Psychological, Cognitive
c. Literal, Physiological, Cognitive
d. Psychological, Literal, Visual

2. The elephant illusion is caused by the brains tendency to
a. Fill in blanks
b. Use contours to recognize objects
c. Form a figure-ground relationship
d. Make assumptions from past experiences because it is known that elephants have four legs

3. Negative after-images are caused by
a. Bleaching or tiring of photoreceptors
b. On-center receptor cells inhibiting neighboring cells
c. Off-center receptor cells exciting neighboring cells
d. Miswiring of the visual cortex

4. Where are cones densely populated?
a. In the ventral stream
b. In the dorsal stream
c. The retina's periphery
d. The fovea

5. Rhodopsin, the visual pigment is rod cell is made up of a protein retinal and a light absorbing portion known as opsin.

6. Lateral inhibition occurs in both rods and cone cells.

7. The ventral stream is known as the "what" pathway, while the dorsal stream is known as the "where" pathway.

8. The first step for hyperpolarization in photoreceptors is the change in configuration of the visual pigments caused by light.

9. Receptor cells tend to be much larger in the fovea, which is why visual acuity in the fovea is so high.

10. Photoreceptors convert visual information into sensory signals that the brain can interpret.

1. Briefly describe each of the three classifications of optical illusions.

2. How does a negative after-image effect occur? There are two ways, but just explain one.

3. Explain how the Herman grid illusion works at a neurological level.


1. Literal illusions are images, which are different from what the original objects are meant to portray. Physiological illusions are illusions that are caused by a physiological defect occurring, typically due to an over-stimulation of a certain type of information. Cognitive illusions occur because the brain makes an a false assumption about an object that is being seen by the retina.

2. Negative after-images occur due to a bleaching effect. Bleaching occurs when there is an excessive amount of light being hitting the visual pigment in a photoreceptor. This excess light deforms the protein portion of the visual pigment, opsin.
Negative after-images occur because photoreceptors get tired or adapt to light. When these photoreceptors adapt, they decrease their firing rate causing the opposite rod or cone to fire.

3. The Hermann grid illusion occurs because of a process known as lateral inhibition. On-center receptor cells have an outer ring, which inhibits activity of the cell when light hits it. The middle portion of on-center cells leads to activity. The intersections create gray dots because there is more light, white space, for the inhibitory outside portion of the cell to fall on. The inhibition counters the activity of the light falling on the center of the cell. This process is known as lateral inhibition and it is the reason for the appearance of gray dots.

Suggested Readings
This link briefly explains the different types of illusions and shows an example of each kind.

This web site has some helpful images and great explanations of lateral inhibition, photoreceptors, and the Hermann grid illusion. Click the links on the left side of the page.

This online text provides detailed easy to understand explanations of all the types of cells in the retina. These can be found under section 3, The Eye.

This site has a plethora of illusions with brief explanations of the basics on how each illusion works.

This is a fun video from Beau Lotto, an illusion expert.

If the Beau Lotto video was entertaining, this link contains his research on the topics he talked about in his presentation.

Kandell, ER., JH. Schwartz, and TM. Jessell, eds. Principles of Neural Science. 4th ed. New York City: McGraw-Hill Companies, Inc., 2000.

Pelli, D., Majaj, N., Raizman, N., Christian, C., Kim, E., & Palomares, M.Grouping in object recognition: The role of a Gestalt law in letter identification. Cognitive Neuropsychology, 26(1), 36-49. doi:10.1080/13546800802550134. 2009.

Gregory, RL. Knowledge in Perception and Illusion. Phil. Trans. R. Soc. Lond. B (1997) 352, 1121–1128. 1997.

Gregory, RL. Perceptual Illusion and Brain Models. Proc. Royal Society B 171 179-296.

Eagleman. DM. Visual illusions and neural biology. Macmillan Magazines Ltd. 2001.

Shimojo, S. Y. Kamitani, S. Nishida. Afterimage of Perceptually Filled-in Surface. Science. 1677-1680. 2001

Coren, S. Relative Contribution of Lateral Inhibition to the Delboeuf and Wundt-Hering Illusions. Perceptual and Motor Skills. 88, 771-784. 1999.

Chiangzi, MA, A. Hsieh, R. Nijhawan, R. Kanai, S. Shimojo. Perceiving-the-present and Systematization of Illusions. Cognitive Science.

Corney, D. J. Haynes, G. Rees, RB. Lotto. The Brightness of Colour. PLoS ONE. 2009.

Lotto, RB. Visual Development: Experience Puts the Colour in Life. Current Biology. 14: R619-R621. 2004.

Bach. M. 88 Visual Phenomena and Optical Illusions. 2010. http://www.michaelbach.de/ot/

Hubel, D. Eye, Brain, and Vision.http://hubel.med.harvard.edu/book/bcontex.htm

The Negative Afterimage Stimulation http://www.unm.edu/~toolson/435_afterimages_introduction.html

Optical Illusions. Dir Jounral Info Blog. http://www.dirjournal.com/info/optical-illusions/

Psychological Article: Types of Online Optical Illusions. Baby Boomer Yearbook. http://boomeryearbook.com/blog/2009/03/15/psychological-article-types-of-online-optical-illusions/

Douglas, K. Retinal Illusions.http://www.nku.edu/~issues/illusions/Retina.htm

Opitcal Illusion. http://en.wikipedia.org/wiki/Optical_illusion