This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.

Contribute Thoughts | Search Serendip for Other Papers | Serendip Home Page

Biology 202
2004 First Web Paper
On Serendip

The Mind's Eye? A Look at Optical Illusions

Ghazal Zekavat

Epicharmus, a Greek poet and originator of Sicilian Comedy (1) is credited to have said that "the mind sees and the mind hears. The rest is blind and deaf." (2) Although Epicharmus' idea was conceived around 450 BC, it is interesting to apply it to our modern understanding of optical illusions, if we understand an optical illusion to mean a "false visual perception" (3). One type of optical illusion that specifically interests me elicits the illusion of motion. These "motion perception" (4) illusions provide exceptionally striking visual effects, usually of a stationary figure appearing as a circular-rotating figure. Using motion perception illusions as a model, can we use Epicharmus' notion that the "mind sees" and the "rest is blind" (in this case, the eyes) to explain the phenomena of optical illusions?

It will first prove helpful to understand how the eye works. When we see an image, our eyes are actually receiving light, which enters through the cornea. The cornea bends the rays of light before they reach the pupil. The rays of light then pass through the lens and bend toward the retina. (2) The retina, however, captures an inverted image. There is a layer of photoreceptors (among other types of neurons) on the retina which are used to measure light intensity in a way that then allows the rest of the nervous system to understand the signals. In humans, as well as most animals, nerve cells found in the eye are organized into a "lateral inhibition network." Before the signals are sent to the brain through the optic nerve, the lateral inhibition network, along with other organizations of neurons on the retina, process them. (5)

The lateral inhibition network actually "throws away" a significant amount of information. So, is lateral inhibition helping or hurting our ability to see? Lateral inhibition consists of excitatory input from some photoreceptors and an inhibitory input other photoreceptors. The same levels of illumination of excitatory and inhibitory photoreceptors generates the same output signal. However, when there is a contrasting dark/light border, different output signals are generated. (5) In general, lateral inhibition is able to "fill in" much of the information that it "throws away." In this case, lateral inhibition does not hurt our ability to see. On the other hand, sometimes the wrong information is filled in, and we see the illusion of another image.

The motion perception model (also called the "peripheral drift illusion") that I would like to discuss is called "rotating snakes." (6)Here, the viewer sees the image of rotating coils of snakes, whereas the actual image is quite stationary. It is important to note that there exist many different regions of color contrasts in this illusion, and that it heavily relies on peripheral drift. In general, illusory motion has a pattern of moving from a black region to an adjacent dark gray region, or from a white region to an adjacent light gray region. Factors such as curved edges and shorter edges enhance the peripheral drift. (7)
Although Epicharmus could not explain the phenomena of peripheral drift or the lateral inhibition network, his idea that the "mind sees" and the "rest is blind" still raises some interesting points. With regard to the peripheral drift illusions, the image the viewer sees is in large part a product of the "brain." The eyes, therefore do not behave as a camera does; they cannot simply capture an image independent of a lateral inhibition network, independent of the brain's involvement. However, simply because the brain may be involved in our sight does not mean that seeing is necessarily a "conscious" effort. For example, the lateral inhibition networks work as a part of the unconscious brain (5). In effect, no matter how hard one tries to avoid being fooled into motion perception, one cannot do it (unless one is an appreciable distance from the image, therefore lessening the strength of its light/dark regions).

So, while it is the mind, or part of the mind that is deciphering the rays of light picked up by the eyes into meaningful images, it may be working semi-independently from other parts of the brain which are used for logical thinking or problem solving. In other words, when it comes to peripheral drift illusions, we cannot think our way out of seeing something that is not there. On the other hand, we can know that what we are seeing is in fact, an illusion (although not necessarily instantaneously).

Clearly, no two brains are alike, so we can infer that no two people see something in one way. By and large, however, the patterns of vision are similar, especially with regard to motion perception illusions because of the way the eyes (and brain) work. Knowing that what we see is not exactly a snapshot of the world can be a disheartening notion. However, realizing that the way in which we view the world is unique and subject to a system as complex and evolved as the human brain, our view of the world does not seem so disheartening, after all.


1)Encyclopedia Britanica: Epicharmus
2)Are you seeing what I'm seeing? By Keith Gaudet, A simplified explanation of how the eye works and perceives illusions.
3) dictionary definition of "optical illusion"
4)Sap Design Guild's Optical Illusions, A nice web resource for the different types of optical illusions
5)Serendip: Lateral Inhibition, A rich resource from Bryn Mawr College about how the eye works
6)Optical Illusions: Rotational motion, A website containing rotation motion optical illusions, namely the "Rotating Snake Illusion"
7)Phenomenal Characteristics of the Peripheral Drift Illusions: Vision: Vol. 15 No. 4 261-263, 2003, An article from the Journal "Vision" explaining the phenomenon of peripheral drift.

| Course Home Page | Course Forum | Brain and Behavior | Serendip Home |

Send us your comments at Serendip

© by Serendip 1994- - Last Modified: Wednesday, 02-May-2018 10:53:06 CDT