Neuroscience For Kids

Visual Illusions: Explained?

By Annie Yang, Neuroscience for Kids Guest Writer
April 28, 2024

Is seeing really believing? What happens when what we see doesn't necessarily reflect reality?

Visual illusions have been around far longer than the science that has existed to explain them. One of the first examples of visual illusions in architecture came from the roofs of Greek temples which appeared curved though they were in fact linear structures. The Airavatesvara Temple in India holds one of the most famous and oldest examples: a sculpture appearing to be both a bull and elephant, depending on the observer.

Illusions result from a mismatch between reality and our subjective perception of our surroundings, events, or an object. We can organize visual illusions into several categories: literal, physiological, and cognitive.

Literal Illusions

Look at this drawing on the right. What do you see? Do you see a vase? Or two faces? Or both? If you can discern both images but not simultaneously, you are likely witnessing a literal illusion. A literal illusion occurs because our brains do not perceive information in individual chunks; this is also known as Gestalt psychology. There are many principles of Gestalt psychology including the principle of similarity, proximity, depth, and figure-ground. We can apply the specific Gestalt principle of "figure-ground" here to analyze what's going on. The principle states that our brains focus on a main object (the figure) and will tune out the surroundings (the ground) in order to avoid sensory overload and to direct our attention to only what's necessary. In this image, there are two main objects (the vase and the faces) and when we focus on one it is the figure, and the other naturally becomes the ground.

The Italian painter Giuseppe Arcimboldo was famously known for blending illusions with his art. Have a look at this painting on the left and see if you can explain to yourself what's going on in your brain!

Physiological Illusions

Hermann Grid What do you see in this image on the right? It appears to be a simple pattern with black squares and intersecting white lines. How many grey dots can you count at each intersection? If you find that there are many grey dots that all seem to disappear when you try to focus on them, you are not alone. This picture is famously known as Hermann's Grid. It's an example of a physiological illusion, which causes the observer to witness an object or image that isn't actually present in reality; however our brain fills in and provides us with extraneous or incorrect information.

Let's examine our visual system at the cellular level. This diagram below shows how a retinal ganglion cell might respond at each of these locations in the grid. (A ganglion cell is a type of neuron found in the retina that receives cascades of visual input from other cells. It is organized into a "center" (inner circle) and a "surround" (outer circle).

In the diagram, the color green indicates areas in the ganglion cell's receptive field that have been stimulated by light and yellow represents non-stimulated regions. However, an important distinction to make is that only the cells in the center are excited by light; cells in the surround that have been stimulated actually act as inhibitory signals. Ganglion cell 1 has 10 out of 16 total inputs that have been stimulated by light. Looking at the center we see 8 excitatory inputs and 2 inhibitory inputs in the surround, which marks a net of 6 stimulated excitatory neurons. As a result, we see bright white lines here. On the other hand if we take a look at ganglion cell 2, there are no inputs that have been stimulated by light, so we see a black background.

Lastly, ganglion cell 3 at the intersection shows 12 out of 16 total inputs that have been stimulated by light. Looking at the center we see 8 excitatory inputs and 4 inhibitory inputs in the surround. Recall that ganglion cell 1 had a net 6 stimulated inputs but here there is only a net of 4 stimulated neurons, leading the intersection to appear darker than it actually is.

Notice that if you try to focus on one of the intersections, the grey dot disappears. This is because the point of our central vision (the fovea) has extremely high resolution. Ganglion cells close to the fovea have smaller receptive fields and therefore fewer inhibitory inputs. As a result, we are able to see the black and white more clearly.

Cognitive Illusions?

Penrose Triangle Lastly, we will discuss cognitive illusions. Unlike literal or physiological illusions, cognitive illusions result not from any faulty transmission of information or subjective perceptions in our brains, but rather because they confuse our understanding of physics and math. The figure on the right shows one such example known as the Penrose Triangle.

Try to imagine rolling a marble along the Penrose Triangle. You will find that following any path will bring you to each and every side, although looking at the whole figure that shouldn't be possible.

The Purpose of Visual Illusions

While visual illusions are fun to play with, there is actually a practical reason for their perception. For instance, if you were to perceive every object in your view as a "figure" you would be overwhelmed with information. This would be extremely unhelpful if you were running from a predator because your brain would register each and every detail as equally important. This is difficult for us to picture in our brain because we are incredibly accustomed to focusing on one thing at a time, even if you are not aware of it. The very existence of visual illusions provides us with great insight into the visual system about how our brain processes information from the outside world. So the next time you see something that isn't there, stop and ask yourself: Am I really seeing it or is my brain playing a trick on me?

More activities and resources:

Color Giuseppe Arcimboldo's "Summer" (1573)
Step-by-step instructions to draw the Penrose Stairs
American Museum of Natural History - Optical Illusions
National Eye Museum - Optical Illusions
Neuroscience for Kids - Sight (Vision)
How Website Designers use Gestalt Principles
How your mind plays tricks on you
When circles are finally squared - Kokichi Sugihara

References

Adelson, R. (2002, June). Figure this: deciding what's figure, what’s ground, Science Watch American Psychological Association. (Original work published 2002).
Coren, S., & Girgus, J. (1978). Seeing is Deceiving: The Psychology of Visual Illusions (1st ed.), A Brief History of Visual Illusions. Routledge.
Editors of Encyclopaedia (2024, April 11). Gestalt psychology, Encyclopedia Britannica.
Howell, A. (n.d.). What Causes Optical Illusions?, Arizona State University for You.
How Optical Illusions Work, (2023, February 14). CooperVision Live Brightly.
Lotto, R. B., & Purves, D. (n.d.). An empirical explanation of color contrast, Proceedings of the National Academy of Sciences.
Making Sense of the Hermann Grid Illusion, (n.d.). Ross Lab - University of Pittsburgh.
Optical Illusions: Tracing their origin and evolution, (2022, November 28). Hindustan Times.
Van den Berg, C. (2019, October 24). How does an optical illusion work?, Queensland Brain Institute.