Optical Illusions and Our Tricky Visual Perception ~ الأوهام البَصَرية
The eye is an extraordinary sensory organ that detects light through photoreceptors and converts it into electrical signals. These signals are then transmitted via the optic nerve to the brain, where they are transformed into images that the brain perceives and interprets, resulting in vision or sight.
At times, images can create illusions that deceive both the eye and the brain, causing a mismatch between what is seen and what is actually presented. When the brain processes visual information based on past experiences and expectations in a way that doesn’t align with reality, visual or optical illusions occur.
The winged bull (Lamassu) represents one of the earliest geometric illusions crafted by the human mind. Unlike other optical illusions that deceive the eye by distorting shapes, colours, or lines, the Lamassu illusion is dynamic in nature, changing according to the viewer’s position relative to the statue. From the front, Lamassu appears to have two legs (or two forelegs), evoking a sense of stillness and majesty. From the side, however, it shows four or even five legs, depending on the angle of view, giving the impression of motion and power, as one of its forelegs visually overlaps the other, making them appear as a single limb. The fifth leg is a clever visual trick employed by the Assyrian sculptor to serve a dual purpose: to depict stillness when viewed from the front and movement when seen from the side—both within a single, unified form. These statues were carved during the Neo-Assyrian period, in the 8th century BCE, and placed at the entrances of palaces and temples in cities like Nineveh and Khorsabad, symbolising divine protection and royal authority. Photo credit https://www.artstation.com
In addition to the enjoyment of observing them, optical illusions serve as a valuable tool for researchers exploring the human mind. Some illusions assist in diagnosing mental disorders, while others are used in fields such as fashion design, camouflage, vehicle concealment, and traffic planning. People perceive and experience illusions in different ways for reasons that continue to intrigue psychologists, philosophers, and doctors alike. These reasons include individual and cultural differences, genetic composition, and the principles of psychological gestalt, which involve concepts like similarity, proximity, closure, figure/ground and more.
Below are examples of some interesting optical illusions:
Müller-Lyer Illusion is an optical illusion in which lines of equal length appear to be of different lengths. For most observers, lines with inward-facing arrowheads appear shorter than those with outward-facing arrowheads. This illusion was created by the German psychologist Franz Carl Müller-Lyer in 1889. People from different cultures, as well as those from rural versus urban environments, may perceive the Müller-Lyer illusion differently. Additionally, individuals are more likely to experience the illusion when they simply observe it, but are less likely to “fall for it” when they consciously consider how to describe it.
Hermann Grid Illusion is characterised by “ghostlike” grey blobs (dark spots) that appear at the intersections of a white (or light-coloured) grid on a black background. The grey blobs disappear when you look directly at an intersection. These grey blobs don’t actually exist; they are visual “artifacts” created because our visual system enhances the edges of objects. Detecting edges is crucial for the brain’s ability to understand and define an image. The illusion was first reported by German physiologist and speech scientist Ludimar Hermann in 1870.
The Moon Illusion occurs when the full Moon appears larger near the horizon compared to when it is higher in the sky, even though its actual size remains unchanged. This was first observed in a cuneiform inscription on a clay tablet from the Royal Library at Nineveh, dating back to the 7th century B.C. A satisfactory explanation for the phenomenon is still lacking. Additionally, there is an optical contradiction: when the Moon is near the horizon, its apparent size should theoretically be smaller, since the Moon is a few thousand miles farther away compared to when it is overhead. However, the angle the Moon subtends at the eye remains the same, roughly 1/2 degree (0.52°).
Kanizsa Triangle is an example of an illusory contour where a triangle seems to form even though it is not physically present, created by the edges of three shapes. The illusion consists of a white equilateral triangle that appears to be surrounded by three Pac-Man-like shapes. These shapes are black with white crescents facing inward. In reality, the three Pac-Man-like shapes are made up of three partially overlapping circles with white edges, which together create the illusion of a triangle. Our brain fills in the missing information by connecting the three white crescents of the circles, completing the shape. The Kanizsa triangle was first described by the Italian psychologist Gaetano Kanizsa in 1955.
The Ames Room Illusion is an incredible optical illusion! A room with distorted perspective can make people appear to shrink or grow depending on their position inside it. When viewed through a hole in the front wall of the room, the observer sees the person standing at one end of the room as a giant, while they appear small if standing at the other end. As the person moves from the far corner to the nearer one, they seem to grow, while they appear to shrink when moving in the opposite direction. The first to notice the importance of perspective in depth perception was Hermann Helmholtz in the late 19th century, and the first to design such a room was American ophthalmologist Adelbert Ames Jr. in 1935, after whom the illusion was named. The room is built in such a way that it appears from the front to be a normal rectangular room, with a back wall and side walls that are perpendicular to the floor and ceiling, which are horizontally flat. However, the room’s actual shape is trapezoidal. When a person moves to the left side of the room, they are actually farther away, and the ceiling is higher, making their image on the retina smaller. The opposite effect occurs on the right side of the room. This illusion demonstrates the effect of experience/learning on perception. We prefer to perceive the room as normal, as we are used to it in our daily lives, and we see people as being of different sizes.
Rubin’s Vase, also referred to as Rubin’s Face, is a figure-ground illusion; when two fields share a common border, one is perceived as the figure and the other as the ground. The illusion features an ambiguous image that can be interpreted as either a vase or two faces, depending on the viewer’s focus. This highlights the brain’s ability to shift between two interpretations of the same image. Described by Danish psychologist Edgar Rubin around 1915, the oldest known version of this image appears on a German postcard from 1888.
My Wife and My Mother-in-Law Illusion is another example of figure-ground perception. The ambiguous image of a woman can be interpreted as either a young woman or an old woman. Created by American cartoonist William Ely Hill, it was published in Puck, an American humour magazine, in 1915. Similarly, the Rabbit-Duck illusion also demonstrates figure-ground perception.
Impossible Figures, also known as impossible objects or undecidable figures, are geometric shapes that defy the laws of physics. While they can be drawn in two dimensions, they cannot exist in the physical world. Classic examples include the Penrose Staircase and the Impossible Trident. The Impossible Trident was created by American psychologist D. H. Schuster, based on an advertisement he saw in an aviation journal, and was first published in 1964. Mathematicians have studied impossible figures to develop formulas for applications like computer vision. Cognitive scientists, artists, and philosophers have also shown interest in these figures.
Motion Aftereffect, also known as the Waterfall Illusion, occurs when stationary objects appear to move in the opposite direction after staring at a moving image. The phenomenon was first noted by Aristotle around 350 B.C., though he didn’t specify the direction of the motion. The motion aftereffect was clearly identified by Jan Evangelista Purkyně in 1820, after he observed it following a cavalry parade. In 1834, British philosopher Robert Addams described the waterfall illusion after witnessing it at the Falls of Foyers in Scotland. This illusion is believed to result from motion adaptation.
Motion Induced Blindness occurs when stationary dots disappear from view for brief, random periods while focusing on a flashing dot or a moving pattern. This happens because the brain prioritises moving objects, temporarily “ignoring” stationary ones in a winner-takes-all manner. This phenomenon can occur in various daily activities such as walking, driving, sailing, flying, hunting, biking, and skiing. Also known as Bonneh’s Illusion, it was discovered by Bonneh, Cooperman, and Sagi in 2001.
Rotating Snakes Illusion features concentric circles that appear to rotate, even though there is no actual movement. This is a type of peripheral drift illusion, where the centre of the figure remains motionless, while the peripheral areas seem to move. Developed by Japanese psychologist Akiyoshi Kitaoka in 2003, it builds on earlier work by Fraser and Wilcox (1979) and Faubert and Herbert (1999). Philosophers have explored what such illusions reveal about the nature of experience.
The Scintillating Starburst Illusion features a starburst pattern that appears to have bright rays radiating from its centre, even though these rays do not actually exist. Created by visual artist Michael Karlovich for his design company, Recursia Studios, in 2019, the illusion arises from how the brain perceives light and contrast.
The Expanding Black Hole Illusion makes a black hole appear to gradually expand, even though its size remains unchanged. It was created by Japanese psychologist Akiyoshi Kitaoka in 2003, and was recently studied by scientists at Flinders University in Australia. These scientists developed a mathematical model and found that high-contrast images like this one cause retinal neurons to misinterpret contrast, perceiving it as movement. The brain’s cerebral cortex then interprets this as an expanding hole instead of a static image.
The Spinning Dancer, also known as the Silhouette Illusion, was created in 2003 by Japanese web designer Nobuyuki Kayahara and originally distributed as a GIF animation. The dancer appears to spin either clockwise or counterclockwise, depending on the viewer’s perception. Some may even see the direction suddenly reverse. This illusion demonstrates the brain’s ability to interpret motion in different ways and is thought to be due to the lack of visual depth cues, and is possibly linked to spontaneous brain fluctuations.
The Hollow Face Illusion occurs when a concave mask appears to protrude outward, creating the illusion of a convex face. This happens because the brain is accustomed to perceiving faces as convex and thus tends to misinterpret concave shapes as protruding outward. It was first reported in 1916 by American lighting engineer and psychologist Matthew Luckiesh in his book Light and Shade and Their Applications. The illusion is weaker among individuals with schizophrenia or psychotic symptoms.
The Münsterberg Illusion, also known as the Café Wall Illusion, is a visual phenomenon in which straight horizontal lines appear curved or slanted due to the alternating arrangement of light and dark tiles. The first version of this optical illusion was described by the German psychologist Hugo Münsterberg in 1894. Later, in 1898, American psychologist A. H. Pierce described it and referred to it as the “kindergarten illusion.” The version known today was observed on the wall of a café, which is where it got its name. It is associated with British psychologist Richard Gregory, who discussed it in 1973.
Curvature Blindness Illusion occurs when the brain has difficulty perceiving the curvature of lines or shapes, even when they are clearly present. The curvature appears distorted, and the observer may perceive it as straighter or differently shaped than it actually is. It typically arises when a curved object or line is embedded in a complex visual environment, highlighting the brain’s reliance on context and surrounding visual information. The illusion was first described in 2017 by Japanese psychologist Kohske Takahashi.
The Helmholtz Square Illusion is when a square filled with vertical lines appears wider than taller, while a square filled with horizontal lines appears taller than wider. This illusion was first noted over 150 years ago by German physicist and physician Hermann von Helmholtz. Fashion designers may use this technique to emphasise certain parts of the body or draw attention away from others.
The Checker Shadow Illusion occurs when two squares of the same colour appear different due to their surrounding context, causing one to seem darker than the other. Crafted by MIT professor of vision science Edward H. Adelson in 1995, this illusion demonstrates how our visual system interprets light, shadow, and colour in complex scenes. It involves a checkerboard pattern partially covered by the shadow of a cylindrical object. Two squares on the checkerboard, labeled A and B, appear to have distinctly different shades—one looks dark and the other light. However, when isolated from the surrounding context, both squares are the same shade of grey.
The Jastrow Illusion occurs in the real world and typically involves two identical curved shapes, such as toy railway tracks. When one figure is positioned slightly above or below the other, the top figure appears larger, even though both are the same size. This illusion is also known as the Ring-Segment Illusion, Wundt Area Illusion, or Wundt-Jastrow Illusion. The earliest reference to it can be found in The World of Wonders, published in 1873. In 1892, Joseph Jastrow introduced his version of the illusion.
The Ebbinghaus Illusion (or Titchener Circles) is an optical illusion that shows how perception of size can be influenced by surrounding shapes. An object surrounded by larger shapes appears smaller than when it is surrounded by smaller shapes. Named after German psychologist Hermann Ebbinghaus (1850-1909), the illusion has been used in the fields of abnormal and clinical psychology. Interestingly, our genes may play a role in how we perceive this illusion.
1 Comment