A guide to detecting visual illusions and deceptive cues
To the readers, please note that having an experience related to the one described does not suggest any visual defect; rest assured your vision is normal. Do not approach these illusions as a test of sight.
Often, we describe illusions as the incursion of a metaphysical world that claims no place in our reality. We imagine they belong to our dreams that we encounter while enduring a disturbing sleep. Suffice to say, what our unconscious delivers can cause rivalry to our current understanding of reality; a reality constructed through our sensible perception of the world. Ironically, there are some instances where the distinction between reality and illusion becomes blurred. Consequently, we lose the trust in our perception as we experience a dismantling against the dominance of the unconscious mind. A unique example is a Necker cube, which describes an object with twelve equal sides drawn on a flat surface. Despite its simple shape that adheres to steady standards, the observer encounters a challenge to determine its orientation on the paper. A single viewer interprets its orientation as either right-oriented or left-oriented, but never both at once. Insanely enough, the geometric proportions of the cube disappear in the eyes of the viewer, because the goal becomes to decide its orientation on a dimensionless paper. Until that consensus is reached, the object is highlighted to be a visual illusion featuring mind trickery and abstract position. A decision the viewer attributes to the ambiguity of the flaws and simplicity of human perception and visual limitations. An explanation that encompasses how illusions are embedded in human neurophysiology solves the Necker cube paradigm, provided that humankind has contemplated vision since thousands of years ago.
Since 400-500 BCE, philosophers became conscious of the vitality of human vision, as they delved deep into life, physics, metaphysics, and democracy. Partly due to the influence of thinkers like Democritus, Aristotle, and Plato, society was maturing to witness the development of several disciplines, including medicine. Starting with the invention of the terms ‘atom’ and ‘corpuscles,’ the thinker Democritus envisioned the world based on small, undestroyable entities. He expanded on his theories by defining vision as a product of atomic waves that originate from the object and expand through perpetual motion. A student of Democritus noted the communication between the physical soul and the object to be fostered by the emerging atoms and particles. Yet, Plato imagined the eye as the bridge between the spiritual soul of the living entity, where he pictured the eyes as a light source emitting rays to enact human vision. Harbouring different thoughts on the subject, Aristotle proposed the occurrence of vision outside the human eye within the medium where light travels.
While none of these predictions proved to be accurate, they have retained their place in history as the emerging attempts to understand perception and vision. This topic was later carried to the Middle East and adopted by the polymath Ibn Al-Hatham, to record the closest scientific explanation of vision. Also known as Alhazen, Al-Haytham emphasized the role of the eyes and brain in interpreting visual stimuli in his book, /The Book of Optics/. After examining the behaviour of light around lenses through the ‘dark room’ experiment, Al-Haytham discovered a sensitive crystalline humour that interacted with reflected light from objects, creating its upside down projection. Similarly, he postulated that eye movements are required to perceive a complete image of the object. His impressive work on optics proved to be a good explanation of the contradictory orientation of the Necker cube. Facilitated by constant eye movements, the visual cortex in the brain pictures the image from different angles. For example, one stimulus received from an upward angle suggests a cube oriented to the right. A stimulus received from a lower angle would imply a left oriented cube. Eventually, the observer would settle on a single orientation, and may or may not share the same description of the cube as others. Al-Haythem was aware of the inevitability of the formation of different stimuli but was intrigued by the retention of a single stimulus. Accordingly, he assumed a unique connection between the information carried by this stimulus and the observer of the object. Specifically, the process of making an educated guess to refute all other incoming stimuli had to occur in a timely manner through the unconscious. More specifically, through perceptual inference the visual cortex must use the observer’s past experiences and facilitate the retention of the stimuli holding the answer of the observer.
Perceptual inference claims that vision involves the use of knowledge acquired throughout the person’s life. Because the images formed on the retina remain ambiguous without further processing, the unconscious mind retains virtual aspects of the real world to determine possible entities to reference the object later. The unconscious mind guides our inferential ability to eliminate unnecessary visual clues attained through retinal ganglion cells within the eye. The visual cue that holds the most logical explanation will become the only representative of the object within the observer’s visual recognition. Similar to the one representative cue, the unconscious may allow several visual cues to become a representative of a single object, creating a phenomenon called bistable perception.
In bistable perception, several cues have the potential to stand out, consequently deceiving the observer. A picture can be interpreted as vases if focused on the figure, while the background implies two men facing each other. Another example: a picture may display a man playing a saxophone from one angle, but a woman from another. To resolve the transition between different visual representations of the same object, the picture can be facilitated with auxiliary sounds that communicate with the viewer unconsciously, raising their confidence about certain stimuli. For example, if the saxophone was left to play in the background, the observer would become biased to perceive a saxophone player ignoring the stimulus that implies a woman standing in the picture.
Another illusionary trick, known as binocular rivalry, demonstrates the effects of exposing different eyes to different visuals and representations. One eye may enjoy better vision than the other and may provide stronger evidence of its visual stimuli. Despite the contradiction between the images, the brain will focus on the picture that was generated by the stronger monocular. Similarly, if images of the ceiling and ground were to be separated by the two binoculars, the brain would eliminate the ceiling image and simply perceive the ground. Here the brain ignores the visual cues received from the eye facing the picture with the ceiling. This bias is due to our natural adaptation towards processing the ground, as humans failed to inhabit the ceiling due to gravity.
More recently, scientists are interested in computationally analyzing the instances humans encounter visual illusion. Examples include the visual illusions that promote the activation of certain neuronal pathways in the brain. With that, an ambitious response to a certain image became the evidence of the underlying work of the brain. The emergence of several illusions that may distract observers looking at different patterns may not imply a defect in human neurophysiology. But it can serve as evidence to solve the intricate assembly of the human mind.