The Visual System in Humans
The visual system is a complex apparatus through which living organisms absorb waves of light (photons) to interpret the physical world through the sense of sight. This system encompasses the eye, the retina, the optic nerve, the optic tract, and the visual cortex. The following information will pertain to normal-sighted humans. Photons in the visible spectrum (400–700 nm) enter the eye and are refracted when passing through the cornea, which is the clear layer covering the eye. The cornea aids in focusing the entering photons and protects the inner eye from the outside world. After passing through the pupil, the light is additionally refracted by the lens. The size of the pupil increases and becomes dilated when there is a limited amount of light present. Conversely, when there is a significant amount of light present, the pupil size will decrease and constrict to mitigate the amount of light entering the eye. The size of the pupil changes based on the level of light present in an environment and can also change as a result of an emotional response.
The iris is the colored portion of the eye, and muscles connected to the iris control the size of the pupil. Light passes through the lens, which is clear and curved and works in conjunction with the cornea to focus light. The lens projects an inverted image onto the fovea of the retina, which is a sheet of light-sensitive neurons called photoreceptors at the back of the eye. Two types of opsin proteins in photoreceptor cells, called rod opsins and cone opsins, have unique roles in generating a representation of the surrounding environment. Cones, which function in bright light, help humans distinguish colors, perceive detail, and aid in sensing spatial resolution. They are mostly located in the fovea. Rods are found across the retina and function in low light, providing the ability to see in relatively dark conditions as well as detect periphery movement. Through a signal transduction pathway, an opsin transduces absorbed photons into electrical pulses and then transmits a signal to the photoreceptor cell. This decreases the membrane potential — the difference in electric potential between the interior and exterior of a cell — of the photoreceptor, through a process called hyperpolarization. These signals are transformed into electrochemical signals to be sent by the optic nerve through the optic canal to the thalamus. The optic nerve and the thalamus are connected at the lateral geniculate nucleus (LGN), a part of the thalamus. These signals are sent through axons to the primary visual cortex referred to as V1.
To summarize, the visual system collects data and the brain processes data. Errors occur in the visual system when artificial stimuli, processed as a natural phenomenon, delude the brain. Such occurrences are deemed visual illusions. Studying the mechanisms behind visual illusions can provide insight into both veridicality and illusoriness.
Defining visual illusions
It is important to note that labeling a phenomenon as a visual illusion is paradoxical because it could be (and has been) argued that all subjective perceptions are illusions. However, for the purpose of understanding how and why the brain misinterprets sensory data reported from an object or occurrence, the term “visual illusions” will be used in this context. Nuances in the definitions of visual illusions alter how they are classified and distinct word choices alter how narrow the definition is, though most hold the same general meaning. Stephen Macknik and Susana Martinez-Conde provide a broad definition in The Neuroscience of Illusion, a Scientific American article: “Visual illusions are defined by the dissociation between the physical reality and the subjective perception of an object or event.”
In An Introduction to Perception by Irvin Rock, a psychologist who studied visual perception, Rock similarly delivers a broad definition of a visual illusion: “a sensory impression or perception that is false or incorrect. By incorrect [it] is meant that what we see … does not correspond with the objective situation that can be determined by other means, e.g. measurement” and “non-correspondence between perception and the objective situation.” Rock also gives a narrower definition shortly after, writing, “Most of the well-known illusions can be considered to be either perceptual distortions of magnitude (length or size) or perceptual distortions of direction of lines.”
British psychologist Richard Gregory broadly defined visual illusions as “Disagreement[s] with the external world of objects” or as “departures from truth, or from physical reality,” and then more narrowly as “…departures from the world of behavior. On this account they [visual illusions] are not departures from deep accounts of physics, but rather from behaviorally accepted objects, with characteristics such as lengths and angles and curvatures, which are measured very simply.”
Ross Henry Day, an American psychologist, defined visual illusions as “Consistent and persistent discrepancies between a physical state of affairs and its representation in consciousness.”
From the multiple definitions listed above, it is evident that broader descriptions of visual illusions are simply generalizations of narrower definitions. The broader definitions can be used to describe general characteristics of illusions without providing details regarding the specific attributes of an object or event being distorted. In my own words, visual illusions occur when perceived reality diverges from physical reality. Common visual illusions occur when an individual perceives fallacious properties of a given object, such as distorted visual attributes (size, shape, position, color, orientation), imposes properties that do not exist, or when an individual does not perceive a present attribute entirely. Studying the mechanisms behind visual illusions can provide insight into both veridicality and illusoriness. Research in this field will further reveal how errors in the visual system occur when interpreting sensory data and will lead to a more comprehensive understanding of how the visual system provides a subjective perspective of the physical world.
Classifying visual illusions
Illusion-inducing designs, created by both artists and researchers alike, have been of interest for over a century. A non-exhaustive list of such creators includes Oppel, Kundt, Müller-Lyer, Brentano, Zöllner, Poggendorff, Hering, Wundt, Ebbinghaus, Münsterberg, Delboeuf, Jastrow, Judd, Baldwin, Fraser, Ponzo, Sander, Ehrenstein, Orbison, and Escher. These individuals created, among others, geometric-optic illusions and geometric-photometric illusions, relying on distinct processes in the visual system to generate a specific illusion. Classifying illusions is the first step in understanding their enigmatic nature. An opinion paper published by Richard Gregory in 1997 offers various categories for visual illusions based on how they deceive the brain.
Gregory posits there are two overarching types of illusions: physical (illusions with physiological causes fall under this category) and cognitive. Aptly named, physical illusions can be explained in the realm of physics while cognitive illusions are related to misapplications of rules or knowledge. Gregory analogizes physical and cognitive illusions to hardware and software.
Physical and Cognitive Illusions
Physical illusions are caused by an optical disturbance between a given object and the retina or influenced physiological signals in the eyes or brain. Cognitive illusions occur when preconceived general rules are wrongly applied or when specious knowledge of objects is utilized. Gregory further suggests that illusions be categorized by appearance. The names of these categories are based on the names of errors of language: ambiguities, distortions, paradoxes, and fictions. The visual system is an intricate network with innumerable tasks being carried out simultaneously, including integrating the separate data received from each eye into one image (monocular neural representations), evaluating distances, deducing depth by stereopsis, perceiving motion, color vision, and recognizing patterns, among many others. The brain is incredibly good at making predictions, filling in missing information, and using previously learned rules to reach conclusions. Illusions may deceive any one of the numerous components of the visual system, resulting in an erroneous perception of the physical world.
Examples of Illusions
The Necker Cube, published in 1832 by Louis Albert Necker, can be classified as a cognitive ambiguity of a geometric figure. This visual illusion is a two-dimensional figure of a cube, taking the form of a wire frame. As such, the drawing contains no visual cues to indicate its orientation. The viewer of this illusion may perceive the cube in two ways: the lower-left square as the front face, or the upper-right square as the front face.
The Fraser Spiral was first proposed by James Fraser in 1908. This visual illusion is known by multiple names, including the false spiral or the twisted cord illusion. It is classified as a physical distortion of a signal. When looking at the Fraser Sprial, the viewer perceives a black-and-white patterned spiral. In actuality, the pattern consists of numerous concentric circles — circles with a common center and different radii. This illusion is generated by a series of overlapping strands appearing as a twisted cord, which deceives the brain into recognizing a nonexistent spiral pattern.
The café wall illusion is another example of a physical distortion of a signal. The lines dividing the rows of black and white checkers are parallel even though they appear to be slanted. This has to do with how the checkers are staggered, deceiving the viewer into thinking the lines are curved. The illusion isn’t as prominent when the colors contrast less, as can be seen in the image with pink and blue checkers.
See other visual illusions here!
Harnessing Visual Illusions to Understand Processes in the Brain
When presented with a visual illusion, the brain responds as it would if there was no illusion present. This means that rather than unveiling faults of the visual system, one may view perceptions of visual illusions as indicators of the adaptability of the brain. Illusions are based upon assumptions the visual system exercises to make sense of sensory data. As written by Day, “It is argued that for the most part these discrepancies [visual illusions] occur mainly as a consequence of the activation of perceptual processes by contrived, artificial stimuli. When activated by their normal or natural stimuli the processes result in veridical perception. When activated by artificial or unnatural stimuli the same processes result frequently in nonveridical, illusory, perception.” Consequently, Day argues, studying illusions offers the opportunity to understand sensory and cognitive processes involved in veridical perception. Research in this field will further reveal how errors in the visual system occur when interpreting sensory data and will lead to a more comprehensive understanding of how the visual system provides a subjective perspective of the physical world.