Visual perception
Visual perception is the ability to detect light and use it to form an image of the surrounding environment. Photodetection without image formation is classified as light sensing. In most vertebrates, visual perception can be enabled by photopic vision or scotopic vision, with most vertebrates having both. Visual perception detects light in the visible spectrum reflected by objects in the environment or emitted by light sources. The visible range of light is defined by what is readily perceptible to humans, though the visual perception of non-humans often extends beyond the visual spectrum. The resulting perception is also known as vision, sight, or eyesight. The various physiological components involved in vision are referred to collectively as the visual system, and are the focus of much research in linguistics, psychology, cognitive science, neuroscience, and molecular biology, collectively referred to as vision science.
Visual perception involves not only what we see but also how our brain process information, which is also adaptive and influenced by both lifelong experience and changing cognitive capacities.
Visual system
Most vertebrates achieve vision through similar visual systems. Generally, light enters the eye through the cornea and is focused by the lens onto the retina, a light-sensitive membrane at the back of the eye. Specialized photoreceptive cells in the retina act as transducers, converting the light into neural impulses. The photoreceptors are broadly classed into cone cells and rod cells, which enable photopic and scotopic vision, respectively. These photoreceptors' signals are transmitted by the optic nerve, from the retina upstream to central ganglia in the brain. The lateral geniculate nucleus, which transmits the information to the visual cortex. Signals from the retina also travel directly from the retina to the superior colliculus.The lateral geniculate nucleus sends signals to the primary visual cortex, also called striate cortex. Extrastriate cortex, also called visual association cortex is a set of cortical structures, that receive information from striate cortex, as well as each other. Recent descriptions of visual association cortex describe a division into two functional pathways, a ventral and a dorsal pathway. This conjecture is known as the two streams hypothesis.
Study
The major problem in visual perception is that what people see is not simply a translation of retinal stimuli, with the brain altering the basic information taken in. Thus people interested in perception have long struggled to explain what visual processing does to create what is actually seen.Early studies
There were two major ancient Greek schools, providing a primitive explanation of how vision works.The first was the "emission theory" of vision which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If an object was seen directly it was by 'means of rays' coming out of the eyes and again falling on the object. A refracted image was, however, seen by 'means of rays' as well, which came out of the eyes, traversed through the air, and after refraction, fell on the visible object which was sighted as the result of the movement of the rays from the eye. This theory was championed by scholars who were followers of Euclid's Optics and Ptolemy's Optics.
The second school advocated the so-called 'intromission' approach which sees vision as coming from something entering the eyes representative of the object. With its main propagator Aristotle, and his followers, this theory seems to have some contact with modern theories of what vision really is, but it remained only a speculation lacking any experimental foundation.
The most decisive development of the intromission theory came from the work of the 11th-century scholar Ibn al-Haytham. In his Book of Optics, he rejected both the extramission theory of Euclid and Ptolemy and the purely speculative account of Aristotle. Through systematic experimentation, he demonstrated that vision occurs when light rays reflected from objects enter the eye, where they are focused by the lens onto the retina. This empirical approach marked a turning point: Alhazen not only provided the first correct explanation of vision in terms of intromission but also introduced experimental methods that influenced later European scholars such as Roger Bacon, Kepler, and eventually Newton.
Both schools of thought relied upon the principle that "like is only known by like", and thus upon the notion that the eye was composed of some "internal fire" that interacted with the "external fire" of visible light and made vision possible. Plato makes this assertion in his dialogue Timaeus, as does Empedocles.
Alhazen carried out many investigations and experiments on visual perception, extended the work of Ptolemy on binocular vision, and commented on the anatomical works of Galen. He was the first person to explain that vision occurs when light bounces on an object and then is directed to one's eyes.
Leonardo da Vinci is believed to be the first to recognize the special optical qualities of the eye. He wrote "The function of the human eye... was described by a large number of authors in a certain way. But I found it to be completely different." His main experimental finding was that there is only a distinct and clear vision at the line of sight—the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the modern distinction between foveal and peripheral vision.
Isaac Newton was the first to discover through experimentation, by isolating individual colors of the spectrum of light passing through a prism, that the visually perceived color of objects appeared due to the character of light the objects reflected, and that these divided colors could not be changed into any other color, which was contrary to scientific expectation of the day.
Unconscious inference
is often credited with the first modern study of visual perception. Helmholtz examined the human eye and concluded that it was incapable of producing a high-quality image. Insufficient information seemed to make vision impossible. He, therefore, concluded that vision could only be the result of some form of "unconscious inference", coining that term in 1867. He proposed the brain was making assumptions and conclusions from incomplete data, based on previous experiences.Inference requires prior experience of the world.
Examples of well-known assumptions, based on visual experience, are:
- light comes from above;
- objects are normally not viewed from below;
- faces are seen upright;
- closer objects can block the view of more distant objects, but not vice versa; and
- figures tend to have convex borders.
Another type of unconscious inference hypothesis has recently been revived in so-called Bayesian studies of visual perception. Proponents of this approach consider that the visual system performs some form of Bayesian inference to derive a perception from sensory data. However, it is not clear how proponents of this view derive, in principle, the relevant probabilities required by the Bayesian equation. Models based on this idea have been used to describe various visual perceptual functions, such as the perception of motion, the perception of depth, and figure-ground perception. The "wholly empirical theory of perception" is a related and newer approach that rationalizes visual perception without explicitly invoking Bayesian formalisms.
Gestalt theory
working primarily in the 1930s and 1940s raised many of the research questions that are studied by vision scientists today.The Gestalt Laws of Organization have guided the study of how people perceive visual components as organized patterns or wholes, instead of many different parts. "Gestalt" is a German word that partially translates to "configuration or pattern" along with "whole or emergent structure". According to this theory, there are eight main factors that determine how the visual system automatically groups elements into patterns: Proximity, Similarity, Closure, Symmetry, Common Fate, Continuity as well as Good Gestalt and Past Experience.
Language model
Following in the footsteps of George Berkeley, the Australian philosopher Colin Murray Turbayne argued in favor of an alternative to the classical "geometric model," of visual perception by asserting that aspects of it have needlessly clouded our understanding of vision since the time of Euclid. Quoting the sculptor Naum Gabo he notes: "Lines, shapes, color and movement have a language of their own, but reading takes time. It is not enough to look. you must see and "see" means "read". Turbayne argued that a "language model peculiarly illuminates this ancient problem of how we see, shedding a bright light on dark areas dimly light by its great rival." Specifically, he highlighted the limitations found within a purely mechanistic explanation of vision by arguing that several cases of "visual illusion" can be more adequately explained through the utilization of the terms found within such a language model. With this in mind, he presented a comparative analysis of specific examples of visual distortion including: the "Barrovian Case", the case of the "Horizontal Moon" and the case of the "Inverted Retinal Image."Analysis of eye movement
During the 1960s, technical development permitted the continuous registration of eye movement during reading, in picture viewing, and later, in visual problem solving, and when headset-cameras became available, also during driving.The picture to the right shows what may happen during the first two seconds of visual inspection. While the background is out of focus, representing the peripheral vision, the first eye movement goes to the boots of the man. Eye movements serve the function of attentional selection, i.e., to select a fraction of all visual inputs for deeper processing by the brain.
The following fixations jump from face to face. They might even permit comparisons between faces.
It may be concluded that the icon face is a very attractive search icon within the peripheral field of vision. The foveal vision adds detailed information to the peripheral first impression.
It can also be noted that there are different types of eye movements: fixational eye movements, vergence movements, saccadic movements and pursuit movements. Fixations are comparably static points where the eye rests. However, the eye is never completely still, and gaze position will drift. These drifts are in turn corrected by microsaccades, very small fixational eye movements. Vergence movements involve the cooperation of both eyes to allow for an image to fall on the same area of both retinas. This results in a single focused image. Saccadic movements is the type of eye movement that makes jumps from one position to another position and is used to rapidly scan a particular scene/image. Lastly, pursuit movement is smooth eye movement and is used to follow objects in motion.