Photoreceptors are specialized light-detecting cells. One possible evolutionary road of the eyes is the following: photoreceptors became concentrated in groups, they began to form light detecting organs and this light-detecting organ developed until it became the eye as we know it today.

The front of the eyeball is covered by the cornea, a transparent tissue. Here the light is focussed. Behind the cornea is the iris. Inside the iris is the pupil. Behind the iris is the lens, which adds to the focusing process began by the cornea. The lens is adjustable and the cornea is not. Light forms an image of the object on the retina. The image is on the retina is upside down, but the retina’s function is to trigger patterns of activity in neurons running to the brain.

The process by which a stimulus from the environment generates electrical changes in neurons is called transduction. Transduction is the function of photoreceptor cells. There are two types of photoreceptor cells on the retina:

1. Cones
   These permit sharply focused colour vision in bright light. They are most concentrated in the fovea, the area of the retina that is in the most direct line of sight.
2. Rods
   These permit vision in dim light. They exist everywhere in the retina except the fovea.

At the place on the retina where the axons of neurons converge to form the optic nerve, there is a blind spot. We normally don’t notice this. There are two separate, but interacting visual systems within the human eye:

1. Cone vision (photopic vision)
   This is focused on high detail and colour perception. There are three types of cones, each with a different photochemical that makes it most sensitive to the light within a particular band of wavelengths.
2. Rod vision (scotopic vision)
   This is focused on seeing very dim light but lacks the capacity to distinguish colours.

The three-primaries law states that three different wavelengths of light can be used to match any colour that the eye can see if they are mixed in the appropriate proportions. The primaries can be any colour, as long as one is from the longwave end of the spectrum, one from the shortwave and one from the middle. The law of complementarity states that pairs of wavelengths can be found that, when added together, produce the visual sensation of white.

There are two theories of colour vision:

1. Trichromatic theory
   Colour vision emerges from the combined activity of three different types of receptors, each most sensitive to a different range of wavelength.
2. Opponent-Process theory
   Colour perception is mediated by neurons that can be either excited or inhibited, depending on the wavelength of the light and complementary wavelengths have opposite effects.

Most of what new-born sees is out of focus. Convergence (both eyes looking at the same object) and coordination (both eyes following a moving stimulus in a coordinated fashion) are poor at birth but develops rapidly. Synapses are formed and maintained when an organism has species-typical experience as experience-expectancy processes. As a result, functions will develop for all members of a species, given a species-typical environment.

The primary visual cortex is responsible for vision. Here, millions of neurons are involved in analysing the sensory input. In the primary visual cortex there are neurons called bar detectors, neurons called edge detectors and neurons called feature detectors.

The feature-integration theory, also called the two-stage theory states that any perceived stimulus consists of several distinct primitive sensory features (e.g: colour and the slant of its lines). The essence of this theory is that the detection and integration occur sequentially in two fundamentally different steps or stages of information processing.

1. Detection of features
   This occurs instantaneously and involves parallel processing. Parallel processing means that this step operates simultaneously on all parts of the stimulus array (our visual system picks up all the primitive features of the objects whose light rays strike our retinas at once).
2. Integration of features
   This requires more time and leads eventually to our perception of objects. This step involves serial processing, which occurs sequentially, at one spatial location at a time, rather than simultaneously (e.g: we can integrate the features of X and then an instant later the features of Y, but we can’t integrate the two sets of features simultaneously).

The Gestalt psychology stated that the whole is greater than the sum of its parts because the whole is defined by the way the parts are organized, not just by the parts themselves. They assume that we automatically perceive whole, organized patterns and objects (e.g: people perceive and recognize the chair as a whole before noticing its arms, legs and other components). The Gestalt principles of grouping are the following:

1. Proximity
   We tend to see stimulus elements that are near each other as parts of the same object and those that are separated as parts of different objects.
2. Similarity
   We tend to see stimulus elements that physically resemble each other as parts of the same objects and those that do not resemble each other as parts of different objects.
3. Closure
   We tend to see forms as completely enclosed by a border and ignore gaps in the border.
4. Good continuation
5. Common movement
   When stimulus elements move in the same direction and at the same rate, we tend to see them as part of a single object.
6. Good form
   Things that are simple, uncluttered, symmetrical, regular and predictable are more likely to be seen as a single object than as two objects.

Besides those six principles of grouping, we tend to automatically divide any visual scene in figure (the object that attracts attention) and ground (the background). This is mostly determined by circumscription. We tend to see the circumscribing form as ground and the circumscribed form as figure.

Once your visual system has hit upon a particular solution to the problem of ‘what is there’,
it may create or distort features in ways that are consistent with that inference. This is called unconscious inference, your visual system uses the sensory input from a scene to draw inferences about what is present.

Control that comes from higher up in the brain is called top-down control and they refer to control that comes more directly from the sensory input as bottom-up control. Perception always involves interplay between bottom-up and top-down control. Bottom-up processes bring in the sensory information that is actually present in the stimulus and top-down processes bring to bear the results of calculations based on that sensory information plus other information, such as that derived from previous experience and from larger context in which the stimulus appears.

The recognition-by-Components theory states that in order recognize an object, our visual system first organizes the stimulus information into a set of basic, three-dimensional components, which are called geons, and then it uses the arrangement of those components to recognize the object (e.g: an aeroplane, no matter how it is positioned relative to our eyes, always consists of the same set of geons). The theory posits that recognition of an object occurs through the following sequence:

Pick-up of sensory features -> detection of geons -> recognition of objects

People with visual agnosia can still see, but can no longer make sense of what they see. There are several types of visual agnosia:

1. Visual form agnosia
   People with this condition can see that something is present and can identify some of its elements, such as its colour and brightness, but cannot perceive its shape.
2. Visual object agnosia
   People with this condition have no problems seeing shapes, but they are unable to identify the object.

The visual areas beyond the primary area exist in two relatively distinct cortical pathways or streams, which serve different functions:

1. “What” pathway
   This stream runs into the lower portion of the temporal lobe and is specialized for identifying objects.
2. “Where-and-how” pathway
   This stream runs upward in the parietal lobe. This stream is specialized for maintaining a map of three-dimensional space and localizing objects within that space. This pathway is also crucial for the use of visual information to guide a person’s movement.

There are several cues for seeing depth. There are binocular cues and monocular cues.

Binocular cues:
There is binocular disparity, which refers to the slightly different views that the two eyes have of the same object or scene. This disparity can serve as a cue for seeing depth. The less the disparity, the greater the distance. The ability to see depth from binocular disparity is called stereopsis.

Monocular cues:
One of the monocular cues is motion parallax, which refers to the changed view one has of a scene or object when one’s head moves sideways. The smaller the change when moving your head, the greater the distance. Motion parallax depends on the geometry of true three-dimensionality. It cannot be used to depict depth in two-dimensional pictures.

There is a way to see depth in two-dimensional pictures. These cues are called pictorial cues for depth and there are several of them:

1. Occlusion
2. Relative size for familiar objects
3. Linear perspective
4. Texture gradient
5. Position relative to the horizon
6. Differential lightning of surface

The ability to see an object as unchanged in size, despite change in the image size as it moves farther away or closer, is called size constancy. Previous knowledge of the object’s usual size may contribute to size constancy.

Multisensory integration refers to the integration of information from different senses by the nervous system. When sight and sound are put in conflict with each other, vision usually wins. This is referred to as the visual dominance effect. The McGurk effect occurs when the sound is not correct according to the vision.

Multisensory integration is most apt to be perceived then the individual sensory stimuli come from the same location, arise at approximately the same time and evoke relatively weak responses when presented in isolation.

Synaesthesia is a condition in which sensory stimulation in one modality induces a sensation in a different modality.