question: How does our brain combine the different images from our two eyeballs?
comment: There's a really deep neuroscience explanation that's way over my head, but basically, we just evolved that way. We developed eyes because it's super advantageous to be able to see. The better eyesight you have the better your chance of survival, so over millions of years we learned (evolved) to make sense of what our eyes were telling us.
passages: Retinal correspondence
  
  Retinal correspondence is the inherent relationship between paired retinal visual cells in the two eyes. Images from one object stimulate both cells, which transmit the information to the brain, permitting a single visual impression localized in the same direction in space.
  
  Section::::Types.
  
  Normal retinal correspondence (NRC) is a binocular condition in which both foveas work together as corresponding retinal points, with resultant images fused in the occipital cortex of the brain.
   The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain:
  
  BULLET::::1. M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus;
  
  BULLET::::2. P cells, with smaller center-surround receptive fields that are sensitive to color and shape;
  
  BULLET::::3. K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
  
  BULLET::::4. another population that is intrinsically photosensitive; and
  
  BULLET::::5. a final population that is used for eye movements.
   Multiscopy
  
  Unlike conventional 3D stereoscopy, which simulates a 3D scene by displaying only two different views of it, each visible to only one of the viewer's eyes, 3D multiscopy displays more than two images, representing the subject as viewed from a series of locations, and allows each image to be visible only from a range of eye locations narrower than the average human interocular distance of 63 mm. As a result, not only does each eye see a different image, but different pairs of images are seen from different viewing locations.
   The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the hypothalamus of the brain. At this point the information coming from both eyes is combined and then splits according to the visual field. The corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively, to be processed. That is, the right side of primary visual cortex deals with the left half of the "field of view" from both eyes, and similarly for the left brain. A small region in the center of the field of view is processed redundantly by both halves of the brain.
   In vision, binocular disparity is the difference between two retinal images. This disparity serves as the basis for stereopsis, one of the most important depth cues in human sight. In Campus' video, however, the two disparate images are mixed on a single monitor, rendering stereoscopic perception of the image impossible.
  
  Section::::Chapters.:3. Convergence.
   In order to balance the input of visual information from each eye to the brain, the data is presented in such a manner that the user needs to use both eyes to see the complete scene. Furthermore, the stimulus offered to the weaker eye may be stronger, for example with stronger contrast, than the stimulus for the stronger eye.
  
  Some of these methods involve the use of dichoptic computer games (such as the game Tetris). A dichoptic presentation of popular movies has also been proposed.
  
  Section::::See also.
  
  BULLET::::- Cheiroscope
  
  Section::::Further references.
   Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium.
  
  Section::::Types.:Compound eyes.:Superposition eyes.
  
  The second type is named the superposition eye. The superposition eye is divided into three types:
  
  BULLET::::- refracting,
  
  BULLET::::- reflecting and
  
  BULLET::::- parabolic superposition
   Section::::Physical basis.
  
  The biological foundation of the mind's eye is not fully understood. Studies using fMRI have shown that the lateral geniculate nucleus and the V1 area of the visual cortex are activated during mental imagery tasks. Ratey writes:
   Section::::Binocular interaction.
  
  Apart from binocular summation, the two eyes can influence each other in at least three ways.
  
  BULLET::::- Pupillary diameter. Light falling in one eye affects the diameter of the pupils in both eyes. One can easily see this by looking at a friend's eye while he or she closes the other: when the other eye is open, the pupil of the first eye is small; when the other eye is closed, the pupil of the first eye is large.
   There is a creation of intermediate and far-intermediate distance zone of binocular fusion, allowing the merging of images from each eye in the brain so no dissociation occurs between the eyes as with monovision. Due to increased depth of field in both eyes, the distance vision is significantly better than would otherwise be anticipated.
   Some educational theorists have drawn from the idea of mental imagery in their studies of learning styles. Proponents of these theories state that people often have learning processes that emphasize visual, auditory, and kinesthetic systems of experience. According to these theorists, teaching in multiple overlapping sensory systems benefits learning, and they encourage teachers to use content and media that integrates well with the visual, auditory, and kinesthetic systems whenever possible.
   Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360° field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images.
   Section::::Types.
  
  Section::::Types.:Vision.
  
  In many ways, vision is the primary human sense. Light is taken in through each eye and focused in a way which sorts it on the retina according to direction of origin. A dense surface of photosensitive cells, including rods, cones, and intrinsically photosensitive retinal ganglion cells captures information about the intensity, color, and position of incoming light. Some processing of texture and movement occurs within the neurons on the retina before the information is sent to the brain. In total, about 15 differing types of information are then forwarded to the brain proper via the optic nerve.
   Section::::Defining criteria.:Brain regions.
  
  The thalamus is a section of the brain responsible for relaying sensory and motor signals to the cerebral cortex. As stimuli pass through the thalamus, there are specific regions dedicated to each sense, and therefore is able to sort out the multiple parts of an environment an individual experiences in a given moment. Two of these regions are specific to vision and hearing respectively, which may be responsible for the order in which sensory information is coded and then perceived within the cerebral cortex.
   When very different images are shown to the same retinal regions of the two eyes, perception settles on one for a few moments, then the other, then the first, and so on, for as long as one cares to look. This alternation of perception between the images of the two eyes is called binocular rivalry. Humans have limited capacity to process an image fully at one time. That is why the binocular rivalry occurs. Several factors can influence the duration of gaze on one of the two images. These factors include context, increasing of contrast, motion, spatial frequency, and inverted images. Recent studies have even shown that facial expressions can cause longer attention to a particular image. When an emotional facial expression is presented to one eye, and a neutral expression is presented to the other eye, the emotional face dominates the neutral face and even causes the neutral face to not been seen.
   People with sufficient control over the parallax of their eyeballs (e.g. those who can easily view random-dot stereograms) can hold up two paper printouts and go cross-eyed to superimpose them. This invokes deep, fast, built-in image comparison wetware (the same machinery responsible for depth perception) and differences stand out almost immediately. This technique is good for finding edits in graphical images, or for comparing an image with a compressed version to spot artifacts.
   The crossing over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric visual field from both eyes. Superimposing and processing these monocular visual signals allow the visual cortex to generate binocular and stereoscopic vision. For example, the right visual cortex receives the temporal visual field from the left eye, and the nasal visual field from the right eye, which results in the right visual cortex producing a binocular image of the left hemispheric visual field. The net result of optic nerve crossing over at the optic chiasm is for the right cerebral hemisphere to sense and process left hemispheric vision, and for the left cerebral hemisphere to sense and process right hemispheric vision.
   Researchers investigating precise properties of these neurons with respect to disparity present visual stimuli with different disparities to the cells and look whether they are active or not. One possibility to present stimuli with different disparities is to place objects in varying depth in front of the eyes. However, the drawback to this method may not be precise enough for objects placed further away as they possess smaller disparities while objects closer will have greater disparities. Instead, neuroscientists use an alternate method as schematised in Figure 2.
   An object at some distance from an observer will reflect light in all directions, some of which will fall upon the corneae of the eyes, where it will be focussed upon each retina, forming an image. The disparity between the electrical output of these two slightly different images is resolved either at the level of the lateral geniculate nucleus or in a part of the visual cortex called 'V1'. The resolved data is further processed in the visual cortex where some areas have specialised functions, for instance area V5 is involved in the modelling of motion and V4 in adding colour. The resulting single image that subjects report as their experience is called a 'percept'. Studies involving rapidly changing scenes show the percept derives from numerous processes that involve time delays. Recent fMRI studies show that dreams, imaginings and perceptions of things such as faces are accompanied by activity in many of the same areas of brain as are involved with physical sight. Imagery that originates from the senses and internally generated imagery may have a shared ontology at higher levels of cortical processing.
   Section::::Face processing.
   Parallel processing (psychology)
  
  In psychology, parallel processing is the ability of the brain to simultaneously process incoming stimuli of differing quality. Parallel processing is a part of vision in that the brain divides what it sees into four components: color, motion, shape, and depth. These are individually analyzed and then compared to stored memories, which helps the brain identify what you are viewing. The brain then combines all of these into the field of view that you see and comprehend. Parallel processing has been linked, by some experimental psychologists, to the Stroop effect. This is a continual and seamless operation.
   Two different methods have been used to identify the regions involved in visual perception and visual imagery. First, functional magnetic resonance imaging (fMRI) is used to measure cerebral blood flow, which allows researchers to identify the amount of glucose and oxygen being consumed by a specific part of the brain, with an increase in blood flow providing a measure of brain activity. Second, an event related potential (ERP) can be used to show the amount of electrical brain activity that is occurring due to a particular stimulus. Researchers have used both methods to determine which areas of the brain are active with different stimuli, and results have supported the dual-coding theory. Other research has been done with positron emission tomography (PET) scans and fMRI to show that participants had improved memory for spoken words and sentences when paired with an image, imagined or real, and showed increased brain activation to process abstract words not easily paired with an image.
   Since a human's two eyes are approximately 6.5 cm apart, they see the world from different viewpoints: the image projected on the left retina is slightly different from the image projected on the right. This difference (known as binocular disparity) is the fundamental cue underlying stereoscopic depth perception . The importance of stereoscopic perception is familiar to anyone who has ever attempted to thread a needle with one eye closed; and when two slightly disparate photographs are viewed through a stereoscope (a device that makes it easy to fuse the two images), the fused scene takes on a three-dimensional appearance.
   Section::::History.:Visual test.
   Neuroplasticity is a key element of observing many pixel images. While two individuals will observe the same photons reflecting off a photorealistic image and hitting their retinas, someone whose mind has been primed with the theory of pointillism may "see" a very different image as the image is interpreted in the visual cortex.
  
  Section::::Techniques.
  