Each incoming light pattern is magically aligned to produce a coherent image: a physiological unattainability that is taken for granted as being physiologically accomplished.
Creating the mental movie that constitutes vision requires mixing a hodgepodge of sequential images into what seems a static frame, then connecting frames. Human vision processing is so demanding that 1/3rd of the brain is intensely active during image fabrication.
A different picture is projected in each eye, offset by the space between them; binocular vision. Spatial orientation is derived from processing these separate images. Those projections are simultaneously passed to the visual cortex for assimilation, accounting for the parallax from eye placement.
One aspect of that assimilation is depth perception, which arises from a variety of cues. The impression of depth (stereopsis) as a distance cue most readily arises from binocular vision. The eyes converge on a single object for stereopsis. Visual convergence is the binocular oculomotor cue for perceiving distance. This convergence stretches extraocular muscles, providing kinesthetic sensations which frother cue the mind-brain.
The cue of stereoscopic convergence is effective for a focal plane less than 10 meters. The angle of convergence is less when the eyes focus on faraway objects.
There are many more depth perception cues that are monocular, including motion parallax, kinetic depth, crash distance, blurring, perspective, relative size, familiar size, interposition, shading, aerial perspective, texture gradient, peripheral vision, and accommodation.
Motion parallax: indicates distance by discrepancy in relative motion when one is moving. Many animals move their heads to gain different viewpoints to get better depth perception from motion parallax.
Kinetic depth perception: the opposite of motion parallax in accounting for objects moving without one moving. Receding objects typically become smaller, and vice versa.
Crash distance: countdown to contact at a certain velocity. The mind can even account for variances in velocity, which is instantaneous 2nd-derivative calculus; hence the ability play “catch” and drive in traffic.
Blurring: moving objects blur. The mind calculates size and distance information from this cue by incorporating offset from the instant focal plane.
Perspective: when parallel lines converge in the distance.
Relative size: when 2 objects known to be of absolute selfsame size show their distance by relative size.
Familiar size: as the visual angle of an object decreases with distance the mind instantaneously performs algebraic trigonometry to determine depth.
Interposition: occlusion hints at distance as objects in front block ones behind.
Shading: the relative way light falls on an object, and the shadow cast, is an effective cue of shape and position.
Aerial perspective: owing to atmospheric light scattering, objects at distance have lower luminance contrast and color saturation. Objects in the foreground appear in higher contrast.
Texture gradient: textures are sharper close-up, becoming blurry at distance.
Peripheral vision: like a photo from a fish-eye lens, parallel lines curve at the outer extremes of the visual field. Though not of objects in direct visual focus, this distortion yields visual information of depth.
Accommodation: this oculomotor cue for depth perception comes in from the mind-brain accounting for the ciliary muscles, which stretch to change lens focal length, allowing focus on faraway objects.
In short, depth perception is a convergence of multi-vectored mathematical factor analysis simultaneously performed by the mind instant by instant. This is on top of rod/cone input-filtering and image-mixing.
Vision is an extensive exercise in working memory. The details of known objects are filled in from experience, supplemented with current awareness. Discrepancies focus visual awareness, as do new objects for which no memories exist.
The tendency to shortcuts in object recognition creates an illusion of detail without actually sensing that level of detail. This accounts for the vagaries of visual memory that are common in later recall.
Dreams are illustrative. Engaging the sensation system used for vision, dreams are visually sketchy, with only focal objects rendered with any detail. Yet dreams can create such an immersive experience as to give the impression of reality. This occurs because dreams mix sensation with emotion-laden content. Dreams feel real.
Awareness while awake is similar in this regard: a pastiche of perceptions mixed into a sensory stew. Emotions create an especial intensity to an experience that renders it vivid.
Mentally processing visual text does not require vision for its input. It doesn’t matter if the person is reading with the eyes or the hands. The mind treats the pattern as if it were seen.
As with other senses, touch and sight are intertwined in the mind-brain. When looking at an object, the mind not only processes what the object looks like, but also how it feels.
Given all this, it is easy to understand why vision processing is the most intensive constant cognitive activity, and why the respite of sleep is necessary. Yet, during dreaming, the visual processing part of the mind-brain is given a workout. Perhaps dreaming provides practice for the mind to make its vision processing more efficient – learning how to better render the visual world ‘real’. That infants and children spend so much time dreaming lends support to this hypothesis.
A supposed visual snapshot is actually a time-varying composite, as the eyes constantly move (saccade), even staring at a single spot. Hence, what is mentally construed as a static visual image is a pastiche of collated patterns.
While anatomical study has rendered a detailed picture of the tissues and cells involved, and studies on visual acuity have discerned processing factors and tradeoffs, how detailed panoramic imagery is construed from a deficiently sampled collage of light input remains an enigma. Physiologically, sight is impossible.