Vision
"We do not see with our eyes; we see with our brain." Jeanette Norden
Of all the sensory systems available to us, vision [52, 57, 133] is undoubtedly the most impactful. It allows us to observe, analyze, and interact effectively with our surroundings. Without it, our primary window to the world would be closed.
Reception :
Anatomy :
The receptor organ for vision is the eye (eyeball) [43, 45]. This spherical organ is composed of three layers [42], which are, from the exterior to the interior: the sclera, the choroid, and the retina.
The sclera (the white of the eye) [146] is a white, tough envelope that maintains internal pressure and protects the eye against mechanical damage. It continues anteriorly as a thin, transparent, non-vascularized, and richly innervated envelope: the cornea [64]. The cornea bulges at the front of the eyeball.
The choroid [5, 64, 146] : A black, richly vascularized envelope that nourishes the photoreceptor cells of the eye and maintains the interior of the eye as a dark chamber. The choroid continues anteriorly as the ciliary body [94, 122, 147] and the iris [133] , which gives us our eye color and delineates an opening: the pupil. Together, these vascular structures form the uvea [72].
The retina [57]: A thin membrane of about 0.5 mm [154] that is highly vascularized. This is the nervous tissue that continues as the optic nerve at the level of the optic disc (optic papilla). The retina is the sensory part that contains the photoreceptor cells [39, 94].
Behind the iris lies the lens (crystalline lens) [99, 148], a transparent, biconvex structure involved in focusing light beams onto the retina.
The interior of the eye is filled with substances that maintain its globular shape: in front of the lens is the aqueous humor [104, 148] , a transparent liquid of low viscosity that nourishes the cornea. Behind the lens is the vitreous body [116, 148, 149] , a transparent gelatinous substance that holds the retina in place against the ocular wall and absorbs a large amount of ultraviolet rays.
Optics :
The cornea is the first surface that light must pass through to reach the retina; it is curved and therefore contributes significantly to the convergence of light rays.
The pupil is the diaphragm of the eye [52]; it regulates the amount of incoming light by changing its diameter in response to light intensity, thanks to the antagonistic muscular system of the iris [150]: radial fibers dilate the pupil, and circular fibers constrict it.
The lens is the objective of the eye [43]. Its biconvex shape and flexibility allow it to modify its curvature to focus on objects at various distances; this phenomenon is called accommodation [3].
The eye thus functions like a camera [43] with a diaphragm (the pupil) [52] for automatic exposure control, an objective (cornea and lens) [43] in autofocus mode, and a photosensitive surface (the retina). The resulting image is focused on the retina (in an emmetropic eye), where it is reduced and inverted.
The retina :
Retinal cells :
The retina [3] (nervous tissue originating from the diencephalon during development [41, 72]) consists essentially of three layers of nerve cells [1, 70, 151]. Moving from the inner surface of the eye toward the back, we first find the ganglion cells [4, 99, 152]. Their axons bundle together to form the optic nerve, totaling about 1 million fibers per eye [75, 96, 133, 149, 152].
Behind them are the bipolar cells [153], which bridge the gap between photoreceptors and ganglion cells, forming the intermediate layer. Paradoxically, the photoreceptor neurons [99], the cells that actually detect light, form the deepest layer at the very back of the eye, sitting against the choroid.
Two other types of neurons also exist in the retina: horizontal cells and amacrine cells [99]. These interpose between the three layers of the retina and help improve the contrast and definition of the image transmitted to the brain.
Photoreceptors :
There are two types of photoreceptor cells: cones and rods. Each human eye contains about 125 million photoreceptors [99], of which only 5 million [75] are cones.
Cones :
Although they are far fewer in number than rods, cones determine our visual acuity [48]. In fact, the central area of the retina (macula) [154] contains a central region (fovea [4, 39, 148]) that is completely devoid of rods [38, 39]; only cones exist at this place.
Cones provide us with color vision [1, 39] thanks to three pigments (opsins [38] ) sensitive to blue, green, and red. Each cone contains only one of these pigments. Cones allow us to perceive image details [1, 39] because each cone is linked to a single bipolar cell, which in turn is linked to a single ganglion cell [41]. So, each cone has its own dedicated "line" to the brain.
Rods :
Rods [153] are extremely sensitive to light [1]; a single rod can absorb and react to a single photon [5, 41]. They contain an essential pigment (rhodopsin [57]); each rod contains about 1,000 discs with 40 million rhodopsin pigments [155].
Rhodopsin contains a molecule (retinal, a derivative of vitamin A [4, 42]) that changes shape whenever it absorbs light, triggering a chain reaction that hyperpolarizes the membrane and stimulates a bipolar cell.
Rods are distributed mainly at the periphery of the retina and allow us to detect the movement of objects [149]. Their high sensitivity enables us to see in the dark (scotopic vision) [41], unlike cones, which provide photopic vision.
The optic disc (optic papilla):
In a fundus examination [67], the area where all the nerve fibers gather to form the optic nerve is clearly visible; this area is called the optic disc (optic papilla) [148]. Since there are no photoreceptors at this level, it is a blind spot [41]. Why then do we not notice any gap in our visual field? The answer is that the brain fills in the missing information by using data collected from the surrounding areas [39].
Retina and visual hemifields :
The retina is divided into two parts [72]: the nasal retina and the temporal retina. The visual field is also divided into two hemifields, each corresponding to the region of the retina that receives it [41]. Light from the right visual field strikes the nasal retina of the right eye and the temporal retina of the left. For the left visual field, the opposite is true.
Transmission :
All the nerve fibers from the ganglion cells form the optic nerve [41, 116]. This is the only nerve in the body that belongs to the CNS and not the PNS [41].
From an anatomical point of view, it is the only nerve surrounded by the three layers of the meninges [64]. Embryologically, it develops from the diencephalon [50]. At the cellular level, it does not contain Schwann cells but rather oligodendrocytes, which is why it is often affected in multiple sclerosis.
The optic nerve originates behind the eye and terminates at the optic chiasm, just above the pituitary gland.
The optic chiasm [119] is a crossover point where fibers from each nasal retina cross the midline to join fibers from the temporal retina of the contralateral eye, thus forming a pair of optic tracts. Each optic tract contains information from the contralateral visual hemifield [50].
The two optic tracts curve around the brainstem (cerebral peduncles) and reach the lateral geniculate nuclei of the thalamus. From there, several bundles of fibers (optic radiations) [41, 149] project onto the primary visual cortex in the occipital lobe, as well as other structures like the superior colliculi in the brainstem, where certain reflex phenomena are processed.
Perception :
The primary visual cortex (V1, Brodmann area 17) [49, 73], located in the occipital cortex, is the first cortical relay for the nerve fibers of the visual system. It receives visual information and handles its primary processing.
Each visual cortex analyzes the contralateral visual hemifield. There is a retinotopy [5] with a very large area of the visual cortex corresponding to the fovea (the central area of the macula).
From the primary visual cortex, other nerve fibers project to other regions of the cerebral cortex known as the secondary visual cortex [57]: V2, V3, V4, V5, MT... [57] in order to analyze visual properties such as color, shape, texture, movement, and depth.
Other fibers project to distant regions of the cortex called associative areas [57]. There are two main types of these projections [57]:
The dorsal stream (the "where," or occipito-parietal pathway): this system analyzes movement, depth, and spatial orientation, allowing for the localization of objects.
The ventral stream (the "what," or occipito-temporal pathway): it analyzes shape and color, allowing for the recognition and perception of objects.