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The Senses
Sub-Topics
Vision

Linked
Help Link : The Eye Link : Do You Wear Glasses? Link : ANATOMIE DE L'OEIL
Link : The Eye (animations) Link : Animation : Visual Pathways in Human Brain
Original modules
Experience Module : The Blind Spot The Blind Spot
Tool Module : Optics   Optics

Six oculomotor muscles are attached to each of your eyeballs and let you make various types of movements with your eyes. Some of these movements are slow, such as when your eyes follow an object passing in front of them. Others, called saccades, are very rapid. One good example of saccades is the movements that your eyes are making while you are reading this page. Your eyes stop for a moment at one point in a sentence to analyze its image, then jump ahead very quickly to the next point.

Link : Movement perception Link : Extraocular Muscles Link : Eye Movements Link : Types of Eye Movements
Link : Oculomotor System Link : Les capteurs du système postural Research : Daniel Guitton Research : Douglas Munoz

THE EYE
THE TARGETS OF THE OPTIC NERVE THE VARIOUS VISUAL CORTEXES

As can be seen in the cross-section here, the first structure through which light passes into the eye is the conjunctive—a thin, transparent membrane that covers the front of the eye and then folds over to line the inside of the eyelids.

 

Next, before light can reach the various parts of the retina, it must pass through the other optical components of the eye: the cornea, the aqueous humour, the pupil (at the centre of the iris), the lens, and, lastly, the vitreous humour, a slightly more viscous fluid than the aqueous humour.

The cornea is continuous with the white of the eye, or sclera, which forms the hard outer wall of the eyeball. Three pairs of muscles are inserted into the sclera. It is these ocular muscles that enable the eyeball to move in its socket in the skull.

Between the sclera and the retina lies the choroid, a richly vascularized layer that delivers nutrients to the iris and the retina. The choroid contains many dark pigments that make the inside of the eye, as seen through the pupil, appear black

The aqueous humour and the vitreous humour play an essential role in focusing the image on the retina. Because light travels at different speeds in these two humours compared with its speed in the air, they bend any light rays that enter the eye at a non-perpendicular angle, so that these rays strike the proper place on the retina. This process is called refraction.

The curvature of the cornea also accentuates the refraction of the virtually parallel light rays that reach the eye from very distant objects. Some of these rays strike the cornea's centre. Hence they are already perpendicular to the cornea, and their angle does not change. They continue straight ahead to the centre of the retina. But other rays from these same distant objects strike the curved parts of the cornea. These rays are bent inward so that (provided the person has no vision defects) they arrive at exactly the same central point on the retina and form a focused image.

Where distant objects are concerned, the cornea does most of the refracting of light rays to make them converge at a single point on the retina; the lens also contributes, but to a lesser extent. For closer objects, however (starting about 9 metres from the eye), the lens plays a far more active role in focusing the image on the retina. The light rays reaching the eye from closer objects diverge more, so they must be refracted more in order to converge on the retina. The lens alters its own shape to provide this additional refraction.

 


 

The lens is attached via the suspensory ligaments to the ciliary muscles, which in turn are attached to the sclera. These muscles form a ring around the inside of the eye, so when they are relaxed, the tension on the suspensory ligaments is greater. Consequently, the lens is flatter and refracts less. But when the ciliary muscles contract, they relieve the tension on the lens. It therefore tends to return to its natural, more convex shape, which refracts light rays more, so that they converge more. The increased refractive power that the lens thus acquires allows a crisp image of close-up objects to be formed on the retina. This phenomenon is called accommodation.


       

Linked
Link : Visual Neuroscience Link : CENTRAL VISUAL PATHWAYS Link : The Visual Cortex (animations)
Original modules
Tool Module : Cybernetics Cybernetics

Even though the nerve fibres that run from the left and right eyes to each LGN connect to different layers within it, each of these layers still maintains retinotopic mapping (as is also found in the superior colliculus). What is more, the maps in the six layers of each LGN coincide, so that a line running through all of them perpendicularly passes through neurons that are sensitive to the same points in the binocular visual field. Hence these points are perceived simultaneously by both eyes.

THE TARGETS OF THE OPTIC NERVE
THE EYE THE VARIOUS VISUAL CORTEXES

The axons of the retina's ganglion cells pass through the optic nerve, the optic chiasm, and the optic tractus. They wrap around the midbrain and cross the medial surface of the temporal lobe, and 80% of them then terminate in a synaptic relay called the lateral geniculate nucleus (LGN), located in the dorsal part of the thalamus. The LGN is thus the major target for each optic tract.

In cross-section, the left and right LGNs are seen to be organized into six distinct cell layers. When examined in three dimensions, these six layers look like a stack of pancakes folded around the optic tract in a way that resembles a knee joint, whence the name "geniculate", from the Latin for knee.

The distribution of the LGN's neurons into various layers suggests that some distinct aspects of the visual information from the retina may be processed separately in this synaptic relay. And that is exactly what has been demonstrated experimentally.

Because the right LGN processes all of the visual signals from the left visual field (and vice versa for the left LGN), it receives axons from the left nasal retina and the right temporal retina. Research has shown that the axons from the ipsilateral eye (in this case, the right eye) synapse on cell layers 2, 3, and 5 in the LGN, whereas those from the contralateral eye (here, the left one) project to layers 1, 4, and 6.

Examination reveals that the LGN's ventral layers (1 and 2) contain larger neurons than its more dorsal layers (3, 4, 5, and 6). The two ventral layers are therefore called magnocellular (M) layers, while the four others are called parvocellular (P) layers, just like the type M and type P ganglion cells. In fact, it has been shown that it is precisely these type M ganglion cells that project to the magnocellular layers of the LGN, and the type P ganglion cells that project to its parvocellular layers. The parallel processing in distinct information channels that begins in the retina thus seems to be maintained through the LGN.

Even the small neurons that form the koniocellular layers on the ventral side of each of the six numbered layers of the LGN receive connections from the non-M non-P ganglion cells in the retina. This observation confirms the segregation of information from different types of ganglion cells.

Despite the great influence that innervation from the retina has on the structure of the LGN, about 80% of the excitatory inputs to the LGN come not from the retina but from the primary visual cortex and lower centers in the brains! The primary visual cortex thus appears to exert a significant feedback effect on the LGN. In other words, the LGN's main target may in turn modify the LGN's own visual responses.

Another observation lends weight to the idea that the LGN, just like the other subcortical structures involved in vision, does more than just passively relay information from the retina to the cortex: the LGN may be activated by brainstem neurons whose activity is associated with vigilance and with processes related to attentiveness. These neurons seem to modulate the response of the LGN neurons, which tends to confirm that the LGN is actually the first location on the visual pathway where particular mental states can influence our visual perception.


       

Linked
Link : The Eye and Sense of Vision Link : More Than the Sum of Its Parts Link : The Infinite Mind: Vision Link : Central Visual Pathways
Link : The Strange Symptoms of Blindness to Motion Link : A Hot Spot in the Brain's Motion Pathway Link : Integrating Information About Movement Link : The "Standard Model" of object recognition in cortex
Link : The Visual Perception of Object (animations) Link: The Visual Perception of Motion (animations) Link : Eye Movements (animations) Link : Conscience et cerveau : le système visuel
Original modules
Tool Module : Cybernetics Cybernetics
Tool Module : Brodmann's Cortical Areas   Brodmann's Cortical Areas

Unlike people who have colour blindness and confuse certain colours, some very rare individuals cannot see any colours at all. This disorder is called achromatopsia.

Achromatopsia can be inherited—for example, when there is a genetic defect in the cones of the retina. But it can also be acquired—as the result of a stroke, for instance. Brain imaging studies in persons with achromatopsia have confirmed the presence of substantial lesions in area V4 of the visual cortex , which is known to be involved in processing colours. These people describe their visual perception of the world as dull and grey, something like an old black and white television screen.

Achromatopsia is also usually accompanied by some difficulty in recognizing the shapes of objects, which is consistent with the shape-recognition role that also is played by the ventral system.

THE VARIOUS VISUAL CORTEXES
THE EYE THE TARGETS OF THE OPTIC NERVE

Hierarchical relationships between a primary cortex and secondary cortexes are found in several parts of the brain, including the sensory, motor, and auditory cortexes. The secondary areas of these various cortexes then converge onto what are called associative areas, which perform a more global level of information processing. They progressively associate signals from other sensory modalities to create an integrated, multisensory representation of the world.

To date, researchers have discovered nearly 30 different cortical areas that contribute to visual perception. The primary area (V1) and the secondary area (V2) are surrounded by many other tertiary and associative visual areas: V3, V4, V5 (or MT), PO, etc.

From all this complexity, however, a general pattern does emerge. There appear to be two major cortical systems for processing visual information: a ventral visual pathway that extends to the temporal lobe, and a dorsal visual pathway that projects to the parietal lobe.

 

The basic function of the ventral visual pathway seems to be to let us consiously perceive, recognize, and identify objects by processing their "intrinsic" visual properties, such as shape and colour. The basic function of the dorsal visual pathway seems to be to let us exercise visual-motor control over objects by processing their "extrinsic" properties—the ones that are critical for handling them, such as their size or their position and orientation in space.

In the cortical areas that contribute to the ventral system, increasingly complex, specialized representations of the outside world are elaborated.

Area V3 receives major connections from area V2 and sends projections to areas MT and V4. Many of the neurons in area V3 have properties similar to those in V2. For example, most of them are selective for orientation. But much remains unknown about area V3, and it also has some cells with more complex properties. For example, some of these cells are sensitive to colour and movement, traits more commonly analyzed in subsequent stages of visual signal processing in the brain.

After passing through areas V1, V2, and V3, part of the visual information continues ventrally to area V4 on its way to the temporal cortex. Area V4 receives information from the blobs and interblobs of the striate cortex, via a relay in V2. Like the cells in all of the other visual areas besides V1 (also known as the "extrastriate areas"), the cells in area V4 have larger receptive fields than those of the striate cortex. Also, the receptive fields of V4 are often sensitive to both colour and orientation. The exact role of area V4 is still under debate, but it is probably involved in recognizing shapes, and it appears to be essential for perceiving colours.

Area IT gets its name from the inferotemporal cortex, where it is located, and comprises areas TEO and TE. The cells of area IT receive many connections from area V4 and respond to a very wide range of colours and simple geometric shapes. These cells appear to play an important role in visual memory, in addition to being a key locus for object recognition.

Neurons have been found in area IT that respond specifically to images of faces. Initially discovered through intracellular recordings in monkeys, the existence of these cells has been confirmed in human beings through functional magnetic resonance imaging (MRI) studies. This discovery is of some interest to neuropsychologists, who have long known of a rare syndrome called prosopagnosia, in which patients have difficulty in recognizing faces, even though the rest of their vision is normal. This syndrome appears following lesions to the extrastriate areas of the visual cortex, which strongly suggests that these neurons in area IT may be involved.

Link : Faces, Faces Everywhere Tool Module : « Cellules grand-mère » ou décharges synchrones de neurones
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