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The Senses
Sub-Topics

Linked
Help Gross and Microscopic Anatomy of the Eye Electronic Atlas: eye Defects of the eye
LA MYOPIE ET LA PRESBYTIE The Eye (animations) Animation : Visual Pathways in Human Brain Animation : The Evolution of Eyes
Les réflexions d’un philosophe sur ses interactions avec un spécialiste des neurosciences — L’hypothèse des deux systèmes visuels
Original modules
Experience Module : The Blind Spot The Blind Spot
  Experience Module : Proving That the Periphery of the Retina Is More Sensitive to Light   Proving That the Periphery of the Retina Is More Sensitive to Light
Tool Module : Optics   Optics

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

 

The blood vessels on the retina's surface can be seen with an ophthalmoscope (the instrument that physicians use to examine the back of the eye through the pupil). These blood vessels enter the eye through a colourless region of the retina, known as the optic disc. The optic disc is also the head of the optic nerve; it is here that the axons of the retina's ganglion cells converge and exit the eye.

Because there are no photoreceptors in the optic disc, this part of the retina is insensitive to light, just as it is at the location where the largest blood vessels pass. For this reason, the optic disc is also known as the blind spot. But you do not experience any interruption in your field of vision at your eyes' blind spots, because the brain has a way of "filling in" your visual perceptions at these locations (see the first Experiment module link to the left).

At the centre of each retina there is also a darker area called the macula that is practically devoid of blood vessels, in order to optimize central vision (as opposed to peripheral vision). At the centre of the macula, a small depression creates a black spot about 2 mm in diameter. This is the fovea—the part of the retina that is composed exclusively of cones and where visual acuity is greatest.

 

When light rays do not converge on the retina precisely, several different types of vision defects may occur.

For example, if the eyeball is too short from front to back, the rays converge at a point beyond the retina. This defect is called hypermetropia, or far-sightedness. It can be corrected by eyeglasses with convex lenses, which increase the rays' convergence and thus pull the focus back onto the retina.

If the eyeball is too long from front to back, then the rays converge in front of the retina. This condition is called myopia, or near-sightedness. It can be corrected by eyeglasses with concave lenses.


Another vision defect, presbyopia, is due to hardening of the lens, associated with aging. As the lens hardens, it becomes less elastic, so that it can no longer assume a sufficiently convex shape during accommodation or a sufficiently flat one during relaxation. The eyeglasses prescribed for this condition contain bifocal lenses, where the top half is concave for distance vision, and the bottom half is convex for close-up vision.

 


The visual field or receptive field of an eye is the area in space that is imprinted on the retina when the eye is focused on a distant point. The visual fields of the two eyes overlap to a large extent, but the right eye's field extends farther to the right, and the left eye's extends farther to the left.



The figure above also shows how the right eye's visual field (in yellow) is analyzed in the left visual cortex, and vice versa. The central area where the two eyes' visual fields overlap is the binocular visual field.

 

       

Linked
Voyage fantastique entre l'oeil et le cerveau LES SYNCHRONISEURS EXTERNES The lateral geniculate nucleus (LGN) The Visual Cortex (animations)
Streams for Visually Guided Actions (animations)
Experiment
The pulvinar and visual salience A subcortical pathway to the right amygdala mediating “unseen” fear

The numerous connections among the various areas of the brain that are involved in processing visual information (such as the visual cortex, basal ganglia, pons, cerebellum, and oculomotor nuclei) are generally reciprocal. This reciprocity creates feedback loops which vividly demonstrate just how much the visual system is intrinsically interlinked with the operation of the nervous system as a whole.

The evolutionary history of the human brain sheds much light on the reason for all these neuronal feedback loops in our visual system. In the reptilian brain, for example, vision is tightly linked with reflex defence responses. A deafening noise, a new tactile sensation, or an object approaching rapidly from the edge of the organism's field of vision will make it turn its head quickly toward the new stimulus so that the eyes can assess how much danger it actually represents. Even though the visual system of humans and other primates has become far more sophisticated to let us acquire a conscious, detailed vision of the world around us, these old circuits are still useful, have been preserved by evolution, and are still at work in the human brain.

Tool Module : Cybernetics
THE TARGETS OF THE OPTIC NERVE
THE EYE THE VARIOUS VISUAL CORTEXES

The axons of some of the ganglion cells of the retina diverge from the optic tract to project to structures other than the lateral geniculate nucleus, which is the main relay between the retina and the visual cortex.

One of these structures is the hypothalamus, and more specifically its suprachiasmatic nucleus, which receives a certain number of connections from retinal axons. The suprachiasmatic nucleus is considered the primary site of the body's internal biological clock. The visual signals that it receives from the retinal axons keep it continuously informed about the darkness or lightness of the environment, thus enabling it to synchronize a wide range of biological rhythms, including sleep and wakefulness, that are linked to the cycle of day and night.

Axons from some other retinal ganglion cells project to the pretectum, a part of the midbrain that controls the opening of the pupil and certain eye movements.

Lastly, about 10% of the axons emerging from the retina project to a part of the tectum (roof) of the midbrain called the superior colliculus. This pathway is relatively large. It comprises about 150 000 axons, equivalent to the total number of ganglion cells in the retina of a cat! In fact, the superior colliculus corresponds to the optical tectum in all non-mammalian vertebrates, in which this retinotectal projection is the main efferent pathway from the retina.

Because of the way that receptive fields overlap one another in the retina, a beam of light shined on the retina activates a large population of neurons in the superior colliculus. These neurons trigger movements of the eyes and head, via the motor neurons of the brainstem, to try to bring the image of the light beam onto the fovea. Thus the retinotectal pathway is involved in orienting the eye toward a stimulus that initially appears in its peripheral field of vision.

Like the lateral geniculate nucleus, the superior colliculus also receives connections from the primary visual cortex. The neurons of the superior colliculus in turn project their axons to subcortical structures such as the reticular formation, the inferior colliculus, and the spinal cord. The neurons of the superior colliculus also influence two structures that are involved in vision: the lateral geniculate nucleus and the pulvinar.

The pulvinar is a nucleus in the posterior portion of the thalamus. It receives afferences directly from the optic tract (via collateral axons) as well as by way of the LGN. Like many other thalamic nuclei, the pulvinar was long regarded as a relatively passive relay for information en route to the cortex, where the real information processing was assumed to take place. But this view of the pulvinar has been altered radically by the accumulation of data showing that its neurons display sophisticated visual responses of which formerly only the cortex was thought to be capable.

 

Now the pulvinar is instead believed to be an image-interpreting centre that plays an important role in visual attention and motion perception. For example, the pulvinar is thought to help maintain the stability of the body's visual environment by compensating for the effects that body movements have on the position of images on the retina. Hence it is no surprise that the neurons of the pulvinar have been found to project to the secondary visual areas involved in detecting motion.

 

       

Linked
VISION AND MOVEMENT IN THE CEREBRALCORTEX A Hot Spot in the Brain's Motion Pathway Vision Bases neurales de l'imagerie mentale
Mécanismes de la perception visuelle du mouvement A Dichotomous Visual Brain? Functional analysis of primary visual cortex (V1) in humans Consciousness, Coordination, and Two Visual Streams
The Visual Perception of Object (animations) The Visual Perception of Motion (animations) Eye Movements (animations) Conscience et cerveau : le système visuel
The Primary Visual Cortex Physics Makes a Toy of the Brain The Primary Visual Cortex Physics Makes a Toy of the Brain
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Researcher
J. Anthony Movshon
Experiment
Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey Dorsal and ventral visual stream contributions to perception-action interactions during pointing Functional properties of neurons in macaque area V3

Visual agnosia, which often occurs following bilateral occipito-temporal lesions, makes people incapable of discriminating the shapes of objects, though these people may sometimes retain good abilities to discriminate colours and textures. People with visual agnosia perform very poorly when asked to recognize the shapes of arbitrary geometric objects, letters of the alphabet, and black and white drawings, yet have no trouble in moving their hand toward an object, turning their wrists to insert an object in a slot, or arranging their fingers in the right way to pick up an object. Visual agnosia syndrome thus impairs conscious visual awareness but leaves the ability to manipulate objects intact.

In the opposite syndrome, optical ataxia, people with damage to their parietal lobe can recognize objects but cannot pick them up and use them appropriately. Observations of such individuals have helped researchers to uncover the differing roles of the ventral and dorsal visual pathways.

Link : Les agnosies visuelles

In 1983, Joseph Zihl and his collaborators published an article in Munich about a 43-year-old woman who had become totally incapable of perceiving movement, following a stroke that had damaged both sides of area V5, the part of the extrastriate cortex that is involved in recognizing movement. This patient was thus suffering from the strange syndrome known as akinetopsia, or motion blindness. For several seconds at a time, she would see nothing but a still image and have no conscious awareness of any movements in her environment. Crossing the street, for example, was very dangerous for her, because a car that she had last seen "stopped" a good distance away might suddenly appear right on top of her after she had begun to cross. Pouring a glass of water could be almost as big a problem. Since she saw the water as frozen in space rather than flowing, she could not tell that she had overfilled the glass until she suddenly saw a puddle of water on the table.

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

Following the groundbreaking studies published by Leslie Ungerleider and Mortimer Mishkin in 1982, scientists distinguished two major pathways for the cortical processing of visual information: the ventral visual pathway, for identifying objects, and the dorsal visual pathway, for determining their position in space. Various subsequent studies, however, have raised some questions about this dichotomy. Some of these studies have involved making selective lesions in each of these pathways in monkeys. Others have involved observing humans who had suffered brain injuries that affected only one of these pathways (see sidebars).

Today it is believed that the main function of the dorsal visual pathway is to guide in real time the actions that we direct at objects in the visual world. Most of the processing done by this pathway is believed to be unconscious. The dorsal pathway could thus be described as an "action pathway", because by integrating the spatial relationships between our bodies and our environment, it lets us interact with this environment effectively. The ventral visual pathway, on the other hand, seems to be involved in forming conscious representations of the identity of objects. Thus, in addition to the functional dichotomy between the dorsal and visual pathways, there would appear to be another dichotomy, between unconscious and conscious vision.

The dorsal pathway comprises several cortical areas, including the medial temporal area (MT or V5), the medial superior temporal area (MST), and the ventral and lateral intraparietal areas (VIP and LIP).

Area V5 (or MT) seems to contribute significantly to the perception of movement. This area receives projections from V2 and V3. It also receives projections from layer IV B in the primary visual area (V1)—a layer that, interestingly enough, is part of the magnocellular channel involved in analyzing the movement of objects. This channel also maintains its specificity for movement in area V2, where it is concentrated in the thick stripes that contain large amounts of cytochrome oxydase.

It has therefore been suggested that the segregation between the magnocellular and parvocellular signals persists up to the highest levels of visual analysis. The marked functional difference between the ventral and dorsal pathways might also be attributed to a preferential contribution from the P-IB channel in the former case and from the M channel in the latter.

For the cells in area MT, the movement of an object seems more important than its nature—so much more, that this area of the cortex is organized into columns that code for the orientation of the movement, just like the line-orientation columns in V1.

Some cells in area MT even seem to respond not to the actual direction of movement, but rather the perceived direction. For example, two groups of illuminated lines, each moving at 45 degrees to either side of the vertical, will give the impression of a vertical movement as they intersect. In area V1, the cells that have a preference for 45-degree angles will respond best to this kind of stimulus. But in area MT, many cells that normally show a selectivity for vertical directions will respond convincingly to two stimuli moving at 45 degrees—in other words, to the apparent direction of movement.

Beyond area MT, there are other areas involved in analyzing movement, such as area MST. The cells in this area are sensitive not only to linear motion, like the cells in area MT, but also to radial motion (to or away from a point) and circular motion (clockwise or counter-clockwise). They are also selectively activated by complex configurations of movements, such as occur in your surroundings as you walk through them.

Certain neurons in the superior temporal polysensory area (STP) even respond selectively to biological movements that may have proven essential for individual survival, such as by recognizing a member of the same species from the way that he or she walks.

Your brain may use this large volume of movement-related data gathered by the dorsal pathway for many purposes: to extract relevant information about the objects flowing across your field of vision as you move, in order to guide your movements; to orient your eye movements; or to identify moving objects around you that have implications for your survival.

In addition to the serial path that visual signals follow up through each of the hierarchical levels of the visual system, there are also numerous channels that process information in parallel, thus forming a network of highly complex circuits. This complexity is at least partly attributable to the numerous feedback loops that each of these areas forms by returning connections to the areas that send connections to it. Another aspect of this complexity comes from the projections to subcortical structures such as the lateral geniculate nucleus and the superior colliculus.

Tool Module : "Grandmother Cells", or Synchronous Discharges of Neurons? Tool Module : Cybernetics
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