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The Senses

Help Webvision Facts and Figures concerning the human retina Anatomy and Physiology of the Retina
Electronic Atlas: eye Structure of the Retina Anatomie comparée et évolution du système visuel primaire des Vertébrés Building Images with Amacrine Cells
Morphology and Circuitry of Ganglion cells Animation : Receptive Fields in the Retina
Original modules
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

In addition to using ordinary chemical synapses to transmit nerve impulses through its circuits, the retina also uses electrical synapses to transmit large volumes of information more rapidly, especially in the pathways that start in the rods. In addition, researchers have also discovered that a great deal of neuromodulation takes place in the retina—diffusion of substances over large distances to influence a large number of neurons at once.

Lien: Neurotransmitters in the retina

The ganglion cells are the last link in the chain of neurons in the retina. This chain begins in each retina's 125  million photoreceptors, which gather information and channel it, via a small number of synaptic connections, to the retina's 1  million ganglion cells. The axons of these ganglion cells form the optic nerve that carries the action potentials out of the eye to the brain.

By the time these signals leave the eye, the information that they carry is thus far more sophisticated than a mere point-by-point representation of the world encoded by the photoreceptors. And the retina is thus much more than a simple layer of photosensitive cells. It is actually more like a small brain outside the main one. In fact, in the development of human embryos, the retinas are originally part of the brain and detach from it subsequently.


Just like the rods and cones, whose structure and function are oriented entirely toward converting light energy into nerve impulses, every other type of cell in the retina is located and connected so as to perform some initial step in the processing of visual information.

Source: University of Kansas Medical Center


While the other neurons in the retina emit only graduated electrical potentials, the ganglion cells are the only ones that send out neural signals in the form of action potentials. When you consider that it is the ganglion cells' axons that form the optic nerve and thereby transmit information from the retina over large distances, the significance of the generation of action potentials in these cells becomes apparent. Note that these potentials are generated spontaneously; it is the frequency at which they are discharged that is increased or decreased by the appearance of light in these cells' receptive fields.

Though most ganglion cells have either ON-centre OFF-surround receptive fields or the reverse, there are other criteria that define other categories. On the basis of overall appearance, neural connections, and electrophysiological traits, at least three such categories of ganglion cells have been distinguished in the retinas of macaques (short-tailed monkeys whose retinas are very similar to our own).

The small parvocellular (or "type P") ganglion cells (from the Latin parvus, meaning "small") represent about 90% of the total population of ganglion cells. Large magnocellular (or "type M") ganglion cells (from the Latin magnus, meaning "large") account for about 5%. Non-M, non-P ganglion cells, which have not yet been well characterized, account for the remaining 5%.

In addition to being larger themselves, type M ganglion cells have larger receptive fields, propagate action potentials more quickly in the optic nerve, and are more sensitive to low-contrast stimuli. In addition, the positive response of an M cell to a stimulus consists of a brief salvo of action potentials, whereas the response of P cells is more tonic, continuing as long as the stimulus is active.

The most commonly accepted theory is that M cells are particularly involved in detecting movement in a stimulus, whereas P cells, with their small receptive fields, would be more sensitive to its shape and details.

Another distinction is essential for colour detection: most P cells and some non-M non-P cells are sensitive to differences in the wavelengths of light. Most P cells are in fact "single colour opponent cells", which means that the response to a given wavelength at the centre of their receptive fields is inhibited by the response to another wavelength in the surround. In the case of a cell with a red ON-centre and a green OFF-surround, red cones occupy the centre of the field and green cones occupy the surround. The same thing goes for cells with blue-yellow opposition, in which blue cones are opposed to red and green ones. Type M ganglion cells do not have any colour opposition, simply because both the centre and the surround simultaneously receive information from more than one type of cone. Also, there are no M cells in the fovea, which confirms that these cells do not play a role in processing colour.

Thus ganglion cells bring the brain information that has already been partly processed as regards areal comparison of the processes of light-dark, red-green, and blue-yellow opposition.

Like many other systems in the brain, the visual system processes information in parallel. Your two eyes first provide two parallel streams of information, which your brain then compares to obtain an estimate of the depth of a given object in the scene in front of you.

The ganglion cells also appear to transmit independent flows of information about the amount of light at each point in space.

Type M ganglion cells can also detect subtle contrasts, thanks to their very large receptive fields, while P cells, with their small receptive fields, seem better adapted to discriminating small details. Lastly, P cells and non-M non-P cells are specialized in processing colours.



Neuronal Constituents THE FEEDFORWARD MODEL The Visual Cortex (animations)
Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

The centre-surround structure of the receptive fields of retinal neurons results from the way that the horizontal cells connect photoreceptors and bipolar cells. This structure makes it possible to augment the contrasts of objects in the visual field. Information fed back from the inner plexiform layer also influences the activity of the horizontal cells. These cells, in return, modulate the signals from the photoreceptors under various light conditions, thus making the process of transduction less sensitive in bright light and more sensitive in dim light.

Bipolar cells, like all the other neurons in the retina except the ganglion cells, transmit nerve impulses not by means of action potentials, but in the form of simple graduated potentials. Nevertheless, we speak of an ON response when depolarization increases the amount of neurotransmitters released and an OFF response when hyperpolarizaton reduces this amount.

The question that then arises is whether the connections between photoreceptors and bipolar cells are excitatory or inhibitory.

In the absence of light, photoreceptors release their neurotransmitter, glutamate, continuously. Consequently, the glutamate receptors of OFF-centre bipolar cells are excitatory, because the absence of light must stimulate them. Similarly, the receptors of ON-centre bipolar cells are inhibitory, because light striking the photoreceptors at the centre of their receptive fields hyperpolarizes them and reduces their release of glutamate. Since glutamate is an inhibitory neurotransmitter, reducing it excites the bipolar cell. Thus it is the excitatory or inhibitory nature of the glutamate receptors that determines the type of receptive field for bipolar cells.

ON-centre Ganglion Cells

Like bipolar cells, ganglion cells have circular receptive fields, with centre-surround opposition. In addition, the ON or OFF characteristic of a bipolar cell is passed on to the ganglion cell to which it is connected. Most ganglion cells are not very sensitive to light stimuli that strike both the centre and the surround of their receptive fields. Hence, in total darkness or uniform light, they emit few action potentials. However, these cells are highly sensitive to differences in illumination at different points in their receptive fields, such as when an area of light or darkness sweeps across one side of a receptive field but not the other.

The information conveyed by the action potentials from ganglion cells thus has more to do with the contrasts in illumination between light and dark areas than with the absolute degree of luminosity. The perception of light and darkness therefore is not absolute, but relative.


In the visual cortex, in addition to the simple and complex cells in the primary visual area (V1, also known as Area 17 or the striate cortex) and in secondary visual area 18 (V2), there are hypercomplex cells in secondary visual area 19 (V5 or MT) that respond only if a light stimulus presents a given ratio of lit surface to dark surface, or is coming from a given angle, or includes moving shapes. Some of these hypercomplex cells also are sensitive only to lines of a certain length, so that if the stimulus extends beyond this length, the cells' response is reduced.

Hypercomplex cells occur when axons from several complex cells with different orientations and adjacent visual fields converge on a single neuron. These hypercomplex cells provide yet another level of information processing. At every level, each cell "sees" more than the cells at the levels below it, and the highest-level cells have the greatest power of abstraction. This capability is generated by the neuronal connections at every stage along the visual pathways from the eyes right up to the various visual cortexes in the brain.

These levels of abstraction can be summarized as follows: the retina and the LGN "see" the position of an object, the simple cells see its axis of orientation, the complex cells see the movement of this axis, and the hypercomplex cells see the object's edges and angles.

Receptive Fields of Hypercomplex Cells



Some reflections on (or by?) grandmother cells How Do We See Colors? Anatomical Substrates for Functional Columns in Macaque Monkey Primary Visual Cortex The "Hypercolumns" myth
The Visual Cortex (animations)
David H. Hubel - Autobiography Torsten N. Wiesel - Autobiography


Brodmann Areas

In addition to sending projections outside the primary visual cortex, the axons of the pyramidal cells in all of its layers also send out branches that make local connections with one another. Most of these connections are radial: they are made perpendicular to the surface of the cortex and pass through its various layers while remaining within the same column, thus preserving retinotopy.
However, the axons of certain pyramidal cells in layer III send out branches that are horizontal rather than vertical and hence make their connections across columns in layer III.

These radial and horizontal connections play distinct roles in the analysis of visual information.

In the visual systems of newborn infants, the input pathways that convey information from the two eyes to the brain converge on the same target cells. But just a few weeks after birth, a segregation occurs, and the connections are thenceforth made according to which eye the input comes from. Following this synaptic reorganization, each layer of the lateral geniculate nucleus and each ocular dominance column in the striate cortex receives inputs from one eye only.

In order to study the effects of sensory deprivation during critical periods of development, a number of experiments have been conducted in which either one or both eyelids of cats and monkeys have been sewn shut, or in which the animals have been given strabismus surgically. These studies have shown that the normal development of the connections of the visual cortex depends not so much on the activity of a particular neural pathway as on competition between the relative activities of different pathways.

After the right eye of a young cat is sewn shut during the critical period for the establishment of the ocular dominance columns in the primary visual cortex, a process of competition causes the surface area of the columns innervated by the visual pathways of the sutured eye to decrease relative to the corresponding area for the intact eye. This process seems to work as follows. First, the axons projecting to the cortex from the LGN cells that receive connections from the closed eye regress, leaving neurons on the cortex vacant. These neurons are then innervated by collateral branches that develop from the axons of the cells of the LGN of the intact eye.

Experience Module : How the Brain Keeps Information from the Left and Right Eyes Separate


So much research has been done and published on the primary visual cortex that we can now appreciate its cell architecture in all its beauty and complexity.

First, there is the horizontal stratification of the visual cortex into various types of neurons that specialize in receiving or sending neural information.

Next, the cortex is also divided radially, into a multitude of columns in which all the neurons respond to the same characteristic of a given point in the visual field. The columns thus form functional units that run perpendicular to the surface of the cortex.

In addition, if we insert a microelectrode perpendicularly through the various layers of the visual cortex, for example, all of the neurons that it encounters will have the same orientation preference, regardless of whether they have simple or complex receptive fields. As a corollary, if we insert a microelectrode parallel to the surface of the cortex, so that it passes through several columns in the same layer, we will see the orientation preference change as the microelectrode progresses. Hubel and Wiesel showed that the orientation preference was reversed by 180 degrees on average when the electrode moved about 1 millimetre in layer III.

The ocular dominance columns can be said to represent a third dimension of the cell architecture of the primary visual cortex. These columns are located in layer IV C and take the form of regularly spaced bands 0.5 mm wide. In fact, experiments with tracers (link to Experiment module from the sidebar to the left) have shown that these bands represent the nerve endings of the left and right eyes and that they thus alternate between one eye and the other, in a pattern something like a zebra's stripes.

And as if all this were not enough, in the late 1970s, other researchers, using a stain called cytochrome oxydase, revealed the presence of another kind of columns, spaced at regular intervals and running through layers II, III, V, and VI. These columns, which look something like a leopard's spots when viewed tangentially, are called blobs. These blobs are arranged in lines and centred on an ocular dominance band in layer IV C. Between the blobs are areas called interblobs whose neurons do not have the characteristics of these blobs.

What is special about the blob cells is that they are sensitive to the wave length of light—in other words, its colour. In addition, they are monocular, and they do not have any orientation selectivity; instead, they have circularly symmetrical receptive fields. Some blob cells even have the same centre-surround colour opposition structure as the P ganglion cells where this pathway originates (see box below).

Hubel and Wiesel also showed that every point in the visual field produces a response in a 2 mm x 2 mm area of the cortex. Such an area can contain two complete groups of ocular dominance columns, 16 blobs and interblobs that may contain more than two times all of the orientations possible across 180 degrees. This region of the cortex, which Hubel and Wiesel called a hypercolumn (or, more generally, a cortical module) seems both necessary and sufficient for analyzing the image of a point in visual space. Because the cortex is a continuous cellular layer and because it is very hard to establish the boundaries of these modules physically, their existence from a functional standpoint is still the subject of debate.

In the early 1960s, David Hubel and Torsten Wiesel (who won the Nobel Prize for Medicine in 1981) were the first to use microelectrodes to explore the receptive fields of the neurons in the lateral geniculate nucleus and the visual cortex. First, Hubel and Wiesel showed that the neurons of the lateral geniculate nucleus behave practically the same way as the ganglion cells in the retina. Then the scientists discovered the existence of three relatively independent pathways in the processing of visual information, each of which takes care of a different aspect of vision.

The first is the M (magnocellular) channel, which begins in the magnocellular ganglion cells of the retina, passes through the lateral geniculate nucleus, and projects into layer IV Caof the striate cortex. In this layer, the receptive fields of the neurons in this pathway are no longer circular (as they are in the retina and LGN), but instead are somewhat oblong.

The cells in layer IV Caproject to the neurons of layer IV B. These latter neurons also have simple receptive fields, but often respond to stimuli from both eyes, contrary to the cells in layer IV C, whose receptive fields are monocular. Thus the neurons in layer IV B begin the process of integration that is necessary for human binocular vision .

The cells in layer IV B are also selective for direction, but only if the straight-line stimulus is moving in a particular direction. For this reason, the M channel is thought to specialize in the analyzing the movement of objects.

The second pathway for transmitting visual information is the P-IB (parvocellular-interblob) channel, which starts with the parvocellular ganglion cells in the retina, passes through the LGN, and ends in the cells of layer IV Cb, which respond like the parvocellular LGN cells from which they arise: they have small, monocular, circular receptive fields, most of them displaying red-green colour opposition.

The axons from the cells in layer IV Cb then project to the interblob areas in layer III. The complex cells in that layer are even more specific than simple cells with regard to the orientation of stimuli, which suggests that the P-IB channel specializes in analyzing the shapes of objects.

The third pathway for transmitting visual signals is the blob channel, which passes through the parvocellular and koniocellular layers of the LGN and then converges on the blobs in layer III. Besides certain neurons in layer IV C, the blobs contain the only colour-sensitive neurons in the striate cortex. The blobs are thus probably used to analyze the colours of objects.

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