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The Senses


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Each eye can be said actually to contain not one retina but rather two retinas superimposed on each other. One is composed of rods, which are sensitive to the low levels of light that we experience at dusk and dawn, for example. The other is composed of cones, which can detect colour and are sensitive to broad daylight.

The retina is not designed to record the absolute intensity of the light reaching it, but rather to detect the differences in the intensity of the light striking it at different points.


For you to see anything, your eye must first form a precise image of it on your retina. Then the light energy striking your retina must be converted into nerve impulses by the retina's photoreceptor cells.

The image can then be processed by your nervous system. This processing does not start in the brain, but instead starts immediately in the retina itself. In fact, anatomists regard the retina as a part of the brain that is located outside it, somewhat the way you may regard your home satellite dish as an integral part of your television receiver.


Physically, the retina is a thin layer of nerve tissue with the consistency and thickness of a wet cigarette paper. The neurons of the retina are arranged in 3 main layers separated by 2 intermediate layers whose main purpose is to make connections among the various neurons.


The deepest layer of neurons processes the light first. These neurons are the photoreceptors, the only cells in the retina that can convert light into nerve impulses. The photoreceptor layer then transmits these impulses to the bipolar neurons in the second layer and on to the ganglion neurons in the third layer. It is only the axons of these ganglion neurons that exit the eye and carry the nerve impulses to the first visual relay in the brain.

In addition to this direct pathway from the photoreceptors to the brain, two other kinds of cells contribute to the processing of visual information in the retina. The horizontal cells receive information from the photoreceptors and transmit it to a number of surrounding bipolar neurons. The amacrine cells receive their inputs from the bipolar cells and do the same thing to the ganglion neurons: activate the ones that are in their vicinity.


Link : Simple cell movie Link : Animation: Center-surround receptive field

Each of the neurons in the various layers of the retina "covers" an area in your field of vision. This area in space where the presence of an appropriate stimulus will modify the activity of this neuron is called the receptive field of this neuron.

The receptive field of a single photoreceptor cell, for example, can be said to be limited to the tiny spot of light, within your field of vision, that corresponds to this photoreceptor's precise location on your retina. But in each succeeding layer of the retina, the receptive fields become increasingly complex, and they become even more complex when it comes to the neurons of the visual cortex.

Here is an example of this complexity. The receptive fields of bipolar cells are circular. But the centre and the surrounding area of each circle work in opposite ways: a ray of light that strikes the centre of the field has the opposite effect from one that strikes the area surrounding it (known as the "surround").

In fact, there are two types of bipolar cells, distinguished by the way they respond to light on the centres of their receptive fields. They are called ON-centre cells and OFF-centre cells.

If a light stimulus applied to the centre of a bipolar cells's receptive field has an excitatory effect on that cell, causing it to become depolarized, it is an ON-centre cell. A ray of light that falls only on the surround, however, will have the opposite effect on such a cell, inhibiting (hyperpolarizing) it.

The other kind of bipolar cells, OFF-centre cells, display exactly the reverse behaviour: light on the field's centre has an inhibitory (hyperpolarizing) effect, while light on the surround has an excitatory (depolarizing ) effect.


ON-centre Bipolar Cell


OFF-centre Bipolar Cell

Receptive Field of a Ganglion Cell




Just like bipolar cells, ganglion cells have concentric receptive fields with a centre-surround antagonism. But contrary to the two types of bipolar cells, ON-centre ganglion cells and OFF-centre ganglion cells do not respond by depolarizing or hyperpolarizing, but rather by increasing or decreasing the frequency with which they discharge action potentials.


That said, the response to the stimulation of the centre of the receptive field is always inhibited by the stimulation of the surround.



The receptive fields of the neurons of the primary visual cortex are not circular, but rectangular. They respond especially well to rays of light that are oriented in a particular direction. The cells whose receptive fields thus respond to light with a specific orientation are called simple cells.

These rectangular receptive fields often have an ON centre band that responds positively to light flanked by two OFF side bands that respond to darkness. The diagram here shows that when the beam of light is not oriented to follow the ON band precisely, the stimulus is simply not effective for this cell.


Simple Cell Receptive Fields

The simple cell receptive fields in the primary visual cortex are thought to be the result of the convergence of several adjacent receptive fields of cells in the relay that precedes it, the lateral geniculate nucleus. Note, by the way, that the receptive fields of this nucleus are still circular, like those of its source, the ganglion neurons in the retina.

Other cells in the primary visual cortex have "complex" and "hypercomplex" receptive fields with properties that are even more selective.


Original modules
Tool Module : Brodmann's Cortical Areas Brodmann's Cortical Areas

The primary visual cortex is the first relay in the visual pathways where information from the two eyes is combined. In other words, a single cell in this cortex may respond just as much to the stimuli presented to one eye as to those presented to the other.




In the visual cortex, the cell bodies of the neurons are divided into six layers that typify the primate neocortex. In this thin envelope of grey matter, about 2 mm thick, the six layers are numbered from I to VI, in Roman numerals, starting from the outside (the layer in contact with the meninges). Each layer is distinguished both by the type of neurons that it contains and by the connections that it makes with other areas of the brain.

Layer IV, for example, contains numerous stellate cells, small neurons with dendrites that radiate out around the cell body and receive connections from the lateral geniculate nucleus. Thus this layer specializes largely in receiving information.

Pyramidal cells are found in several layers of the visual cortex and are the only type of neurons that project axons outside it. Each pyramidal cell has one large dendrite, called the apical dendrite, that branches upward into the higher layers of the cortex, and other dendrites that emerge from the base of the cell. Of course, each pyramidal cell also has an axon, which may be very long to reach distant areas of the brain. Layers III, V, and VI contain large numbers of pyramidal cells and consequently serve as output pathways for the visual cortex.

Layer I contains very few neurons. It is composed of axons and dendrites from cells in the other layers.

With the development of improved staining methods, some of the six layers in the visual cortex have now been classified into sub-layers.

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