The Brain-Part2
Functional organisation of the brain.
Cerebral cortex.
Sensory systems.
Sensory coding.
Somatosensory cortex.
Vision system.
Visual pathways.
Primary visual cortex.
Higher order visual areas.
Functional organisation of the brain.
The Brain consists of several functional systems that are relatively autonomous.
There are distinct functional areas for each of the senses.
Touch.
Vision.
Hearing.
Taste.
Smell.
Different types of movement, also have different functional areas.
The primary motor cortex, and other higher order motor areas,
control and co-ordinate movements in the body.
The functional systems of the brain act, in a collaborative manner.
There are many association areas, in the brain.
All the sensory systems, and the motor system, are interrelated,
in higher order areas, of the brain.
Neural pathways in the brain, specially in the cortex,
are specifically designed to interconnect, the related functional areas.
In the sensory system, the sensory neurons connect,
directly or indirectly, to regions in the spinal cord, brain stem, and thalamus.
The thalamus connects to the primary sensory areas, of the cortex.
For example:
The primary visual cortex.
The auditory cortex.
The somatosensory cortex, etc.
The primary sensory cortices, in the cerebral cortex,
connect to other functional areas.
The brain has specialised functional areas, for most of its functions.
But the brain, essentially works, as a whole.
The inter relationships are complex, which makes a holistic understanding,
of brain functions, a challenging task.
Information is transmitted, from the sensory organs to the brain,
along functional pathways.
Information travels along the functional pathway, through synaptic relays.
Information may be amplified or attenuated, at each synaptic relay.
A single neuron can receive signals, from thousands of pre-synaptic neurons.
The summation of all these inputs, determines the output of the neuron.
In this way information gets pre-processed, before it reaches its final destination.
Typically, each successive neuron, in a functional pathway,
transmits processed, and more complex information, to the next neuron.
This provides one dimension, to the working of the brain.
This dimension is hierarchical.
Neurons can also be laterally connected, to other neurons,
specially in higher regions of the brain.
This provides a lateral dimension to information processing.
To have a complete understanding, of the brain, we need to appreciate,
the networked nature of the brain.
The functional pathways, of the sensory system, is well organised.
Information from the sensory organs, like the eye, ear, and skin,
are topographically organised through successive stages of processing.
For example, neighbouring groups of cells, in the retina,
connect to neighbouring groups of neurons, in nuclei, of the thalamus.
These neurons in turn, connect to neighbouring neurons of the visual cortex.
Each relay in the functional pathway, form an orderly neural map,
of information from the receptors surface.
These neural maps reflect, the spatial map of the receptors, like in the skin.
They can also reflect the sensitivity of the area.
For example, the fovea has the highest density of photoreceptors in the eye.
Areas corresponding to the fovea, have a similar density of neurons,
in the visual cortex.
Body movements are controlled from the primary motor cortex.
The primary motor cortex, maps the body.
The density of neurons, in this map, is proportional to the fineness of control,
required for that part of the body.
For example, our fingers are represented over a disproportionately larger area,
in the motor cortex.
This is the reason why we can exercise, fine control with our fingers.
The skin and areas in the body are disproportionately represented,
in the primary sensory cortex.
The face and fingers, for example, have a much larger representation,
in the somatosensory cortex.
Information is processed hierarchically, in the sensory and motor systems.
In the cortex lower order information processing, takes place, in the primary cortex area.
Secondary and tertiary cortex areas, are involved in higher order processing.
Information for the primary sensory cortex areas, originate in the sensory organs.
The information is mediated, in the synaptic relays.
Most of this information is routed, via the thalamus.
The thalamus sends the information to the primary sensory cortex areas.
For example, the primary visual cortex, and the primary auditory cortex,
and the somatosensory system.
The primary visual cortex is located in the occipital lobe.
The primary auditory cortex is located in the temporal lobe.
The primary somatosensory cortex, is located in the parietal lobe.
The secondary and tertiary cortices process, higher order information.
The secondary and tertiary visual cortex process, more abstract,
higher order visual information.
Lower order regions process information, like orientation.
Higher order regions process information, like form and figure.
The primary sensory areas, conveys information to adjacent higher order areas.
At very advanced stages of visual information processing,
neurons are responsive to complex information, like the shape of a face.
Similar hierarchies exists for other sensory systems.
In sensory systems, the primary cortex, is the first site of information processing.
The primary motor cortex, mediates voluntary movements, of our body.
Neurons in the primary motor cortex, innervate neurons, which activates muscles,
resulting in body movement.
In this case the primary motor cortex, is the final site of processing.
Higher order thinking, of whether to move, what to move, how to move, etc.,
are carried out in secondary, tertiary motor cortex areas.
Higher order sensory areas, send information to other,
major multimodal association areas.
Multimodal association areas integrate information, from two or more sensory regions.
This integrated information is coordinated with plans of action.
Complex sensory information is also sent to higher order motor regions.
These regions compute movement strategies, which are conveyed,
to the primary motor cortex for implementation.
The brain is bilateral.
It has two cerebral hemispheres.
Most pathways in the central nervous system, are also bilateral.
These pathways cross over to the opposite side of the brain.
The opposite side is called contralateral side of the brain.
As a result sensory and motor activities, of one side of the brain,
are mediated by the cerebral hemisphere, on the opposite side.
Movements of the right hand, are controlled by the left primary motor cortex.
Pressure on the left foot, is sensed by the right somatosensory cortex.
Cerebral Cortex.
The highest level of information processing, happens at the cerebral cortex.
The four lobes of the brain, houses the outer layer of the cerebral cortex.
The lateral sulcus separates the temporal lobe, from the frontal and parietal lobe.
The central sulcus separates the frontal and parietal lobes.
The cerebral cortex, has a highly convoluted shape.
This facilitates packing a large number of neurons.
The cerebral cortex is about 2 to 4 millimetres thick.
Human beings have a dramatically large surface area, of cerebral cortex,
compared to other animals.
This provides us with a larger number of neurons, to enable the thinking process,
that we are capable of.
All the higher order functions, are located in specialised areas,
in the cerebral cortex.
Sensory systems.
Overview.
A Human being’s awareness of the environment, is dependent,
on the information that is received, from the sensory systems.
The basic sensory systems, originate from :
The skin.
The muscles.
The eyes.
The ears.
The nose.
The tongue.
Sensory organs.
The sensory system, is a part of the nervous system.
The sensory organs have specific receptors to stimuli.
For example, the skin has receptors to sense,
heat, cold, pain, touch and pressure.
The receptors are very specialised.
Each type of receptor, senses one type of stimuli.
The CNS and the brain, receives stimuli from the external or internal environment.
Neural pathways conduct information, from the receptors to the brain.
These pathways are typically nerve fibres.
Nerve fibres extend from all parts of the body, and specially from the sensory organs.
The nerve fibres that carry the signals, from the receptors, are afferent nerve fibres.
Signals from these receptors, finally reach the brain.
Different parts of the brain, specialise in processing different types of stimuli.
For example, the special areas in the occipital lobe, in the brain,
processes visual information.
Information processed by a sensory system, is called sensory information.
Sensory information, may or may not, result in conscious awareness of the stimuli.
If the information reaches consciousness, it may be called as a sensation.
The understanding of the sensations meaning is perception.
For example, pain is a sensation.
Awareness that our toe, hurts is a perception.
Perceptions are formed by neural processing, of sensory information.
We can take an analogy, by comparing the ear and the telephone.
The telephone converts sound waves to electrical signals.
The signals are transmitted say by a wire.
To this extent the analogy with the telephone works.
But it ends here.
The receiver telephone, converts electrical signals to sound waves.
The brain does no such thing.
The brain just perceives the coded information from the ear, as a sound.
How exactly it does that, is not clear.
Neural pathways.
The starting point of a sensory signal, is always a sensory receptor.
A single afferent neuron, with all its receptor endings, is a sensory unit.
In general, the peripheral end of an afferent neuron,
divides into many fine branches.
Each branch terminates at a receptor.
All the receptors of a sensory unit, are typically sensitive,
to the same type of stimuli.
For example, they might all be sensitive to pressure.
The central process, of the afferent neurons,
terminate in several inter neurons.
More than one afferent neurons, can terminate in one neuron.
The neural pathways which go to the brain,
are made up of parallel chains, of inter neurons.
Typically each pathway conveys only one type of sensory information.
For example, one pathway might be only for mechanoreceptors.
Another pathway might be only for thermo receptors.
Most of these pathways pass through to the brain stem, and thalamus.
The final neurons, in the pathway, go from here,
to different areas of the cerebral cortex.
Different areas in the cerebral cortex, specialise in different functions.
Somatosensory cortex.
Somatic receptors are present in all parts of the skin, in our body.
The skeletal muscles, tendons and joints, also have somatic receptors.
All the pathways from the somatic receptors, finally terminate,
in the somatosensory cortex.
The somatosensory cortex is in the parietal lobe.
It is situated just behind the parietal lobe’s junction with the frontal lobe.
Visual cortex.
Visual is a sense of light.
They originate from the eyes.
Specific pathways from the eyes reach the visual cortex.
Auditory cortex.
Auditory is a sense of sound.
They originate from the ears.
Specific pathways from the ears reach the auditory cortex.
Taste buds.
Taste buds are sensors for taste.
They are mainly located in the tongue.
Specific pathways from the taste buds, pass to the cortical areas,
adjacent to the face region of the somatosensory cortex.
Olfaction.
Olfaction is the sense of smell.
They originate from the nose.
There is no known area of the cortex, to represent olfaction.
We have to keep in mind, that a specific cortex area,
in the brain can connect to other association areas in the brain.
Sensory coding.
The sensory pathways, convey information in more than one way.
The main ways, that sensory information is coded are :
Stimulus type.
Stimulus intensity.
Stimulus location.
Stimulus type.
Stimulus type is indicated not only by which receptors are stimulated,
but also by the specificity of the pathway to the brain,
and by the brain region.
For example, if the neural pathways, from the eyes are stimulated,
the brain perceives light, even if the eyes are closed.
Stimulus intensity.
A stimulus generates an action potential.
The action potential travels along a nerve fiber.
The amplitude of the action potential is always constant.
The frequency of the stimulus, varies with the intensity of the stimulus.
Increased stimulus strength, results in higher frequency of action potential firing.
When a stimulus strength increases, more receptors, also begin to fire.
The action potential generated by several receptors, propagate along the branches,
to the main afferent nerve fiber, and increases the frequency of the action potential.
Stimulus location.
The brain not only needs to know, the stimulus type, and stimulus intensity,
it also needs to know the stimulus location.
There are specific inter neurons, for specific locations in the body.
This results in specific pathways, for different locations.
If the stimulus comes from a specific pathway, the brain is able to identify,
the specific location, from where it came from.
The brain has a map for different locations of the body.
The number of receptors, in a given area, is called receptor density.
Receptor density differs in different parts of the body.
For example, the receptor density in the thumb, fingers and lips are very high.
When a stimulus is received, over a larger area, more receptors,
activate more neurons.
A group of neurons, are associated with a specific location.
A specific nerve fibre, and neural pathway,
If we hold a glass of water, signals will come from a lot of receptors,
resulting in firing of multiple neurons.
Stimulus control.
All information coming into the nervous system, is subject to extensive control.
This control is exercised at synaptic junctions, before it reaches higher levels,
of the central nervous system.
In general, the CNS has an inhibitory system, which will attenuate the incoming signal.
Inhibition helps to suppress unwanted information.
In most afferent system, the stronger inputs are enhanced,
and the weaker inputs of adjacent sensory units are simultaneously inhibited.
This dual method helps to locate a stimulus more accurately .
Somatic sensation.
Somatic sensation is the sensory function of the skin and body walls.
Skin receptors give rise to sensations of touch, pressure, heat, cold, and pain.
It also gives us the awareness, of the body position, and movement.
Each sensation is associated, with a specific receptor type.
There are distinct receptors for heat, cold, touch, pressure, joint position, and pain.
The sensory pathways from somatic receptors, go to the cerebral hemispheres.
The pathways from the left side of the body, go to the right cerebral hemisphere.
The pathways from the right side of the body, go to the left cerebral hemisphere.
The left brain feels and controls, the right side of the body.
The right brain feels and controls, the left side of the body.
Somatosensory cortex.
The somatic sensation, goes to a specific part of the brain cortex,
called the somatosensory cortex.
This is located in the border of the parietal lobe, just behind the frontal lobe.
Within the somatosensory cortex, different parts of the body, are mapped.
There is a complete map of the body, in the somatosensory cortex.
Areas of the body which are more sensitive, have a larger representation.
For example, the fingers and the face, have a much larger representation.
Vision system.
The eyes are the sensory organs, which provides us with vision.
The retina in the eye, transduces light signals, to electrical signals.
These signals travel along multiple pathways, to different visual processing regions,
in the brain.
The primary visual pathway, starts from the retina,
and reaches the lateral geniculate nucleus, in the thalamus.
From here the signals travel to the primary visual cortex.
Different classes of information travel along this pathway.
Luminance, spectral differences, orientation, and motion,
are the most primary types of information, that travel to the brain.
From here it travels, to the primary visual cortex,
and then on to other higher order, and related regions of the brain.
Interestingly the vision system, does not see and record the image, like a camera.
The primary purpose of vision, seems to be to “understand” the visual signals,
that comes from the eye.
It is easier to understand the visual system, if we can appreciate vision,
from the evolutionary perspective.
The eye and the brain co-evolved.
This evolution happened over hundreds of millions of years.
As the eye developed, as a more sophisticated sensory organ,
the brain co-evolved, as a more sophisticated interpretation mechanism.
The primitive eye and brain, could see only a vague form.
If the form was like a predator, it meant danger.
If the form was like a fruit, it meant food.
Vision had a direct impact, to the art of living.
Over hundreds of millions of years, the vision system became more sophisticated,
in animals and human beings.
But we still retain, the basic principle of the brain, trying to understand the image.
The eye decomposes the image, into several types of signals.
The brain interprets the signals that it receives.
We could even say that the brain is always trying to understand the image,
rather than mere sightseeing.
The parallel and hierarchical interpretation of visual signals, integration of the signals,
and derivation of meaning, takes place in specialised regions, of the brain.
We share with most animals, the design of our visual system.
There is a great degree of commonality between how a dog sees, and how we see.
The most primitive signals, of the decomposed image, comes from the retina area,
to the primary visual cortex.
Some mediation of the signals takes place, in the visual pathways.
The primary visual cortex, supplies information to other visual areas,
located in the occipital lobe, the parietal lobe, and the temporal lobe.
Visual areas in the temporal lobe or primarily involved in object recognition.
Visual areas in the parietal lobe, are concerned with motion.
Normal vision depends on the integration of information,
from all the relevant cortical areas.
All the processes involved in visual perception, is still being researched.
Visual pathways.
Ganglion cells, receive signals from the retina.
The axons from the ganglion cells, bundle together to form the optic nerve.
The optic nerve exits the eye, via the optic disc.
The optic nerves, from both the eyes, travel and meet at the optic chiasm.
The optic fibres, from the left and right eye, partly cross over, at the optic chiasm.
After crossing the chiasm, it is called as the optic tract.
Each optic tract contains fibres, from both the eyes.
The major destination, for these fibres is the thalamus.
The lateral geniculate nucleus, in the thalamus, is the receiving station,
for these fibres.
The thalamus is the routing station, in the brain.
Fibres from the lateral geniculate nucleus, are called as optic radiation.
These travel to the primary visual cortex, called in short as V1.
It is also called as the striate cortex.
Some fibres, directly connect to the brain stem.
Some of these pathways are related to reflex motor action,
like the pupillary light reflex.
The pupils of the eye, contract and dilate, corresponding to the amount of light,
coming in to the eye.
Some of these pathways are related to motor control of eye movement,
and head movement.
Some fibres, connect to the hypothalamus.
These are involved, in the day night circadian rhythms.
The retina of the eye, has a spatial map of the visual field.
The ganglion cells map these visual fields.
The nerve fibres, from the retina, represent an orderly map of the visual field,
to the target organs, like the thalamus.
Nuclei in the lateral geniculate nucleus, correspond to related ganglion cells,
in the retina.
The brain forms a spatial map, based on every particular nerve fibre,
generating a signal.
We see with two eyes.
We refer to this as binocular vision.
The left half of the visual field, is represented in the right hemisphere,
of the primary visual cortex.
The right half of the visual field, is represented in the left hemisphere,
of the brain.
Both the eyes can see most of the left and right visual fields.
Information from both eyes, are integrated in the brain.
The brain uses, the information from the left and right eye,
to deduce the distance of the object.
This gives us a depth of vision.
The fovea in the centre of the eye, has the highest clarity of vision.
We refer to this as high visual acuity.
The macula region, also has better acuity.
The density of receptors, in the macula and fovea is very high,
compared to other regions.
Correspondingly the neurons and axons, emerging from this region,
have a higher density.
The high acuity areas, the macula and the fovea, are represented,
by a disproportionately large area, in the primary visual cortex.
This means the brain does a lot more of processing, from information,
coming from these areas.
The visual pathway, starts at the retina, travels through the optic chiasm,
and reaches the lateral geniculate nucleus, in the thalamus.
From the thalamus nerve fibres radiate, to the primary visual cortex.
Primary visual cortex.
The visual cortex, is located in the occipital lobe.
The primary visual cortex, is a part of the visual cortex.
Primary visual cortex, is the main receiving site of visual information.
Higher level processing of visual information, starts here.
The primary visual cortex, is called as V1, in short.
Adjacent areas, to the primary visual cortex, process higher level visual information.
These higher level areas, are named as V2, V3, V4, V5 etc.
A certain amount of pre-processing of information happens,
along the visual pathway.
In the initial stages the light signals, correspond to contrast in light.
For example, it uses a centre surround signal, as a coding mechanism.
The image falling on the retina, is decomposed, into some basic elements.
In the visual cortex, basic signals, are processed, to derive higher level information.
The brain seems to look for properties of the image, and more importantly,
the meanings associated with it.
To derive the full meaning of the image, it uses association areas,
in other lobes of the brain.
The temporal lobe, and the parietal lobe, have the major association areas,
which receive visual information, from the visual cortex.
Contrast in light signal, can be used to detect edges.
Edges are elements of a contour.
Contour may be associated with recognition and meaning.
For example, the contour of a cat, means a lot to a mouse.
Neurons in the primary visual cortex, are sensitive to edges,
and orientation of the edges.
Specific groups of neurons are sensitive to specific orientation.
Some other neurons, are sensitive to movement of the edge.
These types of neurons, help to track a moving object.
Nuclei in the brain stem, controls certain motor movements.
Movement of the eye, head and neck help in tracking moving objects,
a predator would be very interested, in a moving prey.
Some neurons are sensitive to, coarseness or fineness.
Some neurons are sensitive to, rate of change of contrast.
These visual primitives are processed first, in the primary visual cortex.
There are further processed in higher level visual areas like V2, V3 etc.
The visual primitives, are recomposed into sets of meaningful properties,
in the visual cortex.
The visual cortex is part of the neocortex.
Like other areas in the neocortex, it has six layers.
Fibres from the lateral geniculate nucleus, terminate in an orderly fashion,
in the primary visual cortex.
A column of the neurons, typically represent a point in the visual field.
Adjacent columns progressively represent adjacent points in the visual field.
Each point in the visual field corresponds to, several columns of neurons,
in the visual cortex.
The primary visual cortex, processes information from both the eyes.
In the lateral geniculate nucleus neurons correspond to signals from one eye.
Signals from both the eyes, terminate in different layers,
in the primary visual cortex.
Integration of this information, takes place in the primary visual cortex.
Most of the cortical neurons, except those in layer 4,
are organised as eye specific columns.
Bringing together signals from both the eyes, or binocular vision,
provides a sensation of depth.
This is called as stereopsis.
The two eyes look at the same object, in slightly different angles.
The central point of the visual field, is called the fixation point.
Objects which lie, in front or back, of the fixation point,
falls on non corresponding areas, in the retina.
The visual cortex, is able to perceive this difference, to give us a sensation of depth.
This is how, we perceive, whether a object, is nearer or further.
Neurons in the lateral geniculate nucleus, also corresponds,
to two major types of neurons in the retina.
The M ganglion cells, in the retina, corresponds to the magnocellular layer.
The P ganglion cells, in the retina, corresponds to the parvocellular layer.
M ganglion cells, have larger receptive fields than, P cells.
Axons of M cells, have faster conduction velocities.
M cells respond, in a transient manner, to visual stimuli.
P cells respond, in a sustained fashion.
P cells can transmit colour information, where as M cells, cannot.
P cells convey colour information, because they are driven by cones,
which correspond to short, medium or long wavelength.
Information conveyed in the parvocellular pathway,
is important in high spatial resolution.
This helps in detailed analysis, of shape, size, and colour of an object.
The magnocellular pathway, are important for tasks,
that require high temporal resolution.
This helps in evaluation of location, speed, and direction, of a moving object.
Higher order visual areas.
Many regions in the brain, are involved in higher level visual processing.
These regions are located, in the occipital, parietal and temporal lobe.
Each of these areas contain a map of the visual space.
The primary visual cortex, supplies the information, to the higher order regions.
These higher order regions, are given names like, V1, V2, V3, V4, MT, IT etc.
Some regions seem to specialise, in a particular aspect of the visual scene.
For example, neurons in region V4, respond more to colour.
Neurons in the middle temporal area, or MT area, respond more to moving edges.
Higher order processing, of visual information, outside the occipital lobe,
takes place in the temporal lobe, and the parietal lobe.
These regions are association cortices.
Association cortices, associate information, from different cortical areas.
For example, an image of an object, can be related to the name of the object.
One system, called the ventral stream, starts from area V1, passes via area V4,
and leads to the inferotemporal cortex, in the temporal lobe.
This system is involved in high resolution vision, and object recognition.
For example, the image of a cat, can be associated with a name “cat”.
Neurons in this region, is sensitive to shape, colour, and texture.
At the highest level, it can be sensitive to a face.
Once a face is recognised, the image of a person’s face,
can be associated, with the name of the person.
Another system, called the dorsal stream, starts from visual area V1,
passes through the middle temporal cortex, or the MT cortex,
and leads to the parietal lobe.
This system is responsive to spacial aspects of vision.
It recognises movement of objects.
This region is sensitive to direction and speed of movement.
If we are playing football, it would be involved in tracking the ball,
and movement of the other players.
As information moves along the visual pathway,
information tends to converge, and become more abstract.
Highest level of processing, typically involves associating information,
from different cortical areas.
Information of sight, sound, smell, memory, emotion etc., can all be associated,
at the highest level of processing, in the brain.
Language helps to anchor, these associations.
When we think of a lion, we can imagine how it looks, how it roars,
how it hunts, and all the events, when we had a chance to see it live, or on TV.
The brain decomposes the visual image we see, and recomposes them,
in a way that it can make sense, and use the information.
In this perspective, vision acts as one of the inputs, to cognition and intelligence.