Sensory system-Part1
Sensory system.
Eyes.
Eye anatomy.
Black and white vision.
Colour vision.
Neural pathways.
Visual cortex.
Ears.
Sound.
Ear anatomy.
Functioning of the hearing system.
Vestibular system.
Neural pathways.
Auditory cortex.
Listening.
Sensory system.
We perceive the world, through our senses.
If we did not have any senses, our mind will be blank.
We will be somewhat like a state of coma.
All of our life’s experiences are the result of sensory experiences.
The human body has special organs, to perceive specialised senses.
Our nose, perceives smell.
Our eyes, perceives vision.
Our ears, perceives sound.
Our tongue, perceives taste.
Our skin, perceives pressure, temperature, pain etc.
These five organs are the traditional sense organs of the body.
We will discuss these organs, in the first part of this module.
To regulate metabolic processes in the body,
the brain has to sense other parameters, like body temperature,
blood pressure, oxygen levels etc.,
We will also discuss these functionalities, in the second part of this module.
Eyes.
The eyes give us a sense of vision.
The eye is the sensory organ, which enables the sense of vision.
The eye senses light signals.
Ambient light.
The image that we see of objects, comes from the light, reflected from the objects.
Without ambient light, we cannot see objects, because there is no light,
reflecting from it.
This is the reason, that we cannot see objects in the dark.
If the object itself is a source of light, we can see it in the dark.
For example, we can see distant street lights, and stars in the night.
This is because, these objects are themselves sources of light.
The sun is the main source of light, in the mornings.
At night, we can use artificial lights, as a source of light.
Visible light.
Visible light is a electromagnetic radiation.
Light comprises of waves, of different wave lengths.
Visible light comprises of a small segment,
of the entire spectrum of electromagnetic radiation.
The wave length of visible light, ranges from,
400 nano meters, to 700 nano meters.
The human eye can only detect, light in this range of wave lengths.
The intensity of the light can vary.
Very bright objects, have a higher intensity of light.
The sun for example, provides high intensity bright light.
The moon provides, low intensity soft light.
Typically objects that we see, are comprised of colours.
Colours are a subset, of visible light.
Different colours, corresponds to different wave lengths of light.
Light with a wave length, in the range of 635 to 700 nano meters,
appears as a red colour.
Light with a wave length, in the range of 400 to 450 nano meters,
appears as a violet colour.
The eye can sense objects with different intensities and colours.
The eye, typically produces, a composite image, of the object, we see.
For many practical applications, light can be perceived,
as light rays, travelling in a straight line.
When we see an object, we can think of it as light rays,
emanating from the object, and entering the eye.
Distant object are perceived by the eye, as smaller objects.
When the objects are closer, they are perceived closer to their actual size.
When the objects are further away, they are perceived smaller than their actual size.
We perceive the objects, with both our eyes.
This helps us perceive the environment, with depth of perception.
Eye anatomy.
The eye ball is roughly shaped like a sphere.
Light enters the eye, through the front of the eye ball.
The image of the object we see,
is projected to the back of the eye ball.
The cornea is located in the front of the eye.
Light enters the eye, via the cornea.
The light rays then pass, through a lens.
The lens helps to focus the incoming light rays.
The rays then reach the retina.
The retina is situated, at the back of the eye ball.
Most of the rear surface of the eye ball, forms the retina of the eye.
The image of the object that we see, is reproduced in the retina.
The retina is composed of photoreceptors cells.
These photoreceptor cells, are sensitive to light.
They convert light signals, into nerve impulses, or action pulses.
Nerve endings are situated near these photoreceptors.
They pickup the impulses, and transmit them, via nerve fibres, to the brain.
Nerve fibres from different parts of the retina, converge into the optical nerve.
The optical nerve transmits all these signals, to the brain.
Some key components of the eye are :
Cornea.
Lens.
Ciliary muscles.
Iris.
Pupil.
Aqueous humor.
Vitreous humor.
Retina.
Fovea.
Cornea.
The lens and cornea of the eye are the optical systems,
that focus an image upon the retina.
Light rays from the environment, enter the eye, through the cornea.
When light rays travel through a new medium, the light rays refract.
When light rays, from the air, enters the cornea, the rays bend due to refraction.
The cornea plays a role, in focusing light rays on the retina.
The surface of the cornea, is curved.
Light coming from a single point source, hit the cornea at different angles.
This causes the light to bend in different angles.
This helps to direct the light rays to a point in the retina.
Lens.
The lens of the eye, helps to selectively focus images, on the retina.
The lens is a transparent and flexible component, located behind the cornea.
Light rays from objects close to the eye, must be bent more,
by the lens, in order to converge on the retina.
The shape of the lens, in the eye, has to be changed,
to focus on objects in different distances.
Ciliary muscles.
The shape of the lens, is controlled by muscles, called as ciliary muscles.
The lens is normally more spherical in shape.
When the muscles pull it, it becomes more flattened, and oval in shape.
The ciliary muscles are controlled by the brain.
The para sympathetic nervous system, controls the ciliary muscles.
This function is autonomic.
This means that we do not consciously control the focusing mechanism,
of the lens in the eye.
The brain, controls the function of focusing, via the parasympathetic system.
Far and near sight.
Under normal conditions, the image of the object falls on the retina.
The Cornea, lens shape, and eye ball length,
determine where the light rays converge, in the retina.
In some cases, the eye ball is slightly longer, or shorter than normal.
If a person is near sighted, or myopic, he will not be able to see,
distant objects clearly.
The images from far objects, in this case falls in front of the retina.
If a person is far sighted, his vision of near objects will be poor.
In this case, the images of near objects, are focused behind the retina.
Corrective lenses, commonly called as glasses, can be used to correct,
defective vision.
Iris.
The amount of light entering the eye, is controlled by a ring like pigmented muscle,
known as the iris.
The iris of the eye is located behind the cornea and in front of the lens.
The iris is circular in shape and is opaque.
The pigmentation in the iris, causes it to be opaque.
The pigmentation also gives the eye a colour.
Black colour, blue colour, brown colour, eyes are due to the pigmentation, of the iris.
The iris partly covers the lens, leaving a circular opening in the middle, called the pupil.
Pupil.
The pupil can contract, or dilate, which will increase or decrease the size of the opening.
When the opening is more, more light is allowed to come in to the eye.
When the opening is small, the amount of light entering the eye is restricted.
In conditions of bright light, the pupil contracts, and allows less light, to come in.
When the ambient light is dim, the pupil dilates, and lets more light, to come in.
The pupil is also controlled by the brain, and the parasympathetic nervous system.
Aqueous humor.
The region between the cornea, and the Iris and pupil,
is filled with a transparent fluid.
This is called as Aqueous humor.
The Aqueous humor contains nutrients for the eye.
Since it is transparent, light rays pass right through it.
Vitreous humor.
The region behind the lens, and before the retina,
is also filled with another transparent fluid.
It is called as Vitreous humor.
About 80% of the eye comprises of Vitreous humor.
It maintains the shape of the eye.
The vitreous humor is transparent, and allows the light rays,
to pass right through to the retina.
Retina.
Light enters the eye, through the cornea, passes through the lens,
and falls on the retina.
The retina has photosensitive cells, which can detect light.
These cells are called photoreceptor cells.
The retina has two types of photoreceptor cells.
They are called:
Rods.
Cones.
In dim light, rods are used for sensing black and white images.
In bright light, cones are used to provide colour vision.
The retina is packed with rods and cones.
The rods and cones, acts as transducers.
Transducers translate one type of signal, to another type of signal.
Rods and cones, are transducers, which translate,
optical signals to electrical signals.
Field of vision.
The area that we can normally see, is called the field of vision.
This field of vision has a left side and a right side.
It also has a top side, and a bottom side.
In this manner, we can conceptualise the field of vision to have four segments.
Top, left and right segments.
Bottom, left and right segments.
The lens of the eye, acts just like the glass lens, that we are used to.
The image that we see is inverted, in the retina.
The image is inverted both in the vertical plane, and laterally.
The inverted image falls on the retina.
The eye gives a lot of importance, to the centre of the visual field.
This area is called as the central visual field.
It is the area which we can see most clearly.
We say that our visual acuity, is best in this area.
The outer areas, in the visual field, are called peripheral areas.
The vision in this area is called as peripheral vision.
The acuity of peripheral vision, is not as good as the acuity of central vision.
Fovea.
The fovea is a central portion, of the retina.
The area of the central vision, falls on the fovea.
The fovea is a special region in the retina.
Rods and cones, are much more closely packed, in the fovea.
This means that much finer detail, of the image, can be discerned by the fovea.
The fovea has much more cones than rods.
Colour vision is best, in the fovea.
Correspondingly, the area of central vision, has also the best colour vision.
The peripheral vision areas have more rods than cones.
Colour vision is not as good in the peripheral vision, as it is in the central vision.
Black and white vision.
The eye is able to identify different colours, by identifying their wave length.
The retina is packed with photosensitive cells.
These cells are called rods and cones.
The rods are sensitive to light, but not sensitive to colour.
Rods are used by the eye, to see in black and white, in dim light.
In very dim light, the eye cannot perceive colour.
The rods help to perceive light, in shades of grey, from white to black.
Using rods, we can see images in black and white.
This is the vision that we use, in dark or dim lighting conditions.
From the evolutionary perspective,
we can say that nature first designed black and white vision.
It later improved upon it, to provide colour vision.
Many animals even now, have only black and white vision.
Colour vision.
Light comprises of waves, of different wave lengths.
The wave length of visible light, ranges from,
400 nano meters, to 700 nano meters.
Each of the spectral colours, corresponds to a specific wave length,
within this range.
For example, the wave length about 530 nano meters, corresponds to green colour.
Similarly, all other spectral colours, have a corresponding wave length.
An object appears to be green, because it reflects the green wave length,
and absorbs wave lengths corresponding to other colours.
Light perceived as white, is a mixture of all wave lengths.
Black is the absence of all wave lengths of light.
A black colour substance, absorbs all wave lengths, of light.
The actual photoreceptors, are molecules called photopigments.
Photopigments absorb light.
The eye uses 4 different photopigments.
The rods use one photopigment called rhodopsin.
Photopigments are proteins, and are collectively known as opsin.
The actual light sensitive part of the photopigment is called as a chromophore.
It is in the retina, and is a slight variant of vitamin A.
The chromophore is the same in each of the four photopigments.
The opsin surrounds and binds the chromophore molecule.
The opsin filters the light reaching the chromophore.
The opsins differs for each type of photopigment.
The different opsins cause each of the photopigment, to absorb light,
most effectively, at a different part of the visible spectrum.
For example, one photopigment absorbs wave length in the range of red light, best.
Another photopigment absorbs green light, best.
The three types of cones, present in the retina,
are commonly called as red, green and blue cones.
Each type of cone, is excited most effectively by light,
of a particular wave length.
It also responds partially to different wave lengths.
For any given wave length, the red, blue and green cone types,
are excited to different degrees.
For example, in response to light in the 530 nano meter wave length,
the green cones respond maximally, the red cones less,
and the blue cones not at all.
Our sensation of colour, depends on the relative signals,
from the three types of cone cells.
Signals from these cells are processed, by higher order cells in the brain.
Humans are unique from the perspective, of colour vision.
Many animals can see only in black and white.
Others can see only limited colours.
Human beings with the aid of red, blue and green cones,
can perceive hundreds of thousands of colour shades.
This is made possible, by interpreting various combination of the signals,
coming from the three cones.
Neural pathways.
Upon exposure to light, a photon of light is absorbed,
and a light signal be generated.
The generated signal does not go directly to the brain.
Some information processing takes place in the retina itself.
These signals reach the ganglion of nerve fibres leading to the optic nerve.
Ganglion cells respond to the activation, by producing action potentials.
The axons of the ganglion cells, form the output from the retina, in the optic nerve.
Optic nerve.
In the eye, the optic nerve is located, behind the eye ball.
The optic nerve, is the cranial nerve 2, which leads into the brain.
The two optic nerves, from each eye, meet at the base of the brain,
to form the optic chiasm.
At this point some of the fibres cross to the opposite side of the brain.
The crossover is partial.
This partial crossover, provides both cerebral hemispheres,
with inputs from both the eyes.
Optic nerve fibres project to several structures in the brain.
The largest number goes to the lateral geniculate nucleus, in the thalamus.
The nucleus transmits the information to the visual cortex.
Visual cortex.
Visual cortex is located in the occipital lobe.
It is the primary visual area of the cerebral cortex.
The visual cortex has several subdivisions.
Each subdivision represents a complete visual field.
The relay of information from the retina to the various areas of the visual cortex,
follows a precise point to point projection pattern.
This means that adjacent retinal regions, project to adjacent regions,
in each subdivision of the visual cortex.
The actual perception of the image, involves a fascinatingly complex process,
in which the image is constructed by high level brain processing,
of different properties, of the image.
It also involves association, with previous memory and learning.
Visual information.
The brain uses the visual information from the retina,
to perform other useful functions.
Some information is passed to the brainstem, and cerebellum.
This information is used in the coordination of eye and head movements,
fixation of gaze, and constriction of the pupils.
Some information from the retina, goes to other parts of the brain.
This information plays a major role, in a variety of functions,
such as biological clocks, locomotion, sleep or wakefulness, etc.
Perception of vision.
The brain has an intricate and sophisticated system to perceive vision.
Visual images have different characteristics such as,
colour, form, depth, movement and texture.
Each characteristics is processed by a separate channel in the visual system.
This segregated parallel processing of information, begins in the retina.
The process continues in the brain at the highest level of visual processing.
All this processing is done in parallel.
The final image that we see, instantaneously is a composite image.
It is natural to compare, the eye with a camera.
To some extent this analogy, is meaningful.
In the eye, the light signals are photons, or light rays.
The image that we see passes through the cornea, lens and falls on the retina.
Up to now the optical principle of light holds good.
Typically, the image that we see, is reproduced in the retina.
The image is inverted both vertically and laterally.
The image on the retina, still reflects the image that we see.
The cells in the retina convert, the light signals to electrical signals.
At this point, the analogy to the camera, should be abandoned.
It is very important to realise that the brain, does not “capture” an image,
like a camera.
It is equally important to know that, the brain does not “see” an image.
The brain only perceives an image.
The inputs to this perception, is electrical signals.
The brain interprets these signals, in a fascinatingly complex way.
The eye independently interprets form, colour, depth, texture, and movement.
Different areas in the brain, process properties of the image.
A composite is then created, of these properties.
It is important to emphasise that the composite, is not like pieces,
of a jigsaw puzzle, put together.
It is like the properties of the image, are independently given,
and a higher functionality of the brain, puts them together,
to form a perception of the image.
To understand what it has perceived, it associates similar properties,
stored in the memory.
This process of visual image processing is complex and fascinating.
It is possible that it is related to the way, functionality of vision,
developed in different forms of life.
Human beings have a highly evolved, sense of vision.
We are able to perceive a rich image, which is a composite of all the properties.
The lateral geniculate nucleus, in the thalamus, relays different kind of information,
from the eye to different cortical zones.
Different properties of the image are relayed to different specialised areas.
Some areas specialise in orientation.
Some areas are more responsive to movement of an object,
across the visual field.
Some areas respond best to colour.
Some areas respond to inputs, from both eyes, related to depth of vision.
The brain does further processing, to understand what it is perceiving.
For this it depends on previous memory and learning.
To identify objects in a visual field, the brain does, some “what” processing.
It is like saying, “what am i seeing” ?
There are pathways in the brain, to transmit information, for “what” processing.
These pathways travel from the visual cortex, to the association areas,
in the frontal lobe, via the temporal lobe.
To locate the objects in the visual field, the brain does some “where” processing.
The visual stimulus travels to the frontal lobe via the parietal lobe,
to do the “where” processing.
It is important to emphasise that all these processing,
is taking place simultaneously, or in parallel, in different parts of the cerebral cortex.
They are reintegrated to produce a conscious sensation of vision.
We must note that the signals, do not form a picture in the brain.
They actually form a specifically coded pattern of electrical signals.
One way to understand the gift of vision,
is to say that the eye “sees“ an image,
and the brain, in some way understands the image.
Ears.
The ears gives us a sense of hearing.
The ears sense sound signals.
The ears also gives us, a sense of balance.
Sound.
Sound propagates in the form of waves.
These waves are pressure waves.
A sound wave has the general properties of a wave.
It has a wave length.
It has a frequency.
It has an amplitude.
These properties determine the nature of the sound we hear.
Our ears can detect an amazing range of sounds.
The frequency of waves can be expressed in hertz.
We can hear sound frequencies,
ranging from 20 hertz to 20000 hertz.
Most of the sounds we hear, are in the range of 1000 hertz to 4000 hertz.
Ear is best tuned, to hear this range of frequencies.
Low frequencies have a bass sound.
This is like a drum, or the left side keys of a piano.
High frequencies have a high pitch, or a shrill sound.
This is like a whistle, or the right most keys of the piano.
Each key on a piano, represents a specific frequency,
corresponding to a specific sound, called a note.
The amplitude of the sound wave, represents the loudness of the sound.
A loud sound, like that from a siren, has a large amplitude.
A soft sound, like that of a whisper, has a small amplitude.
The loudness of the sound, is measured in decibels.
Usually, the limit for the loudness of noise, in a residential area, is about 55 dB.
The ear can detect the frequency and loudness of a sound.
The ear can simultaneously listen to multiple sounds.
The ear can detect and differentiate, the simultaneous sounds that we hear.
For example, when we listen to a symphony, the ear can detect,
the sounds from different instruments.
Ear anatomy.
The ear can be considered, as having 3 basic parts.
The outer ear.
The middle ear.
The inner ear.
The outer ear.
The outer part of the ear is called as the pinna.
Behind this pinna, is the opening called the ear canal.
The ear drum is located, at the end of the ear canal,
The ear drum is stretched across the ear canal.
The middle ear.
Behind the ear canal is the middle ear.
There are 3 small bones, in the middle ear, called the ossicles.
They are called the malleus or the hammer,
the incus or the anvil,
and the stapes or the stirrup.
Behind the middle ear is the inner ear.
The inner ear.
The inner ear has semi circular canals.
It also has the important cochlea.
Inside the cochlea, are 3 chambers.
They are called as the scala vestibuli, scala tympani,
and the cochlear duct.
The organ of corti, is located in the cochlear duct.
The sound sensitive hair cells or located in the organ of corti.
Cochlear nerve extends from the inner ear to the brain.
Functioning of the hearing system.
The outer ear.
Sound waves enter the outer ear, or pinna.
The outer ear helps to direct the sound waves, to the ear canal.
The pressure waves of sound, travel through the ear canal.
The ear drum is situated at the end of the ear canal.
It terminates the sound wave.
The pressure of the sound wave, results in vibration of the ear drum.
The ear drum vibrates slowly, in response to low frequency sounds.
It vibrates rapidly to high frequency sounds.
In this way it reflects, the frequency of the sound.
The middle ear.
The vibrations of the ear drum, are picked up by the ossicles.
These are the tiniest bones in the human body.
The first bone is the hammer.
It picks up the vibration from the ear drum.
The hammer strikes the anvil.
The anvil moves the stirrup.
During this process, the pressure on the ear drum,
is amplified 15 to 20 times.
The inner ear.
Cochlea.
The stirrup is connected to the cochlea, in the inner ear.
The cochlea is filled with fluid.
The cochlea is wound around in a snail like spiral structure.
The stirrup which connects to the the cochlea, transmits a piston like movement.
This causes waves in the fluid, in the cochlea.
These waves travel in the upper chamber, called the scala vestibuli.
The waves return, in the lower chamber, called the scala tympani.
The cochlear duct is situated between these chambers.
The fluid pressure waves are transmitted to the cochlear duct.
Organ of corti.
Inside the cochlear duct, is the organ of corti.
The organ of corti, is the important component, for hearing.
The organ of corti, is lined with hair like cells, called hair cells.
These hair cells are sensitive to the vibration received in the cochlear duct.
The organ of corti, is also wound in the same spiral structure, as the cochlea.
The entry point, is called as the basal end.
The top of the spiral, is called the apex, or the apical end.
If we stretch out the organ of corti, it will be about 3.5 cm.
The basal end, of the organ of corti, encodes the higher frequencies.
The apical end, encodes the lower frequencies.
The full range of frequencies, which is audible,
is encoded between the basal end, and the apical end.
The stretched out organ of corti, is analogous to a piano keyboard.
Each audible frequency from the highest, to the lowest, is encoded in this organ.
This is called as tonotopy .
This tonotopy, is sensed in the auditory cortex of the brain.
Auditory hair cells.
The organ of corti, is lined with hair cells.
These hair cells move, in response to the mechanical pressure wave movements,
in the cochlear duct.
The sound pressure waves that we hear,
is translated into fluid pressure waves, in the cochlea.
The hair cells, sense the mechanical movements and converts them,
into action potentials, or electrical signals.
These signals are transmitted to the auditory cortex,
in the brain, via the cochlear nerve.
A transducer translates one type of signal, to another type of signal.
The auditory hair cells, act like a transducer.
It translates a mechanical signal, to a electrical signal.
This is similar to rods and cones, which are transducers, in the eye.
In the eye, the optical signals, are transduced to electrical signals.
In the case of the hair cells, mechanical signals are transduced to electrical signals.
Vestibular system.
The vestibular system is located in the ear.
The ear performs another important function which is not so obvious.
It gives us a sense of orientation, and balance.
This function is performed by the vestibular system.
The vestibular system has 3 semi circular canals, in the inner ear.
The horizontal semi circular canal.
The superior semi circular canal.
The posterior semi circular canal.
The 3 semi circular canals, are situated adjacent to the cochlea.
Like the cochlea, they are also filled with fluid.
Each semi circular canal, is at an angle of 90 degrees to each other.
They are analogous, to the X, Y, and Z axis, of a 3 dimensional graph.
These axis are aligned to the centre of gravity, of our body.
When we tilt our body, the fluid in the semi circular canals,
move in different directions.
This happens in both the ears.
By sensing the relative motion, of the fluid, between the canals,
and between the ears, the brain can sense the relative orientation, of the body.
Horizontal semi circular canal.
When we rotate the head from left to right side, we create a lateral movement.
It is sometimes referred to as a “No” movement.
This movement is sensed by the horizontal semi circular canal.
Superior semi circular canal.
When we rotate our head, forward and backward, we create a pitch like movement,
it is also sometimes referred to as a “Yes” movement.
This movement is sensed by the superior semi circular canal.
Posterior semi circular canal.
When we move our head, to touch our shoulders, we create a rolling movement,
this movement is sensed by the posterior semi circular canal.
Balance.
Each of the 3 semi circular canals, is a motion sensor, in a particular direction.
Hair cells are located at the base of these canals.
These hair cells sense the motion, in these canals.
These motions are transduced into electrical signals, which are transmitted to the brain.
The brain receives these signals, from both the ears.
By combining and interpreting these signals, the brain is able to accurately,
interpret the position of the body and the head.
Our sense of balance, is provided by the vestibular system.
Balancing, is a reflex mechanism, which happens in the lower brain.
The brain is able to send motor signals to the right muscles,
to enable us to retain posture and balance.
Motion.
The vestibular system also senses motion, and acceleration.
Many motor functions, which control the muscles, involved in movement,
are dependent on the vestibular system, for feedback.
Even a seemingly very natural acts, like walking or dancing,
involves complex coordination, between the motor system and the sensory system.
It is interesting to note, that gravity plays a central role,
in the function of the vestibular system.
in the absence of gravity, like in outer space, it does not function normally.
Vestibular hair cells.
At the base of each canal, there is a bunch of hair cells.
These hair cells , are sensory organs.
They sense the mechanical movement of fluids in the canals.
This is transduced to action potential signals.
These signals are transmitted to the brain, via the auditory nerve.
The brain receives signals from the vestibular system.
The brain processes this information, to give us an overall sense of balance.
This is used by the brain to coordinate daily activities like,
walking, bending, dancing, etc.
The control of body posture, using motor muscles,
is related to information feedback, coming from the vestibular system.
We would have noticed, that when we are about to slip and fall,
the body reflexively rebalances itself, using the muscles, and other parts of the body.
Neural pathways.
Signals are transmitted from the ear to the brain,
via the vestibulocochlear nerve.
This nerve is the cranial nerve 8.
The vestibulocochlear nerve, has 2 distinct branches.
The cochlear nerve.
The vestibular nerve.
The cochlear nerve carries signals, from the hair cells, in the cochlea.
The vestibular nerve carries signals, from the hair cells, in the vestibular canals.
The cochlear nerve, travels to the cochlear nucleus in the brain stem.
From there it travels to the medial geniculate nucleus, in the thalamus.
The medial geniculate nucleus, is a relay station, for auditory signals,
to the auditory cortex.
It is possible that the medial geniculate nucleus,
influences the direction and maintenance of attention.
For example, the brain might learn to ignore certain background sounds,
and pay attention to the class teacher.
The relay stations, in the neural pathway, perform different levels of integration,
and processing, of the auditory signals.
Auditory cortex.
The auditory cortex, is located in the upper part of the temporal lobe.
Not surprisingly, the primary auditory cortex, is tonotopically mapped.
The hair cells, in the organ of corti, in the cochlea, is tonotopically mapped.
The base region encodes higher frequencies.
The apex region encodes lower frequencies.
Signals receive from the cochlea, are also topographically mapped,
in the primary auditory cortex.
Our sensation of sound, originates from the auditory cortex.
The auditory cortex, in each temporal lobe,
receives signals from both the ears.
Integration of these signals, take place in the neural pathway,
and in the auditory cortex.
There is some lateralization of auditory functions in the brain.
Lateralization of brain function, refers to how some cognitive function,
tend to be dominant in one hemisphere of the brain.
Processing of speech, is primarily done in the left hemisphere.
Music with its influence on emotions, is processed in the right hemisphere.
Auditory signals from the brain do not directly come to the auditory cortex.
The nerve fibres from the cochlea, synapse with other fibres.
A good deal of information integration, takes place before the signals,
reach the auditory cortex.
Apart from basic wave length information,
the mind perceives other subtle qualities of the sound.
We can sense inflections, and emotional content of a spoken word.
Sound signals are associated with other senses, and emotions,
in association cortex regions.
Though for convenience, we discuss each sensory system separately,
we must keep in mind, we actually perceive in an holistic, and associative way.
Listening.
Listening is the conscious processing of the auditory stimuli,
that have been perceived through hearing.
There might be some routine background sound, that we hear,
but we do not consciously listen to it.
A child waiting for the mother to come back, would easily perceive,
the sound of the key turning in the door.
This might be one of the many sounds in the room.
So, in one sense, listening is hearing plus attention.
In our day to day life, we tend to pay attention, and listen to the sounds ,
that are important to us.
Listening to speech, typically involves associating with meaning.
We can try to imagine a situation,
where we listen to a person, speaking in a language not comprehensive to us,
as compared to listening to a person, speaking in a language we understand.
This will clearly illustrate how we relate sounds to meanings.