The importance of sense organs in the behaviour of a living organism can hardly be exaggerated. Sense organs are like the doors through which the organism becomes aware of his environment. As we go up the evolutionary stage, sensory mechanisms become more varied and more sensitive. The human organism seems to be well equipped to register his world. The senses provide us with the knowledge of things with which we deal.
In fact, it is impossible to think of behaviour without sense organs as it is impossible to think of it without the brain and the nervous system. At every moment of our life, right from the time we are born till we are dead, we are responding to the physical world around us and to various conditions within our body through the action of our senses.
Our sense organs then, make us aware of our external world as well as the internal processes in our body. The famous British Philosopher John Locke said many years ago that, “there is nothing in our mind that was not first in our senses.”
Our senses are the seat of highly specialized receptors, each capable of reacting to only certain type of changes in the environment or within the organism. These receptors are the nerve endings, which are set in action when a specific energy stimulates them.
When they are stimulated, the nerve impulses generated by the stimulation of the receptors begin to travel through the nerves towards the brain. When these impulses reach certain specific areas of the brain, the brain activity caused by their presence results into conscious experience. Each class of sensory activity has its own specialized centers in the brain, which decode the impulses coming from the receptors of a particular sense organ.
Senses of Man:
It was a traditional belief that the senses a man possesses are five in number. However, research in human physiology has shown that the senses are closer to ten or eleven. They are the sense of vision, hearing, taste, and smell. The skin itself has four senses namely, the sense of cold, warmth, pain and touch. These are known as the external senses but there is also a kinesthetic sense through which we become aware of the position of our limbs and tensions in our muscles.
The vestibular sense located in our ears supplies lots with the information about the movement of our head and helps us to maintain the balance of our body. Besides these, there are other senses located within the body, which give us information about pressure, pain and temperature within the body and hence they are called organic senses. These three are known as internal senses. A minimum list of man’s senses includes- vision, hearing, cold, warmth, pain, touch, organic sensibility, smell, taste, kinesthesis and the vestibular sense.
Each sensory receptor responds to a particular type of stimulation. For example, the visual receptors respond to changes in electromagnetic energy or light. The hearing receptors are stimulated by the sound energy; the taste and smell receptors respond to chemical stimulation; the warmth and cold receptor in the skin are stimulated by thermal energy or temperature changes.
The sense of touch, pain, kinesthesis and vestibular senses are stimulated by some kind of mechanical movement. These different sense modalities show that our sense organs are highly specialized to respond to only some kind of stimulation. The kind of stimulation, which usually stimulates a sensory receptor is called the adequate stimulus.
Another important feature of sensory stimulation is that the stimulus, which excites the receptors in a particular sense organ must be of certain minimum strength if it is to excite them. We are aware that some lights are too weak to be visible or sounds too feeble to be audible.
The minimum strength of the stimulus that is necessary to excite any particular sense organ is known as the lower threshold or limen, sometimes also called the lower absolute threshold. This threshold can be determined by presenting the subject with a stimulus of a given intensity and asking him whether he detects it or not; on the next trail a different stimulus intensity is used and such a procedure is followed through a wide range of intensities. On the basis of certain theoretical considerations, psychologists have agreed to define the absolute threshold as that value at which the stimulus is perceived 50 per cent of the time. Table 2.1 gives the data of absolute threshold for different sense modalities.
Besides the least amount of stimulus energy required to detect a sensation, there is also a certain difference between two stimuli before we can distinguish a difference between them. The minimum amount of stimulation necessary to distinguish between the two stimuli is known as the differential threshold or the just noticeable difference (JND).
For example, two tones must differ in intensity in some measurable amount of stimulation before one can be heard as louder than the other. Like the absolute threshold, the differential threshold is the amount of change in physical energy necessary for a subject to detect a difference between two stimuli 50 per cent of the time.
One of the remarkable features about the differential threshold is that it is not constant. To illustrate this point, if you are in a room, which is illuminated by one 25-watt bulb and if another 25-watt bulb is added, the addition of this amount of extra light will be at once detected because it is well above the differential threshold.
However, if you are in a room, which is illuminated by a thousand 25-watt bulbs, the additional light from one more 25-watt bulb will not be noticed because it is below the differential threshold although the amount of energy added is the same.
From the above illustration it follows that the value of the differential threshold depends upon the intensity of the stimulus to which more energy is added. It has been found that for relatively moderate intensities there is a constant ratio between the amount of energy which must be added, called ΔI to reach the differential threshold, and the intensity, called I of the stimulation. In other words, ΔI/I is constant for the middle range of intensities. This has been called Weber’s law.
An important characteristic in the working of our senses is that they get gradually adapted to continual stimulation. In general, it is true that all senses gradually become less sensitive as they are continually stimulated and more sensitive in the absence of any stimulation. For example, when wearing clothes, our skin senses feel their pressure but soon we become unaware of it because our skin senses: have adapted to the pressure of the clothes.
By the same token, in extreme darkness we are able to detect even a faint glimmer of light because there is complete absence of visual stimulation. These changes brought about in our receptor organs either due to their continual stimulation or absence of stimulation are referred to as the phenomenon of adaptation. There are varying degrees of adaptation in different sense modalities.
For example, the sense of smell shows a high degree of adaptation to continual stimulation. Everyone has the experience how quickly we become habituated even too strong smells, so much so, we may not perceive them at all. Excepting the sense of hearing all the senses show adaptation to a smaller or greater extent.
The process of converting physical energy into activity in the nervous system is known in sensory psychology as transduction. Transduction takes place at the receptors and involves several steps. In general, the specialized cells of the receptor organs act to convert physical energy into a slowly changing electrical potential known as the generator potential. The generator potential, in turn, acts upon the nerve cells and fibres to produce the nerve impulses, which travel through the central portions of the brain and eventually result in an experience.
Sensation and Perception:
The simple experiences, which arise from the stimulation of sense organs have been called sensations. All our experiences of red, white, sound or pain are said to be sensations whereas, more complex experiences, such as house, warning etc. have been called perception.
In simple language, the distinction between sensation and perception is that simple ingredients of experience are regarded as sensations, and the experiences that involve several sensations and their interpretation are regarded as perceptions. A perception, therefore, is the interpretation or the meaning given to the sensation and individual experiences.
Vision has been described as the most important sense in human beings as well as in animals because their survival is closely linked with its normal functioning. In man, this wonderful sense adds colour and movement to life. The experience of the world around us in terms of shapes, sizes and forms in two and three dimensions is made possible because of the functioning of our eye. The electromagnetic energy, which we call visible light, stimulates the specialized receptor cells, the rods and cones of the retina and initiates a series of chemical changes in the light-sensitive substances of these cells.
The result of this series of reactions is an electrical event called the generation potential which causes a barrage of nerve impulses to be triggered and it is this barrage, which becomes the input into the central nervous system, which is responsible for seeing. What we see depends upon the objects transmitting light to the retina.
The Human Eye:
Anyone who is acquainted with the working of a camera will very well see the similarities between the camera and the human eye. Both are dark chambers, which admit light through an opening in front. Both have a lens behind the opening for focusing the images of outside objects on the rear surface.
Both have surfaces behind the lens (film in the case of a camera and retina in the case of the eye) on which the image of an object is projected. Both have images, which fall on their surfaces, which are inverted and turned from right to left. Both can be adjusted to control the amount of light falling on their surfaces, but in the case of the eye, it has its own automatic mechanism for its adjustment.
The iris of the eye serves like a diaphragm of the camera. The iris controls the size of the opening known as the pupil, which enlarges its size when light is dim in order to allow more light on the surface, or contracts when the light is bright in order to allow only sufficient light in the eye.
In spite of these several similarities between the camera and the eye, there are some fundamental differences, which must be carefully noted. Firstly, the eye sees because of the activity in the brain and not because we see the image of the external object falling on the retina. Secondly, the eye has a lens like a camera but it is used only for small adjustments in focusing and the main job of bending the light is done by the cornea. Finally, the eye is far more sensitive to light than a film in a camera.
The eye is roughly spherical in shape and its walls are made up of three different layers. The outer layer known as the sclera is made up of tough fibres that protects it and serves to maintain its shape. The sclera layer bulges out and becomes transparent in front of the eye and this bulge is called the cornea.
Beneath the sclera layer there is another opaque layer known as the choroid coat, which does not allow any light to enter in the eye except through the cornea and the lens. The innermost layer is known as the retina. It is the most sensitive part of the eye, which enables us to see.
Retina, when examined under a microscope, shows that it is made up of several cells on its surface. Two types of cells—rods and cones—are the light sensitive elements. The rods are cylindrical in shape and the cones are conical.
According to a modest estimate there are between 110,000,000 and 125,000,000 rods and between 6,300,000 and 6,800,000 cones. Cones are packed most closely in the central part of the retina known as the fovea. Fovea is the region of the most distinct vision and the part, which we use most when getting clear images of the objects. There are no rods in the fovea.
The cones and the nerves leading from them function only in the light and are responsible for colour vision as well as the sharpness of vision. In darkness, the cones are not stimulated and hence only the rods can function. Through the process of dark adaptation, the rods become highly sensitive and responsive to the smallest quantity of light.
Thus then, cones are a day vision system whereas, rods are a night vision system. The rods do not respond to the colour of objects but can see only white, grey and black. Further, the rods do not record the fine details and shapes of the objects as the cones do.
Another interesting feature about the cones in the region of fovea is that they have separate connections with the optic nerve. This nerve leaves the retina at the posterior side and goes to the brain. The region in the retina where the optic nerve leaves for the brain is insensitive to light and is known as the blind spot.
It is a blind spot because it lacks both rods and cones. As we leave the central part of vision, the fovea on the retina and go towards the periphery, the cones become less and less and the rods become more and more in number.
In most cameras, one has to adjust the focus of the lens for objects at different distances by adjusting the lens back and forth. The lens of the human eye, however, becomes thicker or thinner to focus at different distances. It becomes thinner or flat when focusing on far objects and thicker and curved when focusing on nearer objects.
These changes are called accommodation. The shape of the lens is controlled by the ciliary muscle, which is attached to the lens. The defects in accommodation lead to visual defects known as short-sightedness and long-sightedness. Inability to see objects, which are far at a distance is called short-sightedness and inability to see near objects is called long-sightedness. Such visual defects are usually corrected by using glasses. The accommodation of the lens occurs for far and near objects.
Nature of Visual Stimulus:
The eye is stimulated by electromagnetic energy, commonly called light. However, it is capable of reading only to a small band of electromagnetic spectrum. As a physical stimulus, the light has three different dimensions. The first one is intensity, the second is wavelength, and the third is the wave-length composition. The intensity of the physical stimulus leads to the experience of brightness.
The greater the intensity of the physical stimulus, the greater is the experience of brightness. The characteristic of the wave-length makes us aware of hue or colour and when a mixed light falls on the retina, the experience is of white or grey depending upon the intensity of the wavelength.
The sensation of brightness, which is dependent upon the intensity of light, varies from black to white through various shades of grey. Although we usually talk of brightness as extending from black to white, our experience is that colours can be bright or dark. A weak and strong blue light may have the same wave-length but different brightness.
Brightness then, depends upon the intensity of light and the intensity of light in turn depends upon the amplitude of the light wave. Thus, one and the same wavelength may have different amplitudes; the smaller amplitude usually is intense whereas the larger amplitude is more intense.
Hue is the perceived dimension of colour, which we refer to when we use common colour names like red, green, yellow, blue or combinations of them. When we say that some object is red, we mean that it has a red hue. Our sensation of hue or colour depends upon the wavelength of light. Different wave-lengths produce different sensations of hue. For example, when a light of 760 nanometers in wavelength strikes our retina, we experience the sensation of red hue.
The entire visual spectrum of light extends from red at one end through oranges, yellows, greens, blues’ and violets at the other end. The relation between wave-length and hue is not completely stable and the eye also is not equally sensitive to all wave-lengths.
The relative sensitivity of the eye changes with the intensity of the light stimulation. Hue also depends upon contrast effects. For example, when two coloured areas are adjacent to each other, they may induce what appears as mutual changes in hue in each other near the border between the two areas. This phenomenon is called simultaneous contrast.
When hues are mixed, the resulting colour is different not only in hue but also in saturation. By saturation we mean the purity of colour. Purity or saturation refers to the degree to which a particular colour is diluted or not diluted by greyness or whiteness. Saturation then, refers to the degree of concentration or dilution of the hue or colour. Saturation or purity of colour can be reduced by mixing it with white.
When a hue has no mixture of white, we call it a highly saturated colour. In fact, the most highly saturated colours are made by using only the colour pigments and avoiding any white in the mixture. The deep or strong colours are those which are highly saturated whereas, the weak colours are those which are relatively unsaturated.
Functioning of the Eye:
When the electromagnetic energy or light from the external objects strikes the eye through its opening and the lens and casts its image on the retina, the rods and cones are stimulated. The rods and cones have in them the light-sensitive substances known as rhodopsin and iodopsin respectively. The former is the major pigment found in the rods whereas the latter is found in the cones. These substances start splitting when light falls on them.
When the electromagnetic energy strikes in the visible range, rhodopsin is broken into orange intermediates and then into two substances called retinene and opsin. Retinene gives the yellowish colour, which was originally called visual yellow.
Retinene and opsin spontaneously change back into rhodopsin and a cycle of chemical changes called the visual cycle takes place. In this way, the equilibrium is established between the splitting of rhodopsin and its synthesis from retinene and opsin.
Regarding the cone pigments, three human cone pigments have been identified and these are called blue, green and yellow cones. These changes create generator potential, which triggers the nerve impulses which collectively travel to the brain via the optic nerve. When they reach the visual cortex in the brain and stimulate its activity, we become aware of the things we see.
Our ability to perceive colours depends upon the capacity of the retina to react differently to various wave lengths in the spectrum of light. The cones on the retina are supposed to be specialized structures, which react differently to different wave lengths. Research on colour vision has shown that there are four different primary wave-lengths which correspond to red, yellow, and green and blue which are sufficient to prove all types of colour sensations.
These elementary colours are called psychological primaries. Between these elementary colours are the secondary colours in which the primary components can be identified such as, orange between red and yellow; yellow-greens between yellow and green; bluegreen between green and blue and purples and violets between blue and red. Another set of primaries is called the colour-mixture primaries.
The three colours usually chosen are red, green and blue, which can be used to produce all the other colours by additive mixture. For example, colours between red and green on the colour circle can be produced by additively mixing a pure red and pure green. Similarly, colours between green and blue on the colour circle can be obtained by their additive mixtures, and the same holds good for blue and red. In fact, with additive mixing the entire colour circle can be produced with just three colours.
It is assumed that the cones have different types of photosensitive chemicals, each capable of reacting to different sets of wave lengths. There are red cones, blue cones and yellowish green cones, which react to the red light, blue light and green light respectively to produce various colour sensations.
Theories of Colour Vision:
Psychologists have evolved different theories of colour vision to explain the facts about colour perception, but they do not explain all the facts of colour vision adequately. In Eighteenth Century a theory of colour vision proposed by Thomas Young, an English physicist and later modified by the German physiologist Hermann Von Helmholtz and hence the theory has been called the Young-Helmholtz’s theory.
This theory proposes that there are three kinds of cones: red cones, green cones and blue cones, each of them is highly sensitive to a different wave-length. All other colours are somehow produced by a combined stimulation of these three cones. For example, yellow is produced when red and green cones are stimulated simultaneously. Similarly, white is produced when all three cones are simultaneously stimulated.
Although there is strong support for this theory from the point of view of colour mixture, yet this theory has not been able to explain many other facts of colour vision. For instance, many red-green colour-blind people can see yellow. The theory as originally stated assumes that yellow is due to the mixture of activity in the red and green cones. If this be so, how can red-green colour-blind persons who presumably lack the red and green cones see yellow?
Another colour theory was formulated by Ewald Hering after the above theory. Hering felt that the Young-Helmholtz’s theory did not adequately explain the visual experience. He based his theory on the psychological primaries rather than the colour mixing primaries and argued that yellow is as basic a colour as red, blue or green.
He proposed that there were three types of cones, viz., (1) one that responded to degrees of brightness, the black-white continuum; (2) two colour cones, one provided the basis for red-green perception and (3) the other for blue-yellow. He also assumed that each cone functioned in two ways.
One colour of the pair was produced when the receptor was in a building-up phase (anabolic), and the other appeared when the cone was in a tearing down phase (catabolic). The two phases cannot occur at the same time in a given cone. For example, when a yellow-blue cone is stimulated it responds with either yellow or blue.
It cannot react both ways simultaneously. Hering’s theory has often been called the opponent-process theory. In its modern form this theory assumes that the opponent processes take place not in the cones but in the coding mechanisms closer to the brain in the optic system.
Recent research suggests that both theories may be partially correct. MacNichol, using her procedure called micro-spectrophotometry, was able to identify three kinds of light-sensitive pigments in the cones. One type was primarily sensitive to wave-lengths in the blue band, the second sensitive to green, and a third sensitive to yellow.
DeValois and Jacobs, took recordings with microelectrodes which gave evidence of an “on” and “off” type of process in the bipolar cells and in cells of the portion of the thalamus where visual impulses are relayed to the usual cortex.
They found that some cells respond when stimulated by short wave-lengths but do not respond during illumination with long wavelengths, whereas other cells respond when stimulated by long wavelengths and do not respond when stimulated by short wave-lengths. These findings support the opponent-process theory.
Both the Young-Helmholtz and Hering theories explain another phenomenon of colour vision called successive contrast. For instance, if you look steadily at a bright-coloured spot for a while, and then look at a grey sheet of paper, you may see two kinds of successive afterimages.
The first is called the positive afterimage because it is the same colour as the original stimulation. The second, is the negative afterimage which is the complimentary colour of the original colour. Suppose that the original circle is blue green, the positive afterimage will also be blue green whereas, the negative afterimage will be orange.
It must be noted, however, that not all afterimages are of the complimentary colour. In fact, after staring at a very bright light you are likely to see a whole succession of colours, but seeing the complimentary colour is very common.
Positive afterimages are supposed to be caused by the continuation of the activity after the removal of the physical energy. Regarding the negative afterimages, the Young-Helmholtz theory makes the assumption that the original stimulation fatigues the special colour cones, which are excited. For example, a blue-green stimulus will fatigue the blue and green elements but not the red elements.
Dark Adaptation or Scotopic Vision:
When we enter a dimly lighted room after having come from bright sunlight, we are unable to see anything in the room, but gradually things become visible. You must have experienced that when you have entered a cinema theatre, you could not see the rows of chairs before you.
Similarly, when we come out of a dark room into bright sunlight, our eyes are at first blinded by the light, but in a few seconds we are able to adapt ourselves to the bright light. When we remain in a dark room for a long time, our eyes become highly sensitive to light, so much so, we are even able to see a flicker of match from the distance of fifty miles if there is perfect darkness.
Experimental evidence demonstrates that after an hour of dark adaptation the sensitivity of the eye can increase from 1000 to 1,00,000 times. Light adaptation of photovision on the other hand, diminishes considerably the sensitivity of our eyes.
Colour and brightness are not the only characteristics of objects in our environment. They also have a form. The capacity of our eyes to discriminate between various forms in our environment is called the visual acuity. In other words visual acuity is the ability of the individual to perceive differences in details of the visual environment.
There are several ways to determine and measure the visual acuity but the basic procedure is to present the subject with figures with finer details at a specified distance and the subject is asked whether he can perceive them or not. The charts of letters, which the opticians use to determine the extent of the eye defects are based on this principle.
For instance, if a person can see at a distance of 20 feet what a normal person can see at a distance of 100 feet, he has 20/100 vision which certainly is defective. Similarly, if a person can see at a distance of 20 feet what a normal person can see at a distance of 10 feet, he has 20/10 vision which is excellent. The standard vision is defined as 20/20, which actually means that a person has a normal or average vision.
It is estimated that about four percent of the people in the general population are colour blind and their inability to see certain colours is likely to affect their behaviour in various ways. Colour blindness is not our inability to identify colour by a particular name or even our inability to see colours. Really speaking colour-blindness means mistaking certain colours for some other colours. Colour-blind people usually see a great many colours but they confuse certain critical ones.
Colour-blindness is a defect, which makes a person unable to tell the difference between two or more colours which most other people can easily distinguish. The explanation for colour blindness is that the cones on the retina of the colour blind people are in some way different from those who can see all the colours without any difficulty.
There are various types of colour blindness. Firstly, there are people who are totally colour-blind, who cannot see any colour but perceive the world as a sort of black and white photograph. It is assumed that this defect arises from complete absence of the cones on the retina. Such people seem to rely only on rod vision and are very rare. Secondly, there are people who are called partially colour-blind.
In other words, such people are capable of seeing certain colours but confuse them with certain other colours. Among the partially colour- blinds, there are two common varieties- (1) Those who are known as dicromats whose colour vision is limited to only two colours or hues, namely, the yellows and the blues. These people are also known as Red Green colour-blinds because they usually see these two colours and their shades as some other colours rather than red or green. (2) There are others who confuse very light pinks, greens, tans and browns. Such people are known as suffering from colour weakness called anomalous colour defect.
There are several colour vision theories, which account for colour blindness. According to Ladd-Frankiin theory of colour vision, our capacity to see colours has gradually developed in the course of evolutionary process. In the first phase of development of vision, we are capable of seeing things only in black and white. In the second phase, we develop yellow blue colour vision and at the last phase we develop red green vision.
Perhaps because of this phased development we are more likely to lose our red green vision than the colour vision at the two earlier phases. This explains why there are many more red green blind people than blue yellow; colour blinds.
The most interesting feature about colour blindness is that the colour blind persons or even those around them are unable to detect this defect unless it is highly pronounced. There are several good tests to find out whether a person is colour-blind or not. One such very widely used test to detect the defect of colour blindness is known as the Ishihara’s Colour Blindness Test.
Most types of colour blindness are supposed to be inherited and the defect has been identified as a sex-linked recessive characteristic. It is also assumed that due to the genetic relations involved, colour blindness is more prevalent among men than among women.
The sense of hearing, technically known as audition, is in no way inferior to the sense of vision. Its importance as a means of acquiring knowledge of the external world, can be hardly exaggerated both in the case of animals and human beings.
In the case of animals, hearing is very closely concerned with their survival and, in certain highly developed species of animals, hearing becomes a sort of underdeveloped means of vocal communication.
In the case of man hearing acquires a great importance as a vehicle of communication of language through which we are able to accumulate knowledge. Further, it is through hearing that a man is able to make his visual and other experiences much more meaningful and effective.
The Human Ear:
Broadly, the human ear can be divided into three distinct parts: The external ear, the middle ear and the inner ear. The sound waves are collected at the external ear, which consists of the decorative ear called the pinna and a canal through which the sound waves are transmitted to the ear drum.
The ear drum is a small thin membrane stretched tightly across the inner end of the canal. Inside the ear drum there is a small cavity filled with air and three small bones called ossicles. This is the middle ear.
The boundary of the middle ear extends from the eardrum to another membrane separating the inner ear from the middle ear. It is known as the oval window. One of the three ossicles known as malleus (hammer) is attached to the ear drum. The middle bone, known as incus (anvil) is connected with the stapes (stirrup). The arrangement of the three bones and the two membranes is such that the vibrations caused by the ear-drum are conducted through the bones to the oval window.
The inner ear is more complex in structure. It is a seat of two organs the sense of equilibrium and the sense of hearing. The sense organ of hearing is located in a bony snail like spiral called the cochlea. The cochlea has three different canals which are filled with fluids and are separated from each other by membranes.
On one such membrane known as the basilar membrane there is a tiny structure called the organ of Corti, which is the real organ for hearing. The receptors of hearing known as the hair cells, are located in the organ of Corti. These hair cells are the real receptors of sound energy. When these hair cells are stimulated the generator potential resulting from such stimulations triggers off nerve impulses which are transmitted through the auditory nerve to the auditory area of the brain. It is only when this process takes place that we experience hearing.
The air is a collection of molecules, which are always moving about, colliding with one another and exerting pressure on one another. If these molecules are closely packed, the air pressure will be greater and if they are few, then the air pressure will be less. Such changes in pressure constitute the physical basis of sound perception.
Sound waves are generated by the vibration of a physical object in the air. When an object vibrates in both directions, it results in the condensation and ratification of the air molecules, which create areas of high pressures in the air.
This is what we call the sound wave. When the object vibrates, condensation of molecules takes place and this is known as positive pressure, whereas, when ratification of air molecules takes place, a little vacuum is created and this is known as the negative pressure.
A sound wave moves through the air in much the same way as a ripple moves on the water. Sound waves generated in this fashion vary from each other is three important respects- (1) frequency, (2) amplitude, and (3) complexity. These three properties of sound are important because they are the basis of our psychological experiences called- (1) pitch, (2) loudness, and (3) timbre.
Frequency and Pitch:
The vibrations in the air molecules vary in frequency depending upon the speed of the vibrating object. Experiments have shown that the vibrations may be as slow as one per second and as fast as one million per second. The frequency of a sound wave is measured in terms of cycles per second (CPS) also known as Hertz (Hz).
The human ear is capable of responding only to a small range of frequencies usually from 16 Hz to about 21,000 Hz. The sounds above and below these frequencies are inaudible to the human ear. It is possible that individuals vary in their sensitivity to the frequency of sound wave but the differences are not very large.
Similarly, some animals like dogs, for instance, are capable of hearing sound frequencies beyond human capacity; they can respond to the sound of 40,000 Hz. It is important to note that our experience of pitch (the highness or lowness of a sound) is primarily dependent upon the frequency of sound waves. For example, the higher the frequency of sound, the higher is its pitch.
Amplitude and Loudness:
The second factor in which the sound waves differ from each other is known as the amplitude. Amplitude, is determined by the amount of pressure difference between compression and rarefication of the wave.
Thus then, while the frequency of a sound wave tells us how often the sound wave changes from positive to negative pressure, the amplitude tells us how great the pressure changes are. In other words, the amplitude of a sound wave tells us how intense the sound is. Our psychological experience of the loudness of the sound is primarily dependent upon the intensity of the sound wave.
Scientists have developed special methods of measuring the sound energy. The range of intensity to which the human ear can respond is very great. For instance, the loudest sound that one can hear without discomfort has a pressure of about one million times as great as the weakest sound that is just audible. The intensity of the sound wave is usually measured in terms of decibels. A decibel is equal to the lowest sound pressure that can be recorded by a normal ear.
Complexity and Timbre:
The sound waves produced in the world around us are rarely simple (of uniform frequency or intensity). They are usually made up of several complex waves of all considerable types. However, they may be broadly classified into two categories: periodic and aperiodic. The periodic waves are of various heights and widths. The sound waves produced by various musical instruments are also complex but repeat themselves in regular patterns.
They are usually periodic. On the other hand, there are many objects, which produce sound waves of irregular and unrelated frequencies which produce auditory sensations called noises. This is due to the aperiodic waves.
We are able to tell the sounds produced by one musical instrument from another, only because each musical instrument is capable of producing certain complex and yet characteristic patterns of sound waves. This characteristic quality of a musical tone is called timbre. It is the timbre (tonal quality) of a tone that tells us whether it is being produced by a piano or a clarinet.
What happens when two tones are sounded together is a problem raised. It has been found that they do not lose their identity as colours do when mixed, but they may lead to a fusion that is heard as consonant (pleasant) or as dissonant (unpleasant). The two tones create a third tone based on the difference in their frequencies.
This difference tone may or may not harmonize with the fundamental tones sounded and it is for this reason that some combinations of tones are preferred to others. Musical harmony, for example, depends partly on the interaction between fundamental tones, overtones, and different tones, which combine to make up the complex tonal stimulus.
A noise is sound composed of many frequencies not in harmonious relation to one another. Sound experts sometimes speak of white noise when a noise is composed of all frequencies in the sound spectrum at roughly the same energy level. White noise is like the white light which is composed of all frequencies in the light spectrum.
A bathroom shower, for instance, approximates the sound of white noise. Speech sounds make simultaneous use of both, tonal qualities and noise qualities. When we speak, the speech consists of vowels which have a tonal quality and consonants which have a noise quality.
Functioning of the Ear:
When the ear drum begins to vibrate in response to the pressures exerted on it by the sound waves, the vibrations are first transmitted to the ossicles and from them to the oval window. The oval window conducts the sounds to the cochlea, which is the auditory portion of the inner ear.
Pressure at the oval window sets into motion the fluid inside the cochlea which stimulates the fine hair cells of the basilar membrane in the cochlea on which is located the organ of corti.
The pressure changes in the fluids of the canals cause a shearing movement of the hair cells to produce nerve impulses which are carried over to the auditory areas of the brain through the auditory nerve.
It is only when the nerve impulses reach the brain that we get the experience of sound. The pathways of the auditory nerves are made up of nerve fibers from each ear, which travel to both the cerebral hemispheres and terminate in the temporal lobes.
How the nerve impulses convey the physical characteristics of a sound stimulus like frequency, amplitude and complexity, and how our corresponding experience of pitch, loudness and timbre result, is yet to be fully understood. However, scientists have tried to offer several theories for these questions. The two most accepted theories are known as the Place Theory suggested by Helmholtz and the Telephone Theory suggested by Rutherford.
The place theory assumes that the frequency of a tone is indicated by the region of the basilar membrane that is maximally displayed by the sound wave. The telephone theory, on the other hand, assumes that the cochlea acts like a microphone and the auditory nerve like a telephone wire. According to this theory, pitch is determined by the frequency of impulses travelling up the auditory nerve.
The greater the frequency, the higher is the pitch. Just as when discussing the theories of colour vision, Young-Helmholtz and Hering’s theories were said to be partially correct although they did not offer a satisfactory explanation, in hearing also, pitch discrimination may be partly explained by both the theories.
Both the place of excitation on the basilar membrane and the frequency of nerve response seem to be involved in transmitting the information about the frequency of a tone. Perhaps, it could be said that precise coding of the auditory information takes place in the auditory pathways closer to the brain and in the auditory cortex itself.
It is commonly Believed (hat deafness is simply our inability to hear sounds. However, this is not the complete truth because actually there are many kinds of deafness. In the first instance, there are people who cannot recognize even the faint sounds and such people are regarded as hard of hearing. Technically this phenomenon is known as intensity deafness. Usually people who have to work in deafening noises such as pilots, gunners or factory workers, may develop such intensity deafness.
Really speaking, our ears have protective mechanism which shield the delicate organs in the ear by reducing the intensity of sounds that are allowed to reach the inner ear, but obviously, when these mechanisms are over used, the ability to detect lower intensities of sound decreases.
Sometimes the age factor also seems to be a possible cause of deafness. Research on the relationship between age and hearing has clearly demonstrated that persons below the age of 20 definitely have better hearing than the persons above that age. There is also some truth in the common belief that hearing is very much impaired in persons above 50 years of age.
The second factor which is often responsible for deafness in young people is the stiffening of the ossicles, particularly when the small bone known as the stirrup gets tightly sealed in the oval widow. This may happen due to different causes like the infection in the middle ear or even the rupture of the ear drum. Such deafness is called conduction deafness. The term conduction deafness is used because the origin of the deafness is to be found in the deficiencies of conduction in the ear.
The third type of deafness is called the nerve deafness. This deafness is caused when something is wrong with the auditory nervous system. Sometimes the nerves themselves are damaged or the damage is done to the cochlea or the basilar membrane. In this type of deafness, the hearing loss is much greater at high frequencies.
Such persons are unable to hear high pitched sounds, but can hear low pitched sounds reasonably well. This type of deafness is very common among old people. With advancing age almost everyone shows signs of nerve deafness, but only in a few persons it becomes serious. There is some evidence also to show that nerve deafness is hereditary because it runs through certain families.
Localization of Sound:
Every individual with normal hearing capacity can locate the source of sound by carefully listening to it. Psychological research has shown that this capacity to locate sound depends upon several factors. For example, it has been demonstrated that the sounds which are made directly above our heads are difficult to be localized, but those which are made anywhere on the right or left side can be very easily localized.
Similarly, the sounds, which are made directly in front or behind are also difficult to be localized. Finally, sounds, which have very high frequencies are also difficult to be correctly located.
In localizing the source of a sound we make use of several physical clues but the time taken by the sound to reach the ears seems to be an important factor. For example, when a sound originates on our right side, it first reaches the right ear and then the left ear. This small difference is sufficient for us to locate the source of sound. Similarly, the sound that reaches one of the ears first is slightly louder than when it reaches the second ear.
Such a clue also helps us in locating the sound. Finally, the localization of sound is further helped by the movement of our head and the use of our eyes. Psychologists have devised an apparatus called the sound cage to study the localization of sound. The findings mentioned above are based on such experiments conducted in the laboratory.
The senses of smell (olfactory sense) and taste (gustatory sense) are regarded as chemical senses because the receptors of these senses are stimulated by chemical substances. Both the olfactory sense and the gustatory sense work in close coordination with each other to produce a wide variety of experiences both pleasant and unpleasant.
For some animals, the experiences of taste and smell are as interesting as those produced by the eyes and ears. In the case of dogs the sense of smell appears to be very highly developed.
In most wild animals the sense of smell is connected with their survival because it is through this sense that many wild animals are able to detect the dangers in their environment. In the case of man, both smell and taste have a unique place because they add spice to our lives and make the act of eating either pleasant or unpleasant.
The receptors of the sense of smell are located in the nasal passage leading from the nostrils to the throat. If you look at, you will find that the smell receptors: lie in two small patches, one on the left and one on the right, in the roofs of the passages, and they are known as olfactory bulbs.
They are stimulated when chemical substances in vapour form come in contact with them. When the smell receptors are stimulated nerve impulses are generated which are carried to the brain by olfactory nerves. We do not have adequate information regarding the brain mechanisms involved in smelling due to several difficulties, yet physiologists as well as psychologists have done a lot of research and yet the information is inadequate.
Attempts have been made to discover the basic or primary smells and a German researcher Henning has listed six primary smells as follows:
(1) Flowery (smells of flowers),
(2) Fruity (smells of fruits),
(3) Spicy (smells of spices like cloves and cardamom),
(4) Resinous (smells of pine),
(5) Burnt (smells of tar or burning substances),
(6) Putrid (smells of rotting or decomposing substances).
The industrial chemists, on the other hand, who are concerned with the manufacture of perfumes, prefer a four-fold classification of smells.
According to this classification, the four basic smells are:
(1) Fragrant (musk),
(2) Acid (vinegar),
(3) Burnt (roasted coffee) and
(4) Caprylic (goaty or sweaty).
Whatever may be the number of basic smells, it is interesting to note that the nose is an extremely sensitive sense organ. Only a few molecules of odorous substance are enough to give rise to the sensation of smell. For example, the artificial musk, which is supposed to be the most odorous substance can be smelt when it is mixed with a litre of air in such a small quantity as 0.00004 milligram.
It is believed that smell sensitivity far surpasses the sensitivity, which we find in human beings. The extreme sensitivity of the sense of smell would have been a great nuisance to us had it not been for the phenomenon of adaptation.
Adaptation is a process by which a sense organ gradually ceases to respond to constant stimulation. It is everybody’s experience that smells of soaps, powders or perfumes, which we use constantly are no longer sensed by us. In the same manner we can get ourselves used to very strong foul smells rather quickly. This is only because of very quick adaptation of the sense of smell.
Theories of Smell:
Although there is a disagreement regarding the basic ordours, many scientists have advanced theories to account for the transduction and the psychophysical characteristics of smell. However, none of these theories has gained widespread acceptance. The basic idea is that certain smells are produced by molecules with particular shapes. This theory is called the lock-and-key theory because these molecules are supposed to fit into the “sockets” in the olfactory receptors.
For example, the molecules of camphor, and substances, which have a camphor like smell, are supposed to have a spherical shape and fit into a bowl-shaped depression in the olfactory receptor. This theory considers five of the smells—camphoraceous, musky, floral, peppermint, and ethereal—to be basic because they are supposed to have distinctive shapes.
The other two basic smells—pungent and putrid—are supposed to arise from molecules having special patterns of electrical charge which allow them to fit in the sockets in the olfactory receptor. The evidence for this theory is highly suggestive specially regarding the molecular shapes and the electrical charges but the evidence has not been conclusively proved.
The receptors for taste are usually to be found in little clusters known as the taste buds. Normally there are 245 of these buds located on the top and sides of the tongue, and more sparsely on the back of the mouth and in the throat. Receptor for the sour taste are located mainly on the side of the tongue.
Sweetness is best experienced by the tip of the tongue. Salt receptors are at the top and sides of the tongue and the bitter sensitive areas are at its base. All taste receptors, it has been found, operate with reduced sensitivity after smoking. Smokers who give up smoking often find that foods now taste so good that the habit of over-eating easily replaces the habit of smoking. Old people do not taste as well as do the younger people, and children, with all taste buds functioning fully, seem to have very active tastes.
The stimulation of the receptors in the taste buds can occur only when the substance to be tasted is soluble in saliva. When the taste cells are stimulated by such solution they generate nerve impulses, which are transmitted to the brain.
The areas of the brain, which are involved in interpreting the impulses from taste cells are not yet precisely known. In dealing with smell we had said that there was no agreement among scientists regarding the basic odors. However, we have a better knowledge about the basic taste qualities.
The research evidence in general is that there are four basic or primary taste qualities- salty, sour, sweet, and bitter. The evidence for these qualities comes from the fact that the tongue is not uniformly sensitive to all stimuli. It is true that we taste a lot of other flavours besides the four mentioned above but, this is due to the sense of smell. This in fact is the reason why we find the foods flavourless and tasteless when we are suffering from a heavy cold or when our nose is blocked.
Like all other senses, taste also shows adaptation. In other words, it becomes less and less sensitive during the course of constant stimulation. Research points out that the stronger the stimulation, the greater is the adaptation.
Further, as compared with the senses of vision and hearing, the senses of smell and taste show a very rapid adaptation. Although we believe that we taste with our tongues and smell with our noses, most of us do not realize that we commonly confuse taste and smell. In fact, we often think that we are identifying a flavour by taste when smell is more important.