SENSORY,ATTENTIAL AMD PERCEPTIONAL PROCESSES

Activity . Photochemical Basis of Light and Dark Adaptation : You may wonder why the light and dark adaptations take place. According to the classical view, light and dark adaptations occur due to certain photochemical processes. The rods have a photo-sensitive chemical substance, called rhodopsin or visual purple.

By the action of light the molecules of this chemical substance get bleached or broken down. Under such conditions the light adaptation takes place in the eyes. On the other hand, the dark adaptation is achieved by the removal of light, and thereby allowing for restorative processes to regenerate the pigment in the rods with the help of vitamin A. The regeneration of rhodopsin in rods is a time consuming process.

That is why dark adaptation is a slower process than light adaptation. It has been found that people who suffer from vitamin A deficiency do not achieve dark adaptation at all, and find it really difficult to move in the dark. This condition is generally known as night blindness. A parallel chemical believed to be found in cones is known as iodopsin .

Colour Mixtures There is an interesting relationship among colours. They form complementary pairs. When mixed in correct proportions the complementary colours yield an achromatic grey or white. Examples of complementary colours are red-green and yellow-blue.

Red, green and blue are called primary colours , because on mixing, the light of these three colours can produce almost any colour. The most common example is the television screen. It contains spots of blue, red and green colours. The combinations of these three produce different colours and shades that we see on the TV screen.

After Images This is quite an interesting phenomenon related to visual sensations. The effect of a visual stimulus persists for some time even after the removal of that stimulus from the visual field. This effect is called after image. After images are positive and negative.

Positive after images resemble the original stimulus in terms of hue, saturation, and brightness. They usually occur after a brief intense stimulation of dark adapted eyes. On the other hand, negative after images appear in complementary colours. These images appear when a person stares at the patch of a particular colour for at least seconds, and then transfers the gaze to a neutral background (e.g., a white or grey surface).

If the person looks at the blue colour, the negative after image will appear in yellow. Similarly, a red stimulus will yield a negative after image of green colour. Auditory Sensation Audition or hearing is also an important sense modality that carries great value for us. It provides us with reliable spatial information.

Besides orienting us to certain objects or individuals, it plays a vital role in spoken communication also. Auditory sensation begins when sound enters our ear and stimulates the chief organs of hearing. The Human Ear Ear is the primary receptor of auditory stimuli. While its well-known function is hearing, it also helps us in maintaining our body balance.

The structure of an ear is divided into three segments, called the external ear, the middle ear, and the inner ear (Fig. . ). External Ear : It contains two main structures, namely pinna and auditory meatus .

Pinna is a cartilaginous funnel-shaped structure that collects sound waves from the surroundings. Auditory meatus is a canal protected by hair and wax that carries sound waves from pinna to the tympanum or ear drum . Middle Ear : The middle ear starts with tympanum , a thin membrane highly sensitive to sound vibrations. This is followed by the tympanic cavity .

It is connected to the pharynx with the help of Eustachian tube , which maintains the air pressure in tympanic cavity. From the cavity the vibrations pass to three ossicles known as malleus (hammer), incus (anvil), and stapes (stirrup). They increase the intensity of sound vibrations about times, and send them to the inner ear. Inner Ear : The inner ear has a complicated structure known as membranous labyrinth , which is encapsulated in a bony shell called bony labyrinth .

A lymph-like fluid is found in the space between bony labyrinth and membranous labyrinth. This is called perilymph . The bony labyrinth has three semi- circular canals at right angle to each other, a cavity, called vestibule , and a coiled structure, called cochlea . The semicircular canals have fine hair cells, which are highly sensitive to postural changes as well as changes in the body orientation.

Inside the bony cochlea, there is a membranous cochlea, which is also known as scala media . It is filled with endolymph, and has a spirally coiled membrane, called basilar membrane . It has got fine hair cells arranged in a series to form the organ of corti . This is the main organ for hearing.

Working of the Ear Pinna collects the sound vibrations and serves them to the tympanum through the auditory meatus. From the tympanic cavity the vibrations are transferred to the three ossicles, which increase their strength and transmit them to the inner ear. In the inner ear the cochlea receives the sound waves. Through vibrations the endolymph is set in motion, which also vibrates the organ of corti.

Finally, the impulses are sent to the auditory nerve, which emerges at the base of cochlea and reaches the auditory cortex where the impulse is interpreted. Sound as a Stimulus We all know that sound is the stimulus for ears. It results from pressure variations in the external environment. Any physical movement disturbs the surrounding medium (i.e.

air), and pushes the air molecules back and forth. This results in changes in pressure that spread outward in the form of sound waves, travelling at a rate of about , ft/sec. These changes travel in waves much like the ripples set up by a stone thrown into a pond. When these sound waves strike our ears, they initiate a set of mechanical pressure changes that ultimately trigger the auditory receptors.

The simplest kind of sound wave is one that causes successive pressure changes over time in the form of a single repeating sine wave (Fig. . ). Sound waves vary in amplitude as well as in wavelength.

Amplitude is a general measure of stimulus magnitude. It is the amount of change in pressure, i.e. the extent of displacement of the molecules from the position of rest. In Fig.

. the amplitude of sound wave is represented as the distance of Fig. . : Structure of the Human Ear Hammer External canal Anvil Semicircular canals Saccule Utricle Facial nerve Auditory nerve Eustachian tube Round window Stirrup Tympanic membrane Pinna Cochlea the crest or trough from its mean position.

Wavelength is the distance between the two crests. Sound waves are basically formed due to alternate compression and decompression (rarefaction) of air molecules. A complete change in pressure from compression to rarefaction and again to compression makes a cycle of the wave. sound reflects the complexity of its sound waves.

Most of the sounds found in natural environments are complex. Vision and hearing are generally believed to be the two most highly prized senses. What would your life be if you lost any one of your senses? Which sense would you find more traumatic to lose?

Why? Think and write down. What if you could magically improve the performance of one of your senses, which sense would you choose to improve? Why?

Could you improve the performance of this one sense without magic? Think and write down. Discuss with your teacher.