SENSATION

Our knowledge of the human world depends upon three basic processes, called sensation, attention, and perception. These processes are highly interrelated; hence, they are often considered different elements of the same process, called cognition. Our seven sense organs collect all the stimuli for us in the form of information, each sense organ collecting different stimuli. These sense organs are also called sensory receptors or information-gathering systems because they receive or gather information from various sources.

Sensory channel: The physical stimulations are converted into nervous system activities by the process of transduction. This occurs at the receptor cells which are specialised for the most efficient conversion. During transduction, receptor cells convert physical energy into an electric voltage called receptor potential. This leads to a series of nerve impulses which travel to the brain or spinal cord i.e. the central nervous system. This is the process or channel of sensation.

Five of these sense organs collect information from the external world. These are eyes, ears, nose, tongue, and skin. While our eyes are primarily responsible for vision, ears for hearing, nose for smell, and tongue for taste, skin is responsible for the experiences of touch, warmth, cold, and pain. Specialized receptors of warmth, cold, and pain are found inside our skin.

Besides these five external sense organs, we have also got two deep senses. They are called kinesthetic and vestibular systems. They provide us with information about our body position and the movement of body parts related to each other. With these seven sense organs, we register ten different varieties of stimuli.

The initial experience of a stimulus or an object registered by a particular sense organ is called sensation. It is a process through which we detect and encode various physical stimuli.

Our sense organs function with certain limitations. For example, our eyes cannot see very dim or bright things. Similarly, our ears cannot hear extremely low or high-pitched sounds. The same is true for other sense organs also. As human beings, we function within a limited range of stimulation. The minimum value of stimuli, required to activate a given sensory system is called Absolute Threshold or Absolute Limen (AL).

The relationship between stimuli and the sensation they evoke has been studied in a discipline, called psychophysics.

It is also not possible to differentiate between all stimuli. To notice two stimuli as different from each other, there has to be some minimum difference between the value of those stimuli. The smallest difference in the value of two stimuli that is necessary to notice them as different is called Difference Threshold or Difference Limen (DL).

However, Sensory processes do not depend only on the stimulus characteristics. Sense organs and the neural pathways connecting them to various brain centers also play a vital role in this process. A sense organ receives the stimulus and encodes it as an electrical impulse.

THE HUMAN EYE

Our eye is made up of three layers:

• In the outer layer, there is a transparent cornea and a tough sclera that surrounds the rest of the eye. It protects the eye and maintains its shape.

• The middle layer is called the choroid, which is richly supplied with blood vessels.

• The inner layer is known as the retina. It contains the photoreceptors (rods and cones) and an elaborate network of interconnecting neurons.

The eye is generally compared with a camera. For example, the eye and camera have a lens. The lens divides the eye into two unequal chambers, namely the aqueous chamber and the vitreous chamber. The aqueous chamber is located between the cornea and the lens. It is smaller in size and is filled with a water-like substance, called aqueous humor. The vitreous chamber is located between the lens and the retina. It is filled with a jelly-like protein, called vitreous humor.

These fluids help in holding the lens in its appropriate place and proper shape. They also allow enough flexibility for the occurrence of accommodation — a process through which the lens changes its shape to focus the objects at varying distances. This process is regulated by ciliary muscles, which are attached to the lens. These muscles flatten the lens to focus the distant objects and thicken it to focus the near objects.

The iris is a disc-like colored membrane lying between the cornea and the lens. It controls the amount of light entering the eye by regulating pupil dilation. In dim light, the pupil dilates; in bright light, it contracts.

The retina is the innermost layer of the eye. The word retina means network. It is made up of five types of photosensitive cells among which rods and cones are most important. Rods are the receptors for scotopic vision (night vision). They operate at low intensities of light and lead to achromatic (colorless) vision. Cones are the receptors for photopic (daylight) vision. They operate at high levels of illumination and lead to chromatic (color) vision. Color blind people have deficiencies in their cone functioning. Transduction of physical energy into receptor potential occurs in rod and cone cells.

Each eye contains about 100 million rods and about 7 million cones. The cones are highly concentrated in the central region of the retina surrounding the fovea, which is a small circular region the size of a pea. It is also known as the yellow spot. It is the region of maximum visual acuity. Besides photoreceptors, the retina also contains a bundle of axons of a cell (called ganglion cell) that forms the optic nerve, which leads to the brain. The optic nerve leaves the retina from the area that has no photoreceptors. In this area, visual sensitivity is completely absent. Therefore, it is called the blind spot.

Light adaptation refers to the process of adjusting to bright light after exposure to dim light. This process takes nearly a minute or two. On the other hand, dark adaptation refers to the process of adjusting to a dimly illuminated environment after exposure to bright light. This may take half an hour or even longer depending on the previous level of exposure of the eye to light.

NIGHT BLINDNESS: 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, light adaptation takes place in the eyes. On the other hand, dark adaptation is achieved by the removal of light, 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 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.

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.

• External Ear: It contains two main structures: pinna and auditory meatus. Pinna is a cartilaginous funnel-shaped structure that collects sound waves from the surroundings. The auditory meatus is a canal protected by hair and wax that carries sound waves from the pinna to the tympanum or ear drum.

• Middle Ear: The middle ear starts with the 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 the Eustachian tube, which maintains the air pressure in the 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 10 times and send them to the inner ear.

• Inner Ear: The inner ear has a complicated structure known as the membranous labyrinth, which is encapsulated in a bony shell called the bony labyrinth. It has delicate 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 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 the cochlea and reaches the auditory cortex where the impulse is interpreted. When these sound waves strike our ears, they initiate a set of mechanical pressure changes that ultimately trigger the auditory receptors.

Amplitude and frequency both are physical dimensions. Besides these, there are three psychological dimensions of sound, namely loudness, pitch, and timbre. Loudness of the sound is determined by its amplitude. Sound waves with large amplitude are perceived as loud; those with small amplitude are perceived as soft. Loudness is measured in decibels (dB). Pitch refers to the highness or lowness of a sound. The seven notes used in Indian classical music represent a gradual increase in their pitch. Timbre refers to the nature or quality of a sound. For example, the sound of a car engine and a person talking differ concerning quality or timbre.

The Kinesthetic system

Its receptors are found primarily in joints, ligaments, and muscles. This system gives us information about the location of our body parts about each other

and allows us to perform simple (e.g., touching one’s nose) and complex movements (e.g., dancing). Our visual system provides a great deal of help in this respect.

The Vestibular system

This system gives us information about our body position, movement, and acceleration — the factors that are critical for maintaining our sense of balance. The sensory organs of this sense are located in the inner ear. While vestibular sacs inform us of our body positions, the semicircular canals inform us about our movements and acceleration.