Sensory Processes
Sensory Processes :
The term sensation refers to the process of receiving information in the form of energy (light, heat, sound etc.) from the world outside and sorting it out into the proper sense - vision, touch, hearing. Once that information has been received, we interpret it and arrive at an understanding of what it means, a process referred to as perception. Sensation and perception make up an extensive information gathering system.
Each sense has its own receptors that constantly monitor our environment. All sensory systems have certain characteristics: The sensory system must be selective, which means that only certain types of incoming information are processed. For example, we have more than one kind of receiver for touch. One which responds to changes in temperature and one which responds to damaged cells. The sensory system must have an adjustable speed. Nerve fibers to the ear respond in less than a thousandth of a second because sudden noise does not require analysis, as it does a speedy response. However, the visual system will respond quickly to a blur as something comes towards us, a potential danger, yet it will take it's time when analyzing a complex scene. The system must also be sensitive, but not too much. If our ears were too sensitive we would hear blood running through artery at the base of the ear. Sensory measurement must be reliable. Reliability comes from comparing incoming stimulus with the conditions around us.
Vision
The optic nerve delivers its impulses to a special area of the brain called the visual center. This is where people see objects in the sense of recognizing and reacting to what their eyes look at. In other words, seeing always involves the brain's visual center. Here sensation turns into perception. The brain must learn by experience to analyze correctly the impulses it receives from the eyes. For instance, the lens system of the eye, like that of a camera, transmits its light pattern upside down. The brain has to learn that the impulses received from the upper part of the retina represent the lower part of the object sighted and vice versa.
In the brain also are located the centers that control all the eye's muscular movements, such as the opening and closing of the iris, the focusing of the main lens, and the movement of the eyeball. The eyeball’s movement is voluntary. Other eye adjustments are reflexes. Most individuals use both eyes to see an object. This type of sensory perception is known as binocular vision. Thus two images of the object are formed one on the retina of each eye. Impulses from both images are sent to the brain. Through experience these impulses are interpreted as two views of the same object. Because the eyes are about 2 inches apart from pupil to pupil and therefore are looking at the object from different angles, the two views are not exactly alike. This is known as the stereoscopic effect. If the object is far away, the difference between the images is slight. If it is a few inches away, the difference is very great. The brain makes good use of this phenomenon. It learns to judge the distance of an object by the degree of difference between the images it receives from the two eyes. In the same way the brain perceives what is called perspective.
The Eye
The retina is a soft, transparent layer of nervous tissue made up of millions of light receptors. The retina is connected to the brain by the optic nerve. All of the structures needed to focus light onto the retina and to nourish it are housed in the eye, which is primarily a supporting shell for the retina. When light enters the eye it passes through the lens and focuses an image onto the retina. The retina has several layers, one of which contains special cells named for their shapes rods and cones. Light-sensitive chemicals in the rods and cones react to specific wavelengths of light and trigger nerve impulses. These impulses are carried through the optic nerve to the visual center in the brain. Here they are interpreted, and sight occurs. Light must pass through the covering layers of the retina to reach the layer of rods and cones. There are about 75 to 150 million rods and about 7 million cones in the human retina. Rods do not detect lines, points or color. They perceive only light and dark tones in an image. The sensitive rods can distinguish outlines or silhouettes of objects in almost complete darkness. They make it possible for people to see in darkness or at night. Cones are the keenest of the retina's receptor cells.
Hearing
In hearing the basic energy form is sound waves. Sound waves form at various speeds, or frequencies. The frequency of any given tone is measured in terms of the number of cycles per second. Sound travels slowly compared to light at anything from 20-20,000 cycles per second. The sounds we hear have three basic characteristics. Pitch which is the frequency of the sound. Timbre, determines the tonal quality. The loudness or intensity of the sound wave is measured in decibels. The human ear can pick up sounds just above '0' decibels, otherwise there would be complete silence.
Decibel Table
Decibels Noise…Threshold
40 Quiet office…Normal
60 Normal conservation…Normal
75 Road Traffic…Noisy
100 Subway Train…Potential Damage
130 Rock Concert Human….Pain Threshold
140 Aircraft Taking-off Human….Pain Threshold
The Structure of the Ear
The ear has three separate sections the outer ear, the middle ear and the inner ear. Each section performs a specific function, related to either hearing or balance. The three parts of the outer ear are the auricle (also called the pinna), the external auditory meatus (or ear canal), and the tympanic membrane (or eardrum). The pinna collects sound waves from the air. It funnels them into a tube, the external auditory meatus. This is a curved corridor that leads to the tympanic membrane. The eardrum separates the external ear from the middle ear. The middle ear is an irregular-shaped, air-filled space. A link of three tiny bones, the ossicles, spans the middle ear. When sound waves strike the outer surface of the eardrum, it vibrates. These vibrations are mechanically transmitted through the middle ear by the ossicles, to the opening. This opening is the round window. Like the eardrum, the round window's membrane transmits vibrations. It directs vibrations into the inner ear, where they enter a fluid that fills a structure called the cochlea. This is a coiled tube that resembles a snail's shell. Within the cochlea is housed the true mechanism of hearing, called the organ of Corti. It contains tiny hair-like nerve endings anchored in a basilar membrane, which extends throughout the cochlea. The unattached tips of these nerve endings are in contact with an overhanging membrane, called the tectorial membrane. When vibrations pass into the inner ear, they cause waves to form in the cochlear fluid. Receptor nerve cells in the organ of Corti are highly sensitive to these waves. Other specialized nerve cells send the electrochemical impulses produced by the wave motion into the cochlear branch of the acoustic nerve. This nerve carries the impulses to the brain, where sound is identified.
Taste
It is widely accepted that there are four basic taste qualities, salty, sour, sweet and bitter. It was originally thought that there was a sensory path for each of these tastes. However it appears that there is a pattern of activation in a number of different fibres providing the required sensory input to the brain to distinguish these different tastes. The papillae on the surface of the tongue are the receptors for these taste sensations.
Smell
Deciphering the sensory information for the sense of smell is not dissimilar to that of taste. In the olfactory area the nerve endings grow through the mucous membrane which acts as receptors to determine odors present in the air we breathe.
Touch
The skin or cutaneous sense has some 5 million sensors of at least 7 types throughout the human body. The three major types are Meissner's corpuscles which sense touch. The Pacinian corpuscle's which determine movement and vibration and the Krause end bulbs which sense changes in temperature.
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