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After reading this article you will learn about:- 1. Working of Sensory System 2. Types of Sensations 3. General Characteristics.
Working of Sensory System:
In a way, sensations are purely the result of physical stimuli operating on our nervous system. There is little psychological content in sensations.
Typically the sensory system works as follows:
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A stimulus which may be a light or a sound or heat or even some discomfort inside the body impinges on a receptor or sense organ.
The disturbance is transmitted through the sensory nerves in the form of electrochemical impulses which, in turn, reach a particular area of the brain or to be more exact the cerebral cortex. The activation of the area results in the sensory experience. This is how we experience sensations. Thus, the sensory system involves a receptor or sense organs, sensory nerves as receptor area and a particular area of the brain.
Different sensory experiences are located in different areas of the cortex.The sensory system is linked up with the motor system at two levels of the central nervous system. The first level is at the spinal cord and the second is at the level of the cerebral cortex. Impulses of certain stimuli are not carried to the brain centres but terminate at the level of the spinal cord itself and so result in an immediate response. Such instances are known as reflex actions.
These are unlearnt and primarily concerned with the survival of the organism, avoidance of injury and pain. Reflex actions represent the simplest level of sensory motor behaviour. For example, when you touch a piece of ice, you immediately withdraw your hand.
Other sensory impulses are carried to the concerned brain centre, and are relayed to those centres or areas of the brain connected with action or response. These latter are called motor areas. Activation of the motor areas results in an activation of the motor or efferent nerves.
These motor impulses are transmitted downwards, activate the appropriate muscles and glands which are known as effector organs and result in appropriate behaviour. For example, you see a stone thrown by somebody flying towards you and you immediately move away from its path. This action, unlike reflex action, is a result of learning, past experience and knowledge. Thus, the sensory system works in close conjunction with the motor system and they are together known as the sensory-motor system.
Types of Sensations:
One of the questions which received a lot of attention during the early days of experimental psychology or even earlier was regarding the number of sensations that exist.
For a long time it was believed that there were only five types of sensations:
(i) Vision or seeing,
(ii) Audition or hearing,
(iii) Olfaction or smell,
(iv) Gustation or taste and
(v) Cutaneous or skin sensations (cold, heat, pain, etc.).
Traditional Indian thought, however, postulated two kinds of sensations. The first kind called cognitive senses include the same five mentioned above, the Jnanendriyas. The other five called operative senses, Karmendriyas – include sensations connected with bodily operations and not emanating from external stimuli.
Table showing different kinds of sensations and the type of experiences.
Today the consensus is that there are eight basic sensations involving eight different sense organs and resulting in eight different types of sensory experiences.
The above table shows the basic types of sensations experienced by the human beings. These eight sensations are qualitatively different. The difference in quality depends on the type of stimulus, the receptors stimulated and finally the area of the brain or cerebral cortex involved.
Sometimes, however, there is a fusion among these senses, a stimulus of a particular sense modality being experienced as a sensation of another type. This can be seen for example, in descriptions such as ‘piercing look’ or ‘sweet voice’. This phenomenon is known as synesthesia. The phenomenon of synesthesia probably points to the existence of a particular type of sensory experience where the qualitative differences mentioned above does not operate.
General Characteristics of Sensations:
In spite of the qualitative differences mentioned above, sensations, in general, share certain common characteristics.
These are found to be true of all the above kinds of sensations.
These may be listed as below:
1. Intensity:
Within each modality sensations vary in intensity from low to high in a continuous manner. Thus, we experience mild pain or severe pain, faint light or bright light and so on.
2. Threshold (Absolute Threshold):
For any sensation to be aroused, the stimulus (light, sound, touch etc.) must have a minimum intensity. Stimuli of very low value are not responded to.
3. Differential Threshold or Differential Limen:
When we are listening to a sound, we do not respond to every small change in the sound. Similarly, if a weight of 100 grams is placed on your palm and a further one gram is added we do not feel the difference. But if instead of 1 gram, 10 grams are added we feel the difference.
Changes in stimuli are not sensed unless the change involved is at least of a certain minimal intensity. The minimal change in stimulus intensity necessarily required to produce a change in the sensory experience is known as the differential threshold. For every type of sensory stimulation there is a differential threshold value of the stimulus.
4. Adaptation:
Suppose you are asked to place your hand on a block of ice. In the beginning the cold sensation is very severe but gradually the severity decreases. This is because the sensory system is able to adapt itself. The same thing is true when a person enters a dark room and the eye gets adapted to the dark conditions. This adaptation is a unique characteristic of sensations.
5. After-Images or After-Sensations:
Suppose you look up and stare at the bright afternoon sun for a few moments and then look at other objects around you. You feel that you are still seeing a yellow shining ball. This sensory experience continues even after the cessation of the stimulus.
6. Extensity:
Sensations also vary in extensity or size. Thus, we may see a large patch or a small patch of light.
7. Duration:
Sensations also possess the property of duration. Our sense experiences last for different lengths of time. Sensations like vision and sound arise gradually and also disappear gradually, while there are other sensations which are sudden both in their appearance and disappearance.
The above characteristics are, thus, general characteristics of all sensory experiences. These characteristics have their own advantage. For example, if there were no absolute thresholds we would hear all the sounds in the world. Also, if the phenomenon of adaptation did not exist, people could not work in noisy factory conditions. Similarly, much of our aesthetic appreciation depends on our ability to experience after sensations or after images.