Sensation

Sensation means to sense an external stimulus with the help of sensory organs such as the eye, ear, nose and skin. The term sense means to detect or to become aware of something.

Sensation is defined as the ability of an organism to detect an external stimulus (such as view of something, sound or voice, smell, and touch) with the help of sensory organs such as the eye, ear, nose, and skin.

Our sensory organs have sensory receptors which detect or feel the external stimulus. For instance, the retina, a layer inside our eye, has sensory receptors. When light reflects from an object and falls on the retina of our eyes, the sensory receptors in the retina detect the object which means that sensation occurs.

Our ears also have sensory receptors. When soundwaves strike the receptor inside the ear, it detects the sound which means that sensation occurs. Similarly, our nose has receptors to detect smell; and our skin has receptors to detect touch. Whenever the sensory organs detect an external stimulus, it means that sensation occurs.

The term sensation is sometimes confused with another term perception. It is important to understand the difference in these terms. In sensation, we only detect raw sights and sounds without giving them any meaning. However, these detected raw sights and sounds go through the process of perception to derive the meaning of these detected sights and sounds. It can be said that sensation is the first stage, and perception is the second stage; and both stages, as in a combined manner, help in making a complete sense of the external stimulus.

  Steps involved in sensation

The sensation is not a single-step process. It involves multiple steps. It takes place so quickly (in a fraction of a second) that it seems like a one-step quick process. However, it involves the following three steps: 1) Reception, 2) Transduction, and 3) Transmission.

   Reception 

The term reception means to receive something. It is the first step of sensation where the sensory organs receive external stimuli. The external stimuli can be soundwaves, light waves reflected from some object, voice or sound, smell, and pressure (in case of touch on the skin). As the stimuli strike the sensory receptors in the sensory organs, the stimuli are received by the sensory receptor. It means that reception has occurred. For instance, when light waves, reflected from an object, falls on sensory receptors in the eye, these stimuli are received by the receptors. Similarly, when soundwaves strike receptors in the ear, they are received by the receptors in ear.

An important term here is the reception field. The reception field is the range in space around the sensory organ where the sensory organ can receive the stimuli. The reception field varies for each sensory organ. For instance, for skin, to feel a touch, the stimuli must be in contact with the skin. For ear, the reception is relatively wider which means that the ear can receive stimuli from a source lying at relatively farther distance. For eyes, the reception field is even more wider which means eyes can receive a stimulus from a source lying at even more farther distance. For instance, we can see stars lying far away in the sky.

   Transduction

The term transduction means to transform something from one form into another form. When stimuli are received by receptors, they need to be carried to the brain through nerves. However, the received-stimuli cannot travel to brain in its received actual form; and need to be converted into electrical signals (or electrical impulses) in order to be carried to the brain.

Therefore, the stimuli are converted from their received-form into electrical signals. This process of conversion is called transduction. For instance, the soundwaves are mechanical waves when they are received by the receptors in the ear, they are converted into electrical signals in order to be carried to the brain.

The transduction process takes place in the receptor cells within the sensory organ. For instance, when light falls on the retina of the eye, the retina has receptor cells called rod cells wherein the received-light energy is converted into electrical impulses with help of electrochemical changes in the receptor cells.

   Transmission

The stimuli which are converted into electrical signals (or impulses) are carried to the brain. The process through which the electrical signals reach the brain is called transmission. These electrical signals move through nerves from the sensory organ to the brain. A nerve can be understood as a cable or wire that transmit signals from the sensory organon to the brain. The nerve is a combination of neurons where the electrical energy is processed from one neuron to another and finally reaches the brain.

Different type of stimuli has different centers in the brain where the stimuli (in the form of its electrical impulses) are received. For instance, the electrical signals of soundwaves are sent to the temporal lobe of the brain. Similarly, electrical signals for vision are sent to the occipital lobe of the brain. Likewise, the electrical signals for smell are transmitted from the nose to the olfactory lobe of the brain.

   The role of brain – Interpretation

The above three stages – reception, transduction, and transmission – constitute what is called sensation. To sharply distinguish the term sensation from the term perception, it is generally said these three stages (reception, transduction and transmission) completes the process of sensation, and what follows sensation can be called perception – the stage of interpretation – which takes place in the brain.

When the electrical impulses for different stimuli (sight, sound, smell, and touch) reaches their respective centers within the brain, they are required to be converted into an interpretable form. Therefore, the brain processes the received-electrical impulse to give them meaning. This is because the received-electrical impulses are initially in unorganized form. For instance, both the eyes form slightly different images, therefore to convert them into one image, the brain process the electrical impulses received from both eyes. Similarly, depending upon the distance from an object and position (angle) to look at the object, the image of the object formed on the eye may vary in its size and shape, but the brain processes its electrical impulses to perceive it in its standard size and shape. The brain also works on the color details of electrical impulses (for images) to be perceived in the best way.

In the same way, the electrical impulses for sound are processed in its respective center within the brain to be interpreted properly.