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Sensations & Perceptions of the Body | Part 1

Every one of us is aware that we can see through eyes, smell through the nose, hear through ears. But have we paused for a moment to understand the process of sensing and perceiving? And how does the nervous system internally interprets the various sensory signals as they are? Finally, how are we able to perceive the senses?

First, Definitions

Sensation

 Sensations are the activities of sense organs acquired by our system.

Sensory information is processed through sense organs through a method called transduction. When the receptor cells from our sense organs collect the sensory signals as physical energy,  information from the physical world is modified (or transduced) to electrical signals by the central nervous system.

Perception

Perceptions, on the other hand, are the organisation and interpretation of such bodily sensations.

The converted electrical signals from the sensory organs are then organised and processed further by specific areas of the brain to understand what we are sensing. This is the process of perceiving. However, what our brain perceives is not always perfect. We often get misled by our perceptions leading to illusions.

So to recap… sensations are those physical signals that interact with our sense organs directly. Though the signals from the physical world are always present around us, it is our attention to these sensory signals that we become aware of it.

On the other hand, perceptions are those activities within the brain that process the sensory signals to able to attach a meaning and relevance to it.

For example, let us take an orange fruit. When we attend to the sight of the fruit, we consciously capture the sensory details through our eyes. Our optic nerves convert the retinal image (with various image characteristics) as an electrical signal and send it to the occipital lobe of the brain. After the brain receives the data from the eye, it categorises and organise and compare it to the past data. Finally, we perceive the orange fruit from our past information and current intellect.

Perception is important for our understanding of the physical world around us but- sensation gives us the first knowledge that some object is present near us (which is a crucial step as perception).

Types of Sensations

There are six kinds of sensations- vision, hearing, smell, taste, touch and kinesthesis.

Vision

Technically, the eyes are the type of sensory receptors that help in our vision. The main parts that help to convert physical energy to electrical energy are the cornea, pupil, iris and retina.

The photoreceptor cells transmit information to the brain through optic nerves. Rod and cone cells are very significant photo-receptor cells located in the retina. Rod cells function under low light conditions whereas cone cells work under heavy light. Cone cells also help to detect colour. So rod and cones help in light and dark adaptation.

Where the bundle of optic nerves meet on the retina, there is no visual acuity. This space is called a blind spot. However,  we unconsciously compensate for the blind spot with the other eye or move our head without feeling uncomfortable.

There is another concept in vision- Nearsightedness and farsightedness.

When the eyeball is wide and bigger in shape, the image is focused in front of the retina and not on it. In such a case, near vision is clear but far objects are blurred. This is called Nearsightedness. On the contrary, when eyeballs are short, the image focuses behind the retina screen. So the near vision becomes hazy and the far objects are distinct and clear. This is Farsightedness.

Hearing

For the purpose of the hearing, humans have 2 ears. Ears detect the sound vibrations from the environment around us and measure the air pressure to construct the sound energy into electrical energy. There are 3 parts to the ear that helps in the transduction process- the outer ear, middle ear and inner ear.

There are 3 features to a sound which our ears are detecting quite efficiently.

  1. Pitch- Determines the quality of the sound with respect to the vibration and frequency.
  2. Loudness- Determines the amplitude of the sound
  3. Timbre- Deals with the sound quality with respect to its source

Smell

The smell is another sensory signal that we can detect from the external world. It is one of the senses that allows us to perceive from a distance, similar to vision and hearing. The olfactory receptor cells present within the membrane of the inner nose helps in the process of smelling.

As opposed to other living beings, human beings have a sort of primitive smell features. For the sake of comparison- dogs have 200mn olfactory receptor cells whereas we only have 50mn of them. Our range of smell is only limited to substances with atomic weight from 15 to 300. Similar to the vision chart, we cannot smell anything from below or above the range.

But why is it so primitive? Many animals depend on their smell for survivability. Their sense of smell is thus highly evolved to thrive. But we humans don’t need to depend on smell to a large extent to survive in the environment. We have other faculties evolved high enough to survive in the world and compensate for our lack of smelling faculties.

Taste

We sense taste with the help of our taste buds. Taste buds are located on the upper and side area of the tongue. There are around 10000 taste buds in our tongue- a few bundled together which caters to different types of tastes. The only 4 basic tastes of a human being are sweet, salty, sour and bitter. What makes our taste feel so varied when we eat is the food’s smell, texture, temperature, and pressure that gets added to the taste (which makes us feel we can taste an infinite number of flavours!)

With age our taste buds are said to reduce in numbers.

Touch and other skin senses

We can feel whatever is in direct contact with our body. There are 4 primary sensations we feel- temperature (warmth and cold), pressure, pain.

The most sensitive areas of our body are the hands, fingers, lips and tongue. The least sensitive areas are the arm, legs and body trunk. It is also found that cold receptors are more in quantity than heat receptors.

Some researchers believe that pain in itself is not a sensation. It is just an extreme stimulation of other senses like heat, cold and pressure. It gets activated by tissue injury and is a special receptor property and not a sensation of touch. Some also believe the pain is caused by stimulation of nerve endings. There is a high level of debate on the ‘pain’-ful topic.

Kinesthetic sense

The sense of awareness of our position and movement of the body is known as kinesthesis. Being aware of the movement and keeping body balance falls under the kinaesthetic sense we have while functioning in the physical world.

So these are the sensory organs we have in our body and this is how they function and coordinate with the environment.

Author

Trisha Nandi

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