Other Human Senses

temperature, its pressure on our tongue, and many other sensations. When these factors are removed, we are left with only four basic tastes. Besides, the combination of different flavours in varied proportions results in a different kind of flavour which may be quite unique. .

Touch and other skin senses : Skin is a sensory organ that produces sensations of touch (pressure), warmth, cold, and pain. In our skin there are specialised receptors for each one of these sensations. The receptors of touch are not evenly distributed in our skin. That is why some areas of our body (e.g., face, fingertips) are more sensitive than others (e.g., legs).

Pain sensation has no specific stimulus. Hence, determining its mechanism has been fairly difficult. . 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 in relation to 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. it on to the retina. Retina is divided into two parts: the nasal half and the temporal half.

The inner half portion of the eye (towards the nose), taking the center of fovea as mid-point, is called the nasal half. The outer half portion of the eye (towards the temple) from the center of fovea is called the temporal half. Light from the right visual field stimulates the left half of each eye (i.e. the nasal half of the right eye and the temporal half of the left eye), and light from the left visual field stimulates the right half of each eye (i.e.

the nasal half of the left eye and the temporal half of the right eye). An inverted image of the object is formed on the retina. The neural impulse is transmitted to the visual cortex through the optic nerve where the image is re-inverted and processed. You can see in Fig.

. that 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 .

Adaptation The human eye can function at a very large range of light intensities. Sometimes we have to undergo a rapid change in illumination levels. For example, when we go to a matinee show movie, we find it difficult to see things in the hall on entering into it. However, after spending about to minutes there, we are able to see everything.

After the show when we go out into the open, we find the light outside the hall too bright to see things, or sometimes even to keep our eyes open. However, within a minute or so we feel comfortable, and are able to see things properly. This adjustment is faster than the one made on entering the hall. The process of getting adjusted to different intensities of light is called ‘visual adaptation’.

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 Fig. .

: Structure of the Human Eye Cornea Iris Aqueous humor Pupil Lens Ciliary muscle Retina Blind spot Optic nerve Vitreous humor Fovea Distal stimulus Proximal stimulus 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. There are certain ways in which these processes can be facilitated. An interesting activity is given below to demonstrate this process to you.

Colour Vision In our interaction with the environment we not only experience a variety of objects, but also their colours. It may be noted that colour is a psychological property of our sensory experience. It is created when our brain interprets the information received from the external world. It may be remembered that light is described physically in terms of wavelength, not in terms of colour.

As we have read earlier, the visible spectrum is a range of energy ( - nm) that our photoreceptors can detect. The energy lower or higher than the visible spectrum is harmful to the eyes. The sun light is a perfect mixture of seven colours just like a rainbow. The colours observed are violet, indigo, blue, green, yellow, orange, and red, abbreviated as ‘VIBGYOR’.