The Role of The Sense of Touch

Similar to most humans, you are mindful of the importance of your ability to hear and see in order to function in everyday life- the thought if going blind or deaf is overwhelmingly distressful. But, what about the importance of your sense of touch? If you are walking through your house in the middle of the night, being able to make it through without hurting yourself not only depends on your cognitive map but your tactile sensations. For example, when you are trying to find your vibrating cell phone in your backpack, if you are relying primarily on your vision without using your sensation of touch at all to help you find it, by the time the you find it, it will have stopped ringing.

The sense of touch exhibits not just the existence of an object but also details about an objects size, firmness, shape, texture, which are all essential characteristics for our interactions with the environment and the objects in it. The touch sense could possible serve another important purpose as well. Though it is often considered that vision is the most edifying sense, we should consider the sense of touch to be the most reliable one. If we were to reach into the air, only to feel nothing but air, which of your senses would you rely on? In situations or conflicts such as that, we would most likely trust our sense of touch over the others. The information we take in with the direct interaction with the object (or no interaction) cannot be easily denied.

The sense of touch uses several different types of receptors called mechanoreceptors. These receptors are located in the skin and respond directly to mechanical stimulation or pressure or deformation of the skin. Two of these mechanoreceptors are the Merkel disks and the Meissner corpuscles and they are immediately below the skins surface. These two types of mechanoreceptors have small and punctuate receptive fields, meaning that they respond to touch information from a certain area of the skin. With that being said, these receptors are responsible for encrypting fine details of tactile stimuli like texture. To acknowledge and appreciate the importance of these mechanoreceptors, take into account the density of the Meissner corpuscles which are located on the tops of the eyelids and the fingertips. They decline slowly from forty-fifty per square millimeter of skin during late adolescence to around 10 or so per square millimeter by the time you reach age 50. The decline in these receptors accurately predicts the loss of sensitivity to distinct tactile information that elderly people experience.

Two other types of mechanoreceptors are, the Pacinian corpuscles and the Ruffini endings, and they reside in deeper positions in the skin. These receptors are different from the Meissner corpuscles and the Merkel disks because they have large, diffuse receptive fields and respond to touch information over a much bigger, indistinct area, and provide a “bigger picture” type of information about the nature of a touch stimulus. All four mechanoreceptor types field sizes vary all over the body. When the field sizes are smaller they provide more detailed and precise information, in the parts of the body that are important in the evolutionary aspect for our body such as fingertips and lips. Mechanoreceptors also are different in their rate of adaptation. Both the Meissner’s corpuscles and the Pacinian corpuscles adapt very rapidly. they respond quickly when a tactile stimulus first makes contact with the skin and when the stimulus is removed, but little in between. In contrast, the Ruffini endings and the Merkel’s disks adapt slowly; they respond relatively steadily to continuously applied pressure. An example of the relevance of slow and fast adaptation, picture being in a cold winter morning and you are hurrying to put on your favorite warm jacket. For a moment, you are aware of the clothing pressing against your skin, but eventually you will become unaware of any pressure from the jacket unless you are focusing consciously on it. This situation reflects the response of your receptors adapting. At the onset of the stimulus both slowly and rapidly adapting receptors respond; then, after a brief period of time (around 300-600 milliseconds), the Meissner and Pacinian corpuscles adapt and your tactile experience change.

A good way to experience the importance of rapidly adapting touch receptors would be to close your eyes and try to identify an object by only touching it. Are you able to tell what the object is by only touching it once? Probably not- you need to keep the constant motion so your rapidly adapting touch receptors are able to respond and give your brain the information to tell what that object is. Next we are going to highlight the general principle of perception: to gain the best information regarding a specific event or object, actively exploring it using whichever senses are available to us. Think about this scenario, if you were to be asked to describe an object sitting in front of you in detail, you would not just take a small glance at the object, you would move your eyes all over its surface to explore every detail. Correspondingly to fully appreciate the object’s tactile properties you would also move it around and hold it in your hands, this active exploration using touch is termed haptic perception.

Lenderman (1987) and Klatzky (1990) found that people participate in a series of ritualized and predictable actions called exploratory procedures that reveal different types of signals when examining an object with their hands. Each procedure reveals distinct types of information about the object in question. An example, “contour following” provides data regarding the object’s shape and running of the fingers across an object’s surface deemed “lateral motion” tells us texture information. As long as these and other exploratory procedures are no impeded, humans can identify objects with incredible speed and accuracy by touching alone.

Haptic perception as just suggested, provides much more information about an object than just passive touch. Attempting to perceive an object by simply holding it, without exploration, often leads to mass error in perceptual judgement. Undoubtedly, the dominance of haptic perception must be associated in part to the progressing activity of the detail-oriented, vigorously adapting, touch receptors. Nevertheless, haptic perception beats passive tough even after you take adaptation into account: lateral motion across your motionless hand, fully engaging rabidly adapting receptors, will not generate the same level of perceptual knowledge as active exploration.