The Mechanism Of A Sense Of Touch
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.
Touch Receptors
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 (Mistretta and Thornbury, 1981). 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.
Haptic Perception
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 and perceiving the information it gives you, 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 (Klatzky, Lederman, & Metzger, 1985; Lederman & Klatzky, 2004). 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 (Rock and Victor, 1964). 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.
The reasoning for the difference between advantages of passive and active touch can be unambiguously explained when you call to mind that all of the somatosensory systems work together with one another. Tactile input alone dispenses imprecise data regarding the nature of an object. Feeling a sharp point, by way of illustration, exhibits a small amount of detail about an object if you do not know where the point is positioned in relation to other features of the object, or what actions your fingers may have made to discover the sharp point. To interpret what your mechanoreceptors reveal, you must consider the movements and the relativity of the positions of your fingers. By way of explanation, the process of haptic perception incorporates integrating tactile information with information from the senses regarding kinesthesis and proprioception.
Dermatomes
Receptors for touch synapse with the neurons that carry the touch signals to the spinal cord and then into our brain for processing. It is notable that neurons from individual, identifiable “strips” of the body go into the spinal cord together and travel to the brain as a collective group. These groups are deemed dermatomes. The dermatomes on the right side of the body reflect the dermatomes on the left side of the body. Damaging to the axons entering the spinal cord at even a single level can cause a complete loss of tactile sensation from a single dermatome in the body.
Somatosensory Ascending Pathways
Two of the major neural pathways that ascend into the spinal cord and carry the somatosensory information to the brain are the dorsal-column medial lemniscus system and the anterolateral system or spinothalamic tract. The dorsal-column medial lemniscus system carries touch information to the back portion of the spinal cord. The anterolateral system, transports signals about pain and temperature and ascends it into the front portion of the spinal cord. The anterolateral system is an ipsilateral system, meaning, that the majority of the neurons in the system do not cross to the other side of the body as they go into the spinal cord; the information from the left side of the body is processed in the left hemisphere of the brain and the information from the right side of the body is processed in the right hemisphere of the brain. On the contrary, he dorsal-column medial lemniscus system is a contralateral system, meaning, the neurons from the right side of the body cross from the right side of the spinal cord to the left side and send their information to the right hemisphere of the brain and vice versa.
Cortical Processing of Somatosensory Information
What happens to the information from the somatosensory system transported by the ascending pathways, once it reaches the brain? Wilder Penfield and his colleagues answered this question for us with their research. Penfield decided to see what would happen if they electrically stimulated the brains of their human patients during brain surgery, while they were awake and could speak and recorded the responses that these stimuli produced (Penfield & Rasmussen, 1950; Penfield & Boldrey, 1937). Consequent to the stimulation of the most frontal part of the brain, the parietal lobe, the postcentral gyrus (most central to somatosensory processing, commonly called the primary somatosensory cortex), patients expressed experiencing tactile sensations.
More experimentation by Penfield and his colleagues uncovered two important characteristics of the primary somatosensory cortex. First off, the different regions in the postcentral gyrus correspond to different parts of the human body. The hand and the arm and other adjacent regions of the body receive representations in adjacent regions of cortex. They also discovered that the size of the areas on this cortical map, which was consistent from one person to another, varies per the sensitivity of certain areas of the body to tactile stimuli. The more mechanoreceptors there are in a distinct area, the more sensitive that area, and the bigger the cortical representation tends to be. Parts of the body like the lips and the fingers that are especially important for interaction and adaptation to the environment occupy a larger cortical space than the areas of less importance in adaptation, like the back and feet.
If the brain acquires damage causing the somatosensory cortex to fail to process tactile information completely or adequately, even though it is rare, the failure of the brain to process the stimulation of touch is called astereognosia, sometimes called tactile agnosia. Individuals who suffer from this experience tactile sensations but are not able to identify objects using the sense of touch.
Plasticity in the Somatosensory Cortex
As we have learned from biology classes and other psychology classes over time, the cortex dedicates substantial processing capacity to areas of the body that are evolutionary important for tactile exploration: in humans, being the hands and mouth. This discovery implies Darwinism and that the brain will adapt to the needs of the organism over time. Correspondingly, however the adaptation occurs in a much smaller timeframe. Coinciding evidence indicates that the brain shows momentous plasticity-flexibility in its representation- regarding somatosensory processing.
