PSYC2018 Week 11 Reading The cutaneous senses PDF

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Introduction: - People with a condition that results in losing the ability to feel sensations through the skin often suffer constant bruises, burns, and broken bones in the absence of warnings provided by touch and pain (Melzack and Wall, 1988) - Losing touch makes it difficult to interact with the ...

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Introduction: - People with a condition that results in losing the ability to feel sensations through the skin often suffer constant bruises, burns, and broken bones in the absence of warnings provided by touch and pain (Melzack and Wall, 1988) - Losing touch makes it difficult to interact with the environment because of loss of feedback from the skin that accompanies many actions - Avenanti et al (2005): subjects have hands temporarily anesthetized, the loss of feeling causes them to apply much more force than necessary when carrying out tasks with fingers and hands - Ian Waterman: initial tingling sensation in limbs becoming a total loss of ability to feel touch below the neck, it was determined an autoimmune reaction had destroyed most of the neurons that transmit signals from skin, joints, tendons and muscles to the brain. The loss of ability to feel skin sensations meant he couldn’t feel his body when lying in bed, which resulted in frightening floating sensation, he also used inappropriate force when grasping objects, sometimes gripping too tightly and sometimes dropping them - > Eliminated his ability to sense where hands and legs are relative to each other and to the body, difficult to control his limbs. After many years of practice he was able to sit, stand, carry out tasks such as writing. He had learned to use his sense of vision to constantly monitor the positions of his limbs and body - Ian’s problems were caused by a breakdown of - (1) the somatosensory system which includes: the cutaneous senses, which are responsible for perceptions such as touch and pain that are usually caused by stimulation of the skin - (2) proprioception, the ability to sense the position of the body and limbs - (3) kinesthesis, ability to sense movement of body and limbs Overview of the Cutaneous System: Skin: - Warning function, prevents body fluids from escaping and at the same time protects us by keeping bacteria, chemical agents, and dirt from penetrating our bodies - Provides us with info about the various stimuli that contact it, the sun’s rays heat our skin, a pinprick is painful, when someone touches us we experience pressure - Main experience with skin is its visible surface, which is a layer of dead skin cells. The layer of dead cells is part of the outer layer of skin called the epidermis. Below the epidermis, there is another layer called the dermis. Within the skin are mechanoreceptors. Mechanoreceptors: - Many of the tactile perceptions that we feel from stimulation of the skin can be traced to the four types of mechanoreceptors that are located in the epidermis and the dermis - Slowly adapting (SA) receptors: respond with prolonged firing to continued pressure - Rapidly adapting (RA) receptors: respond with bursts of firing just at the onset and offset with a pressure stimulus - Two mechanoreceptors, Merkel receptor (SA1) and the Meissner corpuscle (RA1) are located close to the surface of the skin, near epidermis. Because they are close to the

surface, these receptors have small receptive fields, a cutaneous receptive field is the area of skin which, when stimulated, influences the firing of the neuron - > An example: a pressure stimulus is presented and then removed. The nerve fiber associated with SA Merkel receptor fires continuously as long as the stimulus is on, the nerve fiber associated with RA Meissner corpuscle fires only when the stimulus is first applied and when it is removed - > The type of perception associated with Merkel receptor is sensing fine details, and with the Meissner corpuscle, controlling handgrip - Other two mechanoreceptors, Ruffini cylinder (SA2) and Pacinian corpuscle (RA2 or PC) are located deeper in the skin, so have larger receptive fields - > Ruffini cylinder responds continuously to stimulation, the Pacinian corpuscle responds when the stimulus is applied and removed - > The Ruffini cylinder is associated with perceiving stretching of the skin, and Pacinian with sensing rapid vibrations and fine texture Pathways From Skin to Cortex: - Cutaneous receptors are distributed over the whole body. This wide distribution, plus the fact that signals must reach the brain before stimulation of the skin can be perceived, creates a travel situation ‘journey of the long distance nerve impulses’ - Signals from all over the body are conducted from the skin to the spinal cord, which consists of 31 segments, each of which receives signals through a bundle called the dorsal root - After the signals enter the spinal cord, nerve fibers transmit them to the brain along two major pathways - The medial lemniscal pathway: has large fibers that carry signals related to sensing the positions of the limbs (proprioception) and perceiving touch. These large fibers can transmit signals at a high speed, important for controlling movement and reacting to touch - Spinothalamic pathway: smaller fibers that transmit signals related to temperature and pain - -> Case of Ian Waterman illustrates separation in function, because although he lost ability to feel touch and sense position of limbs (lemniscal pathway) he was still able to sense pain and temperature (spinothalamic pathway) - Fibers from both pathways cross over to the other side of the body during their upward journey to the thalamus - Most of these fibers synapse in the ventrolateral nucleus in the thalamus, but some synapse in other thalamic nuclei (fibers from retina and cochlea also synapse in the thalamus, in the lateral geniculate nucleus for vision and medial geniculate nucleus for hearing) - Because the signals in spinal cord have crossed to other side of body, signals originating from the left side of the body reach the thalamus in the right hemisphere of the brain, and signals from the right side of the body reach the left Somatosensory Cortex: - From the thalamus, signals travel to the somatosensory receiving area (S1) in the parietal lobe of the cortex and possibly also to the secondary somatosensory cortex

(S2), signals also travel between S1 and S2 and from S1 and S2 to additional somatosensory areas - An important characteristic of the somatosensory cortex is that it is organized into maps that correspond to locations on the body, the existence of a map of the body on S1 was determined in by Penfield while operating on awake patients who were having brain surgery to relieve symptoms of epilepsy (Penfield and Rasmussen, 1950) - > When Penfield stimulated points on S1 and asked patients to report what they perceived, they reported sensations such as tingling and touch on various parts of their body. He found stimulating ventral part of S1 (lower on parietal lobe) caused sensations on lips and face, and stimulating higher on S1 caused sensations in the hands and fingers, and stimulating the dorsal caused sensations in legs and feet - Homunculus: resulting body map, shows that adjacent areas of the skin project to adjacent areas in the brain, and some areas on the skin are represented by a disproportionately large area of the brain. For example, the area devoted to the thumb is as large as the area devoted to the entire forearm - Recent research has shown the S1 is divided into four interconnected areas, with different functions. Example, area in S1 involved in perceiving touch is connected to another area that is involved with haptics (exploring objects with the hand), in addition, there are a number of homunculi both within S1 and S2 Plasticity of Cortical Body Maps: - Basic principle of cortical organization that the cortical representation of a particular function can become larger if that function is used often (experience-dependent plasticity) - Most early experiments that demonstrated this plasticity were carried out in somatosensory system - Jenkins and Merzenich (1987): - Measured cortical areas devoted to each of a monkey’s fingers and then trained monkeys to complete a task that involved the extensive use of a particular location on one finger tip - When they compared the cortical maps of the fingertip measured just before the training to the map measured after 3 months training, found the area representing the stimulated fingertip was greatly expanded after training - In most animal experiments, effect of plasticity is determined by measuring how special training affects the brain - Elbert et al (1995): measured this effect in humans by determining how training affected the brains of musicians. Consider stringed instrument players, a right handed violin player bows with right hand and uses fingers of his or her left hand to finger the strings. A result of this tactile experience is that musicians have greater than normal cortical representation for the fingers of their left hand Perceiving Details: - Example of perceiving details is provided by Braille. A braille character consists of a cell made up from 1-6 dots, different arrangements of dots and blank spaces represent letters of the alphabet, additional characters represent numbers, punctuation marks, and

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common speech sounds and words Experienced Braille readers can read at a rate of 100 words per minute, slower than average rate for visual reading which is around 250-300 words per minute The ability of Braille readers to identify patterns of small raised dots based on the sense of touch depends on tactile detail perception

Measuring Tactile Acuity: - Classic method is the two-point threshold, the minimum separation between two points on the skin that when stimulated is perceived as two points. Measured by gently touching the skin with two points, such as the points of a drawing compass, and having the person indicate whether they feel one point or two - Grating acuity is measured by pressing a grooved stimulus onto the skin and asking the person to indicate the orientation of the grating. Acuity is measured by determining the narrowest spacing for which orientation can be accurately judged - Can also be measured by pushing raised patterns such as letters onto the skin and determining the smallest sized pattern or letter that can be identified Receptor Mechanisms for Tactile Acuity: - Properties of receptors are one of the things that determines what we experience when the skin is stimulated - Merkel receptor is sensitive to details, firing of the fiber reflects the pattern of stimuli. This indicates that the firing of the Merkel receptor’s fiber signals details. For comparison, lack of match between pattern and firing with Pacinian corpuscle indicates that this receptor is not sensitive to details of patterns pushed into skin - Not surprising that there is a high density of Merkel receptors in the fingertips because the fingertips are the parts of the body that are most sensitive to details - A comparison of grating acuity on different parts of the hand shows that better acuity is associated with less spacing between Merkel receptors - But receptor spacing can’t be whole story, because although tactile acuity better on tip of index finger than little finger, spacing between Merkel receptors is same in all fingertips. This means that spacing is part of the answer, the cortex also plays a role in determining tactile acuity Cortical mechanisms for Tactile Acuity: - Just as there is a parallel between tactile acuity and receptor density, there is also a parallel between tactile acuity and the representation of the body in the brain - By comparing two-point acuity thresholds to how different parts of the body are represented in the brain we can see that regions of high acuity like fingers and lips, are represented by larger areas on the cortex - The map of the body on the brain is enlarged to provide the extra neural processing that enables us to accurately sense fine details with our fingers and other parts of body - Another way to demonstrate the connection between cortical mechanisms and acuity is to determine the receptive fields of neurons in diff parts of the cortical homunculus - In monkeys fingers, cortical neurons representing parts of the body with better acuity

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such as the fingers, have smaller receptive fields - this means that 2 points that are close together on the fingers might fall on receptive fields that don’t overlap, and so would cause neurons that are separated in the cortex to fire However, two points with same separation when applied to arm are likely to fall on receptive fields that overlap, and so could cause neurons that are not separated in cortex to fire, thus, the small receptive fields of neurons receiving signals from the fingers translates into more separation on the cortex, which enhances ability to feel 2 close together points on skin as 2 separate points

Perceiving Vibration: - Pacinian corpuscle primarily responsible for this - Recording from fibers associated with the corpuscle shows that these fibers respond poorly to slow or constant pushing, but respond well to high rates of vibration - Why? This is because the presence of the corpuscle surrounding the nerve fiber determines which pressure stimuli actually reach the fiber. The corpuscle, which consists of a series of layers, with fluid between each layer, transmits rapidly applied pressure, like vibration, to the nerve fiber, but does not transmit continuous pressure. Thus, the corpuscle causes the fiber to receive rapid changes in pressure but not to receive continuous pressure - Because corpuscle does not transmit continuous pressure to the fiber, presenting continuous pressure to the corpuscle should cause no response in the fiber. - > Lowenstein (1960) observed this - when pressure applied to corpuscle the fiber responded when pressure was first applied and when removed, but not to continuous pressure. When he dissected away the corpuscle and applied pressure directly to the fiber it fired to continuous pressure/ - > He concluded that this result that properties of the corpuscle cause the fiber to respond poorly to continuous stimulation, such as sustained pressure, but to respond well to changes in stimulation that occurs at beginning and end of a pressure stimulating or when stimulation is changing rapidly, as occurs in vibration Perceiving Texture: - Surface texture is the physical texture of a surface created by peaks and valleys - Visual inspection can be a poor way of determining surface texture, because seeing texture depends on the light-dark pattern determined by the angle of illumination - Thus, although visually perceived texture of two sides can look very different, moving the fingers across reveals that the texture of the two surfaces is the same - Touch, which involves direct contact with the surface, therefore provides a more accurate assessment of surface texture than vision. However, this doesn’t mean that scanning a surface with fingers always results in an accurate indication of surface texture - In 1925, Katz proposed that our perception of texture depends on both spatial cues and temporal cues. - Spatial cues are provided by relatively large surface elements, such as bumps and grooves, that can be felt both when the skin moves across the surface elements and

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when it is pressed into elements - these cues result in feeling different shapes, sizes, and distributions of these surface elements Example of spatial cues is perceiving a coarse texture such as Braille dots or texture we feel when touching teeth of a comb Temporal cues occur when the skin moves across a textured surface like fine sandpaper. This type of cue provides information in the form of vibrations that occur as a result of the movement over the surface - temporal cues are responsible for our perception of fine texture that cannot be detected unless the fingers are moving across the surface Research on perception has until recently focussed on spatial cues Hollins et al (2000, 2001, 2002): show that temporal cues are responsible for our perception of fine textures - called Katz theory that there are two types of receptors involved in texture perception the duplex theory of texture perception Hollins and Risner (2000): presented evidence for the role of temporal cues by showing that when participants touch surfaces without moving their fingers and judge roughness using the procedure of magnitude estimation, they sense little difference between two fine textures. However, when Ps are allowed to move their fingers across the surface, they are able to detect the difference. Thus the movement, which generates vibration as the skin scans the surface, makes it possible to sense the roughness of fine surfaces Hollins et al (2001): used selective adaptation procedure by presenting two adaptation conditions. > First condition was 10 Hz (10 vibrations per second) adaptation, in which skin was vibrated with a 10 Hz stimulus for 6 mins. This frequency of adaptation was picked to adapt the Meissner corpuscle, which responds to low frequencies > The second condition was a 250 Hz adaptation, this frequency was picked to adapt Pacinian corpuscle, which responds to high frequencies > Following each adaptation, Ps ran their fingers over 2 fine textures - a standard texture and a test texture. Ps task was to indicate which texture was finer > Because there were 2 surfaces, chance performance would be 50%, but the results indicate that Ps could tell the diff between the two textures when they hadnt been adapted or had received the 10 Hz adaptation > After they had been adapted to the 250 Hz, they were unable to tell the diff between 2 fine textures >> Thus, adapting the Pacinian corpuscle receptor, which is responsible for perceiving vibration, eliminates the ability to sense fine textures by moving the fingers over the surface. These results support the duplex theory of perception - that the perception of coarse textures is determined by spatial cues and of fine textures by temporal cues

Perceiving Objects: - Active touch: touch in which a person actively explores an object, usually with fingers and hands - Passive touch: occurs when touch stimuli are applied to the skin, as when two points are pushed onto the skin to determine the two-point threshold - Haptic Perception: perception in which 3D objects are explored with the fingers and hand

Identifying Objects by Haptic Exploration: - Haptic perception provides a good example of a situation in which a number of systems are interacting with each other - (1) The sensory system, involved in detecting cutaneous sensations such as touch, temperature and texture and the movements and positions of your fingers and hands - (2) The motor system, involved in moving your fingers and hands - (3) The cognitive system, which involved in thinking about the info provided by sensory and motor systems - These processes working together create an experience of active touch that is quite different from the experience of passive touch - For passive touch we experience stimulation of the skin, for active touch we experience the objects we are touching - Klatzky et al (1985): people can accurately identify most common objects within 1 or 2 seconds - Lederman and Klatzky (1987, 1990): observed Ps hand movements as they made these identifications, they found that people use a number of distinctive movements which the researchers called exploratory procedures (EPs) and the types of EPs used depend on the object qualities the Ps are asked to judge - > Example: people use mainly lateral motion and contour following to judge texture, and they use enclosure and contour following to judge exact shape The Physiology of Tactile Object Perception: - What is happening physiologically as we explore an object? - In order for brain to control every day tasks, such as screwing a lid on a bottle, it needs to have access to info about the size and contour of the lid, and the amount of force needed to grasp the lid - this info is provided by receptors within the body that indicate the position of the joints and by mechanoreceptors in the skin that indicate the textures and contours of the lid - The info for indicating the contours of the lid is signaled by the pattern of firing of a large number of mechanoreceptors - Information on the fingertip corresponds to the EP of pressure - Other EPs such as enclosing the shape with the hand contribute to our...